Art McDonald

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ORAL HISTORIES

Image courtesy of A. Mahmoud

Interviewed by
David Zierler
Interview date
Location
Video conference
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Interview of Art McDonald by David Zierler on April 16, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
www.aip.org/history-programs/niels-bohr-library/oral-histories/47467

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Abstract

In this interview, Art McDonald discusses: careerlong work around tests of the Standard Model; Sudbury Neutrino Observatory (SNO) measurement of neutrino properties that fall outside the original Standard Model; childhood and family history in Canada; master’s work on positron annihilation; time at the Kellogg Laboratory at Caltech with William Fowler; early origins of and motivations behind the SNO project; postdoc at Chalk River Nuclear Laboratory using a particle accelerator to continue experimental work on fundamental symmetries; history of Chalk River; collaborations with George Ewan; collaborations with Hamish Robertson studying the measurement and production of lithium-6; work at Princeton with Will Happer using lasers to polarize nuclei; building a continuous laser beam at Chalk River; work on parity violation in nuclei; work on the Princeton cyclotron; overseeing Kevin Coulter’s thesis project, the first use of laser-induced-spin-polarization of helium; polarized Helium-3’s current uses; technical challenges of building SNO and the transition from construction to operations; SNO and the solar neutrino problem; Herb Chen’s involvement with the design of SNO; SNOLAB; decision take position at Queen’s University; early published findings from SNO; comparison of Super-Kamiokande and SNO experiments; impact of SNO results on the understanding of the Standard Model; winning the Nobel Prize for solving the solar neutrino problem, observing that solar electron neutrinos were oscillating into muon and tau neutrinos; current work with the DarkSide-20k collaboration and how that work led to an open-source ventilator project in the midst of the COVID-19 pandemic; current SNOLAB DEAP experiment using liquid argon to attempt to detect dark matter particles. Toward the end of the interview, McDonald reflects on interrelatedness across disciplines within physics, and his ongoing curiosity in searches for dark matter and neutrino-less double beta decay. 

Transcript

Zierler:

OK. This is David Zierler, oral historian for the American Institute of Physics. It is April 16th, 2021. It’s my great honor to be here with Professor Arthur B. McDonald. Art, it is great to see you. Thank you so much for joining me.

McDonald:

Really, my pleasure.

Zierler:

Art, to start would you please tell me your titles and institutional affiliations? And you’ll note that I pluralized that because I know you have more than one.

McDonald:

Well, I’m the Gordon and Patricia Gray Chair in Particle Astrophysics emeritus at Queen’s University in Kingston, Ontario, Canada. I’m still the director of the Sudbury Neutrino Observatory Collaboration because we continue to do some analysis of the data that we stopped collecting at the end of 2006. I still continue as the scientific director of that activity and we have published 5 new papers in the past several years.

Zierler:

Art, tell me about Gordon and Patricia Gray. Who are or were they and what is their interest in connection with your research?

McDonald:

Gordon is originally from Sudbury as it turns out. He is an alumnus of Queen’s University who is a very successful businessman in Toronto. He had heard about our research a number of years ago, certainly before the Nobel Prize award. He brought his family to Sudbury and we had a tour together of the underground facility. They were very impressed, thought it was wonderful research. They were pleased to see something of this nature happening in the city of Sudbury which has a reputation of being primarily an industrial town, although they have a very high-class science center there as well. So, Gordon and Patricia contacted Queen’s and offered to endow a chair and I was the first occupant of that chair. It was a godsend for me in the sense of helping me to have some teaching relief to be able to continue with research activities at a remote location. The Gray family are wonderful people and it was a pleasure to interact with them through this connection.

Zierler:

Art, we’ll talk later in our discussion about how you got involved with this incredible ventilator project in 2020. But I’d like to ask more broadly as a scientist with collaborators and partners all over the world, in this past year plus working in the pandemic, in what ways has your science suffered and it what ways has it been enhanced by remote working, work from home, not being physically present in the places you normally are?

McDonald:

Well, I received the Nobel Prize in 2015 and that immediately meant a tremendous number of requests for participation in many things all over the world. So, having a year when I’m not travelling is not as bad as it might’ve been for some people.

Zierler:

[laugh]

McDonald:

This is particularly true in the field of particle physics where of course you know, Tim Berners-Lee developed the WWW format at CERN, initially for particle physicists anticipating the need for communication in more than just email. And of course, these days with Zoom or equivalent it’s been possible to continue quite effectively on research over the past year. Where it’s been hampered is where you are attempting to do things on the ground where people are doing installation or where people are doing development work in the laboratory. Some of our laboratories internationally have been able to continue to function. In fact, at SNOLAB in Sudbury many of the experiments have continued operation at a reduced rate but some have not. So, it’s thrown significant delays into a number of these major projects. But in terms of interaction with colleagues planning for the future, discussing and modifying designs, and things of that nature, we’re used to working in this sort of remote environment. Right now a major project I’m working on is the so-called DarkSide-20k project which is to be sited in the underground laboratory at Gran Sasso, in Italy. That’s over 400 scientists literally from around the world with different components of that project coming from around the world. We have been able to proceed with detailed designs and plans for construction in spite of the COVID situation. When you want to talk about ventilators, many of the same people were involved in that activity.

Zierler:

Right. Art, a question that’s very contemporary. We have some wobbling muons at Fermilab that are causing quite a bit of excitement right now. What is your reaction to this and what do you think about the prospects that we actually might be moving beyond the Standard Model?

McDonald:

Well, I think it’s very interesting and the work that’s been done is obviously a very high caliber. It’s still two 3.5 sigma measurements so one needs to be cautious. But the confirmation in the second measurement with a prospect now for significantly higher statistics in future gives you the opportunity to consider that perhaps there is something of a very fundamental nature that is going beyond the Standard Model. In fact, a large fraction of my career I have been involved in what tends to be referred to as tests of fundamental symmetries or tests of the Standard Model in various ways. In fact, the work I was doing which arose from my thesis work at Caltech was studies of isospin symmetry in nuclei, which was an attempt to understand whether there was anything different happening with the electromagnetic interaction in nuclei compared to the effects of the electromagnetic interaction as we normally knew it. The answer was nothing other than the standard electromagnetic interaction to the degree we could test it.

That morphed into work with studies of parity violation in nuclei. Observation of parity violation was a way of identifying the Weak Interaction (the only interaction that violates parity) in the presence of the much stronger Strong Interaction. It was possible to do experiments studying the Weak Interaction and still make valuable contributions, even though you’re dealing with effects at levels of a part in 106 in some cases. The objective of those measurements of the Weak Interaction was again a test of the Standard Model. There’s a mechanism called the GIM mechanism, Glashow-Iliopoulos-Maiani, that restricts the— (I’m now contemplating whether to say “z” or “zed”. I’ve been such a cross border Canada-U.S. scientist that I sometimes use one and sometimes the other. But I’m in Canada right now, so) Zed exchange to being between up and down quarks as opposed to the strange or charm quarks. That was what we were studying in nuclei and we were able to make a whole series of measurements that indicated that there was no substantial deviation from the Standard Model for Z exchange between up and down quarks.

The measurements we made with the Sudbury Neutrino Observatory were tests of neutrino properties where we observed changes in flavor of the electron neutrinos from the Sun. And that is outside of the predictions of the Standard Model. It also enables you to infer that the neutrinos have a finite mass which is also outside the Standard Model as originally formulated. Basically you end up with the conclusion that the neutrinos have a mass generation mechanism that is different than the typical Higgs Boson mechanism that applies to other particles. And so essentially my whole career has been tests of the Standard or other models looking for variations and in some cases very small variations. Therefore, I appreciate the work that’s being done in the g-2 experiment. I also had a personal interest through one of the collaborators, Tim Chupp. I worked with him when he was a junior faculty member at Princeton. At that time we were developing the polarization of helium-3 for studying fundamental symmetries. With his insight, helium-3 has been used in the g-2 experiment for one of the primary magnetic calibrations. It was very nice to see a former colleague actively involved in this beautiful experiment and bringing something to it with which I had familiarity from past work. So, a long-winded answer, but it gives you an answer as to why I appreciate work that’s done to look for stuff that’s in the cracks that then gives you a possible insight into something beyond what we already understand. Now I’m into dark matter which is clearly beyond the Standard Model so, I’ve always been doing this.

Zierler:

[laugh] Relatedly, Art, a very broad historical question given your long research interests with the Standard Model. When in your career did you feel like the Standard Model was still being built and perfected? And when in your career did you feel that the time was right to start looking as you say, between the cracks for evidence that there was physics beyond the Standard Model?

McDonald:

So, just to show you how far I go back. I was a student at Dalhousie in Nova Scotia; I started there as an undergraduate in 1960. And then in ’64 graduated and spent a year doing a master’s degree there. We had a whaling dory that we sailed in Halifax Harbor which we named the “The Quark,” which was a term that was just a couple of years old at that point.

Zierler:

Yes.

McDonald:

“Quiescent arc” was the nickname for it because it wasn’t much of a boat. On the other hand, by the end of the ‘60s there were fairly strong indications that the Standard Model was valid. Into the ‘70s, experiments with neutral current measurements and so on around the world gave considerably more credence to it. So, I would say that it began to be well accepted in the late ‘70s when I was working on parity violation experiments. There was a lot of other work being done in atomic physics and a variety of other things to test it and essentially the Standard Model was being taken as standard. What you were looking for was deviations from it. And so, I would say it was late ‘70s that acceptance came and maybe before that for people who were involved in some of the other direct tests of neutral current processes.

Zierler:

Art, let’s take it all the way back to the beginning. Let’s start first with your parents. Tell me a little bit about them and where they’re from.

McDonald:

Well, my family has basically lived in the Maritime provinces, so-called, of Canada. Nova Scotia primarily, but also Prince Edward Island, New Brunswick, Newfoundland, for about six or seven generations before me. Both sides of my family. My mother’s name was Deroche. Some of my French Canadian relatives were expelled to Louisiana in the so-called expulsion of the Acadians in 1755. My father’s family, the McDonald side of the family, came to Nova Scotia in the 1800s. They were ship builders. Cape Breton Island, the part of Nova Scotia where I lived, is a very Scottish area. My wife’s name was Macdonald before we were married. Her mother’s name was MacDonald with a capital D. They’re all different names among the Scots. So, my family goes back a long way there.

Interestingly, there’s another branch of my mother’s family that came from Sardinia and now for the Dark Matter experiment that we’re doing in Italy, one of the places which is a focal point for part of our work is Sardinia. I was able to go back and trace some roots there as well.

My father was an accountant and went on to be business manager of the local newspaper. My mother in those days was a housewife. In terms of the social mores of the day, she had been a successful secretary before being married but it was expected that she would give up her job to be in the home. She and my father later developed a subdivision in Sydney and my mother was the project manager in much of the work. She was a very skilled individual as well. They both passed away a few years ago, in their 90s. Neither of them went to university. In fact, very few of their generation went to university. My father was overseas during the Second World War. For the first three years of my life he was there. A decorated soldier. And a civic leader in later years as a city councillor. Interestingly, my great-grandfather had two university degrees. One in theology and one in medicine. That part of the family, surname Bruce, came to Nova Scotia from Edinburgh around 1900. So, there was appreciation for university education and my parents always were very encouraging with respect to school and potentially university. But we didn’t have lots of money. So, everyone worked hard for what they were doing.

Zierler:

Did your father ever talk about his military service? What he experienced during the war and after?

McDonald:

In later years. For the first while after he came back he was very quiet on the subject. Twenty years later he was willing to talk about it. It was something he did out of a sense of duty and he certainly didn’t enjoy it. So I did not learn much about his experiences when I lived at home although later I had some conversations with him about it.

Zierler:

Art, were you interested in the natural world as a young boy? Perhaps even before you knew this was science?

McDonald:

We lived on the outskirts of town. I spent a lot of time in the woods so to speak, with friends. We made the mistake one time of observing in this wonderful location we discovered with maple trees about two miles from our homes that we could tap the trees for maple syrup. We did that and carried the sap back. What we didn’t take into account is the factor of 50 associated with boiling it down to make maple syrup.

Zierler:

[laugh]

McDonald:

It was not repeated the next year. So, yes. I was interested in the natural world, but more as a playground than as a thing to study. I enjoyed, particularly in high school, mathematics. I had a wonderful high school math teacher named Bob Chafe who really interested us in the subject. He really went out of his way with extra classes after school and other activities to really get us involved. The overall graduating class was about 120 students, but there were 35 of us in the science and math concentrated class. Three Ph.D.’s came from that group of 35. Two in mathematics and me in physics. Sydney was an industrial town and that’s not the norm. When I hear other people talking about what it is that gets them involved and what they ultimately end up doing, it’s very often a very highly qualified and a strong mentor in their early years. And that was the case for me. So, I went to university knowing that I wanted to do something where I could use math. I tried several sciences in my first year of university and physics just worked for me. I loved it and I was good at it. And I often get asked these days by young people, “How do you choose a career?” And I say, “Well, think of the set of things that you would enjoy doing when you wake up in the morning. And then pick a few of them and try them and see which one you’re good at. And then combine the two.” And that’s essentially what I did and ended up in physics.

Zierler:

Art, was the prospect of going to a larger university farther away from home, was that within the realm of possibility when you were a high school senior?

McDonald:

Well, effectively I did that. I mean, I went to the largest university in the Maritime provinces. Going to something like you know, an Ivy League school or—

Zierler:

Or even Toronto or McGill. Or something like that.

McDonald:

Well, when I was in high school I was always a bridesmaid rather than a bride. [laugh] I was fourth or fifth in my class sort of thing rather than the leader in academics. But I had a good social life and enjoyed the balance at that time. It wasn’t ‘til I got to university that I began to lead classes. When I finished university I actually had the highest marks in my whole graduating class. And that was as much a surprise to me as it was to anybody else. So, the thought of going to Toronto or McGill wasn’t in my mind at all in high school. Dalhousie is well-noted. It’s in the top six or seven in Canada. So, I was going to a good school. But I didn’t think of going further. Besides, I had a girlfriend. [laugh] Later my wife. And she was going to be staying in the Maritimes, so you know, there’s all kinds of ways you get motivated for various things.

Zierler:

Art, what were some of the most exciting things going on in physics from your vantage point as an undergraduate?

McDonald:

Oh, at that point you know, we were very interested in quarks and the eightfold way as it was called. Understanding positron annihilation was an active area of study in the department as was low temperature physics. I had summer jobs where I was working on things that were early days for technology transfer from an academic institution. We measured gravity to a high degree of accuracy all over the roads of Nova Scotia in the summer with a job I had. I was amazed to find that if you measure gravity to a part in a million you can use it very effectively. We detected a gypsum mine that turned out to be very profitable thereafter. So, it was some combination of basic science and applied science that I was exposed to.

For my master’s degree I worked on positron annihilation. These measurements were a tour de force technically in those days. I was measuring the lifetimes of positrons in solids. We were measuring things that were at the nanosecond level looking for the effects on lifetimes in solids in various circumstances. This was 10 years before positron emission tomography became a useful medical tool. We discovered that defects in materials have an observable influence on lifetimes in metals. That result went on to be one of my most cited papers other than SNO in all of the work that I’ve done. It was at the cutting edge but it was a small tabletop experiment, using radioactive positron sources. We had to do it carefully with the type of electronics that we had in those days, which was just at the transition from tubes to transistors in instruments like the multichannel analyzers we were using. Handheld calculators were not introduced until around 1969. In 1964 they had lifted the latest IBM computer, the first one on campus, through the roof of the physics building.

Zierler:

[laugh]

McDonald:

It had nowhere near the capability of this cell phone in my hand. The way Google or Zoom handles things was unknown, but one thing didn’t change. They said to several of us senior undergraduates, “Learn FORTRAN and teach the professors (and everyone else).”

Zierler:

[laugh]

McDonald:

Nobody there had ever programmed a computer at that point. So, actually a friend of mine and I ran a course on FORTRAN after we learned how to use it. That still happens effectively. The young people are the ones that are the experts in computing and I learn from them every day.

Zierler:

Art, were there any professors at Dalhousie that you considered mentors or who were particularly formative in your intellectual development?

McDonald:

There were a number. My first-year professor Ernie Guptill is the one who really got me interested in the subject of physics. And my Master’s thesis supervisor, Innes Mackenzie, on the subject of positron annihilation. He was a very knowledgeable individual and also a person who taught me how to do experimental physics. I also benefitted from the summer work with Professor Blanchard in applied physics measuring gravity for mineral prospecting.

I took a master’s degree in part to flesh out my education a little bit. I did want to go to a more substantial university for my Ph D, particularly in the U.S. or the UK. I was able to do this Master’s work in a year and although that nearly killed me, it helped me get into Caltech. Actually, my friend, Peter Nicholson and I, explored a number of Ivy League schools on the East Coast. We talked the department chair, Ernie Guptill, into supporting us for a trip down the East Coast. He sent letters to all the department chairs saying, “These students want to survey your department.” We made a deal that we would go and get data on these various universities and present it to everyone else when we got back. I can still remember talking to Bob Dicke at Princeton who was an icon in the field. I was very pleased to have him as a colleague when I became a professor at Princeton almost 20 years later. We talked to a number of excellent scientists and in fact, both of us got offers from one or two Ivy League schools when we applied. However, Peter went to Stanford to study Operations Research and I went to Caltech because California just looked like an incredible place and it was. Caltech turned out to be a really good choice for me.

Zierler:

Now did you visit Pasadena before you actually enrolled?

McDonald:

That’s a long way. [laugh]

Zierler:

[laugh]

McDonald:

At one point, I guess when I finished grad school at Caltech, we drove back to Canada. I was going to a position at Chalk River but we went on to Nova Scotia first. I think it was 5,300 road miles for that trip. So, no. In those days I didn’t have a chance to visit Caltech before deciding.

Zierler:

Did you have an idea of who you wanted to work with at Caltech or you were wide open to that?

McDonald:

Charlie Barnes, who was my supervisor, was a Canadian who had been at Caltech for a number of years. My supervisor at Dalhousie and he knew each other. I think they’d worked together briefly many years before. I did have fun phone conversations with Charlie when I was making up my mind. And he was a great supervisor. It was a pleasure to work with him.

Zierler:

What was Barnes working on when you first connected with him?

McDonald:

Well, he was interested in fundamental symmetries. And the topics that I ended up working on for my thesis were related to this question of isospin symmetry in nuclei. Charlie had been also actively involved in measurements relating to the Weak Interaction starting back in the late 1950’s when the Weak Interaction was being studied intensely and parity violation was first observed. I went there in 1965 and worked in the Kellogg Laboratory, headed by Willy Fowler. Willy shared a Nobel Prize in 1983, for having worked out the nuclear reactions that resulted in synthesis of the elements in stars like the Sun and others. There was a lot of work of that nature going on in Kellogg when I was there. Astrophysics was certainly a part of the experimental and theoretical work there. So, I was conscious of that and I even worked on an experiment or two in nuclear physics that related to nuclear reactions relevant to astrophysics.

Ironically, Ray Davis used to visit Kellogg in the summer. He was working with John Bahcall, who was a junior faculty member, on the details of the detection of neutrinos from the Sun with a chlorine-loaded perchloroethylene detector underground in South Dakota. And in fact, one of my grad student colleagues was measuring the energy levels of chlorine-37 to test the theory that was being used to calculate what the detection sensitivity to neutrinos from the Sun would be. Davis’s experiment started underground in South Dakota in 1968—I was at Caltech from ’65 to ‘69—and it was quite apparent from the beginning that the numbers of electron neutrinos that were being detected were smaller by a factor of three or so than the calculations of John Bahcall. John’s calculations were very broad ranging: from the nuclear physics for detecting neutrinos to the modeling of the Sun and how many neutrinos are being produced by the nuclear reactions in its core.

The discrepancy in Davis’ measurements compared to Bahcall’s calculations were a strong motivation for the Sudbury Neutrino Observatory project in 1984. So, the seeds of what I eventually worked on were being sown at Caltech when I was there. Some of the people who worked on SNO, were graduates of Caltech or people who worked on a combination of nuclear and astrophysics projects. I had collaborated with some of them during the ‘70s.

The milieu of what was being discussed in the Kellogg Lab and the nature of the people who were there lent itself very well to providing me with some knowledge about both fundamental physics and astrophysics. There were also other interesting people there at the time. Kip Thorne was a junior faculty member. Dave Schramm was a graduate student in Kellogg when I was there. He became one of the premier astrophysicists, but passed away at far too young an age. There were others that I worked with on SNO such as Bob Stokstad, Hay Boon Mak and Mike Lowry. So, that’s how I got into some combination of nuclear physics, fundamental symmetries and astrophysics. That is what SNO eventually turned out to be.

Zierler:

Art, a broad question on your graduate research. Looking back was your sense regarding the interplay of observation, experimentation, and theory, was experimentation more driving theory at this point or would you say that theory was more driving experimentation?

McDonald:

Well, it depends on the specific topic. You asked about grad work so let me talk about the 1960s.

Zierler:

Yeah.

McDonald:

Theory and experiment were very much coupled in Bahcall and Davis’s work, but also in Fowler’s work on nuclear astrophysics. Fowler was, well, let’s put it this way: Theoretically, he was an experimental physicist.

Zierler:

[laugh]

McDonald:

In point of fact his theoretical capability was very strong as well.

Zierler:

Yeah, yeah.

McDonald:

Experiment was very much coupled to understanding the theory of nucleosynthesis and stars. And so, it really was a two-way street. In addition to that we were studying isospin symmetry for which there were theoretical calculations happening within nuclear theory that related to this particular symmetry. But it was also being driven by access to some very good data on multiplets of higher isospin levels whose energy level and ultimately whose decay properties—which is the two things we ended up studying—related to how the electromagnetic interaction behaved inside a nucleus using standard ways of calculating it. So, we were in close contact with theorists as well as doing experiments. It was a wonderful time for nuclear physics actually. We had a FN Tandem accelerator which was readily available to us in the basement of Kellogg Lab. And we had as much beam time on that machine as our wives could stand, basically. [laugh]

Zierler:

[laugh]

McDonald:

We spent a lot of time collecting data and publishing many papers. It was a fun place to be. Willy Fowler had a very bubbly, friendly personality. Seminars were on a Friday night at 7:30 p.m, usually followed by a party at one of the professor’s houses with the so-called Kellogg band with Charlie Barnes at the piano. A number of my fellow grad students who were more musically gifted than me played various other instruments. Everybody knew everybody else. It was a fun time to be doing science.

I was very interested to observe when I was preparing my own written copy of my Nobel lecture, to look up Willy’s and find that he had a motto on the front page which was, “Ad astra per aspera et per ludum.” If I’ve got the Latin correct it means: “To the stars with hard work and fun.” That was the attitude. You could have fun doing physics. And that’s something that I have always felt. I’ve always enjoyed it. I’ve always tried to inject that into collaborations that I worked on. I’ve always tried to have friendships as a central element of collaborating. Always tried to inject a little bit of humor to break up any arguments that had gone on a little too long. But generally, also socially try to have people enjoy each other’s company, which was the case at Kellogg. It was a great place to do science. Lots of access to equipment and really great scientists to interact with. I took my graduate quantum mechanics course from Richard Feynman, for example. Rudolf Mössbauer used to come there for three months every year. I took a course from him and interacted with many Nobel Prize winners while I was there. So, these were very high-quality scientists in the department in general. It was a fantastic place to be a grad student.

Zierler:

Art, what was Barnes’ style as a graduate advisor? Did you work closely with him or did he more give you a problem and you worked on it mostly on your own?

McDonald:

He was more integrated into the work we were doing. We’d be in the laboratory in the basement every day for a four-day run. He’d be there every day. He didn’t do overnight runs because he had to teach and had other students. But he would drop in and discuss what we’re seeing with us as we were doing it. There was always, “Hey. Maybe we’d better see what Charlie thinks about this.” at a given point, and that was great. He was a very receptive individual who was very much involved in the work and also very interactive with all his grad students of which there were a number at the time. One of them with whom I worked very closely was Eric Adelberger, with whom I later worked extensively on parity violation experiments. In that process when you’re looking for part in a thousand or part in a million effects and have to develop processes for trying to extract the thing you’re looking for from a large background. Typically what you do is arrange for the thing you’re looking for to be modulated and try to arrange for the background not to be, so that you can then extract the signal.

Eric went on many orders of magnitude beyond what we started in parity violation to apply this to measurements of the gravitational interaction. He did absolutely beautiful experiments culminating this year in the Breakthrough Prize in Fundamental Physics for this work by him and his group. One of the things that was being looked for was deviations from the 1/r2 law which they studied in exquisite measurements down to about 50 microns or so separation. Eric was a couple of years ahead of me and I benefitted greatly from interactions with him as well as Charlie. So, it was a good place to be.

Zierler:

Art, what was the process whereby you determined you had enough to write up the thesis and defend it?

McDonald:

Oh, it was four years. [laugh] It was time. And Charlie probably said, “Oh, I think you should write this up now.” [laugh] We had published a number of papers already and he probably would’ve said, “This set of measurements that you did,” which were a set of measurements for nuclei that were relevant to this question of isospin symmetry, “were appropriate to write up as a thesis.” Which I did. So, I went there in the summer of ’65 and I graduated effectively in the summer of ’69. My degree is probably listed as 1970, but I left Caltech in the summer of ’69. So about four years.

Zierler:

By the tail end of your time at Caltech, had the antiwar movement reached the campus at Caltech?

McDonald:

It had certainly reached the campus at Berkeley.

Zierler:

Yes! Yes, of course.

McDonald:

But Caltech, first of all, the grad students at Caltech tended to be so busy with their scientific work that there wasn’t a lot of political activity by the graduate students. But the undergraduates seemed to be somewhat more concerned about—oh, I can never remember. Is it Star Wars or Star Trek that was the TV show that existed at that time and was about to be taken off?

Zierler:

That was Star Trek.

McDonald:

Star Trek. OK. That got protested strongly.

Zierler:

[laugh]

McDonald:

The Berkeley like activity was not a substantial part of the Caltech environment. Big contrast to what was happening at Berkeley.

Zierler:

As a Canadian did you feel more aloof from these very American issues?

McDonald:

I had to be very careful because Canada in general had the attitude towards the Vietnam War which eventually emerged from the activities at Berkeley to the many protest movements that were antiwar. And so, I actually came to Caltech with a bias from my Canadian experience. Canada was not involved in the Vietnam War, but had been in the Korean War. I followed it very carefully—but I tended not to be actively involved in such discussions.

Zierler:

Besides Barnes of course, who else was on your thesis committee?

McDonald:

I believe Steven Frautschi but I’m finding it hard to remember the others.

Zierler:

Any memorable exchanges during the oral defense?

McDonald:

I can’t remember exactly the subjects, but it was a tough grilling where they got me at the blackboard and said, “OK. Derive the following.” And I mean, you really had to be on your mark on that exam. You had to have prepared yourself in all of the courses that you had taken. They were solid questions, not unfair questions. But a real test of your general physics ability and not just what you’d written in your thesis.

Zierler:

Meaning this was not just a formality. You really had to perform.

McDonald:

Oh, for sure. Yeah. I mean, and it wasn’t just recite what you’ve done in your thesis and let me ask some questions about it. You were tested as a person who has learned to be a physicist when you finish and could potentially be a professor, eventually. Did you actually pay attention in your courses? It was a tough exam.

Zierler:

After Caltech, what opportunities were available to you? What was compelling to do next?

McDonald:

Well, I had two offers that I took seriously and investigated. One was the one that I eventually accepted, which was a Post Doc at Chalk River Nuclear Laboratory. That was in fact a continuation of my Caltech work in nuclear physics. It was a type of accelerator I’d been using at Caltech and it was an excellent group of people. I had ready-made colleagues to continue with the work I had been doing. They appreciated the work I had been doing on fundamental symmetries and I also collaborated with people from other universities. Such basic physics was somewhat unusual there because Chalk River primarily was a nuclear reactor development site. However, basic nuclear science had been respected and carried out there right from its start during World War II. It was originally Canada’s equivalent to Los Alamos—Chalk River basically was formed by the British group rather than the U.S. group. Whereas there was never a bomb developed at Chalk River, ZEEP, the first reactor after the Chicago Pile was operated there. Chalk River did go on to produce plutonium that got used in the US to produce atomic weapons, but their primary mission became the production of CANDU reactors for domestic power. CANDU was a heavy-water-based reactor using natural, unenriched uranium. Canada never developed enrichment facilities because they were not needed for CANDU reactors. They did develop facilities to extract heavy water and that eventually became the basis for the SNO experiment. From the beginning, the management recognized the value of the basic science that underlay what they were doing. Nuclear physics was still in its formative days in the early 1970s when I was there. There was quite an illustrious group of scientists who had been there in the early days and there still was a very solid group when I went there. So, it was a good place for experiments and I once again had lots of beam time. Lots of opportunity to do research. I could pursue essentially what I wanted to. I had the same creative freedom that I would have as a university professor.

The other place that I interviewed was Bell Labs, actually in Murray Hill, NJ. A lot of the work that was being done there was quite secretive, and I was a Canadian of course. When I was shown around the person escorting me had to come right into the men’s room with me to be sure I didn’t do anything nefarious while I was there. And they said, “We’ll offer you a position. We’re working on these things that we think have a lot of promise. They’re called LEDs.” [laugh] So, I supposed I could’ve had a career at Bell Labs that would’ve been quite interesting, but I was somewhat turned off by the whole security element of it.

To some degrees I was following my nose by going to Chalk River. But it enabled me to do quite interesting further experiments on fundamental symmetries. I regard my time at Chalk River as a wonderful opportunity to develop some credentials as an experimental physicist. I didn’t have to teach. I had lots of beam time. Eventually I was offered a tenured position at Princeton, and ended up in the senior faculty meeting every two weeks where they were considering tenure for the junior faculty members who had had to develop something fantastic while having all the teaching responsibilities and so on, in a six year period. I had 12 years and I realized the advantages that I had gotten at Chalk River from a professional career perspective as well. At the time the Princeton physics department had only 50% of the department tenured. The rate of success for junior faculty members was roughly one in six which was justified by saying two things. One, everybody knows that when they sign up to work and besides, it’s not very different for other Ivy League schools. And secondly, you will be able to get a good position somewhere else even if you don’t get tenured at Princeton as long as you do a good job during your time at Princeton. I always recognized the advantage I had of working at Chalk River through the 1970s. I was working on things that were of interest to be done at the cyclotron that was in the basement of the physics department at Jadwin Hall in Princeton. And so, I regard myself as very lucky as having in some sense done the right thing at various points in my career to have the opportunity to move on. And that’s kind of a commentary on how I got to Princeton.

Zierler:

Art, was your initial appointment at Chalk River as a postdoc or you were a full-time staff member initially?

McDonald:

My initial appointment at Chalk River was in fact as a postdoc. And ironically, a staff position opened up the following year when a fellow physicist by the name of George Ewan left Chalk River to go to Queen’s University. And George was a wonderful physicist. He has an IEEE Award for being the first person to use a lithium-drifted germanium detector in a nuclear physics experiment. Those detectors revolutionized many things. Nuclear science for one, but medicine as well, trace element analysis and so on. He moved on to Queen’s and that opened up the position for me at Chalk River. We continued to collaborate on experiments thereafter. The reason it’s ironic relates to the future and the SNO project. In 1984 George was one of the two original co-spokesperson for the project. I was involved from the very beginning along with the other 15 on the first design paper, but George and Herb Chen were the Canadian and US co-spokespersons. It’s ironic because when I was hired to Queen’s after my time at Princeton, it was to take a faculty position at Queen’s because George was about to retire. There was a mandatory 65 retirement age at that point. George was the Canadian co-spokesman and I was the US co-spokesman. The. SNO project had not been approved, but they really needed somebody with international experience who also knew the Canadian landscape to come in and take the overall leadership as we approached the final approval stage. They chose me at Queen’s and the collaboration supported me as the overall SNO Project Director. And so, I often say, “If you follow George Ewan around you can do pretty well in your career.”

Zierler:

Art, what was your first project at Chalk River?

McDonald:

Interestingly, other than science, one of the first things was a study which I think was entitled Nuclear Physics at Chalk River in the 1970s to which we later added and beyond. John Hardy, Doug Milton, the department at the time and I edited this study about what the scientific prospects would be if you were able to add on to the accelerator that existed there which was an MP Tandem Van de Graaff. That study was the basis eventually for a superconducting cyclotron that was built at Chalk River and operated starting in the 1980s through the 1990s to do nuclear physics. Much of what was being done was contained within the document that we prepared at the time.

In terms of the measurements that we made on the existing accelerator, I can’t remember which was the first one. I’d have to go back and look at my publication list. I had my own initiated projects and I had ones where I participated as a colleague on ones that were initiated by others. Mine as I say, were aimed at the direction of further measurements of higher isospin levels and also eventually parity violation. I do remember one significant measurement made by Hamish Robertson and myself together with other team members that was instigated by Hamish Robertson. Hamish became eventually the U.S. co-spokesman for the SNO project. In fact, many of my colleagues on the SNO project are people that I had worked with in one way or another before and had good associations and that’s how we built up the collaboration. A number of others that I had not worked with previously, but who were very interested and very valuable came in as well.

Hamish had an idea for an experiment at Chalk River in the mid-70’s that used existing equipment at Chalk River and accomplished two things which to some degree combined the things that we were both interested in at the time. One was a parity violation measurement in lithium-6 which used the Weak Interaction to study neutral currents exchanged between quarks in nuclei. And the other element was a measurement of a major reaction that would have to take place in the original Big Bang involving the production of lithium-6, if the lithium-6 that we now observe was produced there. It turned out that it wasn’t strong enough for that to be the case. And therefore, when you try to use nucleosynthesis in the Big Bang to constrain how the universe evolved back then, you couldn’t use the lithium-6 abundance because it’s dominated by lithium-6 produced by cosmic rays or other than in the primordial process. It was a wonderful experiment. The other person I worked with on the experiment was Peggy Dyer who was a colleague of mine in Kellogg and another student of Charlie Barnes. Shortly thereafter she became Hamish Robertson’s wife. So, they are colleagues for almost all my scientific lifetime. Those were interesting days and that was a very solid experiment at the time.

Zierler:

What was the budgetary environment like at Chalk River? Did the Canadian government support it? You had access to really good instrumentation?

McDonald:

Yeah. It was pretty good in those days and that was another excellent thing. I didn’t have to write grants or anything like that. That was handled by the branch head and the management at Chalk River. And so, within reason we were able to have the latest in equipment and do the latest in experiments. We had to write regular progress reports and show good progress and make the case to the management for equipment and beam time. But it was a very highly skilled and productive group of scientists there and I really enjoyed the way in which we were able to do science.

Zierler:

Who were some of the key collaborators you had during those years who came to Chalk River from the outside?

McDonald:

Well, Hamish Robertson was one example. Eric Adelberger was another. And there were others from within Chalk River who were close collaborators. I worked a lot with Tom Alexander, from whom I learned a lot of experimental technique. Otto Hauser, who went on to TRIUMF and passed away at too early an age was a very solid scientist and had a high reputation for his work at TRIUMF and Chalk River. Also Gordon Ball, who went on to be scientific director at TRIUMF. I never worked directly with John Hardy except perhaps on one experiment, but we were friends and he was a very solid scientist with whom I had lots of constructive discussions. He went on to a professorship at Texas A&M eventually. He has a very good reputation also in fundamental properties of beta decay. Jim Geiger was a very knowledgeable scientist and good friend who went on to be the Branch Head of the group. My daughter is married to his son.

Davis Earle is someone that I worked with on a number of different experiments using the reactor sometimes, accelerator sometimes. We just gravitated to each other as colleagues and he became a principal collaborator of mine on the SNO project and an Associate SNO Project Director. In fact I was at Chalk River in the summer when we both joined the original 16 members of the SNO Collaboration. When I was at Princeton from 1982 on, I often went back to Chalk River in the summer to finish off a project on parity violation that I was working on with Davis Earle. And it was while I was at Chalk River that we heard about the movement in Canada to establish the SNO project and got in on the very first meetings relating to the project. Davis was also one of the founders of that project and brought Chalk River expertise into it.

Zierler:

Art, what were some of your significant visiting positions or sabbaticals during the Chalk River years?

McDonald:

Well, really only two visiting positions. One was a summer spent in Seattle where I was working closely with Eric Adelberger on work relating to parity violation, but also as I remember, finalizing our summary work on the properties of higher isospin levels. Very, very enjoyable experience. That was 1978—in 1981 I spent six months at Los Alamos and that’s notable in several ways. I continued thereafter to work with people from Los Alamos on parity violation experiments at the LAMPF accelerator while I was at Princeton. But it’s also while I was at Los Alamos that I attended a conference ironically in Hawaii. Ironically cause I subsequently very often spent part of the winter at the University of Hawaii. But it was in Hawaii that I was approached by Frank Calaprice about moving to Princeton. We followed that up and then moved to Princeton in 1982. I also made a number of connections with Los Alamos scientists and particularly with Tom Bowles, for example, who was a principal person on the SNO project eventually. Los Alamos became a principal member of the SNO project and in fact, Hamish Robertson had moved there and was one of the people involved. So, these sorts of personal connections I was able to make in these ways contributed later when we were building up the collaboration for SNO.

Zierler:

Art, were you active on the service side of things during the Chalk River years? Were there any significant advisory committees you served on?

McDonald:

Well, I was secretary of the NSERC (Natural Sciences and Engineering Research Council) Grant Selection Committee for sub-atomic physics, for a few years. And that gave me a lot of insight into the Canadian academic funding environment. I wasn’t an applicant because I was at Chalk River, but I learned the Canadian system really quite well and also got to know the people who were involved in the various funding agencies at that time. I served on other review committees in the United States as well and learned about that system. Of course, I learned a lot more once I moved to Princeton. Mostly when I was at Chalk River I was doing research and a not a heck of a lot else. Didn’t have to do much else. [laugh]

Zierler:

Absent your discussions with Frank Calaprice, were you happy at Chalk River? Were you looking for a move or you were fine to stay put?

McDonald:

It came out of the blue. Chalk River provided an excellent environment for doing science. My family was very happy in Deep River which is the residential community related to Chalk River Labs. It’s like the town of Los Alamos or White Rock. Compared to Los Alamos, it’s a smaller residential community of about 5000 people, mostly involving scientists, technicians, engineers from the nuclear facility. Our kids had a wonderful environment there. We lived on a street with 35 relatively new houses and 85 children under the age of 15. So each of our four kids had several close friends within a year of their age. It is still the same and my daughter and her husband moved back there about 15 years ago. She works over the internet with a major international company and he works at the laboratory. Moving to Princeton was actually a bit of shock for our kids, even though Princeton is a small town relatively speaking. It’s in the Boston-Washington corridor and it’s a very different way of life, particularly for kids going to school with the kids of Vice Presidents of major companies in New York or Philadelphia for whom Princeton is a bedroom community.

Zierler:

How old were your kids when you made the move?

McDonald:

We have four children and they ranged in age from about three years old to 14. And so, the oldest was going into high school, the next into junior high school. The older ones had to make the transition socially but got an excellent education there. Life in Princeton was very nice, but it was not something we were looking for. We were quite happy where we were at the time.

Zierler:

So, what was compelling about Frank’s overture?

McDonald:

Oh. The science that could be done there was clearly going to be a good opportunity for me to move beyond what I had been doing in terms of measurements of fundamental symmetries. If you’re dealing with the Weak Interaction you had to look for parity violation and so, having polarized targets was a good opportunity. And when I visited Princeton I met Will Happer who is an excellent atomic physicist. He was an expert on the polarization of noble gas nuclei. We discussed it and he had ideas for how one could use lasers to polarize the outer electron of a rubidium atom and allow it to interact in a gas filled glass cell long enough to polarize the nucleus of atoms like neon and helium-3, which I was interested in. He was already working with Frank Calaprice on xenon. I could see how that had potential for things we would do relating to fundamental symmetries so it looked like a fun new experimental opportunity. I had become familiar with the use of lasers because we had produced a polarized electron source at Chalk River in the experiment I was working on with Davis Earle relating to parity violation in deuterium with a polarized electron beam.

Zierler:

Art, on that point, were you following what was going on at SLAC with polarized electron sources at all?

McDonald:

In fact, yes. We had a high intensity, low energy electron source. A continuous CW source, not a pulsed one like at SLAC. I contacted Dick Taylor at SLAC who became a friend in later years and even more so after the Nobel Prize in which he immediately called me and said he had lots of advice. He was an interesting individual with kind of a gruff exterior with a heart of gold and he told me various things about what it was like to get a Nobel Prize. But one of the things he told me that I try to stick to is, “Everybody thinks you’ll know everything about everything. You don’t.”

Zierler:

[laugh]

McDonald:

[laugh] Anyway, I contacted Dick and told him what I was doing. He was originally a Canadian and that was an additional connection. He said, “I’d love to give you a hand.” So, I went out to SLAC and he introduced me to Charlie Sinclair who was the guru of polarized electron sources at SLAC and eventually at the Jefferson Lab. Basically, I was given drawings of the main heart of the process of generating the pulsed beam of polarized electrons and we had lots of discussions about how to use lasers to produce the light that emits the polarized electrons from gallium arsenide. And so, I built it at Chalk River and we had a continuous beam for the first time. A really substantial continuous beam that was quite reliable.

Zierler:

What was the value of that over the pulse?

McDonald:

Oh. It’s partly just the nature of the accelerators, but in terms of why you have one or the other there was an experimental reason. These count rates we were dealing with were so high that if it was all concentrated in a pulse it would’ve been very difficult for our detectors and electronics to handle it. So, there was an advantage in having a continuous beam. But the source was very intense. It was such that eventually our source when we finished that experiment at Chalk River was “borrowed”. We never got it back, but OK. It was borrowed by the Bates Accelerator at MIT and used for a number of years as the primary polarized electron source for a number of fundamental measurements done there. By working with SLAC and the people there and not trying to invent too many more wheels, but simply take their advice and adapt it to what we were trying to do in a continuous beam accelerator, we were able to get into the business fairly quickly and able to produce something that had good reliability for the longer term. So, yeah, I was quite conscious of what was happening at SLAC. The things that were being done at SLAC were very wonderful experiments in terms of understanding elements of the Standard Model. We were doing things that were relating to measurements in nuclei with a similar sort of objective.

Zierler:

And as you say, this was a collaborative relationship, not necessarily competitive.

McDonald:

Oh, certainly not. I mean, they were extremely helpful to me. We had a very small team, it was basically Davis Earle and myself and several very skilled technicians who were developing this equipment doing these measurements. And you know, compared to the power of the SLAC technical group, we were very small and knew it. And were very happy to learn from them. Dick Taylor was very helpful in all elements of this and we became friends as we did with Charlie Sinclair.

Zierler:

Art, I’ll return to the question that we discussed during your time at Caltech as you’re preparing to arrive at Princeton. And that is the state of the interplay between theory and experimentation at this stage in your career?

McDonald:

Well, I’d had a very strong interaction with theorists on the subject of parity violation experiments which was what I was concentrating on at the time. A particular person with whom I had had a large amount of interaction was Wick Haxton. He is a really excellent comprehensive theorist who was working very extensively in this field and later on solar neutrinos and underground science. There were others who were working at a fundamental level of dealing with the relationship between the fundamental interactions and the Standard Model and what that implied for nuclei. Wick Haxton was very active on the effects in nuclei and Barry Holstein at the University of Massachusetts at Amherst was another person with whom we had a lot of interaction at the time. So, there was a very good collaboration between theory and experiment explicitly on the subject of parity violation in nuclei.

Of course, I also was attracted very much by the quality of the faculty there. The opportunity to be a Princeton professor was very attractive just because of the nature of the department. There was tea at three o’clock in the afternoon every day. You could go down to tea and look for anyone else in the department for a quick conversation on physics. At one point we would seek out Ed Witten for discussions about our experiments looking for axions. When axions were first proposed in the mid-1980s, the original model created opportunities for a number of different experiments ranging over energy scales which included nuclear physics. And so, Aksel Hallin and Frank Calaprice and I had ideas for experiments that could be done in nuclei. Being able to have a discussion with Ed about that at tea was a wonderful opportunity and he was very helpful. So, that aspect of Princeton was very, very attractive.

This was the first time I had taught after 12 years or so of research. I first taught in the group of people who teach first year physics at Princeton, but within a couple of years I was given the overall responsibility for the course. That involved doing the main “gee whiz” lectures with wonderful demonstration material and a special turorial I ran for students getting D or lower on their weekly quizzes. Also working with 14 other Professors — this gives you a measure of Princeton and it’s probably the same now—but in that day it was about 350 students. It was the Principal Physics course for engineers. Fourteen professors teaching tutorials, not grad students, but professors. For a couple of the years I had several Nobel Prize winners teaching with me. I had the responsibility also for dealing with all of the material that got presented in the way of quizzes, exams, and presenting a quality course. And so, I’ve characterized it as sort of going back to school at the professorial level compared to going to school at Caltech at the graduate student level. I learned a lot about being a university professor and also I was the co-principal investigator on the Princeton cyclotron. So, I learned a lot about the U.S. funding system in the process of going after research funding and leading a group involving a number of junior professors. To some degree I knew what I was getting into, but to some degree I was very green. [laugh] And so, it was a very interesting experience.

Zierler:

What was the major project with the cyclotron at that point?

McDonald:

Oh, there were a number of projects that were going on and they tended to be ones that were oriented towards fundamental symmetries. There were ones related to things like these polarized targets we were talking about and ones producing beta decaying nuclei and studying their decay properties. That was the thesis project for Aksel Hallin, a student of Frank Calaprice who later was a Professor at Queen’s and Alberta and became one of my closest friends and colleagues on SNO and other experiments up to the present day. There were some nuclear astrophysics measurements. There was a lot of interest in light nuclei and their properties and exploring more sophisticated aspects of such nuclei with which you could do with certain types of experiments. It was a basic-physics-oriented set of measurements, with a strong flavor of fundamental symmetries. So, not so different from what I had been doing and it was the sort of thing that attracted them to me I guess, because I fit into the nature of the science that they were doing there. I had a fair amount of experience and a pretty good track record on the things I had been doing up to that point.

Zierler:

Did you take on graduate students right away when you got to Princeton?

McDonald:

Yes, I did. I didn’t have a large number, but I had a few who were very good. Kevin Coulter’s thesis was the application of a polarized helium system at Los Alamos for the polarization of neutrons. I started working on polarizing neon, which I wanted for certain experiments at the cyclotron. We recruited Tim Chupp as a junior faculty member and I worked with him for a number of years. He’s an excellent scientist that I mentioned earlier. We started working on helium-3 where he had a substantial interest. Polarized Helium-3 had a lot of very great properties. We worked with Will Happer who had the perspective that you could improve the density of polarized helium by a factor up to a thousand over what was being done at the time so that we could get glass cells with an atmosphere or more of 50% polarized helium. You could flip the polarization of the helium simply by using RF fields and change almost nothing else. And that was exactly what you wanted for doing searches for very small effects that had to flip with the polarization. We could look for very small effects against a background that didn’t change. Low energy neutrons, particularly epithermal neutrons for example, slightly above the threshold, would be polarized in passing through this material because the ones with the appropriate spin would be absorbed and the others would be transmitted. And so, you could flip the spin of a beam of neutrons simply by flipping the spin of the helium chamber. Kevin Coulter’s thesis project, was the first use of this with what we call the laser-induced-spin-polarization of helium at Los Alamos.

We were looking for high mass nuclei where collective effects produced parity violation at up to the few percent level, hoping to find a polarizable nucleus that could then enable us to study time-reversal symmetry through a triple correlation. The idea was that the large parity violation might provide an enhancement in sensitivity. The collaboration was very successful in finding large parity violation, but not in a suitable nucleus for the time reversal measurement.

Polarized Helium-3 has gone on to be used in an enormous number of different ways. It’s been used as a target for particle physics experiments because essentially it’s a polarized neutron sitting in a nucleus. That then became the target for spin structure function measurements of the neutron measured at high energy accelerators. It now is a standard technique. I visited the European Spallation Source in Sweden during a recent trip to Sweden and they are preparing to use these dense helium targets polarized with rubidium that we developed originally. The polarized helium-3 is used to polarize the neutron beam and then to analyze the polarization of the scattered neutron. This is of particular value in studying magnetic effects in condensed matter. After I left Princeton, Will Happer and Gordon Cates developed the use of polarized helium for lung scans which provides excellent resolution compared to normal MRI. Unfortunately, it hasn’t become a clinical application for various reasons. One of them is that the source of helium-3 is the decay of tritium and you have to be very careful you have no residual tritium left when you start trying to do something on with human lungs. The overall availability of helium-3 was another factor. However, this is a prime example showing that if you develop an innovative new technology then you can do many things beyond what you originally intended.

Zierler:

Art, did you, yourself, spend time at the Los Alamos spallation neutron facility?

McDonald:

I did. Yes. I was a part of those experiments.

Zierler:

Was that the only facility like that in the world at the time?

McDonald:

Oh, no. There were others but it was a pioneer in the use of accelerators to produce thermal and epi-thermal neutron beams, predominantly for condensed matter studies. It was one of the best at the time.

Zierler:

Art, tell me about the origins of the SNO collaboration and your central part to that story.

McDonald:

The early days of SNO is a story of two central individuals, George Ewan and Herb Chen, a professor at the University of California at Irvine. But there were 16 of us who were members of the original SNO collaboration that came together in 1984 and a large fraction of those 16 people went on to be major participants in the project. The project came together because it was a great physics idea that had the potential to solve what had been referred to as the solar neutrino problem. That is the factor of three difference between Ray Davis’s experimental results and John Bahcall’s calculations of the number of electron neutrinos coming from the Sun. Electron neutrinos are the only neutrinos that are produced in the Sun since muons and taus are too massive to be involved in the solar processes. Davis’ measurement was of those same electron neutrinos and only one-third were seen compared to the number that were calculated. What was needed was a way to determine whether the electron neutrinos changed to muon or tau neutrinos before reaching the earth, or whether the calculations were correct for how many neutrinos are emitted by the nuclear reactions that power the sun.

George Ewan had been exploring the opportunity of doing physics in an underground environment in Canada for the preceding year or two. He was particularly focused on proton decay which was a major motivation for the Kamioka experiment in Japan, although they eventually switched to solar and atmospheric neutrino measurements in addition. Herb Chen, who had worked with a Canadian physicist named Cliff Hargrove using a heavy water target at Los Alamos, proposed that if you could get enough heavy water then you could in fact do a measurement of neutrinos from the Sun where you measured two things. This could be used to clearly determine whether neutrinos were changing their flavor in Davis’ experiment. It was known that deuterium could be used to measure these two different neutrino reactions but Herb was the one who had the audacity to put this forward as something that could potentially be scaled to a level that you could actually measure neutrinos from the Sun. Just to show you the scale we’re talking about, with a thousand tons of heavy water we observed one neutrino an hour from the Sun over a course of about seven years or so in the SNO experiment.

Herb’s original proposal was for 4,000 tons with the idea that it would be one big chamber with 4,000 tons of heavy water and it would be self-shielding. That is, the heavy water would be used to shield against the radiation coming from outside that you had to get rid of. The requirements for radioactive purity in the heavy water and in the reduction of radioactivity in all the surroundings, including the detector parts, were extreme. In 1984, he contacted Cliff Hargrove and said, “What do you think the chances are of borrowing 4,000 tons of heavy water to do this experiment?” So, Cliff looked into it and talked to people at Chalk River that he knew. Atomic Energy of Canada Limited, that ran the Chalk River Labs were the custodians of the heavy water reserves in Canada. The answer came back, “You realize this is $1.2 billion worth of heavy water that you’re asking for? And besides, it would involve a large fraction of our reserves right now.” On the other hand they said, “If you wanted something a little less than this maybe we could manage it.” The story I heard at one of the early meetings of the SNO collaboration was that Herb was standing with his young daughter—who now is a principal scientist on the James Webb Space Telescope, by the way— in front of an aquarium in California, and said, “Hey. I wonder how they build these aquaria? Maybe we could do that and put the heavy water inside and put ordinary water outside.” And so, we discussed that to our first collaboration meeting in Ottawa in 1984. That got the number of tonnes of heavy water down to about 1,000 tons (still $300 million). But amazingly, through efforts of George Ewan and Cliff Hargrove and others, people at AECL agreed in principle about a loan of that amount of heavy water.

George and Cliff knew all these senior management people at Chalk River personally as I and others like Davis Earle did—these managers were scientists who were intrigued with the idea that you could solve the solar neutrino problem by making two measurements with a heavy water detector. One reaction which was sensitive only to electron neutrinos like the measurements of Ray Davis. The other one was equally sensitive to all neutrino types. A reaction where a neutrino comes in and simply breaks apart deuterium into a neutron and a proton takes place with equal sensitivity for all three neutrino types. And so, if you are able to measure electron neutrinos and all neutrino types separately, if they’re changing their type you should see only one-third of the total being electron neutrinos. You can also test the solar neutrino calculations by measuring all neutrino types regardless of whether they’re transforming. If the calculations are wrong then you could find that all neutrino types are just electron neutrinos and there’s no transformation.

So, it’s a straight forward comparison if you can manage to do the experiment. We had many doubts during those first meetings as to whether we could measure the second reaction which is sensitive to all neutrino types, because radioactivity can induce the same reaction and you really have to control it.

So, Herb put forward the idea and we all bought into it. And amazingly, AECL came through and said, “We think we could probably manage to loan 1000 tonnes of heavy water to you.” George Ewan had developed very good relationships with INCO, the International Nickel Company. INCO owned the deepest mine in North America, in Sudbury, Ontario. There was a senior vice president of INCO, Walter Curlook, who was in charge of research and development and it really caught his fancy as well. So Inco was on board from the very early days as well.

A design was developed, starting with these 16 people and gradually expanding. That was 1984. Very unfortunately, Herb Chen passed away in 1987 from leukemia that developed very quickly. He was only in his 40’s. He didn’t feel well at one of our collaboration meetings in Chalk River Ontario and when he went back to California he was diagnosed with Leukemia. He passed away the following year.

We carried on and have tried very hard to preserve his memory as a founder of the collaboration. When we won the Breakthrough Prize for the SNO Collaboration in 2016, we were allowed one additional person to attend and we chose Herb’s wife, Catherine.

I became the U.S. spokesman for the project in 1987. And gradually, in fact, with help from Fred Reines at Irvine, enlisted others to participate in the project. And so, Gene Beier from the University of Pennsylvania and Hamish Robertson and Tom Bowles from Los Alamos National Lab came on board. David Sinclair had brought Oxford into the collaboration in the very early days. The Canadian collaboration also grew. Through the ‘80s the design progressed and there was a lot of effort to get it funded. We did get some development funding but the overall cost was large. Finally by 1988 we were at a point where people were beginning to take notice of it because the design was looking quite sophisticated. However this was a project that was well above typical Canadian funding for a single project. It was $50-60 million total.

Zierler:

And it was exclusively Canadian funding?

McDonald:

No. About 25% of the total was to come from the U.S. and almost 10% from Britain, but it was predominantly Canadian. By the late ‘80s we were working hard to put together a proposal that could get proper consideration. I actually spent the academic year in 1988 at Queen’s on sabbatical working with a number of other people on preparing the final documentation. A lot of work was done by George Ewan and Dave Sinclair, people at Chalk River and the rest of the collaboration to put it all together. Kevin Lesko and the Berkeley Lab joined and Walter Davidson, from the Canadian National Research Council was another person who was very actively involved in the fundraising, In Canada we had to get funding from other than just normal funding agencies to deal with this scale of project. We managed to get to the point where the president of the main academic funding agency, NSERC, Art May and the president of the National Research Council, Larkin Kerwin, each committed $10 million. They said, “We’ll start with that amount as part of the Canadian support. This is a project we want to see happen.” This occurred just before a final decision by the Nuclear Science Advisory Committee for DOE and NSF in the US. Janet Halliwell, VP of NSERC, presented to the committee and said that Canada was definitely committed to the project and we came out on the top of the recommendations for US support. We were then reviewed by Ed Temple and a group that reviewed all DOE projects at that time, in a joint review between Canada and the U.S. in 1989. We eventually got funding at the turn of the year, in 1990. Essentially full funding including commitments from the UK, Canada, and the U.S.

Zierler:

Art, what were the U.S. funding sources?

McDonald:

It was the Department of Energy, Basic Energy Sciences Division. Same funding agency for example, that funds Fermilab. It also was funding other basic science initiatives in nuclear physics and some in astrophysics as well at the time. Some smaller institutional funding came from NSF.

Zierler:

Art, of course, this is right in the middle of the SSC. Were there any impacts of that with regard to your dealings with DOE?

McDonald:

No is the simple answer. It was unrelated as far as I know. It was also in the middle of a major initiative from TRIUMF for the so-called KAON project which was an upgrading of the TRIUMF accelerator to produce something that would in fact be more powerful than the LAMPF. And so, there were a lot of things on the horizon at the time. KAON would’ve been 10 times the scale of what we were looking for. But it was another major initiative in particle physics which affected things in the Canadian area. But the work of people like Walter Davidson and George Ewan in Canada led to support from the Sudbury community and eventually support from the province of Ontario as well—which had a tendency not to fund basic science projects at the time. They came together with the support from Ottawa to provide funding for the project as a whole. So, that basically is the political, scientific and financial history of how it came together.

We did a number of research projects that related to the question of feasibility. And by 1988-89 we had pretty well convinced ourselves that we could do the second reaction, the neutral current reaction as well as the original one which measures electron neutrinos exclusively. So, we did have a case including that by the time we came to the final funding decision.

Zierler:

Art, what were some of the key technical challenges in the design?

McDonald:

Well, what you were trying to do was to build a detector the size of a 10-story building with ultra clean conditions. In layman’s terms, there should be less dust on the elements of the detector when it was constructed than you could pile on your thumbnail. Less than a gram of dust on the whole detector being constructed in an active nickel mine. We had to accomplish air quality within the construction area which was a factor of more than 104 cleaner than the outside air quality in the mine.

We were also dealing with $300 million worth of heavy water that was going to be housed in an acrylic “bubble”, you might as well say. It was 12 meters in diameter and 5 centimeters thick. First time anything of that scale had ever been fabricated. And it was being done two kilometers underground in an active nickel mine where we were sharing the lift, or the cage as they call it, with miners who were going to work every day. The same cage was being used for the transport of materials to build the detector so the acrylic vessel had to be bonded together in place from 120 pre-formed pieces. The other side of the mine shaft was being used to take 5,000 tons of ore a day out of one of the most active mines that Inco was operating. It is now owned by another company, Vale, but it’s still in operation and very productive.

We also needed very sensitive light sensors. We needed the latest capability in photomultiplier tubes, 10,000 of them. We needed sophisticated electronics and various parts of the collaboration took up these roles. We were very fortunate in having the Pennsylvania group led by Gene Beier, who also became a co-spokesman in the U.S. They had significant electronics expertise and neutrino physics experience. They had been involved in the conversion of the Japanese Kamiokande detector to detect solar neutrinos. The difficulty for Kamioka of course is that Kamiokande was still detecting predominantly electron neutrinos and their measurements of electron neutrinos in the late 1980’s confirmed what Davis had seen. However, you still didn’t know for sure whether neutrinos were changing their flavor or if the solar model calculations for neutrino flux were incomplete or incorrect. The same became true later for other experiments using gallium as a target and for measurements of solar neutrinos with the upgraded SuperKamiokande experiment using ordinary water as a target. They all essentially confirmed what Davis had observed for electron neutrinos but did not have independent sensitivity for other neutrino types.

The solar theory was still needed in order for you to really know whether there was a problem with the understanding of how the electron neutrinos are created in the Sun or whether they are changing their flavor and tricking these experiments because of the oscillation into muon and tau neutrinos. You didn’t have a full answer to the “solar neutrino problem” that says clearly that neutrinos have changed their flavor. With our measurement of the sum of all active neutrino types through our second reaction on deuterium, we were able to provide the answer.

We had a number of other solid technical groups with expertise in the various areas that we needed for the SNO experiment. We had a group from Berkeley led by Kevin Lesko. We had the group from Carleton University which had moved there from the National Research Council in Ottawa, led by David Sinclair and Cliff Hargrove. We had a group from the TRIUMF laboratory with a substantial contributor, Rich Helmer and from UBC with Chris Waltham. We had a group from Chalk River Labs, led by Davis Earle and the group from Queen’s led by George Ewan and later Aksel Hallin. The Oxford group was led by Neil Tanner, Nick Jelley and Steve Biller, the Guelph group by John Simpson and the Laurentian University group by Doug Hallman. We had groups from Los Alamos with Tom Bowles and Jerry Wilhelmy and thereafter also the University of Washington when Hamish Robertson (the other US co-spokesman) and several of his collaborators moved there from Los Alamos. Those two institutions led the development of a set of neutron detectors which provided an explicit way to detect the second of the reactions in the third phase of the project. That reaction is sensitive to all neutrino types. They developed about 400 meters of very low radioactivity proportional counters with helium-3 in them that enable you to make measurements of that second reaction explicitly. Earlier in the project we had looked with pure heavy water and did a hypothesis test where the only legitimate hypothesis that fit the data was that neutrinos were changing their flavor. That’s what we first reported with over 5 sigma significance. Prior to that we had observed a finite effect with 3.3 sigma significance by combining data with the SuperKamiokande solar neutrino measurements that had a small sensitivity to all neutrino types. We subsequently put salt in the water and enhanced the sensitivity for detecting the neutrons from the second reaction on deuterium. David Sinclair (Canadian co-spokesperson) and his group were very largely responsible for the salt measurements. That confirmed the results from the first phase and provided improved accuracy and the ability to extract the rate as a function of energy for the first reaction sensitive only to electron neutrinos. That information provided a further test and constraint on neutrino oscillation parameters as it showed no measurable distortion as a function of energy.

A lot of the technology was first of a kind. The light water outside the acrylic vessel was purified to more than a billion times more than ordinary tap water. And the heavy water was ten times purer than that. We were concerned mostly about uranium and thorium. Those were the naturally occurring radioactive elements that could initiate that second reaction through a gamma ray. Ultimately uranium and thorium decays in the heavy water were fewer than one decay per day per ton of water. That’s a very low rate of radioactivity and it had to be measured. The techniques that were developed to detect the residual radioactivity after you had done the purification were enough to say that we had reduced the radioactivity to about a factor of 10 below what we needed for our signal and then we measured it as well to about 30% uncertainty. So, there was a lot of technology in the water and many of the other systems. That was fun as well as challenging. Not so much fun when things weren’t going well, but ultimately fun when you were able to accomplish it and get the results.

Zierler:

Art, given all of the resources that were poured into this, I suppose it was out of the question to get mining operations to stop?

McDonald:

Well, I don’t know what the daily profit was from the operation of that mine, but it was a relatively rich nickel deposit. They were taking 5,000 tons of ore out of there a day. They had just finished most mining on our level, two kilometers or 6,800 feet down. They were moving their way down and are now mining below 8,000 feet. Apparently the ore is getting richer as they go down. So, there was no thought of stopping the mining. We were there at their pleasure and one of the things we were very scrupulous about is not interfering with mining operation in any way. The same is true for SNOLAB today.

Mind you, it was a tremendous advantage to be working in an active mine because they made sure that all their safety systems were up to date and monitored. And we always had a motto of doing things as safe as Inco or better in everything that we did. Any incident we had that related to safety we reported to them and took action as they felt was appropriate in addition to what we felt was appropriate. So, from that point of view it was valuable to us to be working with a very active, cooperative, safety-conscious mining company.

Zierler:

Art, was the transition from design to operationalizing the laboratory, would you say that was more gradual or abrupt?

McDonald:

First of all let’s define laboratory because initially we were simply building the SNO experiment. And our first results were in 2001 and then even more accurate results in 2002. That’s when we attained the 5-sigma result that is the criterion for a discovery in particle physics. In 2003, the Canadian government put forward a program with $100 million on the table to build three or four new national labs in Canada that would attract international scientists to come and work in Canada. An application was prepared, led by David Sinclair, and we won about $39 million to expand the SNO laboratory and create SNOLAB. It expanded the excavated volume by about a factor of three with several major cavities and a number of other areas that are now housing five or six significant international experiments. That’s the laboratory that now exists and is being very effective in measurements like neutrino-less double beta decay and dark matter searches. SNOLAB is attracting major experiments from around the world. But during the SNO measurements, the experiment and the laboratory were synonymous as far as we were concerned. So you’re really asking about the transition from having constructed the detector to going into operation.

Zierler:

Right.

McDonald:

That occurred in 1998, moving into 1999. It involved simultaneous filling of the inner acrylic sphere that contained the heavy water and the outer area which contained light water as a shielding. As we did that it was still the scientists from the project who were very much involved as the things they had built went into operation. We learned many things as we turned on the detector. At one point there were electrical breakdowns in the connectors we were using that puzzled us quite a bit. That was solved by a suggestion of David Sinclair’s that it was caused by our removal of gas from the light water to reduce radioactivity. When the water was re-gassed with pure nitrogen, the problem went away. We had tested for this but had not been able to carry the testing to the full extent of the degassing in the final experiment, so we learned a general lesson from this regarding the completeness need in quality assurance testing. We also had to figure out how to handle things like occasional flashes of light from the photomultipliers. However, they looked very different from the neutrino signals we were after and could be eliminated. Those are the sorts things that can plague you. Fortunately we had a very skilled team that solved the problems that arose. So, the scientists were very involved as we went into operation. During construction, we had many others on the team, including an external engineering and project management team from the engineering company Monenco. They had just completed the construction of a Canadian-style CANDU nuclear reactor in South Korea.

Zierler:

Art, before we get too far afield in the chronology, we haven’t yet covered your switch from Princeton to Queen’s University. How much of that was about simple geographic proximity for the SNO collaboration?

McDonald:

That was a factor. At that point George Ewan would be retiring in a couple of years. I was one of the U.S. co-spokesmen and had been working very closely with George on various funding agency questions as well as experiment design. I knew the project quite well, particularly having been involved from the start and having spent 1988 at Queen’s on sabbatical. I knew Queen’s well as I had worked on experiments there even when I was at Chalk River. It’s a nice community and our family would be comfortable here. Queen’s is one of the top five or so universities in Canada in terms of research activity and quality of students. There was no doubt that people said to me at the time, you know, “That’s quite a change to give up a tenured professorship at Princeton and go back to a lesser-known university in Canada.” But I knew from the quality of the people I’d be working with that there was lots of very good science that could be done even if we didn’t get the funding for the SNO project. At the time I had to make the decision we didn’t have the funding in place.

Zierler:

So, there was some risk involved here?

McDonald:

Certainly there was. But it was also made clear to me that if someone didn’t come in and be clearly available to replace George when he retired just a couple of years after that, then it would be difficult to get the funding. To get the reviews that you needed in order to make this go forward. I’m not saying I was a principal factor. What I’m saying is if there was a perceived future vacuum in the leadership on the Canadian side because of George’s imminent retirement that would’ve been a detriment to the project. So, that was a factor that other people pointed out to me as I was trying to make the decision. I certainly haven’t regretted the decision. I found the quality of students at Queen’s very solid and the quality of my colleagues at Queen’s and in the project as a whole in Canada very high quality. Whereas at Princeton you know, the pace of the world of physics kind of passed your door daily and you could discuss it at three o’clock tea every afternoon. That still happened at Queen’s, but in point of fact, most of the time I was too busy or on a plane to Sudbury once I transferred to Queen’s. I still taught. But when I went to Queen’s I think there were two of us teaching 350 students in a first year class, rather than 14 or so at Princeton. Towards the end I got a large amount of teaching relief and I’m very grateful to my university department colleagues for doing extra teaching to enable the relief I received. At the end I was teaching a fourth-year laboratory which I would teach on Monday afternoon and Friday afternoon. On Monday evening I would fly to Sudbury and Thursday evening I would fly back to Kingston. So, I spent many years, probably six or seven years spending midweek each week in Sudbury.

Zierler:

Art, tell me more about this critical period between 1999 and 2001 where you began data acquisition to when you first published the scientific results. What was the first indication that you had something exciting to report?

McDonald:

We were faced with a real impetus to publish because we were hosting the so-called Neutrino 2000 conference in Sudbury, a leading conference in the field. We had clear spectra from the first of the two reactions sensitive to the electron neutrinos alone as opposed to all the neutrino types. It’s partly a question of where you set your threshold in the experiment and the threshold gets set just above the events that come from background. But at the time of that conference we had not accumulated enough statistics and background knowledge to be definitive about whether neutrinos change their type or not. By the following year, 2001, we had enough statistics from the first reaction to combine our results with results that Super-Kamiokande had obtained with their work on solar neutrinos. They had a small 14% sensitivity to the second reaction and by combining the two results you could separate the two reactions. Our first publication in 2001 was in combination with—not collaboration with—but combination with the Super-Kamiokande results. And the result was a 3.3 sigma significance result suggesting that neutrinos had changed their flavor. By the following year, 2002—in fact, really just within six months of our first paper—we had pushed the threshold down and got the information about the second reaction out of our own data and could have a 5 sigma result just from our own data showing clearly that neutrinos had changed their flavor. You always worry when you’re trying to combine two experiments that you don’t totally understand fully the systematics of either experiment when you try to make an absolute comparison between the two.

At the time of the Neutrino 2000 conference we had blinded ourselves to the final result so that nobody doing the analysis could know what the result is. So, you couldn’t be led by your nose to your preconceived idea of what the result is and adjust your analysis to make that happen. At the time of the conference we had a bit of a quandary because everybody in the community was expecting that we might have an answer. But we knew we didn’t have enough statistics to even do this comparison with the Super-Kamiokande results. And so, we weren’t prepared to publish something until we had an answer that we could publish.

What we did at the conference was to show the data with no normalization on it. And the people at the conference were extremely impressed by the degree to which we had been able to suppress background to get an absolutely clean measure of this electron neutrino measurement. It was showing them that we would be able to do the experiment, but simply didn’t have enough statistics as yet to give a final result.

Zierler:

So, this is essentially proof of concept at this point?

McDonald:

What we showed at the conference was proof of the concept, but not a final result. But needless to say, lots of discussions, lots of opinions. That’s where it’s great to be as friendly as you can with your colleagues.

Zierler:

[laugh]

McDonald:

And to have fun and to take advantage of the fact that you’re doing good stuff. And you can be a little patient. And that was the approach we took.

Zierler:

What was the next step after this?

McDonald:

Well, the next step after this was to really get to the point where we could make at least a 3 sigma result combined with the Super-Kamiokande result. Without unblinding we could determine what the accuracy was going to be statistically without knowing what the answer was going to be. It was actually great fun. I mean, you really end up with a Eureka moment virtually, because people were not all in Sudbury at the time. There was a collaboration meeting going on when we unblinded, but not everybody was there. We ended up unblinding and everybody at the same time had a realization that, at least at 3 sigma significance at that time, we had a significant result and then we had the same experience in the next phase a year later at 5-sigma. And then lo and behold, when we did it again with salt enhancing the results from the second reaction. We had better accuracy but the same result. When we did it with neutron detectors, same thing. So, it was a wonderful experience collegially as well.

Zierler:

Art, if you could just explain the Eureka moment, what was not known before the moment and what was known as a result of the moment?

McDonald:

What was not known was whether electron neutrinos change their type as they come from the Sun to the Earth. Only electron neutrinos can be made at the temperatures in the sun. We were measuring the number of solar electron neutrinos interacting in our detector with first reaction. The second reaction enabled us to measure a number that corresponded to the sum of all neutrino types that interacted. We knew with great accuracy what the probability of interaction with deuterium was from accurate nuclear physics calculations.

We found that the number that were still electron neutrinos was only one-third of the total. And that was very clear with five standard deviation significance. There was only a chance of one in 10 million that neutrinos did not change their flavour. What we were able to show therefore is that neutrinos change from electron neutrinos to muon or tau neutrinos. We don’t know which type, but we could show that they changed to other neutrino types before they reached our detector. In addition to that, the sum of all neutrino types —even though this changing was going on— is the equivalent of the number of electron neutrinos produced in the Sun. And that number was within the uncertainty of the number that John Bahcall had calculated.

I have a wonderful video of John Bahcall being interviewed by a New York television station. And they said to him, “How do you feel?” And he said, “Well, I feel like dancing, but even more so I feel as though the DNA evidence has just come in to exonerate me!”

Zierler:

[laugh]

McDonald:

[laugh]

Zierler:

That’s great.

McDonald:

“After many years of people questioning my calculation.” And that was the feeling at the time. It was a great feeling of exhilaration and wonderful to share it as a collaboration.

Zierler:

Art, a counterfactual: if Super-Kamiokande never was, how would that have changed things from your vantage point?

McDonald:

It would not. We were very independent experiments. The Kamioka experiment, the original one, pioneered underground large-scale water detectors. And the University of Pennsylvania group that developed the low threshold electronics for that experiment were very valuable to us and we learned a lot from working with them. We also learned a lot from how they built that in the first place. But in terms of the results of Super-Kamiokande, we simply went on doing our own thing. And really we ended up measuring essentially two separate things. They measured neutrinos produced in the atmosphere and oscillating as they traverse the Earth. And so, they were measuring the disappearance of muon neutrinos as they traverse the Earth. We were measuring the disappearance of electron neutrinos, but we were also measuring their transformation to muon and tau neutrinos in that our second reaction was sensitive to all neutrino types. And therefore, in terms of this process of transformation of neutrinos, it was clear from our experiment that they not only disappear, but they get transformed into other neutrinos. And so, you couldn’t explain it simply by for example, the decay of neutrinos from the source to the result. So, they were somewhat independent. In addition to being different neutrino types being studied, they were somewhat independent in the nature of the experiment.

Of course, the beautiful measurements by SuperKamiokande of solar neutrinos also allowed us to obtain our first observation of flavor change of solar neutrinos with 3.3 sigma accuracy by combining our results with theirs.

Zierler:

To return to an earlier question from our discussion. As a result of understanding these neutrino properties, what happened to the Standard Model?

McDonald:

Well, the Standard Model works extremely well in many ways. But what had been postulated is that because of the observed handedness of neutrinos, you don’t have right-handed neutrinos showing up in what we normally observe in nature here on Earth. There are many theories as to what is happening. One of them is that the so-called right-handed neutrinos are very massive and that there is some mechanism that makes that mass difference as opposed to them not existing at all which is effectively what would be assumed in terms of a zero-mass hypothesis. That is one of the leading candidates for where the mass of neutrinos comes from, referred to as the seesaw mechanism. The Standard Model prior to this, for lack of information, primarily postulated that the mass would be zero for neutrinos. And that worked in all ways until a combination of SuperKamiokande and our measurements showed that in fact, they transform by mechanisms that arise because they have a finite mass. And so, the mechanism for generating neutrino mass has got to be different than the mass generation mechanism for the remainder of the objects in the Standard Model which relates to the Higgs boson.

SNO doesn’t really show what the final mechanism is for neutrino mass generation. And it’s a big theoretical topic. It’s being explored in various ways experimentally. In fact, the SNO experiment is being converted to a neutrino-less double beta decay experiment. That’s something that I have also been working on. I’ve been working somewhat more on dark matter measurements in recent years. But back in the 2000s, even before the completion of the SNO experiment there were a number of us that were working on what the potential for converting the SNO experiment to a double beta decay experiment would be. Mark Chen, who is now the director of the project, SNO+, was very active on this. I worked on it as well, particularly during a sabbatical I took in 2003-04, including 3 months spent at the Physics Department in Hawaii. The SNO detector, being a very large liquid scintillator detector in ultralow background, is an excellent detector for observing the very rare process called neutrino-less double beta decay which can only occur if the neutrino is its own antiparticle and has a finite mass. What we were considering is to dissolve in liquid scintillator in the SNO central acrylic vessel, appropriate nuclei that can undergo this neutrino-less double beta decay process. We started with neodymium, but later switched to tellurium as a material. We now have the SNO experiment converted to SNO+ and filled with liquid scintillator. In order to do that we had to work out a way to hold down the central sphere rather than holding it up. It was obviously heavier than the surrounding water for SNO but that is no longer the case with liquid scintillator. That all had to be worked out and it took a while. However, final preparations are now being made to install several tons of tellurium metal in which the 130Te isotope occurs naturally at a 35% abundance. Measurements of neutrino-less double beta decay will start about a year or more from now.

That’s one way in which you try to study further properties of neutrinos in order to understand whether they are their own antiparticle (Majorana Neutrinos) or Dirac particles. If they are Dirac particles they would not participate in neutrino-less double beta decay. If you can observe neutrino-less double beta decay (which is a very daunting prospect and many experiments are trying to do it) then you can get additional information to test theories by studying the angular distribution of the two electrons that are emitted in the process. But that’s many years away. So, we know there is an adjustment that has to be made to the Standard Model, particularly to deal with neutrinos. We don’t quite know yet what it is. But then I think most scientists you talk to feel that there’s very likely to be other additions to the Standard Model because for one thing we don’t have gravity in there yet.

Zierler:

That’s right. [laugh]

McDonald:

Also, things like supersymmetry are ways of going beyond the Standard Model and may even give us candidates for dark matter particles.

Zierler:

Art, a classic question that can always be posed in moments of fundamental discovery. Now that there’s this understanding of the properties of neutrinos, what new questions can be asked that were not possible prior? Particularly in the realm of astrophysics and even cosmology?

McDonald:

It’s very interesting actually that the influence of neutrinos in structure formation in the early universe gives you limits on absolute neutrino mass that are somewhat model dependent in an absolutely remarkable Lambda Cold Dark Matter Model. It has six parameters that explain astronomical data including many things such as structure formation very well. But one of the parameters in there is in fact the sum of the masses of the neutrinos. We know only the differences in mass of the three active neutrino flavors from oscillation measurements; we don’t what the base mass is. You can infer that with some uncertainty if you have a successful observation of neutrino-less double beta decay. You can also measure it directly in experiments like the KATRIN experiment studying tritium beta decay. SNO collaborators like Hamish Robertson and others are very heavily involved there. The question of the neutrino mass is significant in cosmology as I mentioned. If in fact a laboratory measurement could be made of it and it could be fed back into the cosmological modeling that’s being done, then I’m told by the people doing the modeling that it could be an advantage in pinning down other parameters that they’re interested in.

What’s interesting today is that the sensitivity is very similar in the cosmological models and the direct terrestrial experiments. And there are different systematic uncertainties based on the models you use to do it. So, determining the absolute mass of neutrinos is very much a cosmological question as well as a particle physics question.

Determining the mass ordering and determining whether or not there is CP violation in the oscillations of neutrinos are also important questions in figuring out what theories are going to be the ultimate accepted explanation for neutrino mass. In addition to that, there is another question of cosmological interest. It is thought that you should have roughly equal numbers of neutrinos and anti-neutrinos created during the Big Bang where energy is converted into real particles and anti-particles.

However, it appears that almost all antimatter has decayed away such that we have a matter dominated universe. Now this requires several things. The more accepted theories that are put forward as to how this could occur involve what they call leptogenesis, in which matter creation occurs through leptons with neutrinos having a big part in it. If neutrinos violate CP, the particular symmetry which is a matter/antimatter symmetry, then there are mechanisms that can be hypothesized that would result in the decay of the antimatter in the early universe. So, measurements of the CP violation in the neutrinos we can get our hands on, the lighter active neutrinos, are of considerable importance these days. We should be careful to say that the CP violation in the early universe would be for heavier versions of neutrinos, but if CP violation takes place in neutrino oscillations of the lighter types we could infer that it also occurs in the early universe. This is the motivation for long baseline neutrino measurements like the DUNE experiment at Fermilab and the Hyper-K experiment in Japan along with definitive determination of the mass ordering for the three types of active neutrinos.

So, as usual you answer one question and the next layer of the onion is in front of you and you work hard to try to answer the next questions and put it all together.

Zierler:

Art, as you’ve indicated, it’s not just the fact that this project was supported so nicely by so many agencies, it’s really the quality of the people, the key people involved in the collaboration as you’ve emphasized. I wonder if you can reflect on that. What was so special about the collaboration, the dedication, the collegiality, that was fundamental to the collaboration’s success?

McDonald:

Well, I think we tried as much as possible to make decisions based on consensus. In other words, we discussed them to the point where if people had contrary opinions they could have an opportunity to put them forward. But ultimately they could perceive that the prevailing opinion in the collaboration may not have been exactly what they were saying. But you have to make a decision in a timely way. That, coupled with strong expertise in a wide variety of areas creating respect on the part of other collaborators was something that I think resulted in a lot of good decisions. And ultimately, collegiality due to the fact that we all had bought into a big project with a major scientific objective and had to work as a team to make it happen.

The Nobel Prize was something that was wonderful but we really were most satisfied with contributing to our knowledge of the Universe in a significant way with our scientific results.

I went out of my way to bring as many of the collaborators as I possibly could to Stockholm with their spouses in as many cases as possible. Spousal support is very important in these long-term experiments with hard work throughout. I managed to take 35 people to Stockholm although I was originally offered only 14 positions. The way in which that was accomplished was by having people sharing tickets to the various events that take place. I was able to use the prize money to have them all stay at the Grand Hôtel in Stockholm. And so, husbands and wives all came. I’m very sorry to say that the principal people in the project were all male. That’s no longer the case. Among our students and postdocs that have graduated and taken up faculty positions, which is 25% or so of the total, about 35% of them are women. And that’s unusual in physics these days. Everybody who came to Stockholm had a wonderful time to celebrate.

But that wasn’t the main point of our work. The major satisfaction was at the time of these Eureka moments, that’s when we had the opportunity to celebrate. Because we knew, with a solid basis with 5 sigma significance and better later, that we had done a significant scientific measurement. Which is what we set out to do. And we did it as a highly collegial collaboration. Hard times along the way, but ultimately the enjoyment of that scientific experience I think went through the collaboration.

That’s the way in which a high-quality collaboration of people who are willing to be collegial and take that as an approach to science can really do wonderful things. It was great at the time of Stockholm to be able to take as many people there as possible. And after Stockholm I went out of my way to visit every one of the major institutions. I’ve visited almost all of them in person and gave a talk that points out the contributions that were made at that particular institution. The senior administrators there should know how strong their scientists are, but it’s also a chance to say thank you to the people who made the contributions.

Zierler:

Art, of course you could never know how these decisions are made behind closed doors. But at what point did the buzz, for lack of a better term, at what point did the buzz surrounding the possibility that this collaboration would be recognized by the Nobel committee, at what point was that sort of too loud to ignore?

McDonald:

[laugh] Uh…never. In the sense that the history of it is that somewhere around 2007 Yoji Totsuka (the leader of the SuperKamiokande collaboration at the time, who passed away in 2008) and I were put on the Reuters website as possible winners of the prize. This website also measures numbers of citations of papers and these predictions are based on numbers of citations largely. So, for the next couple of years I would get a phone call the night before the Nobel award from somebody from one of the press agencies like the American Press saying, “Would you mind giving me your cell number just in case you win the Nobel Prize tomorrow morning? Have a good sleep.” [laugh]

Zierler:

[laugh]

McDonald:

But by 2015, I was having lots of good sleeps because you know, the prospects perhaps were not as substantial after whatever it was, 13 years from our definitive results. Within the community there was always a feeling, I think, by neutrino physicists that potentially there was another Nobel Prize for oscillations. The prize that was given to Davis and Koshiba in 2002 was stated to be for pioneering efforts in the field. And there wasn’t a specific mention of oscillations in the citation. And so, people in the community seemed to hold out hope that the observation of oscillations would be recognized and very often they would have Super-Kamiokande and SNO and very often KamLAND as well, as a part of it. I think that would’ve been quite legitimate—but the regulations of the committee I subsequently learned, say that you can only award it for two separate experiments. Not three. You can give it to three people, but only for two separate experimental efforts. And so, they had to pick two. And they picked Super-Kamiokande and SNO. KamLAND was often mentioned by the community as a potential prize winner. It was a beautiful experiment as well. Shortly after our observations of flavor change for neutrinos from the Sun, they observed very clear-cut flavor oscillations for anti-neutrinos on Earth from reactors. These had essentially the same oscillation parameters for electron anti-neutrinos as we saw from a combination of what we observed and models of the Sun.

Zierler:

Art, where were you when you got the call from Stockholm?

McDonald:

Oh, I was asleep at home. [laugh]

Zierler:

Another good sleep?

McDonald:

Well, yeah. I was having a good sleep. It was 10 after five in the morning. And the phone rang and my wife picked it up first. She thought someone in the family had died. Then she thought it was a crank call and was about to berate the caller. I picked up the extension around that point and recognized a Swedish accent. The chairman of the committee told me that I was a co-recipient of the prize. Then I was congratulated by all of the committee members, including at the end Lars Bergström, who’s actually a neutrino physicist. He was the secretary of the committee at the time. And he said to me, “Congratulations. When we last spoke in Stockholm, (which had been a couple of years before), we had a conversation about hockey as well as neutrinos.” He’s a big hockey fan—Sweden and Canada are very, very big hockey nations. And he comes from the hometown of several very eminent international hockey players. I said, “The captain of my favorite team since my teenage, the Toronto Maple Leafs, Mats Sundin, from Stockholm, just retired. I wish he hadn’t ‘cause the team’s not doing well right now.”

I wondered why Lars did it and thought that perhaps a personal comment from a committee member is done in order to ensure that it's not a hoax. There have been hoaxes in the past. I mentioned that when I was interviewed by our local Kingston paper. And the headline in the paper the next day was not “Kingston Person Wins the Nobel Prize.” The headline was “Not a Hoax.”

Zierler:

[laugh]

McDonald:

[laugh] And there was another headline somewhere else which was “Typical Canadian, He Learns He Wins a Nobel Prize and All He Wants to do is Talk About Hockey.” [laugh] Anyway, I did ask Lars later and he said, “Well, part of it was to calm you down, but part of it was to authenticate it.” So, anyway, it was an interesting experience. I gave my wife a hug and said, “Wow. What’s next?” And it really has been a wonderful experience, but very busy for five years up until COVID. The pandemic actually enables me to travel less and that was welcome.

Zierler:

Art, in preparing for the Nobel lecture. Of course, this is an opportunity to talk about the science to a much broader audience. An audience that has very little understanding of what it is that it took to get to this moment. What were some of the things that were important for you to convey to that larger audience?

McDonald:

Well, I think the important aspect of it was to be able to convey the way in which these measurements fit into our opportunity to understand the universe as a whole. The sorts of things we were talking about earlier in terms of how neutrinos fit into our understanding of how the universe has evolved since the Big Bang. The importance of perseverance and a strong team and sticking with a problem over many years in order to try to do something that’s extremely difficult to do. It’s not hard to show that with the sort of things we had to do and the type of detector and underground environment. But the value of fundamental science is mainly in terms of understanding the world around us more completely. And of course, the more you do that the more you are able to use this knowledge of science for the benefit of humans as well. There are a number of instances where that takes place. Some examples from technical developments that we have made, some that are still being made. I often point out that Time Magazine put Albert Einstein on its cover as the person of the 20th century. Not because he invented the laser or the computer or transistors — even GPS uses general relativity. Not because he invented all those things directly, but because the basic physics he did at the turn of the century was the basis for much of the technological revolution that occurred in the 20th century. That’s the value of basic science and it continues. So, those are the sorts of themes that I try to include in such discussions with the general public.

Zierler:

Art, you alluded to it earlier in some of the advice that Dick Taylor gave you. As you well know, of course, the Nobel Prize gives a platform that is really unrivaled in any other category. And on that platform you have the ability if you want to talk about things that may have nothing to do with the research. To what extent have you used that platform to talk about things beyond your specific area of expertise or not?

McDonald:

That only occurs for a short period of time if you don’t know what you’re talkin’ about. [laugh]

Zierler:

[laugh]

McDonald:

And that of course, is Dick Taylor’s basic message. However, the Nobel has enabled me to be involved in things that I found valuable. One in particular. In 2016-17, I was asked to serve on a nine member panel that was asked to review the entire academic funding system for Canada. It was called the fundamental science review. The Liberal government had just become elected on a platform in which one of their themes was evidence-based decision making. They clearly respect science and the methods of science. And so, I thought that if we could do something concrete then it would be recognized by this government and perhaps acted on. It was an excellent experience. I had a lot of experience in academia and in funding mechanisms associated with it over the years. And we came out with a set of 35 recommendations. Some of them were quite significant in terms of adjustments to the funding system in Canada. About half of them have in fact, been taken up by the government who have changed things. So, that sort of thing where I do have some experience and am willing to apply that experience to something of value is what I’ve been trying to do since the Nobel.

Another example of that is this ventilator project that we had in the last year. I am a member of the DarkSide-20k collaboration working to build an experiment to detect Dark Matter particles in an underground lab in Italy. It is a 400 member collaboration around the world with a significant component in Canada as well. The Spokesperson of the project, Cristiano Galbiati, was locked down by Covid in Milan in March of 2020.

Zierler:

Yeah.

McDonald:

I have worked with him for many years and he contacted me and said, “The sort of expertise we’ve developed for handling argon, separating it from other gases, handling it in basically high-tech ways, could well be applied to the construction of a ventilator.” His family are medical doctors. The situation in Milan was very severe, even at the beginning of the pandemic. He said, “Would you be willing to help?” I could see where we had capabilities in Canada to do this, even beyond our scientists working on the DarkSide-20k experiment. I called the director of TRIUMF laboratory, the director of the Chalk River Laboratory, the director of SNOLAB and the director of the McDonald Institute at Queen’s which actually extends across the country. (I didn’t establish the institute, that was Tony Noble, but they named it after me.) But it includes academics across the country working on particle astrophysics.

We were able to pull together a team quickly that over a course of the next six months or so developed a new style of ventilator working with manufacturers in Italy and in Canada. We participated in a competition and managed to get a significant contract from the Canadian government to manufacture what has turned out to be 7,300 ventilators that have been added to the Canadian stockpile. There are discussions going on for their donation to underdeveloped countries where there is a great need for ventilators right now.

At the time, knowing who to talk to about what was happening in terms of the development of equipment for COVID-19 within Canada enabled me to get the information for us to become a part of the review process that the government was putting forward to try to build more ventilators. And our collaboration team worked very fast to have a prototype working within about 10 days. The ventilators previously being built in Canada had many parts, making them hard to build and expensive, especially with supply chains affected by COVID. There were patents involved as well. What we developed was open source. We published our design immediately and it’s still open source. We were able to build something that was cheaper, with fewer parts, by concentrating on adult, intubated COVID patients in ICUs. We had about fifty rather than over 1,000 parts so supply chain problems were less of a problem.

Zierler:

Art, was it a totally different kind of satisfaction doing this ventilator project? Going from basic science to something that had such immediate and important human significance?

McDonald:

I think that was the case for all of the team—more than 200 people by the time we were finished—all of the team basically dropped other things they were doing with the approval of their directors or heads of their companies. They did this because in this pandemic. You say to yourself, “I’m a particle physicist. What the heck can I do to help?” or, “I’m an engineer doing something else. What can I do to help?” They were pleased to see that their talents could be applied as a part of a team to make a difference in the pandemic. They worked night and day for seven days a week for a long period in order to make this happen. And all of them were motivated basically by being able to make a contribution in something that was very important for the world. So, yes. There was an immediate understanding of what the significance might be if you could make it happen.

Zierler:

Tell me about the evolution of SNOLAB. Now we have SNO+. What is going on with that?

McDonald:

The SNO detector is now filled with liquid scintillator and soon an organic compound of tellurium will be added. It has the potential to measure a number of other things, but it’s principal objective is neutrino-less double beta decay with Tellurium dissolved in the scintillator. It will have better sensitivity than the best measurements that have been made so far on that topic. The other experiment I’ve been strongly involved in at SNOLAB has been the DEAP experiment which is using liquid argon to attempt to detect dark matter particles. Liquid argon has extremely good properties that enable you to use the timing of the light output from it to eliminate important radioactive backgrounds. DEAP has three tons of liquid argon. The DarkSide experiment at the Gran Sasso underground laboratory in Italy will have over 50 tons. Our same international collaboration is projecting in the longer term to use 400 tons in SNOLAB for our future experiment using this technology. Ironically it will push to the point where the interfering background will be neutrinos. So, I’ve gone from neutrinos being the object of the experiment to neutrinos being the interfering background in the future work that I’m doing. These are all similar projects where a group of people (in this case the collaboration is over 400 scientists) who are very skilled and can push things right to the frontier. There are a number of other experiments looking for dark matter that are very valuable in their different configurations because we don’t know exactly how it interacts with matter, so it requires a variety of experiments to cover the landscape.

Zierler:

We’ve come right to the present, so I had one more broadly retrospective question and then we’ll end looking to the future.

McDonald:

OK.

Zierler:

Art, that question is, the really broad retrospective question is, in all of the writings about you, you have been variously identified as an astrophysicist, a neutrino physicist, a particle physicist, and a nuclear physicist.

McDonald:

[laugh]

Zierler:

I wonder if the lesson from all of that is the legacy of your career is that all of these disparate disciplines are not really as far apart as the nomenclature might suggest. I wonder if you could reflect on that?

McDonald:

It’s absolutely true. And I brought a little bit of all of those things to the work that I’ve been doing. These days I would refer to myself as a particle astrophysicist. And that’s actually the title of the chair that I occupied at Queen’s and am now emeritus. I think that’s true of many of my colleagues, that they have worked on a wide variety of different things and bring these things to the table for their current work. What’s changed in the field of physics since I started my master’s degree is enormous. We were using slide rules at that point. You have to be able to follow new trends, to learn new things, and in the process develop the skills that enable you to make progress.

Zierler:

You’ve been involved in so much fundamental discovery which as you say, peels back a layer of the onion to look at new mysteries of the universe. With time as a precious resource and all that there is to discover, what are you most curious about? What do you want to be involved in for as long as you want to be active in the field?

McDonald:

[laugh] Well, that’s turning out to be a combination of searches for dark matter and searches for neutrino-less double beta decay. Those are the two things that I am involved in now and will continue to be involved in going forward. Both would be very valuable if we are able to make observations that are well proven and tell us what this dark matter really is and how it fits in the components making up our Universe. Or tell us whether the neutrino is its own anti-particle and possibly thereby tell us what the mass of the lightest neutrino might be. We know the mass differences between the three types from oscillation measurements; we just don’t know the mass of the lightest one to give us a mass of all three active types. They’re solid fundamental measurements that I enjoy participating in.

Zierler:

Art, this has been a tremendously rich and captivating discussion. I want to thank you so much, so deeply, for spending this time with me and for contributing to the historical record. So, I really appreciate it. Thank you.

McDonald:

Well, thank you. I’ve enjoyed talking with you.