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Interview of Allen Odian by David Zierler on May 12, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
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Interview with Allen Odian, Permanent Staff Physicist Emeritus at SLAC. Odian discusses his current work on the EXO 200 double beta decay search for xenon, and he recounts his Armenian heritage, his upbringing in Boston, and his early realization that he wanted to be a physicist. He describes his undergraduate work at MIT, and he explains his decision to remain there for graduate school to work at the synchrotron laboratory run by Louis Osborne. Odian discusses his thesis research on proton pairs under the direction of Al Wattenberg, and he describes his postdoctoral work in pulsed electronics at the University of Illinois. He explains his decision to pursue a Fulbright scholarship to work on the 1 GeV accelerator at Frascati, Italy, before returning to take a job at SLAC just as the lab was coming together. Odian conveys the frenetic pace of building and research during SLAC’s early years, and he describes Shelly Glashow’s direction to look for charmed mesons. He discusses his work on the streamer chamber, and he describes the interplay of theory and experiment for SPEAR. Odian describes his work for the SLC positron source and his advocacy for a streamer chamber at the SSC. He explains the significance of the Askaryen effect, his involvement in the development of the Fermi telescope and his research on the inverted polarized electron gun. Odian discusses the SLC’s value for millicharged particle research, he explains the origins of EXO 200 and his work on the heavy photon search at JLAB. At the end of the interview, Odian reflects on how his experimental work has provided guidance to theorists, he conveys the centrality of Panofsky’s vision and leadership at the center of SLAC’s success, and he explains his ongoing curiosity about the possible existence of Majorana neutrinos.
OK, this is David Zierler, Oral Historian for the American Institute of Physics. It is May 12, 2021. I'm delighted to be here with Dr. Allen Odian. Allen, it's nice to see you. Thank you for joining me.
Allen, to start, would you please tell me your most recent title and institutional affiliation?
At SLAC, which used to be Stanford Linear Accelerator Center, and now it's SLAC National Accelerator Laboratory, and SLAC stands for nothing. My title, for most of the time that I was at SLAC, was permanent staff. It was a position that had tenure that doesn't exist, I don't think, anymore in the laboratory. It started, although I didn't understand it as such, when I first arrived in 1962, that we did facility design for SLAC before it was constructed. And at some point, a year or two later, they changed the position to Beam plus three, when Beam came on, plus three years. So at that point, I realized that, although I had been an assistant professor at the University of Illinois earlier, and then spent three years in Frascati, two of them on a Fulbright, and one from the Italian government, that I was back to a post-doc, or something like that. [laugh] But I was convinced that I would get a permanent job. And I did.
Allen, as emeritus, of course, you still have an official affiliation with SLAC. And so, I wonder, to your knowledge, since you were there right at the very beginning in 1962, can anyone match your tenure at SLAC? Has anybody been at SLAC for as long as you have?
There is someone, Greg Lowe. He was involved as the head of the electronics division and then went into management. He's still around, but I don't know that he goes into SLAC at all or is involved. But he's, I think, the only one left.
In what ways have you remained connected with SLAC, since you're retired? Or more generally, what's been happening in physics that you continue to follow?
Well, the major experiment I've been involved in since retirement was EXO 200. It's a double beta decay search for xenon. But that experiment's over. In nEXO, a proposed experiment, there'll be what used to be called a down select in July. We expect to win that. And that goes from the 200 kilograms of the xenon in EXO 200 to five tons of separated xenon in nEXO. And I've been very involved in it. I designed large parts of EXO 200, and I have designed parts of nEXO as well. And I guess my role now is a guru, although the young people know all sorts of things that I haven't got the slightest idea about. I know a lot more about classical physics. It's not such a strong part of their education. And also, I guess I've made 100,000 mistakes in experiments and don't like to repeat them. And I find that everyone makes the same sorts of mistakes that I've made. Plus, I've specialized more on exotic hardware. I invented the readout system for EXO, the charge readout, which has been incorporated in NXO, and I invented a purifier for gaseous or liquid xenon that they haven't yet decided to put in, but they should.
Well, Allen, let's take it all the way back to the beginning. Let's go to Massachusetts and even before that. Tell me about your parents and where they're from.
Both of my parents were born in the Ottoman Empire in what's today Turkey. My father came from Ankara. His father, my grandfather, had a factory that made cloth of various forms, including women's dresses with gold thread in it and a cloth that was used throughout the Middle East called Kesseh, which is an abrasive cloth that they made a mitten out of. And when people took a bath, they rubbed themselves with this somewhat abrasive cloth, and it was used throughout the Middle East. But my father had two brothers and two sisters. The two brothers were in the United States. He finished college there in 1914, and as an Armenian, he escaped the genocide that started in 1915. So, he came to the United States to go to Harvard Medical School, and entered the class of 1916, and graduated in 1920. On my mother's side, she had, I think, three sisters and two brothers. In 1912, they left because Turkish friends of my grandfather said, "It's not healthy to be here for long. Get out." And so, they left, and they went to New Jersey. My mother was the second-oldest. The oldest was already married, and her husband and she stayed. They both got killed. But we learned later on that three of their children escaped and ended up in the Middle East in Syria and Lebanon.
Where did your parents meet?
In Boston. Although my mother lived in New Jersey, in the 1920s, there was someone who wanted to marry my mother. My mother said, "I want to marry an educated man," and he wasn't educated. And they asked my grandfather, and in those days, all marriages were arranged. And so, she said, "Look, I can't stay here. I will go to some relatives in Boston." And she went there, and she met my father.
Do you know what language they initially communicated in?
In Armenian. They both spoke some Turkish as well. My father spoke Armenian, French, and some Arabic, I guess. And Turkish. I think they spoke mostly in Armenian. I think I spoke Armenian up to the age of 4 only. I entered kindergarten at 4, and I found that at school, nobody else understood me. So, I rapidly switched to English. And from then on, my parents would always talk to me in Armenian, and I would answer in English.
What were some of the Armenian customs or holidays that stick out in your memory from childhood?
I guess the biggest holiday was Easter. And my mother would get up very early in the morning and bake a sort of long, twisted sticks of bread. And we would color eggs, and everybody had their group of eggs. And you would hit eggs and see which one broke. And that was the biggest holiday. There were a variety of others. My family was not very religious at all. Religion was a social construct.
Do you have any memories of the Great Depression and economic hardship?
Oh, yes. My father was a doctor at the Soldiers' Home in Chelsea, Massachusetts, where I was born. He was there, and I guess it was for Civil War retirees. And then, he left there, and I'm not sure exactly what he did for some years, but he had a house in Dorchester, a two-family house. And the people upstairs couldn't pay rent. And the Depression was already underway. So, my father lost the house and moved to the south end of Boston. And that was a somewhat more depressed area. He did all right, but patients didn't pay in dollars. Some of them brought fruit and vegetables for payment. And we lived in a house that I saw on television in a series called St. Elsewhere.
And in the beginning of the show, they would pan the camera around Blackstone and Franklin Park. And there was a white building. And that's where I lived. But we had a big house, and I know that during the Depression, people would come knock on the door saying, "I'm hungry. Can you give me something to eat?" And my mother would make a sandwich. And then, to sort of make it right, would say, "This is work, not charity. Why don't you wash a window?" So, she would have them wash a window. She said, "It's always good to keep people's sense of dignity up." And so, the place actually really deteriorated. And I left in 1955, after I got my PhD. So, I lived at home until I got my degree, and I went to MIT for both undergraduate and graduate school.
Were you specifically interested in science as a kid? Did you know you wanted to pursue that as a career?
Yes. I knew at the age of 12 that I wanted to be a physicist. I had at home a room that started with one of these Gilbert chemistry labs. And one of my mother's sisters had given me the biggest chemistry set. And I knew, learning on my own, more or less what freshman chemistry at MIT was. So, I knew that. But I also had the feeling that chemistry involved memorizing lots of things. And although I could remember all of those things, when I had physics in high school in my junior year at Latin school, I found that things like F=MA– from a few principles, you could do everything. And I guess my real interest came in freshman algebra at Latin school. That, I found fascinating.
So, math and physics were aligned. I entered that school, it goes from the 7th grade on through high school, and I had five years of Latin. I could avoid it the sixth year by having both chemistry and physics. And I could do Latin, but it seemed really unfair to me. I had a Latin teacher who graded on what he called the absolute grade. And so, there was one month that, in the whole class of, I don't know, 25 or 30, one person passed. And I came and was in second place, and they graded on the numerical scheme. 60 was passing, and I got 59. And we had a report card every month. And when I brought that home, my father looked at it and said, "Well, you don't go out for a month. You have to do Latin as well as everything else." And more so, my sister, who's two years older than I and was at girls' Latin school on the other side of the street from the boys' school, had to help me. That was the biggest pain. But anyhow, I survived all of that.
Who were some of the big-name professors in physics when you were an undergraduate? Who sticks out in your memory?
Well, of course, there was Einstein and Fermi.
I mean at MIT.
Oh, well, there was Zacharias and Martin Deutsch, who discovered positronium. There were theorists, like Feshbach. And then, there was William Kraushaar in cosmic rays. But the people who impressed me were Jerrold Zacharias, who did molecular beam physics, and Martin Deutsch, who was doing nuclear physics. And I was interested in experimental physics. Although, I was all right in math, somehow theory sounds too vague to me. There could be 100 different solutions. How do you know which one's right?
What opportunities did you have as an undergraduate for laboratory work or larger experiments during the summer, perhaps?
That didn't happen until, I guess, between my junior and senior years at MIT. Then, I had a job looking for the lifetime of the pi meson. I became a scanner. They had a place where they stopped pi mesons in a scintillator, and you could see the particle entering with a flash of light, and then when the pion decayed, you saw a second flash of light. And you had an oscilloscope with two pulses, and I was measuring the distance between them. That's what I did one summer. And then, after graduating and my first year of graduate school, I was involved in a scanning of some emulsions. And actually, I had done a little bit of scanning in the junior to senior year before graduate school. Before that, I had summer jobs as a counselor at a camp.
What considerations did you have between staying at MIT for graduate school or perhaps going somewhere else?
I applied to one school besides MIT. Caltech. And I got into Caltech, but financially, I could live at home, and the tuition at MIT the first year was $700. Sort of surprising. But financially, I thought it would be better to stay at MIT. Although, I would desperately [want] to go to Caltech and California. I didn't make it immediately; it took some number of years before I came out here.
Now, given you were an undergraduate at MIT, did you know who your thesis or graduate advisor would be, even before you started at graduate school?
No. I got an assistantship at the synchrotron laboratory that was run by Louis Osborne. And so, I went there. Although I understood chemistry well, I really found that I didn't know anything about experimental physics. I was really green, especially in designing mechanical things. That hadn't been my strong point. Someone called me once a theoretical experimentalist. I like to design equipment on paper, but from first principles and knowledge. Getting all the screws right is the challenging part. But it took me a while. And after the first year of sort of roaming around, learning different things, Al Wattenberg took me on as a student. And from my second year on, I started doing experiments on the synchrotron.
What was Wattenberg doing at that point? What was his research?
He was interested in protons being ejected with hard x-rays from complex nuclei. And the first experiment I got involved with, running shifts on and taking care of the apparatus, was a 300-MeV electron synchrotron that produced Bremsstrahlung beam at an energy spectrum that falls as one over the energy to the cutoff. And so, if every photon energy had an equal probability of transferring all of its energy to a proton, you would see the same spectrum of protons coming out. But the spectrum of protons coming out sort of cut off at 150 MeV, and it was a 300-MeV experiment. So, if the maximum energy going in was 300 MeV and the highest energy proton that came out was 150, there was another 150 going somewhere. Or else, the probability suddenly dropped that ejecting protons happened at 150 MeV, which no one believed.
So, we said, "What else could come out that's taking the rest of it?" Well, supposedly there was a theorist who taught that a big nucleus, there are inside pairs of protons and neutrons. And when you bring in a high energy x-ray, you do like the photo disintegration of deuterium, and there's a neutron that comes out at the opposite direction to balance momentum and energy. And so, after we saw that cutoff, sort of my experiment of building a neutron counter and looking to see if the proper angle that in deuterium you would find a neutron, and I could rotate the angle to see if there was a peak in the coincidences between a proton and a neutron. And yes, there were.
Actually, that wasn't my thesis. That was the second experiment. And my thesis ended up being how many proton neutron pairs there were, and it seemed like if you had a mixture of protons and neutrons, the probability of a proton and a neutron being close together in that model, I tried to collect all of the neutrons. Because they were moving inside the nucleus, so the target was in motion. And so, that smeared out the angles some. So, I had to build a big neutron counter that took all of them. And I had to prepare the scintillator, purify it, and then run it with some number of photons that worked. And it came out just right. And so, I got my degree.
What were some of the theories that may have guided your research? What was going on in theoretical particle physics that may have been relevant for you at this time?
Well, at the time, we knew of the pion and the muon. And then, I guess, while I was in graduate school, the ka was also discovered. But nobody understood why, and the question was, why is the muon as well as the electron? Nobody understood that. And in fact, one experiment I did with Osborne before I got my degree was to produce muon pairs. Like an x-ray could make an electron-positron pair, likewise, you should be able to make a mu+, mu- pair. But quantum electrodynamics at that time was the only theory that was around. So, it was bound by the mass of the electron divided by the muon squared. So that's 40,000 times smaller. So, it's rare. We didn't reach that. At the time, people thought maybe there was something about the muon that it didn't interact with other particles, but with the opposite charge muon, maybe there was a strong force.
And then, the cross section would be big. But we didn't see any. So that experiment interested me. But it was one of some number of upper limit experiments that I've done looking for something that hadn't been found. Some still not found. So, at the time, except for quantum electrodynamics, there was no theory. And after I got my degree, my job at the University of Illinois didn't start until September, and I got my degree in June. And Dave Ritson had exposed the edge of a whole pile of emulsions to a positive momentum-analyzed beam at the so-called Bevatron at Berkeley, and they had developed the film of the emulsions, and you could hold the emulsion up and see where all protons stopped. The range of protons was about a centimeter into the emulsion, which was sort of like the size of a sheet of paper, 8.5 x 11. So, they were big emulsions. And you could see about a centimeter into a dark area where all protons stopped. And Ritson had told the scanners to look beyond where the protons stopped. And so, lighter particles appeared in the emulsion because of the momentum of the magnet selected. And there were darker ionizing and minimum ionizing. The darker ionizing things were what people call Ks. And the minimum ionizing were pions and muons.
Well, he told the scanners, "Take the darker ionizing, and look at the end product." And some of them ended in three particles, and some ended up in one visible in the decay. And this was the so-called tau theta. And he said, "Go read an article written by an Englishman called Dalitz," and so it was all about geometry. The British liked to do things in geometry. And then, he told me, "Look at the events with three tracks, and in the microscope, measure the endpoint of the three tracks." And they were pions. And with a k+, you would have two pi+ and one pi-. And you could tell that it was a minus, even although there was no magnet on the emulsion because it got absorbed by nuclei and blew it up. And then, I could measure, by the range, the energies, and by the angles, the momentum sharing of the three particles. And I put it on a Dalitz plot. And the distribution on the Dalitz plot would tell you the spin parity of the k decay. And the k’s that decayed in one visible track had a different spin parity than the ones that decayed into three.
So, the question was, were there two kinds of kaons? Or was spin parity not preserved? And so, I analyzed 50 of these three track events, and plotted them, and they didn’t have the same spin parity as the single visible track events. But we said, "Well, is parity conserved?" It was 1955.
Well, we published at some point, but I had left for U of I by the time the article was published. But I remember being at the University of Illinois as a post-doc. I asked a theorist then, Francis Low, "Is parity always conserved?" And he pounded the table and said, "Always." [laugh] And so, in either late '56 or early '57, I saw an article by I think TD Lee saying, "Well, we have a charge multiplets. There are positive pions, negative pions, neutral pions. And kaons, there are charged plus, minus, and neutral. Maybe there were parity multiplets. The same particle could come in two different versions of parity." I said, "Oh, that's a smart idea." And then, a few months later, all of a sudden, there was a parity violation. And I don't know whether I would've been smart enough to have looked for angular distributions of muon decay to electrons, but I missed it by a long distance.
Did you have a good experience at Illinois?
Yeah, I worked with an Italian, Gilberto Bernardini, who, after several years, before I left, went to CERN and became the head of the neutrino group or something like that. I hated electronics when I was a graduate student. And when I became a post-doc, they told me my job was to do the electronics. And it was vacuum tubes in those days. And so, I built a circuit, plugged it in, turned on the switch, and smoke came out. And so, I was working 16 to 18 hours a day, trying to understand why. And I had quite a bit of help from a graduate student and a post-doc who knew a lot about electronics, and they taught me a lot. And so, eventually, I succeed in doing it and realizing that, at that time, most of the books on electronics were on communication electronics in the so-called frequency domain, but it was all about AC circuits.
And at Illinois, I learned about pulsed electronics, as we called it. And the whole math for it was much simpler and transparent. And I had learned at MIT that the theorists always said, "You shouldn't be ‘anschaulich,’ a German word for ‘needing a picture.’ You shouldn't want to picture quantum mechanics. Forget it." So, there was all of that. But for me, to be able to see something and predict out of it, I needed a picture to understand. And Matt Sands, during World War II, had written one of the MIT radiation series books on pulse electronics. I think it was radar. And I learned a lot from that. That was the only book that had something about pulse electronics. Later on, it became digital electronics. But there was nothing of that around at the time. I became an assistant professor in 1957, two years after I started, and at that point, I got involved in PSSC. After Sputnik, there was a big push to get physics education right.
And at MIT, Zacharias had pushed a national program for having a high school physics book that represented modern physics rather than the pulleys, which I learned in my high school physics courses. And I spent some time on that. But then, at Illinois, Bernardini left, Hansen was busy elsewhere, and I found that, as an assistant professor, I was the big shot in the lab. And I had always said I prefer to be a little fish in a big pond than a big fish in a small pond. And so, I tried to propose some experiments. But I couldn't find anybody else who would join with me. Experiments, in those days, needed five or six people, not the hundreds nowadays, but I couldn't even muster that. Nobody was interested in doing rare decay experiments.
And so, I got sort of bored. People were doing what I considered stupid experiments. And so, I had an Italian colleague who worked on a different experiment, and he left, and he said, "Why don't you come to Italy? We're building a 1 GeV accelerator." And at Illinois, there was a 280 MeV betatron. So, I was looking for the higher energies. So, I applied for a Fulbright and got it. And it was supposed to last one year, but I loved Rome. It was the years of “La Dolce Vita.” The Sweet Life. I remember seeing Burton and Liz Taylor hand in hand, walking down Via Veneto when she was still married to Fisher, I guess.
Did you take a leave from Illinois? Or resign your position?
No, I asked for a sabbatical because I had been there for four years. They said, "No, you have to be there for seven years." I said, "How about a half-sabbatical?" They said no. So, I said, “All right.” So, I took a leave of absence. I found, at the synchrotron at Frascati, that the Italian that had been at Illinois, planned an experiment to photo-produce muon pairs. This was the experiment I had tried once, and had set an upper limit for, but I wanted to succeed. And I built a lot of that experiment, and it succeeded, and we found muon pairs. And the number was within 5% of what was predicted from quantum electrodynamics. The production rate was about 40,000 times smaller than that of electron pairs. So, a muon looked just like an electron with 200 times the mass of it. But I was happy to do an experiment. I was unhappy that we didn't find a deviation from quantum electrodynamics.
And spark chambers had just come out, and I'd started playing with them. And then, all of a sudden, I wanted to come back to the US. And my father died in 1960, so I came back for the funeral and tried to see if I could find a job at Brookhaven. But coming from out of the cold without all the proper things, Brookhaven wasn't interested. So, I would’ve stayed in Italy at that point. But I got a surprise letter from Joe Ballam at SLAC, offering me a job, saying that I’d been recommended by David Ritson. And Ritson had been in Frascati for a sabbatical, I guess, from Stanford.
And this letter would’ve been when, 1961?
It was in the beginning of ‘62. But I was busy in the middle of an experiment. I wrote back, “Yes, I will come, but it will be later on.” And they said, “OK.”
What was Joe Ballam’s position at this point?
He was head of the research division of SLAC.
Now, in these very early years, I’m trying to figure out, is SLAC still an idea at this point? Is there any infrastructure?
There was no infrastructure. We were not at the current SLAC site. We were on campus, and we were doing design reports. For instance, the power of the beam could be a megawatt. And how do you end up stopping the beam? You did the calculation, and if you put lead blocks there, not at the surface, but inside, where you had a shower of electrons and positrons, with a megawatt being deposited inside, it would first melt the lead inside and then vaporize it. One pulse of the machine would heat it to melting. The next pulse would blow the brick to pieces. And therefore 360 pulses per second. So, I worked on a crazy idea of mine. It never was done because something else ahead of it came, which was a big tank of water with aluminum pellets in it, spheres. And the water was cooled.
I had the wild idea of something that chemical engineers used to use, which is called a raised bed, where you have a powder in a container, and from underneath, you blow a gas through it that suspends the particles in the bed. This reduces the density of the powder which has an enormous surface area for fast chemical reactions. So, I said, "Well, carbon can stand high temperatures. So, if I have a carbon outer frame tank, and fill it with carbon powder, and blow helium underneath, if it's long enough, it will dissipate and make the carbon red-hot. But that gets transferred to the helium, and the helium carries it off.” Well, it was only on paper at that point, when the other one succeeded. So, I've had some wild ideas that never were tried, actually. It was all on paper.
What was your first project when you got to SLAC?
After facility design, I went with Hobey DeStaebler, and Joe Ballam to the CEA, Cambridge Electron Accelerator to test if Sid Drell’s so called “Drell Process” was valid. If true, SLAC could provide beams of pions, kaons as well as muons, electrons, and positrons. We showed that it was valid, and these beams were made available.
We wanted to do photo production. There was a theory called vector dominance, that a photon converted into the vector mesons the rho, omega and phi. And so, we wanted to photo produce these particles and study that. And first people were proposing to use spark chambers. SLAC’s energy was wonderful, 20 GeV in those days, but the beam came in 360 bursts that lasted about a microsecond. So, all your events came at once. So, the electronics of experiments jammed. Scintillation counters had thousands of counts per microsecond. They'd pile up on each other. So, I said, "You need a visual detector like a bubble chamber." But a bubble chamber could only cycle once every few seconds or once a second and could not be triggered. So, I had heard about the wide gap spark chamber. The spark chamber gaps were one centimeter, while the wide gap chamber had 30 centimeters, so I wanted to do it with wide gap chambers.
Soon, I learned that a Georgian had invented the streamer chamber. He had a little chamber with neon gas in it, with two mesh plate electrodes. And after a particle went through the neon gas, you applied a very high voltage pulse to it. From each of the ionization electrons you'd start a spark coming. But if the pulse that you applied was very short in time, the spark would be arrested in space. And if you looked through the electrode, it would look like a bubble chamber. You would see little sparks in space, not going from electrode to electrode. We built a double-gap chamber, because if you pulse a single gap with high voltage it would cause tremendous electronic pickup. I wanted a 30-centimeter gap not 10, and it requires 20 kilovolts per centimeter. With 30 centimeters, it needed a 600-kilovolt pulse that would last 12 nanoseconds. A really short, very high voltage pulse. Well, we couldn't do that in those days. So, I started learning about high voltage technology, and I learned about, from the British, who were doing these things for nuclear weapons. They could make x-rays with kilo-amperes of current at the hundreds of kilovolts range to simulate the EM radiation from a nuclear weapon. They wanted to harden all electronics from nuclear weapon explosions. And they talked about a Blumlein—a device that could generate short duration High Voltage pulses. So, I took that device and developed it, and we made a streamer chamber. It was like a bubble chamber. We had three cameras above in stereo inside of a magnet with no upper pole-piece that you could look through and photograph. And you could pulse it five times a second. Also, you could trigger it. So, you waited for an event, with scintillation counters outside. When you got the right event, you triggered the Blumlein and you took pictures.
There were scanners that picked good events and measured them. Because one could wait for a trigger rather than pulse during every beam burst, you could search for things that were much rarer. And we did that and had a collaboration with a variety of universities and BNL to study neutral k long decays. We made a neutral k long beam, and watched them decay in the chamber. At the time, V minus A for weak interactions was not well established, and we were looking to see if there was any tensor, not just vector and axial vector in beta decay. No, there wasn't.
After that, we tried to do inelastic muon scattering. That's because of a ratio called scaling in inelastic electron scattering was observed. At SLAC, this idea by Bjorken, BJ for short, was a mathematical construct of which I didn’t understand the significance. Soon Feynman came to SLAC and gave a seminar explaining in very simple terms that scaling was really a consequence of scattering on the quarks inside of a proton. All inelastic scattering at SLAC up to that time used incident electrons. They only looked at the electron after it scattered. We could look at a muon after it scattered as well as at all the other particles that came out as well. We could look at what the final state was. Were there quarks coming out? No. We didn't see anything.
Then I went to a conference where Glashow gave a talk. It was a meson spectroscopy conference. He said, "Stop doing all this nonsense, and look for charmed mesons." And he said, "Charm mesons production should have a cross section for pion beams of one millibarn. Pion total cross sections were 20 millibars, so 5% of all interactions should produce a charmed particle. Charm decays into strange particles. Among them are k shorts that make Vs. Now, in our inelastic mu scattering, we worried about pions being in the muon beam. So, we did a pure pion beam run, and looked at interactions. So, we interrupted the mu scattering experiment. We triggered the Streamer chamber when a pion went into our Hydrogen target and a muon of either polarity came out at a large angle to the beam. And so, if we put a pion in and looked for a muon out, that could be a charm decay. And we looked for Vs. There was nothing special there. And this was 1973. And we didn't find it. Instead of one millibarn, we set an upper limit of five microbarns. The highest energy pion beam we got at SLAC in those days was 18 GeV. Someone later on found that at 18 GeV, the cross section for charm was one microbarn, a thousand times smaller than Glashow had predicted. Our upper limit was a factor of five higher than the production of charm. We missed that one.
I went to Munich for a year to develop streamer chambers for use at DESY. And while I was there is when SLAC found what they called the psi, and Ting called the J.
Oh, so you missed the November revolution?
Yes, but I heard about it. And when I got back, we looked at the data we had collected when we were looking for charm. Since we triggered with either a positive or a negative muon, we asked for triggers with both mu+ and mu-?" And we saw rho, the omega, and the interference between the rho and the omega mass plot seemed to have no events above 1.6 Gev/c2. We had fantastic energy resolution. The programmer making the plot stopped the graph at 2 Gev/c2. Since by then we knew that the J/psi had a mass of 3.1 Gev/c2, we extended the graph of our data to higher mass. We found to our surprise, one event at 3.1 Gev/ c2. We missed that discovery by our stupidity.
Now, you said you were in Munich working for CERN. What was the institution in Munich?
The institution in Munich was the Max Planck Institute (Heisenberg) The experiment was done eventually at DESY. Maybe I'm wrong.
Perhaps at Hamburg.
And I learned something about plotting things to the last event from that fiasco. And I also learned, earlier when we were looking for charm, not only did we look for Vs, we had multi-prong events. And it was in a magnet, so we had the momentum of tracks. We didn't have particle identification, so we took tracks two at a time, all tracks, and did masses. Then we took three at a time, four at a time, and did masses in 1 MeV intervals. And there were so many combinations. And we had so much data that we made what we call the moving average of bins, and we looked at 11 bins. We were looking at the one in the center. We'd use the other ten for the background. And then, we moved that to look for a peak above the background. And in that experiment, we had tens of thousands of one-sigma, thousands of two- or three-sigma, we even had two five-sigma events. And so, I learned that when people talk about the standard for discovery is five-sigma, that is wrong. It should be low probability. And if you do enough experiments, you will find a random bump that is five-sigma. So that's another little thing that I learned.
It's out of chronology, but I wonder if you might comment on the muon anomaly at Fermilab right now. Because a lot of people are comparing the number 4.2 with 5.0, where 5.0 is that certainty. But you're saying something different.
Well, if you do one experiment, only one experiment, and look at only one bin, and not many ways of making that bin, then five-sigma is a low probability. But if you have lots and lots of different channels, and you're not looking at one channel–at 1 MeV, if you look at a GeV band, you have 1,000 channels already. So, if you find a peak in that thousand, not in a specific one, then the probability of randomness producing your peak gets bigger and bigger. So probability is the proper measure, and not sigma because sigma is just a peak somewhere, and it doesn't tell you exactly how many bins you looked at or how many experiments were done.
By 1978, what would you say were some of the major accomplishments of the streamer chamber?
We did the rho vector photo production, and we did the final state in inelastic scattering. Showed that deeply inelastic events didn't produce quarks out. There were no visible, very low ionizing particles. The streamer chamber became obsolete when drift chambers came. The moment I saw Charpak's results, I said, "OK, that's the end." And we were planning on doing an experiment, looking for Ws at PEP, at SLAC. That was with higher energy electrons and positrons. The theorists were telling us that the W mass could be 20 GeV or 30 GeV. The theorists had a thing, they were betting on where the W and the Z mass would be. We were looking for the Z. And PEP was 15 on 15 GeV, so you could see 30 GeV virtual photons, or Zs. And we made a proposal for the first set of experiments, and we got turned down. Everybody else was looking for it. They didn't find anything either.
And so, we got disturbed and said, "We're going to go back to SPEAR." That's the low end where the J/psi was found. And as Mark II had went to PEP, SPEAR was empty. They found gold there, and there was a lot of it. And drift chambers had just come out. So, I designed some drift chambers to put into a new detector Mark III, an upgrade from what they had in Mark II. And we found lots of that was a bread and butter thing. We found all sorts of decay modes of all sorts of particles. Nothing spectacular, but after the J/psi and the tau, there was nothing really big that came out. So, at that point, SLC was the next accelerator.
But first, there's Mark III at SPEAR.
Yes, that's the one that did bread and butter [experiments]. We published a large number of papers on all kinds of decay modes, some D physics, and looked for D mixing. Didn't see that.
At SPEAR, what interactions, if at all, did you have with theorists?
Generally, they were the ones who pushed us into generic things. E+e- goes to vector plus scalar, or vector vector, or classifications in that, and we compared different vectors, rho omega as vectors versus pseudoscalars, and so on. And we found their productions and the various decay modes. But I guess it added to the list of things that theorists used for chromodynamics. But I didn't have any direct coupling with the theorists. We thought we found a scalar particle. They were very interested in that, if it could be the Higgs. And that scalar particle was called the sigma. And theorists have had models of the sigma for a long time, all kinds of things. And no one has confirmed it, but no one's killed it either.
And Allen, what were the circumstances of you joining the SLC positron source in 1985?
In 1980, I went to CERN and worked on a streamer chamber experiment UA5. UA1 was made by Carlo Rubbia, and UA2 was a second experiment that wasn't ready at startup. So, we put in a big streamer chamber at UA2 interaction region, when the proton-proton collider first started. We were forbidden to have a magnetic field or calimetry. We were not permitted to find the W or the Z. So, we saw the p-p collisions in between the two electrodes of the streamer chamber, and viewed all the particles that came out, we measured the angles and the multiplicity of tracks. There was a wild rumor of so-called CENTAUROs that had been seen in cosmic rays, where there were events with enormous multiplicities coming out and no gamma rays. That sounded very strange to us. There are always neutrals. So, we had put some lead oxide plates inside the streamer chamber that could support the voltage to convert gamma rays. So, we could see the charge multiplicities, and we could count the number of gamma rays that converted. And so, we looked at the multiplicity distribution of, I think, 200 GeV on 200 GeV. And the experiment was done with someone from England, and the Germans at Munich supplied the streamer chambers. And there were lots of Scandinavians involved in it. And we found that when the multiplicity and the pseudo rapidity, which is just angle, distribution of all the tracks, when there was a big multiplicity on one side, there was a big multiplicity on the other side. And we saw lots of gamma rays. So, we did the profile of the multiplicity distribution, and the distribution in what they called rapidity, the pseudo rapidity. Rapidity is sort of the transfer's momentum.
Well, we couldn't do momentum. What we did was angle. And so, that's pseudo rapidity. I did a monopole search in 1963, I think. Didn't find them. I guess I always went for the big payoff, unlikely experiments. There used to be an experimentalist at Stanford, Fairbanks. Luis Alvarez commented on Fairbanks. He said Fairbanks was such a great experimentalist that he could take a reaction, which everyone thought was six orders of magnitude too small to be observed, but he'd bring it down to two orders of magnitude too small to be observed. Well, I guess, I ended up in that category as well.
Chronologically, in the late 1980s, were you paying close attention to the developments with regard to the planning of the SSC?
Yes. I proposed a big streamer chamber for the SSC. It didn't fly.
Now, why would this need to be done at the SSC and not just expand what was being done at SLAC?
Well, the energy was huge at the SSC. I wanted to look for a visual trajectory in a magnet with particle ID outside. I wanted to see what sorts of particles could be seen in a direct–I was thinking in terms of a single beam with one of the sources towards a beam dump. I always liked to do beam dump experiments, where you take the cast-off beam after an experiment, on the way to the beam dump, you have a huge beam, and you don't fight with other people for beam time, if it's a major experiment you're behind, in a single beam.
I did that once behind SLC, looking for millicharged particles. To make positrons, you take an electron beam and put it through a target. In that target, you make electron-positron pairs. The electron pairs, they threw away. And they had a magnet. And the positrons were bent around to use for damping them and storing them in a storage ring. But straight ahead of that, gamma rays went. And of course, they had a gamma ray beam stop, and it was underground. So, the muons produced went in dirt. And so, outside, we drilled some holes in the ground within the muon range to find exactly where the beam was. We put some counters down in those holes, found where the muons were. And beyond the muons, there should be no particle there. Except, if you created pairs of particles with a fraction of an electron charge, then they wouldn't lose energy very rapidly. And we made a pit with a huge scintillator in there, 1.5 meters long, and we had one in the center and some that surrounded it to detect a fractionally charged particle. Typically, the theorists said it's 1/100th of an electron charge. So, we looked for one photon produced in this long scintillator. So, we used photo tubes, and had to find how to make them low noise, low dark current. We made them cold, and that brought all kinds of problems with optical coupling between the photo tube, and the scintillator. The optical grease turned opaque when it got cold.
We had to find the right materials, and we did. While SLC was running, we had beam and they had beam all of the time. So, we had a lot of beam and we didn't see any millicharged particles. And I guess that experiment was interesting, at the time in the 90s. But after that, nobody found it to be. And recently, now, people are looking at the paper again because there are new theories that look for millicharged particles coming from a dark sector, from dark matter. They have very weak coupling between the dark sector and our sector, if the dark sector actually exists. But I'm not a fan of dark matter. Everybody's trying to do it. And I say, if people look for ten years, and they don't find something, maybe they should start thinking of another theoretical solution.
Ten years is too generous. It may be more like 30 or 40 years.
But I actually looked for heavy photons. We did an experiment at JLAB. It was hard to get in. JLAB, at the time, was pure nuclear physics. Looking for a heavy photon was sort of like particle physics. But we finally got in, and we haven't found it. I tried two experiments there.
So SLC was a tough grind. They didn't have the luminosity. They worked for years to bring the luminosity up. And if they hadn't gotten the polarized beam, SLC would've been useless. The fact that SLAC achieved a polarized electron beam meant that SLC was relevant. LEP had many more events. But I guess it was the asymmetry that they found at SLAC. They had an asymmetry because of the polarization. I also worked on a polarized gun for SLAC.
Oh, another wild experiment, I read a preprint from people from Hawaii and UCLA who were looking for something they called the Askaryan effect.
What is the Askaryan effect?
Well, it's coherent Cherenkov radiation. Now, we know that Cherenkov radiation, the spectrum falls as one over the wavelength squared. So, it's mostly very, very short wavelength. Longer wavelength is less strong. It also goes as Z-squared in a nucleus. In a nucleus, if you have two protons, that enhances four times. If the incoming particle that's doing Cherenkov radiation is doubly charged, it goes as a square of Z. So then, Askaryan, in the 1960s, in Moscow--he's an Armenian boy like I am, so it was interesting to learn about him--said, "When you have a very high energy cosmic ray interaction that generates pions, among them are pi-0s. And pi-0s decay into gamma rays. So, you have some very, very high energy gamma rays. When a gamma ray interacts in the atmosphere, it produces a shower of electrons and positrons. And that shower starts with one electron, one positron. And as that pair moves through the atmosphere, they interact with air atoms, and they do Bremsstrahlung. They radiate photons. And those photons hit other atoms and make more pairs.
So pretty soon, you have multiplication of pairs, and you have what's called an avalanche. And it grows until a maximum. And that's at what they call the critical energy. And from then on, the electrons and positrons have lost so much energy, there are so many of them, that they lose more energy by ionizing than by creating more pairs. And he said that that bunch of electrons and positrons, and the whole size of the shower, is in the form of a bunch, millimeters in space. And then, he said that positrons die early because positrons can hit electrons in atoms and annihilate. And those gamma rays go off to the sides. And also, the gamma rays that are in that positron- electron bunch–so it’s like one particle. So, they're all going at a relativistic speed. So, it's longitudinally and transversely a millimeter-size bunch. The gamma rays do Compton effect. We live in a matter universe, so there are more negatives. So, the electron bunch picks up electrons from the gamma rays interacting, not making pairs, but doing Compton effect.
So, at shower maximum, there are 20% more electrons than positrons. Now, you can say that bunch is one particle that's a couple of millimeters in size. So, it's got a charge of 20% of the total. Now, the number of particles there is the incident energy of that gamma ray divided by the critical energy, the average energy at shower maximum. So, there are millions of particles. And 20% of millions is a huge number. And Cherenkov radiation goes as a square of the charge. So basically, they said, a very high energy gamma ray coming that makes an electromagnetic shower will make Cherenkov radiation that's enormous. And it easily will become more energy than the energy of the incident particle so there is a cutoff. The Cherenkov radiation becomes the dominant energy loss of a high energy electromagnetic. But you have to go to energies above 10 to the 18th eV, which is 10 to the 15th GeV. There's no accelerator on earth–but there are cosmic rays of that energy.
So, they were looking for this effect at Argonne. They had some electron beam. I called them up and told them they were crazy for trying that experiment at Argonne. The right place to do it was at SLAC, where we could have an x-ray beam that we put a target in to make gamma rays, and then put a magnet afterwards, dump the electrons away, and have those gamma rays go into a big sandbox. And because of the millimeter size of the bunch, the wavelengths that it can do coherently have to be longer than a millimeter. So, the Cherenkov radiation is not in the visible, it's in the microwave range. And Cherenkov radiation is polarized. So, we got a big sandbox that took the gamma rays and made a shower in the sand, and we put cables with an antenna that was maybe three or four millimeters long that fit on the end of the cable in the sand at angles related to the Cherenkov angle, due to the index of refraction of the sand, and we had a very narrow SLC pulse, a millimeter long in duration, so picoseconds long. It went by into the box, and we had a very fast oscilloscope. And we had all these antennas that we put in. And I made a back-of- the-envelope calculation of what they should be. And I said, "Gee, on a 50-ohm cable there should be a one-volt pulse from that antenna." When we turned on the beam, there was a six-volt pulse. It was astonishing.
And we did a whole set of experiments after that. Then, Gotham and Salzberg built in Antarctica, antennae on a balloon that circled around the South Pole, looking for ultra-high energy, 10 to the 20th eV, cosmic rays. There shouldn't be cosmic protons higher than that because protons in interstellar space, interact with the three-degree K cosmic background microwave radiation. A proton hitting that radiation will generate pions. So, the protons coming from outer space, no matter how high an energy they have, after traversing enough space, will lose energy from collisions with cosmic microwave background. And that stops when it hits 10 to the 20th eV. And that produces what the GKS theorists predicted, that there was cutoff on the highest energy cosmic rays. Gotham and Salzberg originally wanted to use the big telescope in Southern California, looking at the moon. When the cosmic rays hit the moon, moon powder would generate Cherenkov radiation in microwaves. So, they looked, but there was too much interference around to see that. So, they tried the ice [at] Antarctica.
When did you get involved with what would become the Fermi telescope?
When, I went into the Fermi Gamma Ray Telescope, (it was called GLAST then), there was a design of the part of the telescope that allowed one to accurately determine the direction that the gamma ray came from. There were lead plates to convert gamma rays into an electron positron pair. These plates were all of equal thickness. Each plate was followed by a sheet detector which recorded the position of the pair. And if the first few plates are thick, the electron-positron pair that you create scatters in them. And then, when it goes into the next one, it scatters some more. So I said, "Make them thin in the beginning, and gradually increase the thickness towards the end, so you have the same amount, but the pointing accuracy is better.” And they wanted to look for gamma ray sources in the sky, so orientation was important. So, I worked to change it, and for NASA at first, it sounded like heresy to propose a change. I found working with NASA people a complete shock. The culture is so completely different. Well, I can understand some of their things, but you can’t fix it after it’s up in orbit. So, you have to build it so it doesn't fail. On the other hand, that made all the gamma ray telescopes before using 30-year-old technology. And there was better technology, silicon strips. So, we had a thin lead plate and silicon strips. So, you got good position accuracy, and by having several in a row, you could point, have good angular accuracy. And so, for NASA, it was already upsetting that there was a new technology. But to change the thickness, they said, "No way." Eventually, they relented and made two thicknesses. But you had to go through committees that took months to change one screw. And I said, "I can't do it. I do wild designs. And after I've gotten one started, I have a better idea, and I'm always improving them." And their culture is not one that I want to live in.
SLAC wanted to make a linear collider, and we needed 200 GeV on 200 GeV or something like that to make the Higgs. But they've never made the linear collider. People are still pushing for it. So, we said, in order to do an experiment, you have to be able to focus the beams to make them collide. You needed beams that were, in diameter, much smaller than a human hair and very short, so that every electron hits every positron to increase the probability of interaction. And the optics of all these storage rings is such that they don't allow you to make such a small focus. So, people said, "Well, in optics, you do higher order. There are small effects that prevent that from you focusing down." And so, they went to higher order, second order optics. Besides magnets, which are dipoles, they had quadrupoles, sextapoles, and octupoles, all to do the higher order thing to be able to focus down. So, I worked on a test beam to see if we could focus electron beams to nanometer sizes. It worked!
And we did an experiment with the focused beam. We shone a high-power laser against this beam. We tried to do the Compton effect, that's the scattering of a photon on an electron. Now, all of quantum electrodynamics is calculated on the basis of alpha, which is a small number, 1 over 137. But if you bring in a large number, more than 137 photons, at once, and they interact together, you get what's called a nonlinear Compton effect. People haven't been able to calculate it. In the strong interactions, quantum chromodynamics is not a complete, detailed easily calculable theory. And quantum electrodynamics becomes like quantum chromo-dynamics non- calculable when the number of photons times an electron is greater than one. So, with the laser and the electrons, we could focus them down so they would collide. And in the collision, since the electron has a high energy, and high momentum, and the laser is a low energy, the scattered photon from that is backscattered. And there's a maximum energy that you calculate that you could get from the Compton effect. But if you put many photons in, a symptom of that to detect it is you find a higher energy gamma ray beyond than the maximum allowed in single-photon. So, we were the first to see nonlinear Compton effect. I don't know whether anything will be made out of it. I see, occasionally, papers refer to it.
How did you get involved in the inverted polarized electron gun?
The gun that SLAC was designing had to hold 120 kilovolts. And it didn't. Marty Breidenbach, who was the head of the SLD, the experiment that did most work at SLC, was getting impatient. He said, "We need the polarized gun. LEP will kill us. They have a tremendously higher intensity. We need a parameter that they don't have." And that was polarization. So, since I had worked on the high voltage on the streamer chambers, he came to me and said, “Are you interested in doing this?” And I said, “Oh, sure.” But this was DC, which I had never worked in. And we started building a gun, but we shot for 200 kilovolts. The other one couldn’t do 120. Others who were working on the conventional gun sent someone to me and said, “This is what we are trying to do. What do you think about it?” So, I guess I believe in belts and suspenders. I thought, "In case we don't make it in time, at least these people, who don't have the slightest idea of high voltage have to succeed if we can’t.” So, I told them, "This is wrong, and that's wrong." They were doing quite a few things that were fundamentally wrong. So, I told them what they should do better, and how they should do it. And then, I went off and continued our work. And at 200 kilovolts, we also were having problems. And that bogged us down. We should've backed down to 120, but in the meantime, the other group succeeded. And so, though the inverted gun is a better way of doing it, there are many paths to success. Also, I learned about a phenomenon that I hadn't understood before.
And what about your subsequent work on millicharged particles? How did that come about?
John Jaros had read a theoretical paper on millicharged particles. And he saw that the best place to look for them was behind the SLC positron source. And so, we talked about a design which used photo tubes. The problem was that most were noisy. We needed eight photo tubes 20-centimeter, eight-inch-by-eight-inch-by-sixty-inch scintillators in an array centered on that beam. The photo tubes were all counting like mad in the dark. Though I'd used photo tubes for years, I'd never studied them in detail. I found that if there was a light flash, I expected one pulse. But following that pulse with a little delay, there were maybe 20 or 30 additional small pulses, and then a delay, and then out to a millisecond, a whole bunch more. So, one big pulse generated something like 1,000 pulses. And the pulse we were looking for was from a single photoelectron. They were all the size of the smallest pulse. And so, I said, "Where does this come from?" Well, some of it is from the photo tube if it has some gas in it, Electron multiplication, generates some positive ions that go back to the photo cathodes, and release more, so they're after-pulses. That was known. And I tried to find out how to tune the photo tube to make minimum after-pulses. So, we found that we had to make the photo tube cold, we had to make the gain of the photo multiplier low and put an amplifier after it. We also learned that practically everything fluoresces. That is it generates light after seeing ultraviolet. And that stumped me. We had thought of using plexiglass as the detector viewed by the photo tube. We found that it fluoresces. We picked plastic scintillator. We were underground with the detector, about ten feet down. Cosmic rays come down and make light in the scintillator. Following all that light, not only was there after-pulsing, there was a steady stream of things. The light that you generate initially has ultraviolet in it.
Ultraviolet excites practically everything, which re-radiates, if it's transparent, at optical wavelengths. So, I knew that plexiglass just fluoresced like mad for a long time. I then thought of liquid scintillator. I needed something to hold it. I went to one Crate and Barrel store locally and bought a square vase that I could put the liquid scintillator in. I said, "But before I put the liquid scintillator in, let me see if the vase fluoresces." And for that, I took the vase, went outside in the sunlight, held it there for 30 seconds, then went and put it in a box where there was a photo multiplier, closed it to make it light-tight, turned on the photo multiplier, and the thing was counting something like ten million counts a second. And it kept counting for hours and hours. Slowly going away. The next day, it was still counting at 100,000 counts per second. I said, "My God. Everything in the box, if it sees ultraviolet, it's going to give you background.” We had to look for things that didn’t radiate. We were able to suppress our background counts to unacceptable level. Because the beam came at a certain time, we knew when to look for it because we detected the muons in the hole in front of the detector. So, we knew exactly when to look for a peak in the count rate from millicharged events. Because of cosmic rays and noise there are counts. But at beam time, no difference, no millicharge.
When you got involved with EXO 200 in 2003, was that right at the very beginning of the project?
What were the origins of that? What idea led to EXO 200?
Giorgio Gratta at Stanford was the one who wanted it. And he talked to Marty Breidenbach and Prescott at SLAC to see if they would collaborate. And so, at that point, I had finished the Askaryan effect and I had dropped out of GLAST, the Fermi Telescope. And that was a group run by Elliott Bloom, and he went on sabbatical. So, I ran the group. I never wanted to be a group leader. I hate bureaucracy and all that. So, I'm happy in the lab. Never wanted to be a group leader. I've thought about that, and the Armenians in Turkey were the prosperous, educated ones, but always had to take a low profile because the Turks would massacre them if they were too conspicuous. And so, I came across the idea that it's always good to be number two. You're not in direct fire.
And what was your role initially with EXO 200? What were you doing?
We were trying to learn about xenon, whether you could drift electrons in xenon and purity. And first, what it took to liquify xenon and what the problems were. So, we had the whole cryogenic thing to do–I had a little bit of cryogenics experience in liquid hydrogen targets. But this was quite different. And we had to learn about the purity of xenon. And in the beginning, Prescott and I worked together to find out how to measure the purity. Rubia invented a purity monitor for liquid argon for his argon detector for neutrino physics. And we copied that and started making modifications, improvements to it, so we could measure the purity. And it wasn't very good. And then, it was a painful thing, the liquid purity monitor. So, I invented a gas purity monitor that's probably good to a part-per-trillion of negative impurities.
And we found that when we had a pump–and every pump in the beginning leaked, so we put a pump in the box, put the inlet into the box, and the pump pumped the xenon out of the box. So, if the pump leaked, it was back into the same box that it drew from. And so, we had to learn how to make pumps not leak. And we, then, had fairly pure xenon going through a commercial-size purifier. But with the gas purity monitor, we had a fast reading. We poisoned the Xenon with oxygen, you'd see the purity crash and immediately come back, even when the commercial purifier was bypassed. And minutes later, it came back almost, but not quite, to the original level. And we said, "Where's that oxygen going?" It's going to the walls. So, we realized that oxygen, water, things like that stick to walls. And you'd think you put into a volume pure gas, it comes off the walls. And the amount on the walls is enormous. Someone said if you had a one-liter container, and you had a perfect vacuum in it, and then you removed the vacuum pump and watched what came off the walls, you'd get a fraction of an atmosphere coming from the walls.
So, we learned what it took. And drifting electrons get captured by impurities. So, you need it to be very pure. In EXO 200, we had probably 300 parts-per-trillion of oxygen equivalent in there to have the electron drift something like 18 centimeters. And 3% were attached. So, we lost on the way. So, we had to know the position to correct for that. In NXO, the drift is 120 centimeters. So, we have to be six times purer to have the same. And so, that's one of the things that I've worked on, what I call an electron rain purifier. If electrons drifting get attached by impurities, and if I have a cylinder that the gas passes through, and I have a wire down the middle of the cylinder that's emitting electrons, the electrons get attached by impurities, and the impurities become negatively charged and drift to the wall. And if I put a getter on the wall, they stick. And that's something that I've proposed for NXO, but it hasn't been incorporated yet. It will be.
Now, did you do most of your work searching for heavy photons on-site at JLAB or from SLAC?
Nothing at SLAC. It was all at JLAB. Two different experiments. One by a different group. When we first heard of heavy photons, that sector, we looked to see where we could do an experiment, and JLAB looked like the best place to do that.
And so, because you were retired at this point, you didn't need to take leave from SLAC.
Did you take up residence at JLAB? Did you move?
I was there for shifts. But for the first experiment, the shifts lasted about three weeks. So SLAC paid for that. And SLAC paid for me going to our collaboration meetings. They don't pay me for going to conferences, but for collaboration meetings, since I'm actively involved, SLAC paid. Although, then, after that first one working for a different group with the SLAC group, we mounted a different experiment. And I went twice there. And at that point, they told me, "Please, don't go anymore for shift." Shift is work. And there's some DOE rule that says if you work, you have to get paid.
What they did, though--I went, and it got stuck--they hired me as a $13.57-an-hour worker to pay for my plane fare and per diem at JLAB. It takes bureaucrats to invent these things. They made me a temporary employee to do that. And then, afterwards, they told me, "Don't go anymore."
What were some of the achievements of the heavy photon search at JLAB?
Well, we didn't see it. We set an upper limit. And it's like many of the experiments I was part of, I looked for something that was either not there or much rarer than we look for. But having a wild idea in an experiment is really a lot of fun. And trying to find a way to improve what other people have done in looking for something else, you tend to publish everything that you could possibly look for. You have graduate students looking for those things. But inventing a new experiment, looking specifically for that thing, and using the best available technology.
Just to bring our conversation right up to the present, in what ways is NXO its own project, and in what ways is it an update or extension of EXO 200?
It's its own experiment. I think the EXO 200 flew under the radar of DOE. It was done by Giorgio Gratta at Stanford. He wanted to do it quickly because there were other people looking for neutrinoless double beta decay with germanium. And there had been one crude xenon experiment. And so, we wanted to do an experiment quickly and simply. Well, we did it. It violated all of my principles, doing EXO 200. We did things on paper and built it, and that was the experiment. We didn't build prototypes to see if it actually worked. I like to have an idea, build a prototype, and then improve it. And when I think it's good enough, we go on. But we did things on papers, and I was always the complainer, the one who said, "It's not going to work. You're going to have 1,000 effects that we didn't think about that are going to kill it."
Well, we weathered some very severe ones, but fixed them online and got through them. I found that my appreciation of software went up, being a hardware type. Software people could make corrections for bad hardware. That absolutely astonished me, how well they could do it. So, I learned that. And we have some really good software types. But the number of hardware people wasn't many. And people were going to try to make a bigger EXO 200. And I saw lots of things wrong with that, that you couldn't scale things up. They made more problems and actually made it harder because you could collect the electrons, but they had to drift six times longer distances. You could collect the light, but it's the same light spread over a bigger container, and each light sensor introduces noise. So, the surface area where we collect was nine times bigger, so the noise was nine times bigger. So, we had to change technology to get the noise down. And to measure the charge, the old techniques wouldn't work.
And I invented a completely different charge detector. Half of the collaboration had just barely gotten used to it when I discovered that there was a drawback of it, and I wanted to build something slightly different that gets around that drawback. And I'm having a fight for that now. But that's all right. I don't mind being in a fight. To me, what settles the fight is a demonstration. There are so many theoretical things that sound good and seem to fix lots of problems. There's that theory of supersymmetry. I don't like it. One, it doubles the number of particles that I think is a little bit too expensive, and secondly, more importantly, it hasn't been seen. And people have been looking for it for a long time. It's the same thing I have about dark matter.
To what extent is this simply about not operating at high enough energies? Or are you saying maybe even at higher energies, these things will not be seen?
I don't know. Maybe they'll be seen. I always say it's great to go up in energy. I guess that's what I would've normally done, and I've gone into, now, low energy, looking at 2.5 MeV things and double beta decay. But there's also the role of more intensity, looking in finer detail. I did mu pairs to 5% for quantum electrodynamics. At the time, when I was in Italy in the 60s, I knew Rubbia and Zichichi. Zichichi was a g-2 fan. He's the one who really pushed that. And we used to argue, "Which is better? Mu pairs or g-2?" Well, I guess it turned out he was right. But that was in the 60s, we were arguing about that.
Well, now that we've worked right up to the present, I'd like to ask, for the last part of our talk, a few broadly retrospective questions about your career. Given everything that you've worked on, what stands out in your mind as the most fun, the most enjoyable, in terms of the collaborations or the satisfaction and the success of the experiment?
Well, the millicharge experiment was the most fun. Because I think there were five of us. And in preparation for the experiment, I found all sorts of things that I didn't know about photo tubes, about phosphorescence, fluorescence, and that was fun. Liquid xenon in the beginning, in the EXO 200, was lots of fun. There were lots of do's and don’ts that we learned. It was fun learning a new technology that I hadn't done before. And having something work.
I talked to you earlier about the ways in which, over the course of your career, theorists might have provided some guidance to the experiments you were on. To flip that question, in what ways has your work advanced theory in physics?
Well, in my experiments getting a PhD, I made the pseudo-deuteron model of the nucleus. It more or less said there was a problem with ka decays, tau theta, that was solved by parity violation. At Illinois, we did the Compton effect on the proton. It was never seen before. We saw the scattering of the proton when mesons could be made. There was a giant resonance, (3,3) resonance, as it's called, existed in that scattering. Theory, in those days, was way behind. And experimentalists just found new things. And then, all of a sudden, theory started catching up. I guess the quark model started coming out. And I became an enthusiast of the quark model when I was in Munich that first time. I met Heisenberg there, and the German Physical Society met, and people were talking about the quark model, and Heisenberg didn't believe in the quark model. He was saying that Bjorken had been pushing things like that, and he said, "I can't understand it. Bjorken's such a great physicist, and he believes in things like quarks." And then, later on, Mary Gaillard went to Berkeley and was pushing Ws and Zs before the theorists developed them. I remember her visiting Munich, giving a seminar, and there was a male theorist who kept objecting, finding stupid things wrong with it that weren't wrong. And I was really surprised how she answered the question he should've asked instead of the questions that he asked, not putting him down. And I saw how women reacted differently than many of the men. Millicharge, we didn't find it, so we didn't support that. We didn't find tensor, so that it supported Vector and Axial Vector for the weak interaction. In Mark III, we filled the particle data book with lots of decays, and branching ratios, and production cross sections, and so on, which is bread and butter for theorists. It was all right, but it wasn't the thing that excited me.
I wanted to break some rules. But I never really broke any of them. Came close. If I had seen that plot that cut off at 1.6 GeV, and nothing beyond that, and one event at 3.1, that could've done it, maybe. But now, I've started taking the view that double beta decay is forbidden because the neutrino is not a Majorana particle. All the theorists want the neutrino to be a Majorana particle, that the neutrino and anti-neutrino are the same particle that spin in opposite directions. Now, that's what we're looking for, and if the particle is a Majorana particle, then we should be able to get neutrinoless double beta decay, and the rate of it depends on the mass of the neutrino. But I always say that what we're looking for is forbidden, and I'm a counterweight to everybody who says they want to find it. I really want to find it, but I say I don't because it's also a pleasure to find something that you said didn't exist. I've looked for so many things that so far haven't existed, it would be nice to do the opposite.
Reflecting on your long and perhaps unparalleled tenure at SLAC, in what ways has it provided the ideal academic and administrative home to do all of the physics that you've been involved with over your career?
Well, under the Panofsky reign, SLAC was the best place in the world to work. Panofsky was absolutely phenomenal. He had that technique of managing while walking around. So, while people were working on different projects, about once a week, he would come around and ask how you were doing, what problems you had. And he would sometimes help with technical problems. He was a first-rate physicist, besides being a superb administrator. And if he couldn't help you technically on something, but he saw that you needed more, he would send help. A little more money to buy something. That, you got, if you were doing something that looked good. So basically, he left a lot of freedom, also, to the different groups. You knew that if you succeeded, you'd get more money for future things. You were motivated to succeed. And it was like a family, also. We all went out to lunch together. Pief would sometimes talk about crazy ideas. He had plenty of them.
And afterwards, Burton Richter, a classmate of mine who'd graduated one year after me at MIT became director. He was all right, but he was more interested in what he wanted to do, so he was more selective in resources. And after him came Jonathan Dorfan, and money had to go for building a variety of things. And I guess I was always in the smaller group, not doing what everybody else did. I tended to be the contrarian, going where nobody else wanted to go. So resources tended to diminish all the time. Then, DOE, over the years–I remember seeing LBJ speak at an American Physical Society meeting, where he said that we ought to do less basic science and more applied science. And starting then, money seemed, to me, to slowly go down. And every president afterwards brought it down lower.
Then, DOE became something of a bureaucratic nightmare to me. You could never get the funding you wanted. I don't know how many ideas I threw away because there was no way of doing them. And it became more bureaucratized at SLAC. And it's gotten worse. Maybe Biden's science ideas will do better. Maybe more money will come. SLAC, now, has a small particle physics division. It's mostly x-ray laser and synchrotron radiation. Synchrotron radiation started as a parasite on SPEAR, on the e+e- collider. Pief let people who wanted to do synchrotron radiation use it. As long as they didn't interfere with the particle physics, they had their beams. And Pief encouraged maximum science. That was his thing. Applied or basic, he didn't care. But he wanted to maximize science, except for military applications.
He was the one who turned it down when Congress wanted to make SLAC a secure, gated laboratory like all the other national laboratories. SLAC was the first national laboratory that the guards at the gate would tell you where to go when you were looking for somebody. He didn't want any classified work going on. Now it is 80% applied science. To do R&D for nEXO, the money comes in eyedropper-fulls. The reason I retired is that the management said they would hire two post-docs for my salary. I said, "OK. I have my retirement." And I could still work and use my retirement income. I was 75 then. And they didn't hire two post-docs. They hired zero for us.
Things have changed. When I was a graduate student, physicists would say it was almost like a religious thing. You need a calling to become a physicist. If you don't think that's the only thing you can do in life, then do something else. And you have to have vows of poverty. My first job as a post-doc was $5,100 a year. That was a little more than $400 a month with taxes, and I was paying $100 a month in rent in Illinois. It changed. After Sputnik, my salary went up 10%. So, I said, "Soviet Union, send up another satellite."
For my last question, looking to the future, of all of the things that you've looked for and haven't found yet, what are you most curious about? And as a corollary, what are you most optimistic will actually be seen?
Well, maybe it's nEXO. If Majorana neutrinos exist, we're getting there. Supersymmetry may take a long time, but maybe it has a chance. I started looking into alternatives to dark matter. Dark matter was invented by Zwicky in the 1930s because in the Milky Way, the rotational velocities of the stars didn't fall with distance. The velocity of all our planets decreases as you go around because gravity's weaker, and you need less speed to be going in an orbit. And that didn't happen with the stars in the galaxy. And if you say the mass is in the center, the stars away from the center are rotating too fast to be in a stable orbit. And so, Zwicky invented dark matter, but it's matter that we couldn't see that was in there that made gravity stronger out in high radii in the galaxy that forced them to go faster. They were going too fast.
And in fact, when they plotted it in various galaxies, what they see is that the velocity goes up to some number and flattens out. And so, he said it was dark matter. And somebody introduced me to the concept of MOND. I guess there was an Israeli physicist who invented the idea, saying that what's wrong is not that there's dark matter, it's that Newton's law works when gravity is fairly strong, but when gravity gets down to 10 to the -8 centimeters per second squared in acceleration, gravity, instead of falling as 1 over R-squared, it falls as 1 over R. So basically, gravity is different, and he says that works for all galaxies. I guess he said all galaxies seem to have that same behavior. It goes up to roughly the same velocity, and then it flattens out. And he said, "You need different amounts of dark matter in big galaxies and small galaxies. How come there's just the right amount of dark matter in every galaxy that they look at?" And they looked at close to 1,000 galaxies, and they were all the same.
So, he said, "People say there's 95% dark matter and 5% matter," whatever it is, "so dark matter should be the dog, and matter's the tail. But with the amount of regular matter in the universe, there's always just the right amount of dark matter to make that curve come out flat." And so, astrophysicists have been trying to put holes in it. They've put holes in it, and then somehow, it survives. Variations of it. There's no theory of it. But that doesn't horrify me. I grew up with no theories until quarks came around. There was conservation of energy, momentum, charge, things like that. So, I say, "OK, they need to work on gravity. It hasn't been quantized anyhow. So, who knows what it is?” So, I like to take the alternative source until it gets killed. And I haven't followed it very much in recent days, but you can look up MOND, modified Newtonian gravity. And there are a bunch of theorists who like to do things where, "I'm not like everybody else." There's now this discrepancy in the Hubble constant when you use two different techniques. And I like to take the underdog.
Well, on that note, it's been a great pleasure spending this time with you. I'm so glad we were able to do this so you could share all of your perspective and insight over the course of your career. So, thank you so much.