David Nygren

Zierler:

OK. This is David Zierler, oral historian for the American Institute of Physics. It is May 28th, 2021. I am delighted to be here with Professor David R. Nygren. David, it’s great to see you. Thank you for joining me today.

Nygren:

It’s my pleasure, David.

Zierler:

To start would you please tell me your current title and institutional affiliation?

Nygren:

I am Presidential Distinguished Professor at the University of Texas at Arlington in Arlington, Texas.

Zierler:

How long have you had this title and your affiliation with UTA?

Nygren:

I joined UTA on August 14th, 2014.

Zierler:

I wonder if you can tell me how many other, or if you are the only one, who has the title Presidential Distinguished Professor?

Nygren:

I think there are now about eight. I believe this was a title invented to celebrate members of National Academies. I was the first member of a National Academy to join this particular university, as a member of National Academy of Science. Since then there have been members of the National Academy of Medicine and the National Academy of Engineering who have also joined the faculty at UTA.

Zierler:

David, just as a snapshot in time, what are you working on currently? What’s interesting to you personally and in the fields that you’re involved in?

Nygren:

Well, I am pursuing two, actually three thrusts at UTA. One is part of deeply fundamental physics as I would call it. Almost all physicists would say that all of their physics is fundamental, so I have to be careful here. But this thrust is a strange enterprise using nuclear physics aided by biochemistry to do particle physics partly as an inquiry into cosmology. Another thrust, which is related only in that neutrinos are involved too, is to develop a new technical approach as a candidate technology for the DUNE project at Fermilab, to detect neutrinos from a beam generated in Chicago and sent all the way through the Earth to South Dakota. This is based on electronic ideas which are in a sense very simple, but also I would say perhaps novel. That’s number two.

The third thrust at UTA is that I think that this university, like almost all universities, is structurally discordant or incongruous with the problem that we face today, in this century, in this decade, as to global stewardship of the planet. As we all know, universities now consist of highly specialized, highly honed silos of specialization. Of course this works very well to train people in those specializations. At the same time, it’s my opinion although I could be mistaken, that no university has what you might call thoroughly embraced the global challenges we face — seen most visibly in climate change that humanity faces, collectively, which we can only meet by individual actions taken seriously and appropriately. So, I guess you could say I’m on a campaign. I don’t mean to act as a revolutionary or even a reformer, but to induce or evoke a transition in the university so that it becomes what I call a university for the 21st century. To do this I’m making contact with as many faculty as I can, and I find that quite often there’s a huge latent energy that wants to address these problems. It’s not just global change. Of course there’s the loss of biodiversity. There’s political unrest. There are migrations of desperate peoples due to climate change, and there are technological issues such as modern genetic advances that has led us to genetic modification. There are even concerns about where the pandemic virus came from, whether that was natural or not. And so what you might call the age of unmanageable change is upon us. This is just another perspective on this.

So, to summarize this in a way is, I’m trying to build on the energy that I see within the faculty at the university to build a new vision of what the university should be doing. It’s not to do away with specialization, but to embed it in some sort of larger picture. If you look at, not just look at, but talk to students, they’re very anxious. They don’t think they’re going to have the opportunities that my generation or their parents did. They would like to see something done about it. Basically, the universities—and I do mean all of them, that I’m aware of, even those premiere institutions that have quite substantial activities in climate change or environment and so on—none of them are saying the university really needs to turn around and say, “This is the issue. We’re going to embed this in the curriculum in a way that everybody recognizes that we’re really confronting the really big picture.” This is a wicked problem. We don’t really know how to do deal with it as a university. So, really the response is more or less, “Well, let’s let the faculty do what they want and see if something happens.” I’m not trying to criticize my university because I don’t think it’s unique in this regard. I think that there is going to be a — one could call it a reformation, but I think it will actually be an internal transition, so to speak, in which what you might call global change in the Earth-Human System becomes foremost.

Just look back a moment. If we look at Greta Thunberg who evoked a tremendous response among young people all over the world, well, what happened? I think it’s safe to say, not that much. I certainly don’t mean to demean her intentions or her impact, but really we haven’t been able to harness that energy. Certainly not within the universities or even probably in K-12, as well. So, I think there’s a huge opportunity here. I’m fumbling my way along. I think that we’ll find a solution within which the university can act within the constraints that it sees, you know, as a state university system in a state with mixed politics. Let me put it that way. It’s an experiment. As an experimental physicist, I like to provoke the system and see what happens.

Zierler:

David, because the coronavirus pandemic has turned all of our lives upside down, higher education has not been remote for many of these things. Do you see opportunity in this current crisis in university administration? In starting to rethink the way things are so that it might be more amenable to the kinds of changes you’re envisioning?

Nygren:

Oh, thank you, David, for that question. Sure. Yes. But no good crisis goes unused, so to speak. And so, yes. We are now in a state of churn in every university. The business models are somewhat upended too. There’s going to be changes and some of these changes will be perhaps not so successful. But we know that by the mid-century life is going to look different to us. So, I think that yes, to answer your question quite directly. We have the opportunity now to really take steps forward. Even little incremental steps are what’s necessary. You can’t take a complicated system like a university and make rapid changes because something’s going to break. But you can gradually evoke the energies, I like to say, that allow it to change itself.

Zierler:

Speaking for yourself more personally as an experimentalist. With remote work and the social mandate of not being around your colleagues, how has your science been affected over this past year plus?

Nygren:

Negatively, but not so much as you might think. Even though we do lab work, we were still able to manage to do quite a bit. I would guess that we took maybe a 40% hit in productivity, which is very substantial. But nevertheless, due to the miracle of Zoom, problems notwithstanding, communication has been good. Education suffered mightily though. Lecturing via Zoom is just not going to work as the preferred medium or primary medium of education. We need to be in the classroom together. And that suffered very much.

Zierler:

David, I’d like to ask a very broad question rooted in the traditional way we understand foundational discovery in physics to proceed. And that is, there always needs to be a successful interplay between experimentation and theory. What is your sense generally of where that interplay is in the fields that are most important to you currently?

Nygren:

Oh, it’s a wonderful time of interconnections. In particle physics and cosmology there’s been a tremendous integration of perspectives and expertise, ideas and everything else. The theoretical terrain is quite complicated and it’s an unusual experimentalist who can navigate through and make contributions there. One might say that the era when people like Enrico Fermi could do both experiment and theory at the forefront, well, that may still be here but it doesn’t seem like it. We have become somewhat specialized. I fell into one avenue of specialization which many people would not call experiment per se — but instrumentation. Detectors and stuff like that. I prefer to call it the Art of Experiment because all we have is a toolbox. We don’t have rules or recipes, really, to make new advances. It’s a wonderful time. The particle physics theory has matured greatly, but we remain clueless about some of the most fundamental questions we have, like, what is dark matter? What is dark energy? Why is there a matter-antimatter asymmetry in the universe? And why are there quarks and leptons and all that? We are clueless about these questions and can only hope that the tools we bring and the ideas we bring will maintain the level of pace that we’ve had all through the last century in this field.

Zierler:

On the question of physics beyond the Standard Model, it’s been fun asking eminent physicists their instant reactions to the muon anomaly experiment at Fermilab and all of the excitement that that’s creating. What’s your sense of what’s happening at Fermilab right now?

Nygren:

Well, I think Fermilab has done a fabulous job to take its mandate in this new era where probably there’s no huge new accelerators to be built in this country and really do forefront physics. The anomaly of muons in the g-2 experiment is a playground for theorists because there are hundreds of ways in which this departure from the simple expectations of Standard Model could occur. It’s not so clear how we will investigate that. There are ways to try to see you know, in various leptonic decay modes. I wouldn’t want to speculate on where the most productive avenue will be. But congratulations to Fermilab for producing a seriously new result that says there’s real, new physics out there beyond what we know. We don’t know what the energy domain is or what the mechanism is, but we know that the Standard Model isn’t complete. It’s just great that they have been able to produce a really credible result for the g-2 anomaly.

Zierler:

Well, David, we’ve been talking in the present and future tense. Let’s do some oral history now and go 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.

Nygren:

Well, my parents were born in Spokane and Minnesota. All of my grandparents came from Sweden and emigrated to Seattle because it looked a lot like Sweden, I think. My grandfather even ran for mayor because he thought his Swedish name would win on that alone because there’s so many Scandihoovians in Seattle. He didn’t win — he was too honest. My father graduated from the University of Washington in foreign trade in 1932. What could be worse?

Zierler:

[laugh]

Nygren:

My father worked in my grandfather’s bridge building company for a long time. I worked two summers in Alaska in that business, and that was a wonderful experience for me. My mother was a high school graduate. She worked for doctors, then eventually in the Swedish hospital. Yes, Seattle does actually have a Swedish hospital. It really felt like a family. My sister is three years older, living happily in the Michigan in Fenton near Flint. It’s a success story. She’s had a good life and so have I. Does that help?

Zierler:

Where in Seattle did you grow up?

Nygren:

Well, I went to Magnolia Elementary and graduated from Queen Anne High School in Seattle.. I actually spent some years in California earlier. My father did rise to the point of Chief Examiner for the Port of San Francisco in the US Treasury Department during the war. So, we moved around quite a bit. My father had an illness that prevented him from continuing on that trajectory, but nevertheless I spent most of my maturing years in Seattle. After fourth grade on, I was a Seattleite.

Zierler:

David, as a young boy what are some of the memories of the United States being in World War II that stick out in your mind?

Nygren:

I remember that things were rationed and there were lots of caricatures of Japanese and Germans in the newspapers that one wouldn’t imagine today, but actually did exist then. We saw lots of planes in the air. There were even plane crashes that had to do with typical military operations. It wasn’t particularly scary for me. I don’t think we really had any real deprivation during the war. I was still a little kid and those years have become a little bit of a blur from the standpoint of World War II.

Zierler:

How did you express your early interest in science? What were the kinds of things you did when you were a kid?

Nygren:

Well. I think the one thing to mention is that World War II after the fact was a bonanza. There was a tremendous amount of World War II Army surplus, Navy surplus, and so on. I remember that after high school I would go down to St. Vincent de Paul in Seattle and buy stuff for two cents a pound and then call my parents to come and pick me up. I had all this wonderful stuff to play with and learn. So I had great opportunity to learn. By the time I was in the ninth grade I knew about electronics, Ohm’s law, etc, and I could design circuits.

I had a diathermy machine, which is like a microwave oven on the loose. Doctors did use them to warm people’s tissues by putting the output paddles on their arms and things like that. I can’t imagine people doing that today. With my diathermy machine I discovered that I could put herringbone patterns on every TV set in the neighborhood, on every channel. The FCC was out after me in their trucks with the little circular antennae, but they never caught me. It’s not that I wanted to make trouble, but it was an experiment and successful. Another experiment was done with a friend of mine. We had these World War II walkie-talkies that operated around 150 megahertz. We said to ourselves, “Well, let’s see what the range is.” So we put some great vibraphone music on the input of one walkie-talkie and set out in our car with the other one. And by God, the range was pretty good, even at several miles. Pretty soon another voice came on and said to us, “This is the Federal Aviation Administration. You’re broadcasting on federal frequencies and please stop immediately!”

Zierler:

[laugh]

Nygren:

But we couldn’t because we were miles away. We couldn’t turn it off. We apologized with our other walkie-talkie [laugh] and then drove home. So, there was a lot of hijinks like that that actually were really quite wonderful as learning laboratories. Just having that stuff was really wonderful. I would have to say that the other part of a misspent youth was with automobiles. Those days, you could open the hood of cars and could see what was going on. The engines were self-explanatory. I spent way too much time with my ’49 Mercury Convertible. I put a ’53 Lincoln overhead valve V8 in it. That was a big waste of time in a sense, but I still managed to graduate from high school and go onto college. So, it was a potpourri of various activities that don’t exist today. In those days you could take a phone apart, see what was inside. Don’t even think about that today. So, I think today’s kids have a disadvantage in that they’re awash in technology, but it’s all invisible.

Zierler:

Yeah.

Nygren:

And they’re utterly dependent on it.

Zierler:

That’s interesting.

Nygren:

Like me. Yeah.

Zierler:

In high school did you have a good curriculum in math and science?

Nygren:

I would say that the curriculum in mathematics was really good. Yes. Physics was also very good. For example, I’ll get back to math in a moment, but the physics instructor had a magneto which is what was used on early telephones to ring the bells. He would have every student in the class hold hands and then give the first and last a wire connecting to the magneto. When he cranked it, of course, we all had an unforgettable experience. No teacher would try that today. But in those days we did have that sort of experiences.

In the math department, I was taking advanced calculus from Ms. Sylvia Weinstein, an excellent teacher.. I was bit of a smartass and I thought, “Well, I don’t need to turn in the homework. I can easily get a good grade on the test.” This became a duel between the teacher and me. She finally said, “David, if you don’t do the homework I’m going to give you a C even though you’re getting top grades in the tests.” I didn’t believe her. Well, she gave me a C! Well, this was one of the first times I saw that an adult can really mean what she says.

Zierler:

[laugh]

Nygren:

So, this was the end of the first semester in my senior year. In the second semester I doubled down and made sure I got the highest grade on the test, but still didn’t turn in any homework. Ms. Weinstein said, “You know, if you keep this up I’m going to give you an F.” I didn’t believe her. And she did! So when I got to college and said that I wanted to major in mathematics and physics the professor looked askance at me and said, “Well, I’m looking at these grades on your transcript here. Are you sure you want to major in mathematics?” [laugh] Well, I did major in mathematics and physics, with honors, and I got through OK. Those stories not meant to illustrate how one should lead one’s life, but it’s an example of how I muddled through. And actually I did learn the math pretty well in the end.

Zierler:

Those grades notwithstanding, between your overall academic performance, your family’s financial capacity, and any geographic considerations that might’ve been important, what kinds of schools did you apply to for undergraduate?

Nygren:

Well, I applied to the University of Washington. But really, the story is this. When I was in high school I was randomly assigned to be the host of an admissions counselor. All the nearby colleges and universities would come to the high schools with their spiel, and I just happened to be assigned to one particular college. I led this poor fellow around through all six periods and listened to his pitch six times. So, guess which college I wound up applying to and going to? [laugh]

Zierler:

[laugh]

Nygren:

That’s a true story. I went to Whitman College, which was excellent for me. It’s one of the better small colleges in the country, I think. I graduated with honors and then went to graduate school at the University of Washington, Seattle.

Zierler:

Were the professors at Whitman engaged in original research or their responsibilities were entirely in teaching?

Nygren:

They tried to do original research, but they really had too many classes. I really don’t know how they did it. For the most part the mathematics instruction was great but the physics instruction was a mixed bag at that time. Of course, the faculty has changed and I think that all of the faculty now at Whitman are likely to be absolutely first rate from what I can see during my occasional visits. So, yes. I learned math and physics there but I have to admit that I wasn’t a very serious student. I was probably saying to myself, “Well, I’ll just do what I can to look pretty good, but I’m not really putting in enough work to really master this stuff as I might, as I could.” So, even though I graduated with honors, looking back I just have to admit I was not a serious student, or certainly not a serious scientist at that point.

Zierler:

David, as an undergraduate the Russians launched Sputnik. I’m curious if that had any particular impact on you or if later on you recognized the kinds of opportunities that it may have created?

Nygren:

It had great impact psychologically, of course, to see the Russians do this incredible technological feat. I wasn’t really interested in engineering per se, but I profited from that because a lot of money started flowing as a consequence into physics and other fields related to engineering. It was easy for me to get graduate fellowships to help pay the bills when I was a graduate student. Just walk down the office, say, “Can I have a fellowship?” “Sure!” Not so true today.

Zierler:

Did you have a well-defined appreciation of the binary in physics between theory and experimentation? And did you gravitate toward one or the other by the end of your undergraduate experience?

Nygren:

Yes. I realized that well, I was pretty good at math but my psychological constitution was more hands-on. I liked to feel I was in contact with something where I could do an experiment and see what happened. So that level of scientific curiosity I think was the driving factor in what I did. Of course, a theorist could say that too. But I think I liked tinkering with stuff and seeing how it worked.

Zierler:

Did you want to go to graduate school right away or did you take some time off?

Nygren:

No, I went immediately to graduate school. I couldn’t think of anything else to do. But actually I was…what’s the right thing to say here? I was gradually figuring out that this stuff was really quite interesting and that it was worth really trying to get a good understanding of something. I can’t say I was a serious student yet, to be honest. But my curiosity was very strong. I was driven by curiosity to go to graduate school and that’s one strange way to put it. Yeah. I wanted to know more.

Zierler:

Similar question as before. Did you think about applying farther from Washington State for graduate school?

Nygren:

Yes, I did. I applied to some prestige places back East, but wasn’t admitted. It was then a no-brainer to go to University of Washington which is a very good school.

Zierler:

Who ended up being your advisor at UW?

Nygren:

His name was Robert W. Williams.

Zierler:

What was Williams’ research? What was he doing when you connected with him?

Nygren:

Well, he was doing particle physics with another professor at the university. I have to tell you that when I was a graduate student—if I can back up—that I thought I would go into low temperature physics because superconductivity and all of these things that had happened at ultra-low temperature were fascinating! I went at the end of the first year to the low temperature professor and asked him for a summer job. He said, “Sorry.” So I just went around the corner and knocked on the first door and there happened to be Bob Williams’ office. I asked him if he had any summer jobs and he said, “Yes.” This is more or less how I became a particle physicist. And of course, it turned out to be extremely satisfying to get into the lab and start doing things.

Zierler:

Did you have a sense at the time that this was such a foundational time in particle physics? That exciting things were happening almost every day?

Nygren:

Yes. I really felt that the experiments were discovering things, maybe not fundamental or deep things every time, but it was pretty easy to do an experiment. My thesis had about six people on it, for example, done at the 184-inch cyclotron at Berkeley. It was an absolutely wonderful experience.

Zierler:

You spent a lot of time at Berkeley for your graduate research?

Nygren:

I did. I first came down to help on the other experiments that Professors Williams and George Masek were building at the Bevatron to do muon-proton scattering. I was greatly helped by friendships with other graduate students who were very confident and who became professors on their own merit. It was a wonderful time. I should also add the skiing was good in Seattle and the beer was good too. I would have to admit that I still wasn’t very serious—I made the effort to get good grades so I could think positively about myself. But I wasn’t serious yet. I’ll be honest about that.

Zierler:

To go back to an earlier question. When you were a graduate student working on these experiments what, if any, theory was providing relevant guidance to your work?

Nygren:

I would say that the relevant theory was pretty humdrum stuff. My thesis really didn’t have a lot of fundamental aspects to it. My thesis turned out to be the neutron-neutron S-wave scattering length which involved a simplified effective theory, at that time. The experiment was great, done, as I said, five or six other people at the cyclotron. So, I would say, no, it wasn’t theoretically motivated so well.

But people needed to know the answer because if neutrons did stick together—of course, we know they don’t—but if they did, stars wouldn’t shine like they do and the whole universe would look different and we wouldn’t be here. The very delicate balance in the neutron-proton, proton-proton, and neutron-neutron interaction is amazing. But an MeV here or there would’ve made all the difference. But the neutrons don’t stick together.

Zierler:

Did you recognize at the time the astrophysical implications of this finding or that happened later on?

Nygren:

No, it happened later on. I was just focused on getting through and moving on. So, again, I didn’t have the big picture, which came to me later.

Zierler:

Besides Williams, who else was on your thesis committee?

Nygren:

You know, I don’t remember everyone. George Masek was. There was a theorist and I think another physicist by the name of Ken Young who’s no longer alive. It was four experimental physicist and one theorist. Yes.

Zierler:

Anything memorable from the oral defense?

Nygren:

OK. I have to tell the truth. You know, I was in no great hurry to graduate because it was so much fun. I’d gotten married and life was good. But I was strongly encouraged to graduate, and received a job offer from Leon Lederman at Columbia University. Even though my thesis wasn’t quite finished or typed up, my wife and I set off and drove across the country. We arrived eventually at Nevis Laboratories in Irvington, New York, part of Columbia University. I went in there and I asked, “Well, can I find Leon? Can I be introduced to Leon?” Leon wasn’t there that day, but another prominent physicist, Jack Steinberger, was there. I was introduced to him and Jack said, “Oh. Go out to Brookhaven Laboratory and join this particular experiment.” And so I did. My wife and I went out there and got housing and I joined this experiment. I think it was a couple weeks before I realized I’d been hijacked and kidnapped by Jack and set to work on his experiment rather than Leon’s.

Zierler:

[laugh]

Nygren:

Now, Jack and Leon had this on-going collegial rivalry, and Leon never held it against me. Working with Leon would’ve been great too, of course. I have to admit that that random moment at Nevis Laboratories in 1967 changed my career, and my life. Soon enough, Jack found out that I hadn’t finished my PhD properly, a No-No, for a post-doc and sent me away to finish up. I flew back to Berkeley but realized I hadn’t really prepared for my defense. I think the committee just wanted me to go away, so I soon got a half-page piece of mimeo paper saying I had completed all requirements for a PhD. I should have kept it.

Zierler:

[laugh] What was that accidental experiment that you found yourself on?

Nygren:

Pardon me?

Zierler:

What was that experiment that you accidentally found yourself on?

Nygren:

Right. This was a wonderful beginning of a series of experiments to measure the semileptonic charge asymmetry in neutral kaon decays, the long-lived neutral kaons. The neutral kaons can decay into a positive pion and electron and a neutrino. It could also decay into a negative pion and a positron, of course. And the opposite kind of neutrino. It was thought that these decay rates would be equal. If the rates weren’t equal between the positive and negative that would be evidence of CP symmetry violation. Well, CP violation had been discovered a few years before in the difference between K-long and K-short decays into two pions. That discovery implied that an asymmetry exists in the constitution of the long-lived kaon, between K0bar and K0, the states of definite strangeness. It turned out that the charge asymmetry was indeed there, at about 3 parts per thousand favoring the positron,. We did this with thin scintillation counters, a big helium bag and a Cherenkov counter, Jack’s design. And it worked. We measured the number quite precisely.

We were in competition with Mel Schwartz at Stanford, who was using an electron beam to make kaons, which wasn’t as efficient. We got a positive and quite definitive result and that was great. It was about this time that I realized that being at Columbia around these super impressive people — Jack Steinberger, Leon Lederman, T.D. Lee and others, was a great opportunity. I thought, “You know, this stuff is really interesting. I’d better get serious and learn more about it. If I want to have a career in this I’d better do a good job.” So, it was only then at Columbia that I started to realize that, Wow - this is good.

Zierler:

Being at Columbia in the late 1960s must’ve been very interesting on the social and political side of things.

Nygren:

It was. Later on, when we were doing other experiments, in particular looking for the long-lived kaon to decay into a muon pair, mu+ and mu-, we had to take a huge amount of data because the branching fraction was thought to be a few parts in 10-8. A Berkeley group had not seen this decay mode at the expected level according to the Standard Model. That was a problem for the Standard Model as it existed then. We took a huge amount of data with our great MWPC-based detector. I haven’t told you about that detector yet, but it was a follow-on to the one I just described. We were just taking many tapes of data.

I remember taking boxes of magnetic tape to the high-performance computing center on the Columbia campus. I would get time only at night to run these tapes through that IBM Model 90 or whatever it was. It was a wonderful experience. I learned how to run the computer because the operators were often sleepy and I could run the data through myself. But then they said, “Well, you gotta get these boxes of tapes outta here. They’re kinda gettin’ in the way.” So, late one night I was walking across the Columbia campus at the time of the protests. I was soon arrested by a cop on horseback who said, “Hey you! You’re stealing that box of tapes!” He forced me to come with him to the office of the campus police department and surrender this box of tapes. I was actually very happy to do this because I didn’t want to carry them through the streets of New York at night. So, this was wonderful to be able to give them to the cops.

Zierler:

[laugh]

Nygren:

Then I could go home and rest in peace. So, yes. And I remember one of my questions on an exam was, “If a protestor can throw a brick through a window in the second floor, what’s the velocity that brick must have, leaving his hand?” And that sort of thing. [laugh]

Zierler:

[laugh]

Nygren:

So, yes. It was the days of protests. But it didn’t really touch me very much by that time. Rumors were flying around suggesting we had this number or that, but we didn’t know. When we finally opened the data, a blind analysis — still rare at that time — we saw the decay at just the right rate. We announced the result at an international meeting on the University of Chicago campus in a mock battle with our Berkeley colleagues. I found out later I was on the cover ot the UC Alumni magazine, hiding the as-yet undisclosed results, on paper behind my back, from Carlo Rubbia and Jim Cronin, two Nobel Laureates. The Columbia experiments were transformative for me. I saw how physicists who really knew their craft worked. Working with Jack Steinberger, I got a sense of style that I think was very precious. He was very diligent and very careful. He really forced people to defend their positions and their answers and their data. So, that experience is what really turned me into I think an effective scientist.

Zierler:

David, as a postdoctoral researcher how well aware were you of the major advances that were happening at the same time at places like Harvard, at Berkeley, at SLAC? Were you parochial in your interest at this point or you were aware more generally of what was going on?

Nygren:

Well, there was a lot of rumors that went on. We didn’t have the internet in those days so information travelled haphazardly. And I would say that my interests were certainly parochial when I arrived at Columbia. But by the time I finished there several years later I really was getting the big picture about physics, and I had learned from Jack and other students and postdocs really how to do it. So, the parochialism faded away as I was becoming more serious. I made the effort to go to the library and talk to people and go to conferences. Altogether being at Columbia was a transformation for me.

Zierler:

What were the circumstances of joining the faculty at Columbia? Did somebody specifically ask you to apply?

Nygren:

Well, no. Jack came up to me one day and said, “OK, David, you’ve been a postdoc. Now, I’m going to make you a faculty member.” I was soon teaching an evening class and I wasn’t very good at it. I had a very difficult time balancing time and energies between preparing lectures and research, which was so exciting out at Brookhaven. I really didn’t have any experience or mentoring. I would give myself a B- or C+ in terms of my academic performance. I remember going to a faculty meeting feeling completely insignificant and ignored. I remember seeing I.I. Rabi there, if you can imagine! I actually had that experience — he was in the same faculty meeting. But I really was insignificant as far as the rest of the Columbia faculty. Jack did that to protect me, I think, —that I’d have some sort of advance on my resumé. He didn’t really care about teaching so much. By this time he and I had become friends as well as colleagues. I very much appreciated what he did for me at that time. I thought of myself as a failed academic after that and, in fact, I sought a non-academic job as a consequence. I’m really more of a research person than faculty.

Zierler:

Now, the title associate professor at Columbia, is that a tenured position or no?

Nygren:

No, I wasn’t an associate professor. I was a lowly assistant professor.

Zierler:

Oh, I see.

Nygren:

It was not a tenured position.

Zierler:

And what were the prospects of achieving tenure at Columbia? What was your sense?

Nygren:

Zero, I thought, and I wasn’t really interested. I wanted to get back to the West Coast anyway. As a parochial West Coaster, I really had a hard time adjusting to the busy life of New York City. I wouldn’t say that I got used to stepping over bodies and things like that, but it can be pretty harsh at times. So, I never entertained any hopes of being a faculty member at Columbia.

Zierler:

What were the opportunities that led to your employment at Berkeley Lab?

Nygren:

Well, I think Berkeley had been going through a strong decline after the huge postwar surge in nuclear and particle physics, benefits of the so-called fallout you might say, of the atomic bomb. Berkeley had by this time gone through some very large budget reductions, yet they decided to start something called Divisional Fellow, a kind of glorified postdoc. But it was intended to lead to permanent staff scientist. I loved Berkeley. I mean, it was paradise then. I don’t think of it that way now. In any case, I got a job offer from Berkeley and that was enough. Jack congratulated me because he said I succeeded in getting something that he didn’t, in getting a job at Berkeley. He got fired by Alvarez because he wouldn’t sign the loyalty oath from that era of McCarthy. Jack did have a sense of humor. Many if not most people didn’t think so, but he did.

Zierler:

Was the cyclotron still in operation from your graduate school days?

Nygren:

Yes it was, but it had been converted into a medical treatment machine. In fact, one of my colleagues was successfully treated at the cyclotron for an inoperable pituitary condition. So, it came close to home. Yes. The cyclotron was still running and it was a great machine. I have wonderful memories of working there.

Zierler:

What was your first project when you got set up at Berkeley?

Nygren:

Well, I was given freedom. I thought I would join Owen Chamberlain and his gang for a project they were doing to find out what happens when nuclei strike other nuclei at high energies. This was really the first attempt to do that. The heavy-ion linac HILAC up on the hill had been augmented with a beamline that brought nuclei at MeV energies down through the hillside, through the trees and bushes to the Bevatron. So, the BEVALAC was born. We were colliding deuterons, helium, and other things with other nuclei to see what happened. And measuring total cross-sections. I designed a target changer, pretty boring stuff. I developed a friendship with Owen Chamberlain and that was nice. The first time I went to talk to Owen I went into his office and there he was, behind his desk between two or three tall stacks of papers. Between these paper skyscrapers, so to speak, he was visible. He was a real gentleman and I enjoyed very much getting to know him. Later, he joined me in the era of the time projection chamber project. And there’s a little story there I would like to tell.

Zierler:

Yeah. Well, let’s get to that. Let’s get to the origins of this project. What were the problems that made you think to build this detector?

Nygren:

I think this is the most important moment of my career. After my years at Columbia I had set myself into the same perspective that Jack had adopted, which was to figure out what are the most important questions that can be tackled and then figure out how to tackle them. So, he had defined in a sense my taste in physics. I was prepared to try to find something important to do or fail. People were starting to get worried that I wasn’t really doing anything, just sitting in my office thinking. Meanwhile, a group at Stanford was building the new Stanford positron-electron accelerator ring called SPEAR. I thought I should try to design a detector for that. So I was up in Building 80 sitting at a drafting table and was getting nowhere. I had nothing but a big pile of paper on the floor, feeling quite defeated. I realized something — I was just using conventional ideas. Then I remembered something from graduate school days that had been interesting and strange, something nobody had paid much attention to. That something was a spark chamber operating inside of a strong magnetic field. The sparks were seen to be much more intense and much narrower than when the field was absent. While this effect was useful in the image processing, nobody thought very much about it. I too had thought that was interesting at the time, and I remembered it. What I gathered from that is, as it occurred to me at this moment 10 years later, that Gee, if there’s a magnetic field parallel to the electric field, (of course, a spark chamber has an electric field to generate the plasma, but most spark chambers are not in a parallel magnetic field) somehow the magnetic field prevented the electrons from spreading out. And I thought, Wow — could this really be true? If I could do that somehow, then, if I apply a magnetic field in the same direction, I might be able to take an image of a track, which is ionization electrons in gas, and drift it with an electric field through the gas but retain the information quality, prevent it from being degraded by diffusion. The field direction could be either forward or against, but as long as there was a magnetic field it might work.

I sat there at the drafting table in a trance, for quite a while. I didn’t have any clear way to estimate, so I went down to the library and I found this lovely little blue book called Electrons in Gases, by J. S. E. Townsend, a famous physicist working at the turn of the 20th century. And there on page 20 was a very simple equation. According to Townsend, the diffusion transverse to the magnetic field is going to be suppressed by a factor of one over 1+ omega squared tau squared where omega is the cyclotron frequency of the electron and tau is the mean collision time between collisions of the electrons with the gas molecules. So, the idea is that the electron corkscrews along until it hits something, and then it can bounce a little bit. But overall, the transverse diffusion is suppressed while longitudinal diffusion is not. So, there it was! A very simple formula. I thought, “Wow. This is great!” The amazing thing is that this book was published in 1948 and apparently nobody had looked at that page, right there on page 20, and said, “You know, I can use this.” Georges Charpak had recently developed his multiwire proportional chamber, so people were very much in the game of gas-based detectors. But the more amazing thing is that Townsend in his book referred to an earlier paper of his published in 1912 in the Proceedings of the Royal Academy! I was able to get that and there was that equation again. So, for over half a century this very useful idea that you might be able to transfer images through gas and suppress the diffusion and retain the image quality was just available, but nobody had noticed it. Well, I’m glad I noticed it because it was really a transformative moment. A couple days later I wrote this up as a proposal, in February 1974. Detectors up to that time had many hundreds to thousands of wires going this way and that and were mechanically very complicated.

This was so weird because my idea implied that you could build a powerful detector with nothing in it except gas and fields! And yet it promised to be able to see any number of tracks in a practical sense and capture them unambiguously in 3-D. You would measure the time it takes for each track element to drift in the z-direction to the readout plane, and then measure a 2-dimensional x-y position at the readout plane. So you get 3-D information that way. I’m getting into a monologue here.

Nevertheless, it took about a year for these ideas to sink in to say, “yeah, maybe it really can work.” I initially had the wrong idea about which gas to use. I thought argon/CO2 might be good, but no. Providentially, it turns out that argon/methane is perfect for this. Argon and methane both have a transparency called the Ramsauer-Townsend effect, a quantum mechanical phenomenon. There’s Mr. Townsend again. If you position their energy at just the right point, about one quarter of an electron volt, the electrons can drill through this argon/methane without seeing it. So, you tune the electric field so that the argon/methane becomes transparent to the electrons. And it worked!

Eventually we managed to build this huge detector. It was a cliffhanger. We realized that we weren’t going to be able to build this and demonstrate it in time for SPEAR. But Stanford was planning to build a new, much higher energy ring called Positron-Electron Project, or PEP, so we turned our attention to that.

We were very afraid of the competition from SLAC so we thought “Oh, we must have all of the capability we can imagine”. We decided to pressurize the gas so that we could have better ionization density information and thereby do better particle identification. Of course, the mechanical complication of pressurizing a big detector is enormous. There’s a big story in that.

Nevertheless, just before we had to deliver the proposal in, I think December 1976, we ran our little prototype the size of a suitcase inside a big bending magnet at the Bevatron. We were running out of time. Catastrophe! A spark occurred and a wire broke immeditaely due to metallic dust blown in from the purifier. So we took our prototype out, snipped out the broken wire, put a little five minute epoxy on that sharp point and shoved it back in at the Bevatron (which was running only for us). The detector worked beautifully and we got great data. When the time came, I managed to make a good presentation to the Stanford Program Advisory Committee. We managed to beat out the very powerful Stanford group in the final selection. In fact, Owen Chamberlain, who’d been on the committee, chose to resign from the committee because he wanted to join what we were doing. And he joined and he ran the electric field group.

[phone ringing]

Nygren:

What’s—let me turn this off. Hang on. Hi, Tom and Brenda. I can’t talk now. I’ll call you back. Bye-bye. Sorry. [laugh] Let me turn this off. Sorry about that.

Zierler:

Owen Chamberlain joined the group.

Nygren:

Pardon?

Zierler:

You were saying Owen Chamberlain joined the group.

Nygren:

Yes. And he, in a very scholarly way, he managed the development of the electric field. There’s a story there too, but I think we could go on for too many hours there.

Zierler:

[laugh]

Nygren:

The upshot is that the Berkeley laboratory realized that there’s something really good here and they decided to really get behind it. We had no clear idea how much money we were spending and concluded that we had enough money even though it turns out we didn’t. We went ahead and bought all the electronics. And what that meant is that the laboratory had to scrape up a lot money then to cover our bad checks. In those days a million dollars was a lot of money. I think people at the lab, many people at the lab, didn’t know where their overhead went that year. [laugh] I don’t know if that should be in the transcript.

Zierler:

[laugh]

Nygren:

The lab really supported us strongly. I’m in the process of writing this up because it’s a genuine saga and it is, I think, of genuine historical interest. Nonetheless, we managed to get it all to work finally after many mechanical problems and electrical problems. It worked just as the theory said it should, the simple theory. But that took years and what I would like to mention is that no graduate student quit, as we took maybe two to three extra years to get it to work and lots of extra money. But it finally worked! And these days, time projection chambers and various incarnations are everywhere. I certainly didn’t imagine the uses in which the idea would be put.

One thing that comes to mind at this point is that in 1974, electronics was still rather primitive, relative to today. When we were approved to build this detector we had no way to capture the data. Electronics was still too slow. We had heard about some newfangled thing called the charge-coupled device. Now this was not an imaging device but rather an electrically variable delay line. So it could be operated with a variable clock frequency to adjust the input-output time, just like a delay line might. So, we thought we can capture the data in this device and then read it out more slowly to digitize the signals as the digitizers then were very slow. Well, when we started to use these devices we found that they didn’t work when you changed the drive frequency quickly. We went down to Fairchild Semiconductor to seek a solution. They immediately saw the problem and said, “Oh! Okay, we’ll redesign it for you.” And they did it for nothing. So, then we got CCDs that allowed changes in the drive frequency very quickly from say 10 MHz to 1 MHz or less. That help from Fairchld saved the experiment, really. So, it was a real saga in which we could’ve perished at any moment due to the evil forces out there, so to speak. But we managed to survive.

Zierler:

David, to go back to the beginning when you recognized the value to SPEAR. Did somebody at SLAC commission this work or it was all internal to Berkeley?

Nygren:

It was internal to Berkeley. Berkeley and Stanford had this relationship where it was sometimes we were bitter enemies and competitors and sometimes we were collaborators depending on the circumstances. This is something we did at Berkeley to try to outcompete the Stanford people at their own game.

Zierler:

So, for example, was somebody like Burt Richter, was he involved in what you were doing? Was he aware at least of the value of this work?

Nygren:

No. He was not aware. It was sort of secret for a while. But then, once I wrote this proposal up and put it out there and talked about it at summer studies it became common knowledge. The Stanford people were generally quite skeptical because it was a Berkeley thing and therefore, it can’t be any good. And the Berkeley people thought, “Well, we have something good here. We’re going to beat you at your own game.” And while I describe this in a silly way, the sense of competition was quite real.

Zierler:

For positive benefit, of course.

Nygren:

Well. Sure. Yes. I mean, it was “may the best idea win” …it was all pretty ethical, but serious. The stakes were high. This was to be the major accelerator for the country for the next 10 years, at least. So, we wanted to win. And we didn’t mind if Stanford lost at their own game.

Zierler:

[laugh] David, you mention the electronics more generally in building the detector. In terms of materials, how much could you buy off the shelf and how much did you really need to fabricate because you were doing things that were literally never done before?

Nygren:

Well, we could buy the transistors themselves. Integrated circuits were still a new thing then. To get the level of memory that’s in your phone now, we had huge refrigerator-sized banks of electronics just to store the data that came in off the detector. So, everything was new. There was nothing you could really buy off the shelf that was what you might call a unit that could plug in. The whole thing, everything was new. I mean we could buy ancillary equipment like a computer, cooler or a helium refrigerator to make the superconducting magnet work. Things like that. But nothing in the detector itself existed prior to our developments.

Zierler:

A general question on applications. As you’re building the detector how well-defined is the experiment that you have in mind—and the TPC is built according to that idea? And to what extent is this just a matter of understanding that you have this amazing new technology and it’ll come later on when you figure out how to apply it?

Nygren:

When we made our proposal in 1976, everything was completely figured out as to what we wanted to do. And we built it exactly like that. The problem was that we wanted to do too many new things at once — high pressure, high electric fields, high magnetic fields. Everything. Gas systems that mix argon/methane together. So we had problems that were not intrinsic to the idea, but as a matter of practice. The engineering strength at Berkeley had been somewhat degraded by the budget reductions that Berkeley had to suffer. We had problems with the superconducting magnet. It burned out and the coil had to be rebuilt. Then the mechanical structure that held it in place failed and a new structure had to be built. So, that caused two years of delay. We also found that the electric field was fragile. That you had to really control the potential everywhere in order to get the field you intended. So there was a big rehabilitation project for the electric field as well. So, the two things that we really cared about—the magnetic field and the electric field—we really screwed up. But because of the power of the idea, people stuck with it. And eventually, it all worked just as we thought it would.

Zierler:

What was the greatest moment of concern that the TPC might not be viable when it was in the building phase?

Nygren:

The greatest concern was not about the idea, but about the implementation. That we would be cancelled by external factors due to cost or as I said, the mechanical problems associated with the superconducting magnet. In fact, Pief Panofsky, the director of SLAC, decided that he did need to cancel the project because of the costs and time and so on. He was not hostile to me. He was just a very conservative guy. We had a very good relationship. But he wanted his laboratory to produce rather than just be an R&D site. He called the program committee together to see if they would renounce their earlier decision to approve us. I remember very clearly the day that Jay Marx and I sat in two plastic chairs in the orange room facing the inquisition, so to speak, of the SLAC program committee. I remember it went on all morning and we explained this, that, and the other.

And then, it was lunchtime. The program committee decided to go down to a nearby restaurant, the Velvet Turtle, and have lunch. Jay and I were just left standing around. Charitably, the chair of the SLAC program committee said —“Oh, OK, why don’t you guys come too?” so, we went down with them. That lunch felt like The Last Supper, quite frankly, because we didn’t know what they were going to do after lunch. But it turned out the SLAC program committee strongly affirmed that SLAC should continue to support us. Pief didn’t get his wish to transform us back into frogs. Eventually we rolled in and started to take data. We probably found every bit of physics, along with the other detectors, that could be found in that energy range. But there wasn’t so much. The interesting physics had turned out to be at the SPEAR energies or later at the LEP energies. We were just betwixt and in between at PEP.

Zierler:

I’ll have to check the records, but this must’ve been the only—if not a rare—occurrence where Panofsky was overruled so completely.

Nygren:

I have no idea if it had ever happened before or after. But I have to say that by that time, Jay and I and others realized that we were in the middle of a saga. The heroes must vanquish monsters, or whatever, and face death at any moment, so to speak. I’m not really exaggerating. It was really extremely both exciting and intense and scary — that we would either succeed greatly or fail greatly.

Zierler:

David, prior to its actual application what feedback mechanisms or testing apparatus did you have so that you knew that the TPC was viable?

Nygren:

Well, the first thing I did was, in a way, a bit irrelevant. I built a Coke bottle-sized thing which had an electron source at the top…I have forgotten exactly how I made the electron source for that one… and a little drift region of about 15cm length. And down at the bottom it had a single platinum wire at the end of which I melted into a little ball. And I thought the readout plane could be a honeycomb array of these platinum wires sticking out. The ball tips would attract the electrons to make an avalanche and you could easily sense that. Although that device worked extremely well, it mixed up information because as the electrons come in, they spiral around crossing the magnetic field, and then time with radius become mixed and you don’t really want to do that. Also, I couldn’t figure out how you might make a big array of these platinum wires sticking up — very fragile. So that was a Coke bottle-sized thing. I still have it, or rather the laboratory has it.

The next thing was the size of a shoebox. We built this to go into a beam at the Bevatron and it produced a lot of good data. It had both sense wires and what I call pads underneath the wires to indicate where the avalanche was occurring along the wire, to get that 2-dimensional information in that x-y readout plane. It also could measure the distance of drift or rather the time of drift, using the CCDs. That was the first application with CCDs. We needed to see if we could get that CCD part working. No magnetic field was needed for that. So that was sort of a shoebox-sized thing but the next thing was the real McCoy.

That device worked inside a pressurized vessel, inside a big bending magnet at the Bevatron. This prototype was like a little slice out of the planned full-size detector — it had the same number of wires, 192, and the same number of pad rows, 14 . It was really a proof of principle. This was the detector which we had gotten ready just in time, and which broke a wire. As I said, we managed to snip that wire out and get it to work anyway. I don’t know what happened to that detector. I hope that the lab has kept it somewhere. I did manage to rescue one of the spare sectors that we built for the real PEP-4 TPC. Some parts of the final detector still exist at SLAC. But I think I probably wandered away from the original thrust of your question.

Zierler:

That’s OK. Was it a foregone—

Nygren:

Let me just—

Zierler:

Please.

Nygren:

From the suitcase sized we then went to a 1000 ton detector. That next step was too great. But we had no choice. Otherwise, we wouldn’t have been able to do anything.

Zierler:

Really.

Nygren:

Sorry to interrupt you.

Zierler:

It’s OK. [laugh] Was it a foregone conclusion that the TPC would be applied to the PEP storage ring? Or were there other experiments that it could’ve been used on?

Nygren:

Well, actually, we weren’t even the first to actually get a working TPC for physics. That happened in Canada, at TRIUMF. They’d built one for a muon decay experiment. So, already people were beginning to adapt the idea. We had no other application than PEP in mind at that time.

Zierler:

What were some of the major research questions at PEP and how was the TPC designed to answer those research questions?

Nygren:

We wanted to see if there were new particles that were unstable that could be made in e+/e- collisions at these energies. And nobody knew if there would be or wouldn’t be. But that was probably the main thing. You’d want to be able to see what kinds of particles were created — were there strange particles coming out as well? That would be interesting to know. And heavy quark decays, if they exist. We didn’t have a vertex detector in the way that particle physicists think of them now, looking for new excited states like the J/psi that was so famously discovered at SPEAR. Of course, we expected that breakthrough physics to be repeated at PEP. It was not. There weren’t any of those states to be found. But that’s what we were looking for.

Zierler:

You did say that you found all of the physics that there was to find at the energy that you were operating at. So, the question there is what was the energy and what was the physics?

Nygren:

Well, the energy at PEP was about 30 GeV—15 GeV per beam—30 GeV in the center of mass. And that’s almost halfway between the Z which is around 91 GeV. At SPEAR the energy was 3 GeV, where they found the j/psi. You’d think that a factor of 10 going from 3 GeV at SPEAR to 30 GeV would produce a bountiful array of new physics. There was some new physics, but there was nothing really fundamentally new in that energy. You had to go all the way up to the LEP energy or the SLC energy to find the Z.

Zierler:

Of course—to go back to an earlier question—what theoretical guidance may have been relevant for what was expected to see at those energies? And as the Standard Model was being built, was this something that you were specifically thinking about?

Nygren:

No. I was aware of the developments in the Standard Model as it was emerging at that time, but we were really quite focused on getting this novel detector concept built. We really put those theoretical ideas aside and said to ourselves, “We’re going to build a detector that if there’s new physics to be found, it’s going to be so powerful — can see all tracks, any number of tracks, even what kinds of particles were being created — we will find that new physics.” It was really a search engine, in the sense that we wanted to have maximum performance. So, almost theory independent. There was no particular theoretical model we wanted to test.

Zierler:

Are you aware of Panofsky ever expressing unhappiness that in fact he was overruled? That it was worth it in the end?

Nygren:

No. I don’t think he said anything to anybody about that as far as I know. He was a great guy. I have nothing but admiration for Pief and for the way he built that laboratory and ran it. But there’s lots of stories.

Once when we were…I guess it was a time when we were making some repair to the superconducting magnet. We also wanted to see what was going wrong with our electromagnetic calorimeter which had been losing sensitivity down in one of the bottom units. There were six of these units surrounding the cryostat of the superconducting coil. The calorimeter was a Geiger counter system based on argon and methyl iodide. Now we had been warned that we shouldn’t use methyl iodide because it tended to corrode metals. We did some tests and decided, Nah, we’re OK. We thought what had happened is that water was somehow getting into the gas system, condensing down at the bottom of this detector. We thought, well, OK, let’s drain out the water while we have an opportunity. So, we got a bucket and opened the spigot and liquid indeed started pouring out. But it wasn’t water. I can’t say that I know what it was, but it immediately started forming this white vapor that filled the air. It was hypergolic and exothermic, and people immediately started running out because we didn’t know what was happening. I think in retrospect it was probably trimethylaluminum, which is hypergolic in air and hence spontaneously combust. We subsequently realized that our earlier tests were irrelevant because, exposed to air, aluminum is completely protected by oxide formation. However, inside our calorimeter, there was no oxygen to protect aluminum were stress cracks to form. In that case, methyl iodide would be very corrosive and continuously eat away the aluminum.

However much stuff there was inside this calorimeter was now flowing out into the air as people were running outside. Soon Burt Richter came over on a motor scooter because everybody was evacuating; it was an emergency. We didn’t know whether we had a public health catastrophe or what was going to happen. Or whether we’d have to evacuate Sharon Heights, the nearby community. Burt was there, and somehow it was finally decided that everything was OK after the smoke cleared. Apparently the combustion product of that trimethylaluminum was not that toxic. Or at least nobody got sick from it at the time. But this was another example—I’m telling you, David, this really was a saga. There was not a catastrophe like this one week after another, but we knew that we were skating on thin ice sometimes. Well, this is a bit of a digression, but it’s another example of the mood and circumstance that we had.

When the PEP-4 TPC was first turned on, I saw these lines appearing on the screen. Nobody had really imagined it would be this easy, but the TPC started to work right off the bat. I had to quickly run over to—I’m going back in time a little bit—run over to the liquor store and buy a case of champagne. Of course you can’t drag champagne into interaction regions at Stanford anymore, but we did then. We celebrated the fact that the darn thing just turned on and it was working!

But the tracks were not straight! This was because the field-cages at the active volume surface were not defining the electric potential properly. I’m not going to get into that long story here, how we eventually fixed the boundary conditions. But it was just one little or big thing after another. Successes mixed with disasters. I should add that I am extremely grateful to all the students, engineers, technicians and colleagues for their dedication that brought the PEP-4 TPC to ultimate success.

Zierler:

Were you surprised—please.

Nygren:

I was very surprised that it just worked. I’ll just add one thing. When it finally came time for LEP to accept proposals for their four experiments as a sequel to PEP, two of the four experiments that were selected had time projection chambers as their central element, so whatever problems we had didn’t poison the water, so to speak, for the idea. [laugh]

Zierler:

Were you surprised that the TPC was applied to the PEP for as long as it was? Did you think until 1985 that there was that much science to learn?

Nygren:

Um. No. I think that by that time, honestly, people were exhausted. We’d gotten it to work. It had been a struggle. We just wanted it to run and do its thing, as we were all trying to recover. That era is clouded for me because my wife decided to seek a divorce around that time. And so my personal life got very complicated and I wasn’t really paying that much attention as the TPC wound down. Mike Ronan, who was running it, did find new physics but nothing startling. He led the analysis of much of the data. So, the PEP-4 TPC kept on finding what could be found in that region, but I wasn’t really very involved toward the end.

Zierler:

It’s a counterfactual, of course, because we don’t know what would’ve happened. But absent the TPC, what was capable at PEP? What could PEP have found without TPC?

Nygren:

I think they could’ve found everything. They could have. Yes. I think that the detector that Richter’s group had, and Berkeley with George Trilling had built, probably would’ve found everything that we found. It’s just that there wasn’t anything particularly subtle or tricky to be found. So, the detector worked, but we and they showed that there wasn’t anything to be found that was truly new and unexpected.

Zierler:

David, when did you get involved in SSC planning?

Nygren:

Well, as soon as the SSC became likely I became interested in the problem of how to detect the heavy quarks. We all knew that Higgs would couple strongly to mass. So this, if the Higgs was being involved in a process, would lead to the production of heavy quarks which then would travel tiny distances before they decay into other things. So, the idea of a precision vertex detector where you could get extremely precise measurements of tracks became something I thought was both opportunity and of absolutely vital interest for the SSC in the search for the Higgs. So, I began to work in electronics with engineers and developed ideas for a pixel-based vertex detector. I received quite a bit of support from the SSC grant process to do this. We learned to make integrated circuits with pixels, so called smart pixels in a way that people still use today. Now the idea that eventually came—I don’t want to elaborate—came not from me, but from an engineer I was working with, Jacques Millaud. His idea was to equip each pixel in this 2-dimensional array with knowledge of where it was in x-y. So we didn’t have to do an x-y interrogation to find out which pixel was hit — it would tell you where it was. So only pixels that had been hit needed to be interrogated, so that reduced the data acquisition burden tremendously. That’s an idea that everybody has used. So, our Berkeley work on smart pixel arrays really was, I think, central to the evolution of vertex detectors that found their use, not at SSC, but eventually at the LHC.

Zierler:

Like so many other physicists, did you ever consider joining SSC full-time? Leaving Berkeley?

Nygren:

Briefly, but I decided, No. I thought that my life was pretty much centered in Berkeley. I think that was a problem that the SSC laboratory had in many ways, not just that people didn’t want to go there. They were satisfied with life at the laboratories where they were. There just wasn’t the intellectual depth in the country to build an entirely new laboratory and keep everything else.

Zierler:

What was exciting to you about the SSC in terms of what it could accomplish more broadly? At those moments where it seemed like it was actually going to go through?

Nygren:

Well, it was a new era, a new energy domain. Who knows what it might have produced? It certainly would have easily produced the Higgs. More easily than it was found at the LHC.

Zierler:

What about supersymmetry? Was anyone talking about supersymmetry at that point?

Nygren:

You know, my memory on that is a little hazy. Supersymmetry ideas were around, one of those quite appealing theoretical arenas. Again, I would say that I had decided that the key to success, or where I could contribute, was to develop the capability that in these extremely intense beams, pick out those decays of secondary heavy particles so that you could detect the presence of the Higgs. That was my schtick. And I wasn’t so interested in which theory because I thought the Higgs by itself was likely to be the most central result. And perhaps it is. Who knows?

Zierler:

Conversely, were there any early warning signs that you saw administratively, technologically, politically, that suggested that the SSC was doomed?

Nygren:

Yes. Maury Tigner, who ran the Central Design Group at Berkeley, is an extremely capable guy. You could see problems in the SSC organization—when the director of the laboratory appointment was given to somebody else—that was thought to be a mistake. I think that people were quite concerned that the SSC was not being managed in a way that was likely to succeed politically in the long run. Of course, it’s easier to say that in retrospect. At Berkeley, I think that all of us there thought that Maury had done a very good job and was a natural one to lead the laboratory forward. So, we didn’t understand how the decision had been made. So, yes, there were little hints, but nobody wanted to believe that the SSC would go down in flames. I remember being shocked when the actual vote came in 1994, I think it was. The Senate voted it down after the site was chosen. Before the site was chosen, there were a hundred senators in favor. And after, only two. Yeah.

Zierler:

How easy was it to pivot to the LHC and column-based architecture that you worked on?

Nygren:

Well, I had decided at that point to not pursue the LHC, for a number of reasons. I’d made a contribution to the electronic design for a smart pixel detector that Kevin Einsweiler and others at Berkeley had now taken up. Even though Kevin moved to Geneva and stayed during the whole ATLAS program, the pixel design was completed. I decided that I would move to some new field. I worked a bit on other solid-state issues like developing a lab for semiconductor processing at Berkeley. This was done almost entirely by Helmuth Spieler, but I had some role in starting it. We had gotten money and had built up a wonderful laboratory based on high purity silicon, run by Steve Holland and aimed at SCC challenges. But there was not going to be an SSC.

I realized that we could also use the same capability to make deep-depletion CCDs, in which the entire thickness of silicon would be active. This would greatly improve the sensitivity in the red wavelengths. Saul Perlmutter wanted to build a satellite called SNAP to search for supernova 1A across the universe. For this, you would need to have very good red sensitivity. But ordinary silicon-based CCDs couldn’t go as far down into the red because most of the red light would just pass through the thin sensitive layer. With fully deep-depleted silicon you could extend that sensitivity considerably further into the red. This was really quite an important advance. So, Steve Holland learned, with some processing done at Dalsa, how to make these CCDs. They became quite popular, used in spectrographs and for imaging in many telescopes around the globe. And so, that was the main contribution of our silicon laboratory.

I had started to move away from particle physics in the usual sense at that point. In other words, I was more of an opportunist and if I could see a way to contribute it didn’t have to be in particle physics. But there Saul was and he needed these devices. And I saw that we — Steve Holland — could actually do it. And so, Steve and his team did it. Saul was appreciative of that even though the satellite never got built.

Zierler:

It must’ve been a great opportunity to get more involved in astrophysical research.

Nygren:

Yeah. I mean, those guys, Saul and company, did something great to bring the style, the organizational skills, and the analysis in particle physics into astronomy. They did something great, and it led to a Nobel Prize. I think it was an absolutely wonderful achievement. I never worked directly with those guys, but it was wonderful to have a connection to them, they were just down the hall. It was great.

Zierler:

To return briefly to subsequent developments on the time projection chamber. Were you following what Carlo Rubbia had done with liquid argon and then much later on the way the time projection chambers would be used for dark energy searches? I’m sorry, dark matter searches.

Nygren:

Dark matter searches, yes. I was following that, thinking “Gee whiz, why didn’t I think of that?”

Zierler:

[laugh]

Nygren:

Bill Willis was also one of the early explorers in the liquid argon domain. So, it’s really wonderful because the liquid state of argon and xenon is like a semiconductor; the electrons travel through at much higher velocities than you would otherwise expect. They also travel with very little diffusion. It’s a wonderful example, wonderful realization of the TPC idea to do it in liquid xenon or liquid argon, but I didn’t have any role in that.

Zierler:

Did you interact at all with Carlo Rubbia? With his ideas?

Nygren:

Yes. Carlo and I became well-acquainted and whenever we we’re together we’d always find things to talk about. He is also an electronics guy and we would talk about technicalities. I had the notion that he might want to hire me at some point, but he didn’t offer. But we’ve always respected each other, I think, and seem to find one another at conferences. And, as it’s been said by others, Carlo kind of warps the space around him because of the strength of his personality. But he is a truly competent guy with really diverse ideas and well-thought-out opinions.

Zierler:

How did you get involved in x-ray mammography research?

Nygren:

I just woke up in bed one day and said, “A-Ha. If we turn the silicon around so that x-rays see a centimeter or so of it rather than 300 microns, our detection efficiency will be 100%.” Instead of just adding up energy which is what all of the detectors did until then, we can count the x-rays. This is really important because the low energy x-rays have the highest contrast information. The absorption cross section is changing rapidly in the region around 10-20 kilovolts of energy, falling as 1/E3.5. I attracted Mats Danielsson, from Stockholm, to join me in Berkeley as a post-doc. Working with Bob Cahn and Mats, we figured out that a patient’s dose could be reduced by a factor of about 3 by counting x-rays individually rather than by simply adding up energy. The higher energy x-rays mainly penetrate tissue, just piling on energy without adding much contrast information. Even better would be to measure the energy of each x-ray and weight it accordingly. But that’s harder because of the Compton process.

We developed a little integrated circuit, and, working in the Berkeley nuclear physics laboratory, Mats Danielsson made his own detector. These detectors had stripes, or strips, all keystoned to point back to the x-ray source, like an arch would do. So that way they detect the x-rays that come in and you get the precise 2-D information that way independent of depth of the interaction. With the integrated circuit we showed that we could take wonderful images of, for example, a mouse skull that I’d bought at the Bone Room on Solano Avenue in Berkeley. Much more detailed images and for a much lower dose. I have those mouse skull pictures hanging on my wall. We realized that here was an opportunity to graft particle physics advances into medical technology. So this is another example of opportunistic physics in which an idea was at least at first superficially attractive, then attracting the interest of others who are maybe more competent than I am in certain ways. Mats went back to Stockholm, formed a company, and started making and selling these units. Some countries standardized on that. He sold that company to Philips and they’re in clinics, marketed as the Philips MicroDose System.

Also, because of the slot scan you get almost no scattered radiation. That’s tricky because you have to get the mechanical alignment precisely. So this was an excursion away from the usual topics in physics, but involving practices of particle physics in which you count things rather than just add up energy. So, that was a real contribution, I think.

Zierler:

Have you followed the ways in which this led to better early detection and even better health outcomes?

Nygren:

No, because I got interested in other things. Mats took that idea away and he’s built his career on similar ideas that we had at Berkeley, as well as his own. He’s now a professor at KTH in Stockholm.

Zierler:

David, tell me about your contributions on the neutrino muon detector in the Mediterranean.

Nygren:

Ah, yes. Well, one day my friend Leo Resvanis came to Berkeley as a supplicant, saying, “Dave, we really need you to help us out here. Can you help us?” I had been using CCDs to record data in the original PEP-4 TPC. And I saw that we could use this idea again, but now with an entirely new device conceived and designed at Berkeley by Stuart Kleinfelder. Leo needed to capture data in a detector array, called NESTOR, to be put at the bottom of a deep Mediterranean basin near the coast of Greece. Neutrino astronomy begins with neutrinos interacting with the Earth or water and producing high energy muons (or hadronic showers.) The muon trajectory can be reconstructed through the detected Cherenkov light. Stuart’s new integrated circuit, the Analog Transient Waveform Recorder, could capture analog waveforms at a very high sample rate and then read the samples out slowly for digitization. I thought that this electronics development would be an important springboard to demonstrate our ideas. I decided to do it also, in large part, because I was a friend of Leo. Somehow, I hired an engineer and together we designed and built this new electronics. It would all fit inside a titanium sphere large enough to hold an old-fashioned Intel 384-based PC as well. Leo’s team deployed the detector and laid a cable from that site through the Mediterranean to the shore station near Pylos in Greece. Good data was acquired from the floor of the Mediterranean for a while — until the cable failed. These cables are tricky because they are made by winding up layers of strong wires clockwise and then over counterclockwise, for strength. These cables don’t allow twisting, but in winding them up on a cable reel and then reeling them out from a ship, it is easy to get a kink, like a garden hose. That kink ruins the optical fiber in them. Anyway, the whole system did work for a while and muons were reconstructed. NESTOR was unfortunately not realized beyond this first step. This was my introduction to neutrino astronomy, about which I had no expertise. But now we knew how to make this novel electronics, a system that allowed Leo to be the first to detect muons coming from above through the water and take that data to shore by a cable. So, that was perhaps a minor escapade, but a quite successful one, as this outcome turned out to be a genuine springboard. From NESTOR in the Mediterranean, we successfully promoted the ATWR — improved with an internal digitizer for KamLAND, and then to IceCube. All three of these steps were essential. Its’s a remarkable sequence of increasingly ambitious aspiration.

Zierler:

And what was going on shortly after in work on neutrino oscillation? How did you contribute to that?

Nygren:

Well, I didn’t contribute at all to the discovery of neutrino oscillations. About that time, in the 90’s, I became involved with IceCube, with Francis Halzen and company. It was another one of these random walk processes where you step into the woods and see if there’s a path in there. So I worked with Francis — he is delightful to work with. I thought that I could probably help to make a huge jump from the AMANDA array, an acronym for Antarctic Muon and Neutrino Detector Array. They wanted to go for something like a cubic kilometer, far beyond AMANDA’s scale.

I saw that there was an opportunity that hadn’t been realized in AMANDA, a way to capture all the information content in the signals. AMANDA was very limited in the dynamic range and in the detailed structure of the wave forms it could capture because analog signals were transmitted up through the ice to the surface with very long coaxial cables or even twisted pairs. The signals get vastly attenuated and distorted by that. So, my contribution was to say, OK, we’ll do all of the signal processing that matters right there inside the optical modules looking for Cherenkov light. We will send digital messages like the internet does, where there’s error correction — there’s no loss of information once it’s in a digital format. Our German colleagues were really quite concerned because this was putting all our eggs in a basket that you couldn’t reach once you deployed all this electronics down deep in the ice.

So, there’s a little saga here too. I managed to convince George Smoot to devote $300,000 he had gotten from the NSF to help build the final string 18 of AMANDA as a hybrid between the old analogue approach to signal transmission and the new purely digital format. We decided to do this, start to finish, in less than one year. We designed, built, brought to the South Pole and deployed that hybrid String 18. Really an amazing accomplishment. The electronics had a RadioShack flavor because we didn’t have time to do any real quality control. Nevertheless, we showed that we could time these modules up to two nanoseconds, even over kilometers of twisted pair. And we also showed that we could capture fast signals and transmit the digital data upwards. It was a revelation to see, for the first time, the complex waveforms. We could claim a technical success despite the fact that the reliability of the electronics was poor. So from that wobbly single-string demo in AMANDA we managed to do what I call “the analog to digital conversion of IceCube”. The NSF also was brave enough to go along with it. It was and remains a tremendous success. All those modules with lots of electronics are humming away producing great data. Failures have been very limited even after a decade, less than 1% in the electronics and maybe 1% in connectors. And of course, the great advantage of the signal processing down deep is we could get the whole 15-bit dynamic range with waveform sampling at several hundred megahertz. We captured all of the complexity and information that the Cherenkov signals contained. So, again it was a way of “Let’s really preserve information inherent in the signals. You can see there’s a certain light motif here. In the TPC I wanted to preserve the information by transmitting it through gas, preserving its quality with a magnetic field, then capturing it accurately with continuous waveform sampling. In the case of IceCube I wanted to preserve the data by converting it to a digital format inside this globe of glass, getting us into the modern age, so to speak. So there is a sense of continuity here. And it all worked extremely well.

Zierler:

David, more broadly what was so successful about IceCube? What did it achieve overall?

Nygren:

Well, it showed, first of all, that neutrinos are coming from space at super high energies. They are not just made in the atmosphere by cosmic rays. Perhaps most remarkably, the reconstruction accuracy is good enough that IceCube can point back to a particular point in space and say that a neutrino came from that region, and is probably associated with the activity, for example, in some active galactic nucleus or a blazar. So, I think this is a tremendous accomplishment scientifically, to use the neutrino this way. A neutrino coming from the Sun could pass through a light year of lead, but at the energies that the IceCube is looking for, we see enough of them coming from outer space that we can actually do a form of astronomy. Really this is a new window. It’s a trite phrase, but in this case it’s quite true because the neutrinos carry information from the very heart of a violent astrophysical process like a blazar. So, this is quite new and it’s really wonderful. That the optical modules can absorb the signals and record these tremendously high energies is critical to that success.

Zierler:

It must’ve been tremendously satisfying to be a part of this.

Nygren:

Yes, it was. And I have to give a lot of credit to the engineers. I worked with Jerry Przybylski at Berkeley, engineers in Germany and also at Wisconsin. It was a really wonderful experience where all worked together and we made something great happen. I didn’t do it by myself. Let me make that clear!

Zierler:

[laugh]

Nygren:

But I had the idea to put all that electronics in there and say it’s going to work. And it did.

Zierler:

Tell me about your work with Juan José Gómez Cadenas to develop the NEXT collaboration between the United States and Spain.

Nygren:

So, around 2000, I went to one of the dark matter conferences that Dave Cline was organizing at UCLA. Somehow, while there I came up with this idea that maybe there’s a way to use the gas time projection chamber to look for neutrinoless double beta decay (NLDBD). This was well before I knew J.J. Gómez Cadenas. I had the idea of looking for a kind of curlicue or “dingbat” signature that the electrons would make in a magnetic field from the decay of xenon-136 into two electrons, most if not all of the time, with two neutrinos. But it became clear that there’s just too much multiple scattering of the electrons in the xenon gas to see magnetic curvature. I had to abandon that idea, yet even though that was a blind-end, I continued to think about it. It became clear enough that we could use the TPC in a way that probably would give real advantage compared to other techniques, such as being able to distinguish single- from double-electron tracks. So, in December 2006 I gave a talk at a TPC conference in Paris, organized every other year, about a concept for a high-pressure xenon gas TPC. My wife and I were enjoying Paris. While there I got a phone call from J.J. saying, “Let’s talk.” We did and it seemed that JJ really knew what he was talking about. The next spring I came to Spain and we formed a collaboration between Berkeley and Spain. And it’s been going, building in strength ever since, and producing some very promising results. We hope to get to the several hundred kilograms scale of active mass, where active mass is in the range of 1027 or so atoms. We must be able to see one atom decay into two electrons and, usually, with two neutrinos, unseen but carrying off energy.

The decay produces a barium daughter. The innovation is that we also aspire to detect this barium daughter using single molecule fluorescence imaging, or SMFI. This is a biochemistry technique invented by physicists and to whom the Nobel Prize in chemistry was given in 2014. So, the idea is that, in the electric field of a TPC, the electrons go one way and the ion goes the other. More accurately, the electric field brings the ion to the cathode and the electrons to the anode plane. We must have a way to transport the barium ion at the cathode plane toward a tiny SMFI optical system. If we can do that, we could say, “A-ha!” there is a new barium ion created at just the right time and plausibly in the right place within the volume. Fake events are created by gamma rays from the uranium and thorium decay chains, but gamma rays cannot transmute xenon into barium. These decay chain impurities have been almost impossible to eliminate from the detectors at the level needed. A capability to affirm the presence or absence of barium gets rid of all the background events that could otherwise mimic the real decay. So, here’s an idea, SMFI, which is completely unrelated to particle physics or nuclear physics but perhaps may be a breakthrough.

How could biochemistry become connected to the search for NLDBD? Well, one day I was reading about imaging in the brain, about single calcium ion imaging to see what’s going on inside the neurons. I suddenly realized that calcium and barium are congeners, and that, Gee, if it works for calcium maybe it can work for barium too. And it turns out SMFI can work for barium. You can even develop molecules that are quite specific to barium and therefore give you an additional level of confidence that you’re only looking at what you’re searching for. So this is quite novel to be able to exploit something like biochemistry in nuclear / particle physics.

The question Is the neutrino its own antiparticle? remains open. If the neutrino is its own antiparticle, and has non-zero mass, neutrinoless double beta decay can occur and if the neutrino is not its own antiparticle, it won’t occur. And if the neutrino is its own antiparticle, that state could’ve been forced on today’s neutrinos by heavy neutrinos that existed only at the first moment of the Big Bang, the first trillionth of a trillionth of a second. These ultra-massive neutrinos could not only have forced this condition, being neither matter nor antimatter, on today’s neutrinos, but also could have directly produced the matter-antimatter symmetry that we observe today, about one part in 109 of what originally existed. But that part per billion is all the matter that we see in galaxies, and all the matter that is. So the origin of the matter-antimatter asymmetry is a deeply fundamental question. We use nuclear physics to do particle physics to do cosmology. But to do the nuclear / particle physics convincingly, we want to adapt this biochemistry technique to identify and reject those backgrounds that have been extremely problematic. We haven’t proved that we can do this. We have shown that we can detect single barium ions in high pressure xenon gas. We also have to show that we can transport the barium ions efficiently to the SMFI detector which is a tiny little thing. So, stay tuned for that. But I think it has energized this field and provides an example of a Holy Grail — a background-free experiment — which does not yet exist anywhere.

Zierler:

And when did you realize that this may have been useful for research on WIMP collisions?

Nygren:

Well, about the same time that I was thinking about double beta decay it was obvious that such a detector could also be useful for WIMP collisions too. But the sensitivity of a gas detector is not as convincing as it would be in the liquid state where you easily have much more mass in a practical volume. So, I haven’t really pursued the sensitivity of the gas or liquid TPC for WIMP detections. But other people have, both liquid argon and liquid xenon TPCs of enormous mass. DARWIN would have 50 tons of xenon, and DarkSide is already 20 tons of argon. Very large TPCs to look for WIMPs, indeed, but I’m not part of that.

Zierler:

What were the origins of what would become the SMFI idea? And what’s the current state of play with the DOE NP?

Nygren:

Well, I think I did mention the origins. I only learned about SMFI because of my reading about what people do in brain imaging. And this illustrates something I’d like to say is always good, and I tell students to do this too. Try to learn something outside your own field. Because that’s where the money is. That’s where the new ideas will come. You can’t predict where they’ll come. You can’t guarantee they’ll come. But if you don’t know something other than what you’re specialized in you’re probably not going to find yourself making a breakthrough. So, that happened in about 2015 when I was reading this book, and A-Ha! Why not here too, in particle physics! So, that really was a Eureka moment. I remember putting the book down, saying to myself, “Gee Whiz. I must write this down before I forget it. It’s just too good.”

Zierler:

And so, what’s going on now? What’s the current state of research?

Nygren:

Well, we were able to get good support from the Department of Energy Nuclear Physics. I joke that I’ve had to scrape off my high energy physics tattoo and pencil in NP because now I’m getting money from the Nuclear Physics. It’s been a very productive collaboration with DOE NP. We also have collaborators in this country from Harvard, Argonne, Texas A&M, and Berkeley. So, it’s one of these things that has just grown organically. So, I’m optimistic that we will find a solution. I want to give credit to my young colleague Ben Jones at UTA who has really been able to take ideas, develop his own, and really make things happen. So, it’s been great. The move to UTA from Berkeley has been very productive. Although I wasn’t sure how it would evolve. I just went there as an experiment — “Let’s see what can happen.”

Zierler:

Now, did you go emeritus at Berkeley or this was a transfer?

Nygren:

No. I retired from Berkeley. Emeritus status was not offered, so I have no formal connection now with Berkeley other than as a guest.

Zierler:

And what was the decision making? Did UTA recruit you? Were you looking for a change yourself?

Nygren:

UTA recruited me. I had not thought of moving but they made an interesting proposal. I went, I saw, and thought, Well, this might work. As I said earlier, I never thought of myself as a successful academic but I think I’ve been managing to pull my weight. I invented a class I call Major Milestones in Modern Physics in which I use the Nobel Prizes in physics as a scaffold to ask, “How did these folks do this? What motivated them? What’s their style? Let’s see if we can understand what’s special about this.” It’s been a very popular class. I invite other faculty to take classes on subjects where I don’t have the expertise. They’ve said things to me like, “Gosh, I wish I’d had this class when I was a student.” And I love giving it because it is tremendously interesting to explore how people make discoveries, like the famous discovery of graphene using Scotch tape, for example.

Zierler:

[laugh]

Nygren:

And there’s all kinds of things like that.

Zierler:

Was the expectation at UTA that you would also take on graduate students? That you would sit on committees?

Nygren:

Yeah. Right. I should be a regular faculty member. So, I took on students but I tend to encourage them to work with the younger people and become their students. So, I’m trying to limit my responsibilities in that regard because I’m putting more and more of my effort into what I would call the transition to the 21st century university.

Zierler:

Yeah. Now before you scrape off that high energy physics tattoo so quickly, you were involved in the Q-Pix idea. Can you tell me a little bit about that?

Nygren:

Sure. Q-Pix is a novel idea, with which I must admit I’m quite pleased. Coming back from a wonderful trip to Norway I came down with a terrible cold and was lying in bed for a long time. I was too sick to read but needed something to think about as a distraction. I was not completely happy with the approach that had been taken for the DUNE far detectors and decided to think about that. These are huge liquid argon TPC detectors. And I thought, well, let’s see what else might be possible. Because I was lying in bed doing nothing, and since most of the time nothing happens in the DUNE far detectors too, I thought what’s the minimum action you can take? Well, the minimum you can do is just record the time when something happens. And that’s a very crude idea. Following that idea further I realized that if you just record the time when a certain amount of charge has been integrated, you can reconstruct the signal waveform. Current is usually defined as the amount of charge you get per unit time. Well, another way to define current is how much time does it take to get a unit of charge? It’s the inverse, and yet it works rather well in this case because the signals for minimum ionizing particles that come in from neutrino interactions tend to produce fairly predictable tracks. Fairly predictable higher amounts for nuclear fragments or protons, and so on. So, I conceived an idea which I called Q-Pix, based on recording waveforms in this new way that I call adaptive sampling. Although the waveform will change its width and amplitude depending on how much it has diffused during drift, the Q-Pix circuitry will automatically adapt to give it the same number of samples, with better noise characteristics, at least in principle. I think it’s a very good way to measure tracks so precisely that you can place the track into space, much like a time projection chamber does, but without having to refer to a time of origin. Well, you know when the beam comes from Fermilab and that’s helpful, but a lot of interesting physics can come without the beam trigger, such as a supernova. Who knows? Really, part of what DUNE should be doing is to look for any new physics at the threshold of detection. Q-Pix doesn’t differentiate the signal and integrates over longer times indefinitely. And so, if there’s some—I’m just making stuff up here—if there’s some slowly evolving signal, Q-Pix would see that but other detectors would not. Yes, it’s a fishing expedition, in that way. But it’s also an exercise in diligence to make sure that when we’re using so much of the resources of people in time and money, we don’t miss something.

Q-Pix is designed to mop up all of the possibilities that liquid argon can give. I’m working with Jonathan Asaadi at UTA also looking for ways to use Q-Pix to detect photons. I’d rather not go there because it can get us into a long-winded discussion. But the point here is that Q-Pix has already shown that there’s a different way to do physics here, one that has been easily available to anybody, but apparently nobody’s ever thought of it — to measure varying currents this way. And that could be a benefit. We haven’t proven that. We’re working on the electronics that should prove that or show us what our challenges are. I think that this is a good example of innovation that can occur when you just allow yourself to range freely in what you might call curiosity space.

Zierler:

I’m curious where Q-Pix might go long term.

Nygren:

Well, this is speculation, but we have money in the system now for just one detector from the DOE, one 20 kiloton detector which will be made with wires — not my favorite technical scheme. There could be support from CERN for another detector based on the vertical drift scenario that has recently been evolved. I hope that the third detector might be Q-Pix. Now, who knows if that can happen? I think Q-Pix will have to show that it brings something really new to the table and will be better than what else is being considered. We don’t yet know the answer to those questions. So we’re working hard to obtain experience that is going to teach us what we’ve got to do. Perhaps it’s the beginning of another little saga, to see how all of this will play out. Either you win big or you lose big because there’s no other option—either you build or you don’t.

Zierler:

[laugh]

Nygren:

We’re players in a really big card game here. It’s OK.

Zierler:

David, was there a specific idea or problem or conversation or article? What was the inspiration that started you down the path of rethinking what a university should be in the 21st century?

Nygren:

Well, when I first arrived at UTA the president asked me and another National Academy member to looks at issues connected to the Dallas/Ft. Worth metropolitan area, which is huge. It’s Los Angeles in scale, physically. As we started thinking about that, it became clear very quickly that these problems are not so interesting compared to the ones of the planet itself. The other person more or less retired, so I’ve been picking that theme up and developing the bigger picture. I came up with the idea that we need to frame this discussion as the Earth-Human System because we humans are no longer a perturbation on the planet’s systems. We’re actual participants now in the behavior and evolution of the planet’s systems. The Earth-Human system is a cumbersome phrase that doesn’t have the same zing as some popular memes but it does capture, I think, the true essence of the problem. The University has five guiding themes like Health and the Human Condition and Data Driven Discovery and social topics, as well — I don’t recall them all but they’re all good. However, they lack a sense of unifying principle, which the Earth-human system is intended to provide. I didn’t really intend to have it become my major activity. Yet I’m finding that many faculty are really eager to try to do something other than maybe add a class or a minor. The university has been silent on this because it really doesn’t know how to address it. In my opinion, few universities, if any, do. Any university is structurally discordant with a global framework view for curriculum.

One thing I would like to see is a class that any student can take for credit in any major that addresses historical aspects of energy, land and water use, climate and biodiversity, and their changes in this century. The Class would also address the serious social, political and anthropological issues that I think we face today. And also addressing that everything is now in a state of very rapid change, very difficult to grasp conceptually. It’s a wicked problem, as they say. Universities are shying away from embracing the Really Big Picture. Premiere institutions will say, “Oh, look. We’ve got this great center for the environment. Or Look, we’ve got this great center for climate.” And so on. And they’re all good, but they are isolated, specialized academic fragments, not part of a coherent universal student experience. Students can go through their college experience even at these first-rate universities without feeling that the university is taking planetary disruption seriously.

Our primary goal is the realization of a Center for the Earth-Human System in Curriculum, which will provide financial and other support to faculty, so they are enabled to make the transition happen. I don’t want to use the word revolution or reformation because that’s not what we’re trying to do. But we are aspiring to evoke a transition that will occur through its own energy. The university administration has to be supportive of this and they have to get out in front visibly and credibly and robustly. So, that’s part of my challenge now is to encourage the administration to do that. I want to point out that the current interim UTA administration is very supportive, but we are in an inter regnum.

Zierler:

David, who are some of the key institutional partners beyond UTA as you work on this?

Nygren:

None. I’m working mainly at UTA now. We have a vigorous group that assesses what other universities are doing. We don’t know for sure that what we’re trying to do is unique. But we’re trying to be diligent. I’m not yet working with other institutions because I have my hands full right now at UTA. That’s maybe not a good answer, but I think that UTA could be an exemplar for this whole thing. UTA doesn’t have entrenched entities for this, that, and the other which could be an impediment to this effort.

Zierler:

What are some of the intrinsic strengths that UTA can draw on for this endeavor?

Nygren:

UTA is an excellent institution, with flexible approaches and a very diverse student body and faculty. I don’t mean to offend anybody, but I think it is an excellent laboratory to make this transition. I think the students are very serious. It’s not a party school. They’re here to try to learn and make a good life for themselves. And as I said earlier, they’re anxious and they’re even a little bit resentful. I think that the university needs to get with it and say, “OK. This is a wicked problem but we’re going to step up and confront our obligations to help everybody make the planet more sustainable and more sustained.”

Zierler:

What kind of timescale are you envisioning at least on a preliminary basis?

Nygren:

I’d like to see the university begin a set of lectures, probably biweekly during the fall term of this year — lectures which address these things forthrightly. Even looking at taboo subjects like population — not to take a position on that, but to explore what people are thinking. We know demographic trends indicate that people are having fewer children and that the population may peak around nine billion. We want to look at climate trends for this country and others, particularly in Africa and India and other places where climate change is forcing people to migrate. Migration toward the U.S is happening increasingly in Central America.

I have developed a syllabus, not a perfect one, but I think is a good example. My syllabus attempts, and does, I think, quite successfully, show that we are now in a system in which everything is very tightly tuned and connected. You cannot imagine a lot more freshwater coming into play or more land under agriculture so easily. It’s a little bit like the globalization that we see that now in the pandemic. Normally we can get blueberries from Chile and wine, grapes from South Africa — the 747s are flying all the time, bringing us these things. Suddenly you can’t buy a refrigerator or get electronics because everything is highly tuned, with little elasticity but not so stable either. OK, this is a wicked problem with lots of uncertainties.

There is a chance that the Earth is headed toward a hothouse phase. For the last 1.2 million years the earth has been in this interglacial cycle of 100,000 years, alternately cool and then warm. But apparently, we could diverge now, this century, according to these experts, into a new state where it’s just hot, several degrees hotter. While that tipping point could take a century, it might happen more slowly or more rapidly. Even though we don’t know, we shouldn’t shrink from uncertainties.

So, the main point, again, is that the university — all universities really — just don’t know how to confront this. Higher education primarily relies on faculty to put up more courses on sustainability. We need this, for sure, yet this approach just doesn’t provide the sense that we collectively —as a human species—are doing the appropriate things. So, I’m optimistic that in the timescale of 10 years we will have not only these lectures that I mentioned, and the class that I mentioned, that kids can take for credit in any major, but we might also have a School for Studies of the Earth-Human System. That’s a mouthful and a dream. But why not? Really this is going to be the central issue in the last half of the century. So, I think we can make a lot of progress in this decade toward that goal.

Zierler:

To what extent is this going to require cooperation or support from the federal government?

Nygren:

Well, of course it’s absolutely essential. I think the present US administration is taking all of these things seriously but there’s reluctance as people just can’t deal with rapid changes everywhere . So, there will be tension and stress. Yes, the federal government is stepping up to promote, I think, the sense that it’s a global and urgent issue. We solve problems collectively only by individual commitment. I think we’re on the right track at UTA. Don’t know if it’s too late. Probably not, but who knows?

Zierler:

Well, David, now that we’ve worked our way right up to the present I’d like to talk to you about some retrospective questions about your career. And then we’ll end looking to the future. So, the first thing that I’d like to ask you is if you can broadly reflect on some of the connecting points on all of the projects that you’ve worked on. Your career has been very eclectic, shall we say? You’ve worked on very disparate kinds of projects. I wonder besides having a nose, an intuition for interesting things to work on, if you see some scientific through line that connects all of this research?

Nygren:

Well, I would answer that I was fortunate to have a lot of curiosity that didn’t get blunted along the way. Part of my success I think, quite seriously, was being a not very serious student. In high school I cut classes to go to junkyards and get stuff and do experiments. I had a special experience due to the era. I do want to apologize to my sixth period Latin teacher, Ms. McQuiston, who tolerated my absences. So the fact that I was scientifically curious and somehow just did it was very important. Going to a small college without intense specialization was very important. I think having that kind of broad background where I learned about history, political science, psychology and literature, and so on, was very important. Just to keep that sense that the world is a big place. And then again in Seattle as a graduate student, I was fortunate to work with some exceptional graduate students. As well as having some very good instructors in graduate school. And it was only in Columbia where I saw the value in becoming serious and began to put in the homework to know what I was doing and what I was talking about.

So, one way to kind of weave some continuity in all of this—I was not afraid to fail. Well, of course I was afraid to fail, but I was willing to fail. Let me put it that way. I was willing to say, “Well, I don’t really know what I’m doing, let’s see what happens..” When I came to Berkeley I didn’t have any clear ideas, maybe some poor ideas, but nothing good. I wobbled around for a quite a while before I finally struck something golden. At Columbia and as a graduate student I’d already had the experience of feeling that I can figure stuff out. So, it was this sense of, let’s not miss an opportunity. Let’s step into the woods and see what’s there. Let’s try something and see what happens. And let’s not be too professional about it either. Let’s take time to look at something that’s just out of the ordinary. The x-ray stuff came out of the blue. It just happened because I was willing to think about other things. For me, it’s the absence of specialization and being willing to fail. I was consciously willing to fail at Berkeley because I didn’t want to become only a card-carrying, day-to-day physicist. I wasn’t sure what I would do instead, but I had gotten from Jack a sense that physics can be really quite wonderful if you do it right and picked the best problems. He was much more formal in the way he approached things, but I got that style from him; it was critical to whatever success I have achieved. So, I’m very glad to have been kidnapped by him in 1967. The continuity here is willingness to look around, think about something, take time, even look like you’re not getting anything done. But knowing something about something other than what you are supposed to be doing has been key. I’m not the sort of person I would point to as a role model, but I managed to make it work.

Zierler:

In what ways did Berkeley Lab provide the intellectual atmosphere that allowed you to do all that you had accomplished during those years?

Nygren:

Well, Berkeley was great. I mean first of all when I got there they were trying to pull themselves back into mainstream. And they gave me lots of support. At that time I also somehow said to myself, “I’m going to do whatever I want. Whatever I think is good. Not to be cavalier, but to really pick my own problems.” The lab eventually appointed me Distinguished Scientist. There weren’t many, one or two others. So my sense of just following my nose paid off. They tolerated that, but at a laboratory, people are generally expected to follow directions. Not that folks aren’t supposed to have ideas, but I was a bit more cavalier about just doing whatever I wanted to. But I got away with it because typically I was producing. After the TPC was more or less successful I felt I had license to do what I wanted. The lab was generally supportive — but not always.

Zierler:

I wonder what opportunity you took after receiving the Lifetime Achievement Award from Berkeley and having the symposium the year after, what you wanted to convey during that moment in your career?

Nygren:

Well, I think that science…well, put it this way: Research is what we do when we don’t know what we’re doing. It’s dangerous to say things like that because in the era of really big projects you really have to know what you’re doing. So, there’s a balance to be found between, as I say, wandering in the woods trying to see where the path is and being serious about making these big projects work when so much depends on it. In 2014, I thought my scientific career might be coming to an end because I really had no idea what I would be doing in Texas. I still had this notion that you have to look around and think about something that’s really interesting, something that you can’t wait to get to the lab to do something about. When I got to Texas we had the double beta decay notions but we didn’t yet have the idea of using single molecule fluorescence — which wouldn’t have come about if I’d been just putting my nose to the grindstone. I didn’t have double beta decay in mind when I was reading about brain imaging. It was just interesting. So, you have to take the time to not be so specialized. I think that’s the key thing.

Zierler:

David, I’m struck by your membership to the National Academy of Inventors. Do you think of yourself as an inventor above and beyond being an experimental physicist?

Nygren:

Yes. I think inventor is a pretty good name. I’m also an improvisor. After two glasses of wine I think I might admit to playing the piano. I don’t really, but I improvise. And that’s what I like to say about myself is I’ll take some stuff that I find laying around and I’ll improvise and see what I can do with it. We all have the same toolkit in physics, but it’s what we do with it can make a difference. It’s like an art. And that’s why I named my symposium “The Art of Experiment” because while we have the tools, we don’t have the rules.

Zierler:

What have been over the course of your career some of the advances in technology—either computers, electronics, instrumentation—that you see as most important to what you were able to accomplish?

Nygren:

I don’t know if I can give a good question to that. I think I was galvanized by the revelation that came from Georges Charpak, who became one of my heroes when he developed the multiwire proportional chamber. Now for me one of the interesting aspects of this is that multiwire proportional chambers were around for a very long time. You can find examples of them in the 40s. So, what was new? Well, what Charpak realized is that at that time in 1967 electronics had matured quite a bit. It wasn’t really super mature, but there were transistors and there were the beginnings of integrated circuits in 1969 or something like that, 1968. He realized he could put the electronics close together and put a single channel on every wire, whereas prior to that lots of wires had been ganged together into one channel. So his great advance was to put the wires close together and put a single channel on every wire. It sounds very simple yet it completely changed the way people thought about detectors. The MWPC was the main revelation in that era. The other revelation that happened was the concordance between cosmology and particle physics. that coalesced gradually over a period of time, which has led to the Concordance Model of Cosmology incorporating ideas from particle physics.

I think that most of my contributions have been humble in that things and ideas are available to anyone and can be put together in a way that produces something new. And I admit I’m pleased with myself in that regard. Now, however, we need to have truly new techniques and technologies to go ahead. It’s not getting any easier and I think that we should elevate “experiment” to “art of experiment”. I like to use that terminology because it’s going to take some real insight, some breakthroughs, and some unexpected ideas so that we can proceed into the next era of particle physics.

Zierler:

On the question of insight and breakthrough, for all of the things that you’ve figured out what stands out in your memory as a true eureka moment? In other words, the drama of discovery, not just the incremental advances that take place when you chip away day in and day out at the laboratory?

Nygren:

Well, I love this question, David, but the first A-Ha moment happened to me when I was a graduate student when I realized I could successfully calculate the unintended cross-scattering of one neutron from one detector into a neighboring detector using Monte Carlo methods. And that was wonderful because Ah, I understand this strange effect now. So, I got the idea I could really understand something that was complicated. Then the second and the most important moment was when I realized that a magnetic field parallel to an electric field in a TPC scenario could preserve the quality of information so that I could drift a meter and still have the quality of information as if I’d just drifted a centimeter. So, that was a breakthrough. And it was a paradigm shift for the field of particle physics. Other people then found ways to apply it. Another A-Ha moment was I realized that you could just turn a silicon strip particle physics detector on edge and turn it into a mammography detector. I just remember sitting up in bed one morning and saying, “Wow, that’s going to work. We know how to do this.” And we did. Another A-Ha moment was the Q-Pix revelation when I realized that, with a very simple circuit, a new way to record waveforms was possible, maybe adding some detector capability that didn’t exist before. And that was great. That happened over the period of a week just lying in bed trying to think about things. And of course, the idea to use single molecule fluorescence was an A-Ha moment. I remember saying, “Oh.” I put down the book, leaned back and said, “I have to write this down before I forget it.” Clearly an interesting idea.

I’ve had maybe half a dozen of these moments. I’ve had them only because I’ve been willing to step back a little bit out of the fray and imagine that, well, maybe there’s a better way. There’s a bit of aesthetic element in this too, which I didn’t mention before. Sometimes I’ll look at a problem and say, “There has to be a better way. This is just too ugly.” So, that’s an element too. I know it’s a little arrogant and immodest to say so, but I think it’s real. I think it’s remarkable that I have been able to make some useful contributions without knowing very much!

Zierler:

Conversely, what have been some of the key failures in your research career? Projects where no matter what you did, you hit a wall, or you couldn’t get it to work, or your idea didn’t pan out the way that you thought it would?

Nygren:

Yeah, well, I remember as a graduate student I thought I was going to do this great test while at the 184-inch cyclotron. I put all the circuitry together you know, plugged in the cables and it just didn’t work at all. I’d forgotten something very fundamental. I had to abandon it. So, that was a real wakeup moment of failure. I had one also as a graduate student when I thought I’d measure the charged pion-neutral pion mass difference through time of flight in neutrons and gammas and that didn’t work out at all. So I’ve had some bracing failures and I certainly came close to having major failures with the implementation of the TPC idea. Let’s see if there’s another failure. [pause] Well, you know, maybe I can plead being tired now, but I can’t think of any other failures. [laugh]

Zierler:

[laugh] We’ll go with that.

Nygren:

Sorry.

Zierler:

David, last question looking to the future. You mentioned right at the beginning of our talk that the students that you interact with now have a general sense that they might not have the opportunity of their parents or the next generation behind their parents. If we can bring this back to physics and with your career, obviously timing worked out wonderfully where you came of age during a foundational time of discovery. For students interested in physics specifically and science generally—even if structurally, economically, politically those challenges remain—in terms of the opportunities that science presents for the next generation of physicists and scientists, are you generally optimistic?

Nygren:

I am not generally optimistic. I’ve mixed feelings about the future. I think that we — I wish I could answer differently — but I think that we are now in a state of great sophistication in almost every aspect of human endeavor and may be at a point of diminishing returns. I hope it’s true that I’m wrong in saying so. But I think the problems are very hard now. There’s no low hanging fruit I think in any of these fields. The risks to society in biology are severe. Yet I think that I would say that training in physics is wonderful because it teaches one how to think and address a problem and how to ask the right question and how to avoid the wrong question. I strongly recommend physics as a training ground. My crystal ball is no better than anybody else’s about what’s going to happen in the rest of this century. The problems that we face collectively on the planet surely must have solutions. I have a written about that in my curriculum manifesto. One of the statements I make is we cannot claim to have the solutions now because we do not have them — but we can create the structure within which we can begin to find them. And that’s what I’m trying to do at the university, to say, “Let’s admit that we’re unable to see our path clearly, but let’s get out in front and see if we can find it.”

Zierler:

David, it’s been a great pleasure spending this time with you. I’d like to thank you so much for doing this.

Nygren:

David, it’s been my pleasure and I thank you for the opportunity.