Arthur Poskanzer

Zierler:

This is David Zierler, oral historian for the American Institute of Physics. It is April 1, 2021. I’m delighted to be here with Arthur M. Poskanzer. Arthur, it’s great to see you. Thank you so much for joining me.

Poskanzer:

Fine. Hi.

Zierler:

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

Poskanzer:

Well, now I’m a distinguished senior scientist emeritus.

Zierler:

At Lawrence Berkeley National Laboratory.

Poskanzer:

That’s right.

Zierler:

Art, when did you retire? When did you go emeritus?

Poskanzer:

Well, I actually retired in 2002, but I was publishing papers up until six years ago.

Zierler:

And in the meantime, in what ways have you been connected to the lab, or to physics more generally?

Poskanzer:

I keep up with what’s going on. But I actually moved out of my office and cleaned it out quite a while ago.

Zierler:

How has this past year been for you? Has it been difficult?

Poskanzer:

It hasn’t been much of a change for me, because I’m confined to the house anyway.

Zierler:

Yeah. Have you found that more people being on videoconference, ironically, you might be more connected to some people than otherwise would be?

Poskanzer:

No. [laughs]

Zierler:

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

Poskanzer:

Well, the family originated in Lithuania, and they came over, and they all settled in Albany, New York. My father actually came when he was 10 years old in 1911.

Zierler:

And what about your mom?

Poskanzer:

She came, I guess, when she was one year old, and she came from the Ukraine.

Zierler:

Why did they settle in Albany? Did they have family there?

Poskanzer:

No idea why, but that’s where the whole family accumulated.

Zierler:

And what was your father’s profession?

Poskanzer:

He was a lawyer.

Zierler:

Did you grow up in Albany?

Poskanzer:

No, no, no. My father moved down to New York City to go to Columbia College and then Columbia Law School, and then the family was raised in the Upper West Side of Manhattan.

Zierler:

Oh, so that’s where you grew up, on the West Side.

Poskanzer:

Yeah.

Zierler:

Art, what schools did you go to growing up?

Poskanzer:

Well, I missed the first six years of school because of a sickness, and then I went to Joan of Arc Junior High School, and then Stuyvesant High School, which required an entrance exam. And even though I’d only been going to school for a few years, I passed the exam and got into Stuyvesant. After Stuyvesant, I went, like my brother did, to Harvard.

Zierler:

Now, when you were young and you were not well, were you homeschooled? Did you teach yourself?

Poskanzer:

Some, but very little homeschooling. Mainly, I taught myself.

Zierler:

Art, were you always interested in science, even as a young boy?

Poskanzer:

Ever since I got my first chemistry set. I think it was my 12th birthday. I’m not sure. I was interested then.

Zierler:

Tell me about Stuyvesant. Was it a great education in math and science?

Poskanzer:

Oh, yeah. We actually — everybody who went to Stuyvesant was very attached to it, and we had a 55th reunion not too long ago, because people still remember Stuyvesant, even more so than we remember Harvard.

Zierler:

Was Stuyvesant competitive at all with Brooklyn Tech or the Bronx School of Science?

Poskanzer:

Well, it was much better than Bronx Science. Bronx Science has a swelled head, because some of their people got Nobel Prizes. But actually, Stuyvesant was a much better school.

Zierler:

[laughs] So says the graduate of Stuyvesant.

Poskanzer:

[laughs] Right.

Zierler:

Art, when you were thinking about college, were you thinking about physics already, or that came later on?

Poskanzer:

Well, I was thinking about chemistry, and I applied to Harvard saying I wanted to go into journalism, because that’s where all my extracurricular activities were. And I thought that would be a leg up. But I really didn’t want to pursue that. I tried out for The Harvard Crimson and flunked out very quickly. So, I proceeded to work in science, and I majored in this honors-only combined field of chemistry and physics. So, I did graduate with honors, because I stayed in that major.

Zierler:

Art, who are some of the real luminary professors at Harvard that you remember?

Poskanzer:

Well, my best science course was physical chemistry with George Kistiakowsky. He was the one who, during the war, designed the implosion devices on the plutonium bomb at Los Alamos. I also took courses from Aron Copeland and Thornton Wilder.

Zierler:

By the time you were done with your undergraduate, did you know if you wanted to do more theory, or more experimentation?

Poskanzer:

Experiment. And yeah, I knew I wanted to do experimental science, Ph.D., and I actually got this big fellowship from Columbia graduate school and went there, but left after one year.

Zierler:

Was the intention to stay for the Ph.D., or it was only a master’s program at Columbia?

Poskanzer:

It was a Ph.D. program in which I was supposed to do a thesis with Jack Miller, but I left after one year. I decided before that year that I didn’t like it very much, and Lucille, my Radcliffe girlfriend, and I decided to get married, and she had more years to go at Radcliffe, so I transferred back to MIT.

Zierler:

Now, why MIT and not back to Harvard?

Poskanzer:

Well, there was a famous nuclear chemist at MIT, Charles Coryell. He was head of the fission product work at Oak Ridge during the war. And that was an active nuclear chemistry group, and that’s what I knew I wanted to do.

Zierler:

What were some of the big questions in nuclear chemistry during those years?

Poskanzer:

It was this big controversy between Coryell and Seaborg about where the trans-uranium elements should go in the Periodic Table. Should they be like another actinide series, or should they go higher up in the periodic table? It turns out, Seaborg won. It’s like a new actinide series.

Zierler:

What was your laboratory research like at MIT?

Poskanzer:

Well, I wound up not doing nuclear chemistry for my thesis. I did radio chemistry, that is — the major was physical chemistry, using radioactive tracers. And it was a study of solvent extraction. I figured out what was governing the solvent extraction: it was ionization in the organic phase, which proved the big professor at Oxford wrong — a series of papers he’d published with a different explanation. I sent my thesis to him, but he never replied.

Zierler:

Why did you switch over to radio chemistry? What prompted that?

Poskanzer:

Well, I just — by the time I was married, we had one child, and I just wanted to get out quickly, because I thought everything you do, you do after you have your Ph.D., not before. So, I wanted to get my Ph.D. as quickly as possible. I had it written up in about two- and three-quarter years at MIT.

Zierler:

What options were available after you defended?

Poskanzer:

The nuclear chemistry group at Brookhaven National Laboratory was advertising for postdocs, and I applied there and got one. It turns out the Brookhaven Chemistry Department hired four nuclear chemistry Ph.D.’s as postdocs at the same time.

Zierler:

Art, this being the late 1950s with the Soviet launch of Sputnik and the American response, was your sense that the government’s support for nuclear chemistry increased as a result?

Poskanzer:

Yeah. Sputnik caused a pickup in the funding for science just at that time when I was getting started, so that was good.

Zierler:

And you felt that that created more opportunities.

Poskanzer:

Oh, yes.

Zierler:

What were some of the big projects going on at Brookhaven when you joined?

Poskanzer:

I don’t know any big projects. Each of the postdocs worked independently.

Zierler:

What group were you in?

Poskanzer:

The Friedlander-Perlman group.

Zierler:

What was the focus of that group?

Poskanzer:

I don’t think it had a focus. Friedlander [laughs] himself was doing what’s called Metropolis calculations of the intranuclear cascade on the computer. It was the first important simulation of high-energy reactions in nuclei.

Zierler:

What did the computer look like? This is the very early days.

Poskanzer:

Well, we didn’t have one at Brookhaven then. We had to — we’d pick a day and go into NYU in New York and use their UNIVAC that they had there. And what you would do is, you’d have a stack of punched cards, a box with 2,000 cards. Each card was a line in a computer program. And you’d sit it at the front desk and wait. They would come back and say, “Sorry. The program crashed.” And then you’d have to do the whole thing over again.

Zierler:

[laughs] In those early days, was computer simulation worth it? Did it actually improve the research?

Poskanzer:

Oh, yeah. It really showed us what was going on in the intranuclear cascade of high-energy protons on heavy nuclei, like uranium.

Zierler:

What did you do next? What was your next project at Brookhaven?

Poskanzer:

Well, I did chemistry. The cascade calculations wasn’t my project. That was Friedlander’s project. I did chemistry. The first one was to measure the yields of actinides from high energy protons on uranium, and that involved chemistry. It involved some solvent extraction. And one of the first papers I published was a systematic understanding of a particular solvent extraction procedure, because it was so useful — it separated many products of high-energy protons on uranium.

Zierler:

How long did you stay at Brookhaven?

Poskanzer:

Nine years.

Zierler:

What opportunity came up at nine years for you?

Poskanzer:

A job at Berkeley. Earl Hyde was just finishing a book with Seaborg on a survey of the actinide elements. He spent many years at this and wanted to go back to doing research. And he needed an assistant. So, I was hired as a staff scientist there, as his assistant.

Zierler:

Now, did he recruit you? Was he aware of your work?

Poskanzer:

Yes. Friedlander, when he was visiting the west coast, dropped in on Earl Hyde and told him he had this young scientist who was looking for a job. And since Friedlander wrote the textbook on nuclear chemistry, he was really the senior person in the field. And so, Earl Hyde took his advice, and he hired me.

Zierler:

Was this exciting for you? Had you ever been to California before?

Poskanzer:

When I first got to Brookhaven, the accelerator there — I wanted to do high energy reactions. The accelerator there, called the Cosmotron, broke immediately when I arrived at Brookhaven. So what they did was send me out to Berkeley to use the twin machine called the Bevatron. It was actually a little bit higher energy, 6 GeV, instead of 3 GeV. And I spent probably two visits — one was for the summer with my family, and we rented a house nearby, and I worked the same thing I would have done at Brookhaven, but I did it at Berkeley. And that was the summer that E.O. Lawrence actually died, when I was at Berkeley Lab. And then I went back and analyzed all the data and got a good paper out of it.

Zierler:

Art, what were your initial impressions of Berkeley lab?

Poskanzer:

A lot of bigwigs who [laughs] have their own empires.

Zierler:

Did your family like California? When you told them about this opportunity, were they excited to go?

Poskanzer:

Yes. Yes, they were.

Zierler:

What year did you arrive at Berkeley?

Poskanzer:

That was ’58, for this visit. I arrived to stay here permanently in ’66.

Zierler:

In 1966. Now, were the campus protests…

Poskanzer:

Yes.

Zierler:

…starting up already when you got there?

Poskanzer:

Oh, yes. The year before we arrived, there was the violence on Telegraph Avenue. In fact, one person was killed. But when I arrived, there were still protests. And I marched in one of them. From the lab, we all went down to Shattuck Avenue and walked — marched to People’s Park to show our support for that. At that time, when we marched there, it was surrounded by a chain-link fence, and state troopers were guarding it.

Zierler:

Art, what interactions did Berkeley lab have with the relevant science departments at the university, at UC Berkeley? Was there a lot of interaction between the staff scientists in chemistry and physics professors?

Poskanzer:

No, very little. About 10 percent of the senior staff at Berkeley lab also had campus teaching appointments, but that was a small fraction. And I would say the work at the lab and the campus were sort of independent.

Zierler:

What was your initial research at Berkeley, and what group were you a part of?

Poskanzer:

The Hyde group, which became the Hyde-Poskanzer group. And we set up our research at the Bevatron. For the first experiment there, we collaborated with Joe Cerny, who usually worked at the 88-inch Cyclotron and wanted to see what the same techniques would do at the Bevatron. So, we built a vacuum chamber and put these telescope silicon detectors in the vacuum, looking at a uranium target, and our first experiment with Joe discovered, eventually, lithium-11, boron-14, and boron-15. Very neutron excess light isotopes of the light elements.

Zierler:

I wonder if you can describe what it means to discover these isotopes. How do you know when you find them?

Poskanzer:

Well, the telescope of silicon detectors has a peak for each isotope based on the energy loss and the total energy deposited in the silicon detectors. So, peaks for each isotope. There’s a problem that at the high rates of the protons on uranium where you make everything, you could get pile-up if the rate is too high, giving you false peaks. But we avoided that problem by making the measurements at two different distances. The further distance had a clean spectrum which clearly showed these new isotopes. Previously people used simple reactions, like at the Cyclotron, to make an isotope in a reaction they identified. I pioneered the procedure of using the highest-energy particles on the heaviest targets to make everything, and then sorted out in a very specialized detector technique, so you could see what you had. You made everything — but you could identify what you were looking for.

Zierler:

What were some of the other uses of the Bevatron?

Poskanzer:

Well, there was a medical branch which used some of the beam time, and we kind of objected to that, because it interfered. But they were irradiating pituitary cancers, and that was started by John Lawrence, E.O. Lawrence’s brother, in the medical group.

Zierler:

What was your next research after this? What did you work on next?

Poskanzer:

I stayed with the Bevatron, and then it became the Bevalac. Al Ghiorso had the idea to use the HILAC up the hill; you could build a vacuum pipe and take the beam down to the Bevatron, inject it, and accelerate heavy ions, not just protons. So, that became the beginning of the relativistic nuclear collision field. If you look in Phys. Rev. C now, a huge number of papers are published in this field. When we started Phys. Rev. C did not have this subclass. We published with all the other physics papers, but it soon became clear that this field was growing so fast they needed a subset of Phys. Rev. C, just for relativistic nuclear collision papers.

Zierler:

Now, when you say that the Bevatron became the Bevalac, does that mean physically it was transformed, or it was just a new research group?

Poskanzer:

By combining HILAC, which was a linear — heavy ion linear accelerator, about 100 feet up the hill, with Bevatron, the combination of the two machines, HILAC and Bevatron, became a new machine called the Bevalac.

Zierler:

What were some of the technical challenges associated with this union of machines?

Poskanzer:

Bringing the beam down the hill in vacuum with these magnets out on the hillside, with the deer roaming around between them; it was really an amazing proposal to do this. The idea was Al Ghiorso’s. And the person who completed it, made it a working machine, was Hermann Grunder.

Zierler:

What was Ghiorso’s research? What was his area of expertise?

Poskanzer:

Ghiorso worked at the HILAC. In fact, the HILAC was his machine up the hill. And his sole purpose was to make new super heavy elements.

Zierler:

What kind of administrative responsibilities did you have when you were scientific director of Bevalac?

Poskanzer:

There were all these proposals to do experiments there, and I was in charge of getting their proposals to a program advisory committee to approve a subset of them, and in the meantime, make sure everything ran. I was scientific director of the Bevalac for many years. I wasn’t the engineering director. Other people did that.

Zierler:

What were some of the most significant experiments that happened with the Bevalac?

Poskanzer:

We discovered the collective flow of nuclear matter. No one expected that. People expected that when a nucleus bombarded another nucleus at these high energies, the nucleons would just go through the nucleus and come out the other end. It turns out they didn’t. The two nuclei, where they overlapped, fused, made a very hot, dense glob of nuclear matter, which flowed off at an angle to one side. And we saw this in the azimuthal distribution of the products that came out.

Zierler:

In what ways did this work change nuclear theory?

Poskanzer:

Well, in their calculations, they had to put in more interaction between the nucleons to get this thermalization of the incoming matter and the target matter. And observing this collective flow tells us about the nuclear matter.

Zierler:

Art, when you were reading proposals for research on the Bevalac, were a lot of these proposals coming from outside scientists, or it was mostly from within the lab?

Poskanzer:

Mostly outside.

Zierler:

This would suggest that there’s research that’s possible with the Bevalac that’s not available elsewhere.

Poskanzer:

Yes. The Bevalac was a one-in-the-world unique machine. It’s been copied since in other places, but it was the only place that gave relativistic heavy ions.

Zierler:

This means that you must have had proposals coming in from all over the world.

Poskanzer:

That’s right.

Zierler:

Who were some of the most significant scientists who proposed doing research on the Bevalac at this time?

Poskanzer:

There was a Japanese group. There was a big interest from Germany. But they were, I think, smart in that they collaborated with an in-house group, me, instead of having a completely independent proposal and trying to set up an experimental program at a foreign machine where they didn’t know much about what was going on. So, I got this big interest from Germany to collaborate, and they sent about eight people here to work with me in the relativistic heavy ion program at Bevalac. And that led to them proposing and building a huge accelerator at a laboratory called GSI in Darmstadt, Germany. So, it started their very extensive relativistic heavy ion program in Germany. They trained here. I had lots of visitors. In fact, in my original group at the Bevalac, almost everybody who went home started a group in their home country to do this. So, there was a time when almost every group doing relativistic heavy ion physics trained with me at Berkeley.

Zierler:

Art, tell me about the origins of the Plastic Ball experimental group. When did that start?

Poskanzer:

I’d have to check the date. But that was after we got the support from Germany. We did a few test experiments, a few initial experiments. We wanted to select on high multiplicity of charged particles, and we first built an array of plastic scintillator paddles but later found out that a high hit number in these didn’t tell you it was a high multiplicity. It just told you that a high number of paddles was hit, not that we had produced a high number of particles. It was just statistics. So, we decided you really had to cover the whole outside of the target with detectors in order to get to select high multiplicity. A solid angle is usually measured in steradians, and 4π steradians covers the whole solid angle. So, we tried to get as close to 4π as we could. And we did that by building this spherical array called the Plastic Ball, with no gaps between the modules. Each module could identify the particles because it had two scintillators: a calcium fluoride, and a plastic scintillator, with one output to the electronics. The time sequence of these two signals being different, you could separate them electronically and get the energy loss in the calcium fluoride and the total energy in the plastic scintillator, and do the identification, very similar to the way we did with silicon telescopes early on. And we could build this with no gaps, covering the whole 4π solid angle around the target.

Zierler:

What did the plastic ball look like?

Poskanzer:

It was a one meter sphere, around a target. At the forward angles we tried to get twice the density in the detectors, so there was a protrusion in the front where each detector was half as big, so we could have four times as many.

Zierler:

Art, were you looking specifically for collective flow, or was this sort of an accidental discovery?

Poskanzer:

We were looking for it.

Zierler:

What was the theoretical basis that you would find it?

Poskanzer:

Well, we wanted to see if we had made a hot, dense soup which then broke up, or the particles of the target and the projectile went through like mosquitoes just passing by each other. It wasn’t at all like that.

Zierler:

Did you work with particle physicists at all?

Poskanzer:

Not much.

Zierler:

But they must have been very interested in this research.

Poskanzer:

No.

Zierler:

No?

Poskanzer:

It was a big fight to get resources and beam time for heavy-ion physics, because the particle physicists looked down on it as if it were not really basic physics, but was more like chemistry.

Zierler:

But aren’t you discovering fundamental things about the behavior of things like quarks and gluons? Wouldn’t they have cared about that?

Poskanzer:

No, we were discovering things about bulk nuclear matter, collective flow. They were interested only in one proton on one proton.

Zierler:

[laughs] So, they were a bit more narrow in their focus, if you will.

Poskanzer:

Yes. They thought it was more fundamental, and we thought if you get a big enough collection of particles and a big enough target, things get simple again, because you can invoke things like thermodynamics to understand what’s going on.

Zierler:

Art, tell me about the nature of the collaboration between Berkeley lab and GSI. Who contributed what, and in what way?

Poskanzer:

Well, it was started by Rudolf Bock from GSI and Reinhard Stock from Frankfurt who came to visit me in July 1974 and said, “Can we have a collaboration?” Bock wanted to make sure I had no problems dealing with Germans.

Zierler:

As a Jew, you mean?

Poskanzer:

Yes. And I didn’t. So, we had a very good collaboration. Immediately, Andres Sandoval, a Mexican, and Hans Gutbrod, a German, came and joined the group, and we started experiments, and then more people came. And we had maybe six or eight people supported by Germany in the group at the maximum.

Zierler:

And their funding source was what? A university, the German state?

Poskanzer:

This GSI Laboratory, which in German is “Gesellschaft für Schwerionenforschung.”

Zierler:

But this is a private enterprise? Is it academic?

Poskanzer:

It’s a German government supported lab, just like LBNL is in the United States.

Zierler:

Art, tell me about the discovery of collective flow. How did that happen?

Poskanzer:

Well, it was Hans Georg Ritter who did the analysis, and he showed that when the projectile got heavy enough — not for carbon, but when we used calcium, there was a peak in the distribution of the particles, the energy spectrum of the particles. And we showed that when you add a heavy enough projectile on a similar target, you would get this excess energy at certain angles, which could be due only to the thermalization of the targeting projectile and the re-emission at certain angles. But that’s what we saw in the first measurements, and that’s what led us to build the Plastic Ball.

Zierler:

I see. I see. How long were you leader of the Plastic Ball experimental group?

Poskanzer:

Well, at one point, Hans Gutbrod was making such enormous contributions building the Plastic Ball, I made him co-group leader. It was Hans Gutbrod with Hans Georg Ritter, who is still here, who constructed the Plastic Ball.

Zierler:

Art, tell me about the STAR collaboration. What were some of the basic questions that prompted this collaboration?

Poskanzer:

When the SSC in Texas was being built, they shut down an accelerator at Brookhaven Lab and left an empty tunnel there. And some congressman said: I won’t have an empty tunnel in my congressional district. Let’s build something in it. It was decided to build a relativistic heavy ion accelerator. And that was a national effort. So, national labs decided to contribute to make experiments for that accelerator. About eight places submitted proposals, and we at Berkeley got all the high energy people here together and submitted one proposal for a time projection chamber, proposed by Howard Wieman. And of course, everybody said a time projection chamber would never work, but we showed it would work, and that’s the heart of our detector at the Brookhaven accelerator, which was called RHIC, Relativistic Heavy Ion Collider, and the detector that we proposed was called STAR, Solenoidal Tracker At RHIC. Now, there was another group at Brookhaven interested in building a large acceptance detector also. First of all, I insisted the detector had to have full azimuthal symmetry, so you could see peaks in the azimuthal distribution caused by collective flow. And so, it was clear that we had to combine these two proposals. And so John Harris, the proposed spokesperson for our detector, and I, the group leader, made many trips to Brookhaven, and we actually met once at the airport in Chicago to argue this out. Peter Jacobs in our group finally showed their proposal would not work, and ours was approved. Howard Wieman designed the Time Projection Chamber and Jay Marx supervised the construction of the whole detector. Joy Lofdahl was my administrative assistant.

Zierler:

Art, tell me about the discovery of elliptic flow in STAR. How did that happen?

Poskanzer:

Sergei Voloshin and I were working together to analyze the data and there it was. It became the first paper from STAR.

Zierler:

What were some of the theoretical assumptions that quark gluon plasma existed before it was experimentally demonstrated?

Poskanzer:

Well, [laughs] I object to the question, because you can’t say it exists unless you experimentally demonstrate it. [laughs]

Zierler:

Theorized. Was there a theoretical assumption or a basis for the existence of quark gluon plasma?

Poskanzer:

Yeah, it was predicted that if you get a large enough number of nucleons together at a high enough temperature, that the quarks, 3 quarks in each nucleon, would come out. The nucleons would fuse and release their quarks and gluons, and then you would have a free soup of quarks and gluons.

Zierler:

In what ways was this discovery significant beyond your immediate field? Who else was paying attention, such as astrophysicists, for example?

Poskanzer:

Yes. This definitely was a stage in the creation of the universe. Before the nuclei, you had nucleons, and before that, you had a quark-gluon plasma.

Zierler:

Can you observe quark-gluon plasma in any other experiment or any other way?

Poskanzer:

Outside of high-energy reactions? Well, in cosmology, when you look at the creation of the universe, you need that stage in there to understand what was happening.

Zierler:

Art, when did you start to get involved with research at CERN?

Poskanzer:

When we finished with the Plastic Ball experiment, we kind of shut down the Bevalac program, and we moved the Plastic Ball to CERN. That was quite a project, to get it on a cargo airplane and move it to CERN. Ritter actually went with it.

Zierler:

What were the motivations? Why was it worth the trouble to move it to CERN?

Poskanzer:

We wanted to detect as many particles as possible at CERN, and this Plastic Ball covered the whole backward hemisphere. Of course, we built other detectors for the forward angles. This was the WA80 experiment.

Zierler:

Did you take on new collaborators as a result of being at CERN, or CERN was just the host for the experiment?

Poskanzer:

No, there was — it was a big collaboration, much bigger than we had here, and Gutbrod was the spokesperson. The next experiment, Reinhard Stock was the spokesperson and I was deputy spokesperson. But it was a much bigger collaboration. For CERN experiments, you need 500 people.

Zierler:

Art, tell me about some of your advisory work that you did at Berkeley lab, at Brookhaven. What was most useful to you professionally?

Poskanzer:

I don’t look at it as useful. I think it’s your debt that you have to pay. You know, you have all these people who come to Berkeley to give you advice, to serve on committees. You have to go do that somewhere yourself.

Zierler:

Have you had opportunity to work with graduate students or to teach at all in your career?

Poskanzer:

I’ve never done any teaching. We have had students in the group, some who got a Ph.D. But they had a professor on campus who was nominally in charge.

Zierler:

Tell me about what it felt like when you won the Bonner Prize.

Poskanzer:

That was a big surprise. I was very pleased. I had already retired. I went to my division head, James Simons, and said, “Now can I get a promotion?” And he looked very puzzled. I said, “Yes, well, twice zero is still zero, isn’t it?” And he smiled, because I was already retired.

Zierler:

Tell me about the NA49 experiment. How did that get started?

Poskanzer:

Well, that was Reinhard Stock from University of Frankfurt. “WA” means it’s in the West Area at CERN. “NA” means it’s in the North Area. And we set up this North Area experiment with a huge time projection chamber. The time projection chamber records the three-dimensional trajectory of each particle that goes through it. So, it’s even better than the Plastic Ball, recording the tracks of all the particles. And he proposed that, and we built this huge time projection chamber, and got some good results there.

Zierler:

When did you know it was time to retire?

Poskanzer:

When I was 70. I thought they had to hire some young people. I didn’t want to stop working, but I went on to pension for my salary.

Zierler:

Did that free you up? Was there some research that you wanted to do in retirement that you weren’t able to do?

Poskanzer:

It made no difference. Just the check. Actually, I had a little more take-home pay than before I retired, because I wasn’t contributing to the retirement fund. I just continued to work up until a few years ago.

Zierler:

Art, I’d like to ask some sort of general questions that are not tied to any particular point in the chronology. It’s just whenever you were thinking about these things. So, the first is: the discovery of 28 isotopes. Was there a single intuition you had, or a single technological advance, that allowed for all of these isotopes to be discovered? Or, does each discovery have its own unique story to it?

Poskanzer:

My general philosophy was to use the highest energy particles on the heaviest targets and make everything, and just selectively sort out the results, look for the new products. So, I mainly looked for delayed neutron emitters or delayed proton emitters; I used these telescopes and silicon detectors. All these detectors were very specific in sorting out the results, but the general philosophy was to hit it as hard as possible with a heavy nucleus to make everything, and then see what’s new there.

Zierler:

Was there any one particular isotope that was most meaningful or satisfying for you to discover?

Poskanzer:

For me, the most satisfying one was Helium-8.

Zierler:

Why is that?

Poskanzer:

Because it’s kind of combined physics and chemistry. We had observed previously that when you irradiate plastic, some of the Carbon-11 diffuses out. I figured that helium would diffuse out of carbon fibers. So, I went to a drugstore, and I got some absorbent cotton, [laughs] and we put it in a horizontal cylinder and put the Cosmotron beam through it. And then I had a tank of ordinary helium, and after every beam burst, every six seconds, I would let a burst of helium through the target that would then go through a liquid nitrogen cold trap, which absorbs everything except helium, and then it would go into a pizza box shaped vacuum chamber inside two blocks of paraffin, which contained boron trifluoride detectors, which could observe neutrons. And every beam burst, a multi scalar as a function of time was started, so I recorded the neutron counts as a function of time. And there was a clear decay curve of 122 milliseconds, and that was how Helium-8 was made in the cotton fibers by the 12C+p,5p reaction. And of course, everything else was made, but I absorbed everything — liquid nitrogen cold trap, and I could see the delayed neutron by itself. And that was very satisfying, to use chemical techniques as well as physical techniques to observe a new isotope, which everybody dreamed about, but no one could measure.

Zierler:

Art, in the course of discovering all of these isotopes, were there any isotopes that were predicted to have existed, and as a result of your research, you proved that they did not?

Poskanzer:

Oh, yes. We always listed those in the papers, which isotopes were not there, were absent from the peaks that we identified. There were two that we observed which had been predicted not to exist, but we found them. The theoreticians had to change their calculations to predict them. These were Lithium-11 and Beryllium-14.

Zierler:

Art, what were some of your key contributions in the discovery of how isotopes decay?

Poskanzer:

Well, the work I did with the French group of Klapisch at CERN to look at the sodium isotopes — by the way, that experiment with the PS that I participated in, spending a year there, was the first high energy nuclear physics experiment at CERN. Before that, they would only do particle physics experiments. But we found several new sodium isotopes. Some were delayed neutron emitters. So, we really went out far on the neutron excess side of particle stability with these sodium isotopes. And it stood that way for many years, and no one could get that far out to the neutron excess side.

Zierler:

Art, what was the relevance of this research on solar neutrino experiments?

Poskanzer:

My colleague and good friend, Ray Davis, who won a Nobel Prize for his solar neutrino experiment, was using tanks of perchloroethylene in a mine in South Dakota to search for neutrinos from the Sun. And he didn’t find as many as the theoreticians expected. John Bahcall was the theoretician. And to check on the theory, Bahcall wanted the mirror nucleus of the reaction that Ray used. It was neutrinos on a chlorine isotope. The mirror reaction, with the same decay properties, was Calcium-37. And no one knew about it. It was predicted to be a delayed proton emitter. And I was set up at the Cyclotron at Brookhaven to look for delayed proton activities. We looked for Calcium-37, and we found it. We measured its half-life and decay properties. Ray told me: you have to call John Bahcall and tell him you found it and what the half-life is. So, I called him, and he said, “No shit!” And I said, “Can I quote you on that?”

And he laughed and was very excited. And his colleague, Willy Fowler, a Nobel Prize winner, was about to make a trip to Brookhaven. So, he gave Willie Fowler, a bottle of champagne to bring to us: congratulations for finding Calcium-37. And Fowler arrived in Maurice Goldhaber’s — the lab director’s office with this bottle of champagne. He thought it was going to go to Maurice’s wife, Trudy Goldhaber. But it turned out she had talked about discovering Calcium-37, but had never done it. He finally asked around and found out that it was I who found Calcium-37. So, he called me up, and me and my two postdocs, Ross McPherson and Bob Esterland, traipsed over to the director’s office, and we opened this bottle of champagne.

Zierler:

[laughs] Art, it’s been said of your research that when you obtained some of the first mass measurements of some nuclei, it revealed a new region of deformation, which was interesting because it had occurred at a magic number.

Poskanzer:

Right.

Zierler:

What was that number, and what was magic about it?

Poskanzer:

Well, it was the neutron excess sodium isotopes, which was 28. It was a magic number. But when we looked at the mass results, it didn’t look like a magic number. And Robert Klapisch was visiting me at that time, and I said: let’s go over to the library and look at what masses do at various regions. And we looked, and we saw that in the deformation region, the masses of nuclei behaved in a certain way. And we said: that’s what we have. So, far from stability — we published a nuclear deformation region, a place where previously, people thought it was a magic number. It was a magic number near stability, but deformed not far away. And that was very satisfying.

Zierler:

Why were you compelled to develop new techniques of direct atomic mass measurements of short-lived nuclei? What was missing up until that point?

Poskanzer:

Well, the head of the laboratory at Orsay, Rene Bernas— the French laboratory — trained with A.O. Nier in Minnesota, who was the founder of high-resolution mass spectroscopy. And he set up this group back in Paris to do this, and Robert Klapisch was his student. Robert wanted to do something different. He decided he would make direct mass measurements in-beam at CERN. And he set out to do that, and he did. And the experiment that I joined was actually the thesis project of his student, Catherine Thibault. And I served on her Ph.D. examining committee, in French, which was difficult. And she got her degree, and that was a groundbreaking experiment, where a high-precision mass spectrometer was set in-beam. People didn’t think it could be done, and we had problems that we had to solve. For instance, when the beam — it was the PS accelerator, the proton synchrotron — when the beam hit the beam stop way downstream, after going through our target, the neutrons would thermalize and would come back at just the wrong timescale to give a background in our detector, which had nothing to do with the sodium isotopes. I had the idea to surround the mass spectrometer in a room shielded with sheets of cadmium to absorb the thermal neutrons which filled the tunnel after each beam burst. That reduced the background enormously, and was one thing that made the experiment successful.

Zierler:

What was the value of using differential recoil to study proton nucleus reactions?

Poskanzer:

I think you’re talking about something that I did very early at Brookhaven.

Zierler:

Yes, I believe so.

Poskanzer:

Okay. That was a very crude elementary way to get more differential information, something that solid-state detectors could do much, much better. But we didn’t have them then. This was in 1958 at Brookhaven. I would put a target in the Cosmotron beam with stacks of plastic foils looking at it at various angles, and then take each plastic sheet and dissolve it up in a test tube, and from their radiations try to constrict an energy spectrum and angular distribution of that product from that reaction. I can’t believe how difficult it was.

Zierler:

And then it was later on when you developed the first counter study of nuclear fragmentation, or was that happening at the same time?

Poskanzer:

The counter studies were all done at Berkeley. The telescopes identify the products at various angles and measure their energies. The work at Brookhaven was mainly chemistry using chemical techniques.

Zierler:

What was the value of generating nuclear fireballs and coalescence models to describe the data that you were collecting?

Poskanzer:

Well, people didn’t know. I had no idea how we were going to interpret the results from high-energy heavy ions on heavy targets. Waldek Swiatecki, a local theoretician, proposed the spectator/participant model, which said that after the collision the nucleons were either spectators or participants depending on whether they had collided or not. And in analyzing the data when the German group arrived, we made measurements, and we would meet weekly and sit around and discuss the data. And there was no leader of this, because it was all new. It was new to everybody. So, everybody could make contributions on an equal basis. So, Gary Westfall said: well, what if the nucleons from the heavy nucleus and the target nucleus fuse and make a thermalized hot soup? And he found a way to describe this, and this nuclear fireball model described the data. So, that was new, exciting results. And then we had results for deuterons, tritons, alphas, coming out of the reaction, and we didn’t know where they came from. So, Jannik Johanson suggested they were made from coalescence of protons and neutrons. And that coalescence model worked very well. It described the light fragments from these reactions.

Zierler:

So, prior to this, were there any thermal models to describe the initial stage of relativistic nuclear collisions or not?

Poskanzer:

No. We sort of developed the methods that set the framework for theoreticians to work on afterwards. We published this big paper whose first author was Jean Gosset, a visitor from France.

Zierler:

Did you come up with the terms — when we’re talking about flow, “bounce off,” and “side splash,” and “squeeze out”? Did you invent these terms?

Poskanzer:

Early on, we did, to describe what we saw. But we stopped using them later on. We just now use things like “elliptic flow.”

Zierler:

As metaphors, are they useful? Are they accurate? When you talk about “bounce off,” do things bounce? When you talk about “side splash,” do things splash? And when you talk about “squeeze out,” do things squeeze?

Poskanzer:

Yeah. And those are still descriptive statements about what’s happening, but not as general and as easy to interpret as the way we use now, which is in terms of harmonics of the azimuthal distribution. But when we didn’t have a complete picture, they were descriptive of what we saw, and they served their purpose for a while. But not now, anymore. The present procedure for analyzing these reactions was invented by Sergei Voloshin when he was visiting Berkeley for a year. He and I worked together, and that’s where elliptic flow started.

Zierler:

Art, we talked about computers very early in your career. What were some other major experiments for which computers proved to be very useful for you?

Poskanzer:

Well, analyzing the data requires heavy computer use. Everything we do involves computers, and I spent a lot of time programming computers. So, analyzing data, you have to write the program yourself. And that’s mainly the work you do, analyzing high-energy data, is writing computer programs. I taught myself computer programming during my sabbatical at CERN.

Zierler:

Art, would you like to take a break?

Poskanzer:

Yeah.

Zierler:

Absolutely. We’ll come back in what, 10 minutes?

Poskanzer:

Yes.

Zierler:

Perfect. I’ll see you then.

Poskanzer:

Okay.

[BREAK]

Zierler:

And we’re back.

Poskanzer:

Yes.

Zierler:

Yeah. Art, we’re just about done. I want to ask only a few just retrospective questions about your career. So, I’ve studied your research accomplishments and your CV closely, and it’s really kind of mind-blowing how many fundamental discoveries you were a part of. You seem to have had the Midas touch with all of your experiments. And so, that begs the question: were you ever involved in any experiments that turned out to be a real dud that never went anywhere?

Poskanzer:

Probably. I can’t think of one at the moment. I’m sure I did. But I think the most important part of being an experimental scientist is to pick the project. That’s the most important thing.

Zierler:

How do you know what to pick? How do you pick a winning project?

Poskanzer:

A good scientist has a feeling for it.

Zierler:

You mean it’s more sometimes intuition than data-driven?

Poskanzer:

Yes. And if you look at the Phys. Rev C 50th anniversary edition, every two years they picked a significant paper. And so, they just recently published a list of 25 papers that they have picked out over the past 50 years. And I am an author on four of those 25 papers. So, I seem to have a touch for picking the right topic.

Zierler:

Relatedly, Art, because so much of your work has yielded so much fundamental discovery and resolved so much mystery, what out there remains poorly understood, even 50, 60, 70 years after you first started to think about these things?

Poskanzer:

Well, the quark-gluon plasma needs more quantitative study. And the STAR experiment may give some of those results, but the field is going now to an electron-ion collider. RHIC at Brookhaven collides ions on nuclei. They want to build an electron accelerator there so they can collide electrons on nuclei. And that is bound to give new results. I don’t know what they will be.

Zierler:

Art, your contemporaries in particle physics, when they talk about the discoveries in the 1960s and the 1970s, it seems as if new things were being discovered every day. And so, I’d like to ask: in your field, as you look to the next generation, do you see in the 21st century similar opportunity for fundamental discovery that you enjoyed at the beginning of your career?

Poskanzer:

Yes, always. The story is told that, in maybe 1900 or something, some physicist was asked what’s to do next, and he would say: well, everything’s been done. It’s all been discovered. People today will say the same thing; that’s just false.

Zierler:

[laughs] To reformulate the question, though: is it harder now, because the field has to be more narrowly defined?

Poskanzer:

No, it’s harder in that you need more people to do an experiment. The experiments at CERN have about a thousand people in them. And it’s just bigger and more complicated. You need more people to do the experiment. When I started out, it was me myself. Eventually, I got a few postdocs. But now, you really have 500 to 1,000 people in every experiment. Of course, that makes it hard to assign credit. But the good spokesperson for the collaboration has to give everybody credit for what they do.

Zierler:

Art, of course most of your research has been geared toward basic science, just discovering how nature works. Have there been anything as a result of your research that’s had applications in society?

Poskanzer:

Not that I know of.

Zierler:

Looking back over the course of your research at Berkeley lab, what were some of the things that were so valuable in being a scientist at Berkeley?

Poskanzer:

Well, the engineering support was invaluable. There was a group that just made silicon detectors, which made possible the whole research program that I did when I first came here, and the electronics people — the engineering staff at Berkeley is great. It made all the research possible.

Zierler:

Of course, Art, every national lab reinvents itself over the years. It’s true for Brookhaven, Fermilab, SLAC, Berkeley. It’s true for all of them. What were some of the major scientific shifts that happened at Berkeley during your long tenure there?

Poskanzer:

Well, it started out as a high-energy physics lab, and at one point, the Energy and Environment division was built. Andy Sessler and Jack Hollender started that. And the Lab has diversified enormously with all different kinds of work now. High-energy physics is actually a small part of the laboratory now. So, it’s now a national laboratory doing many different things. It started out as a very focused high-energy physics laboratory looking for the antiproton, and things like that.

Zierler:

Art, for my last question, looking to the future: if you could extrapolate, based on all of the perspective and discovery and wisdom you’ve gained over your career, what advice might you have for younger scientists interested in the kinds of questions that you’ve gained expertise in? What are some of the new frontiers in the field that you might suggest they focus their interests on?

Poskanzer:

Well, you have to look at what’s being built. Like in my field, the new big thing is the Electron Ion Collider at Brookhaven. That’s where the funding is going. That’s where the money is going. It’s best to get involved in something like that.

Zierler:

And in terms of your own curiosity, what might come out of this experiment at Brookhaven that would be compelling or interesting, or even surprising for you?

Poskanzer:

Well, the very inner working structure of nuclei. I’m sure things will come out that will be important to cosmology. I can’t predict.

Zierler:

That’s right. That’s right.

Poskanzer:

We do it to find out.

Zierler:

[laughs] Well put. Well Art, it’s been a tremendous pleasure spending this time with you. I’m so glad that we connected as part of this larger oral history project on the Berkeley lab. Thank you so much for spending this time with me. I greatly appreciate it.

Poskanzer:

It was enjoyable.

By the way, my web page is at http://art.poskanzer.org/Poskanzer/ArtPoskanzer.html

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