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Interview of H. Richard Crane by Charles E. Atchley and Roger H. Stuewer on 1990 June 29,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
Primarily focusses on H. R. Crane's neutrino detection experiments, 1935-1948.
Tell me a bit about your educational background. Why did you choose CalTech? Were there particular people or particular projects that attracted you?
I grew up in Turlock, which is in- the middle of the San Joaquin Valley. At that time they grew nothing but melons. I went to high school where I had a very good physics and chemistry teacher who took an interest in me, probably because I was the only one in the class that was really interested in physics, so he put most of his attention on me. Then, when it came time to think about college, Cal Tech was sending a guy around the country to beat the drums for Cal Tech. And the way he did it was to give a liquid air demonstration at a high school and do tricks with liquid air and tell about Cal Tech. You wouldn't believe Cal Tech would have to do that. But that was a long time ago, 1925. I got interested, and I had to take quite an extensive entrance exam, which I took at my own high school. I remember it took a couple of half-days or something like that.
Did it cover only the sciences, or did it also cover the humanities?
Well, I don't know. But it was not anything like a true-false test, I had to write. I passed the test, went to Cal Tech and did my undergraduate four years. Having graduated, I then went on to graduate school, after taking time off. I went on to Europe, knocked around for a while, then I entered graduate school. Got to working with Charlie Lauritsen. At that time he was working on high voltage x-rays. But that was just the early 30's. To get into nuclear physics, it was an easy conversion from x-ray tubes to ion accelerators, so that's what we did. We put an ion source in one of those high voltage tubes, and then I went on to do my thesis in nuclear physics. I stayed one more year as a research fellow and then went to Michigan. During the time I was a research fellow, we had been doing the disintegration of lithium by protons in which we got very high-energy alpha particles from the reaction, lithium 7 plus a proton to give two alphas.
So you just followed up directly the Cockcroft-Walton experiment?
Well, we had done other things in between.
But that is the Cockcroft-Walton reaction, that's what I'm thinking.
Yes. And I did quite a lot of work on the reaction of lithium with deuterons, with the cloud chamber, observing the high-energy particles that came off and the high energy electrons. 7Li + 2D —> 8Li + p, then 8Li —> 8Be + e + v . (half-life of .5 sec.) with the 8Be splitting instantly to 2 a's.
That gave very high energy beta rays, with two a's.
This was in a couple of your early papers.
It's in that 1948 review paper gave you. I dreamed up a way of making these 8Li radioactive nuclei and squirting them out into the cloud chamber, in order to observe the recoil, or rather the direction of the emitted particles, and to try and see if there was any momentum that wasn't accounted for. That didn't work. I tried it, but there were bugs in it, and we never got it to work.
These were your cloud chamber experiments?
This was still at Cal Tech?
I'm still at Cal Tech. At that time, I was interested in the neutrino and was trying to observe the lack of momentum conservation in the 8Li process just mentioned.
Still at Cal Tech?
What I'm saying is that what I did at Cal Tech didn't work.
This is the work you mentioned in your recent letter to me, the work you did with Lauritsen and Fowler.
Yes, they were able to carry through the experiment and get some results later on. The kind of thing I tried to do, I guess was in 1934, before I left them, so that. I had an interest in neutrinos before I left Cal Tech.
How did this interest come up. Was it a result of your studies, classes that you took, or special seminars on the neutrino or beta decay?
It was like a lot of other things. We had a cloud chamber. We were looking at reactions where high-energy beta rays were emitted. There was an opportunity to look for - momentum conservation.
It was a natural outgrowth then of your previous work?
When I got to Michigan, I didn't start right away looking at neutrino tracks, I did a lot of other things. I set up ah high voltage accelerator tube, and we did various reactions, beta-ray spectra and things like that. Sometime later, it's so far back, I don't always get the chronology right, I tried absorbing neutrinos using a bag of salt. But the main experiment we did on neutrinos was the one with Halpern in which we observed the recoil energy by means of counting the droplets in the cloud chamber made by the recoil nucleus. The recoil doesn't have enough energy to make a track, all it does is make some droplets in the cloud chamber, and if you can count the number of droplets you can get an idea of what the energy's like. So our scheme was to shut off the clearing field in the cloud chamber and let the ions diffuse out until they were separated enough to be counted. Before the expansion of the cloud chamber we left the clearing field off for just a fraction of second to let these ions spread out so the droplet that formed on them would be well separated.
Then you count the droplets?
Then the droplets form around the ions.
Your method of doing the counting was called into question by a couple of people at that time, Wertenstein was one of them. Did these criticisms bother you?
It wasn't the method of counting. He called into question whether the number of droplets was really proportional to the energy. At such a low energy, he put forth a fair question.
You responded by agreeing with him and yet you went on with some refinements in further experiments. That was in 1936. You also did a series of experiments from 1936 to 1938.
That was the main thing we did was on neutrinos. In fact, the only thing. Except that hag of salt experiment which was on such a small scale we really weren't expecting anything.
It was a precursor to others.
It was easy to do. As a matter-o fact, it was a forerunner to the experiments done in the Homestake mine by, I forget his name?
By Raymond Davis.
You had a very early version.
Early and very small version for that kind of experiment.
Getting back to the cloud chamber and recoil experiments, you wrote that linear momentum conservation was valid only if one postulated that a third particle was emitted, the neutrino. It seems to me that this was a "backdoor" acceptance of the neutrino. How did you feel about this? Did you feel that this conservation law had to be valid so that you had to accept this particle? Did this make the neutrino any less real?
I don't know the answer to that. I guess I was a pretty strong believer in the conservation of energy and momentum, and I didn't think there was going to be anything surprising about a particle you didn't see.
That wasn't a problem with you even though other physicists at that time were bothered by an unseen, possibly undetectable particle.
Just because they don't see it, they start doubting the law of conservation of energy. I didn't feel that way. If you don't see it, it's probably there, but you'll have to use some other way to observe it.
So you really had no doubt at all that the neutrino would eventually be detected in one way or another.
Well, I don't know if I'd say no doubt at all. The possibility was still open, but if I had to place any bets, I'd bet on the conservation laws; but that's not physics.
What do you remember about any other experiments in the 1930s when you were doing your neutrino experiments? Were there any experiments on the neutrino you found significant or important?
Well, as I look back on it, there weren't any more tabletop experiments around 1948 or after that. That paper I wrote sort of cleaned up all the small experiments. Allen, down at Illinois, did his "tabletop" experiments before that.
Yes, he did.
So after 1948, it was big stuff, the Savannah reactor.
Yes, experiments went to very large detectors and to nuclear reactors or the sun as sources.
The Davis thing and others were big scale. I don't remember any small-scale things after I wrote my paper.
In the 1930's, nothing else stood out in your mind or impressed you? There were some other recoil experiments going on. I was wondering what you recall from that time.
You summed up things pretty well in 1948. You did tie it together pretty well then in your 1948 review article. You wrote in your letter to me that you were
I don't know of any. In my 1948 paper, I tried to make a clean sweep.
I'm surprised you were asked to do the review article on neutrinos, since you hadn't done any neutrino studies for awhile. How did feel about your qualifications at that time to write that kind of article?
I don't understand. Why did I think I should write that kind of article?
You were saying that in 1947 you were asked to do this article on neutrino work, yet you hadn't done any neutrino work for a long time.
I guess I had maintained my interest in following the stuff.
So even though you hadn't done more active research.
When I was asked to write this article, it was a Festschrift for Millikan.
Oh, the famous Millikan Festschrift in Reviews of Modern Physics?
Having come from Cal Tech and having at least made a try at neutrinos at CalTech, somehow they felt they could ask me to write an article. They didn't tell me I had to write on neutrinos, but that seemed to be an interesting subject.
So the topic wasn't assigned ahead of time?
You chose the response, in other words.
They invited me to contribute a paper to his Festschrift.
And the topic was something you were still interested in.
The topic was something I knew fairly well, so I knew my way around. And I at least had some attempt of my own before. Well, I didn't have to justify my choice.
That's right, they were asking you. In that review article, to quote you, you wrote, "not everyone would be willing to say that he believes in the existence of the neutrinos." This was in your 1948 review article. Do you recall what segments of the scientific community fell into that category, any particular people or groups? You're stating that the acceptance is not universal.
I think there were some who had questions or doubts. I guess if I said that, must have thought that was the case.
No one in particular comes to mind?
No one in particular.
Do you Charles, do you have a particular group in mind?
Offhand, no one group in particular who expressed this view.
I would think if you poke around in papers and speeches at that time, statements that indicate doubt will turn up.
Tolman, at this time, Richard Tolman, you were acquainted with him?
Yes, I knew him.
At this time, 1947 or 1948, he called the neutrino a horrible little particle, because it had only been postulated just so energy and momentum would be conserved. He even stated that perhaps it would be better to abandon conservation of energy and angular momentum, than to believe in some kind of particle we might never see.
I didn't know he said that.
It was in an article on the review of the sciences.
I'm a little surprised, he was a pretty hard-nosed type of guy.
Tell us a little bit about Tolman, in regard to his attitude. I always think of Tolman as a mathematical physicist.
I don't think he did any experiments. At least any I ever heard of. He started out as a chemist, didn't he?
I think perhaps so, though I'm not all that certain myself. Tolman, Zwicky,
Then, he got into this cosmology-type stuff, and thermodynamics was one of his specialties.
Why did you call him a sort of hard-nosed guy. What did you mean by that?
If you wanted to sell him on an idea, you had to give him good reasons. During that time when I was a post-doc at Cal Tech, there was kind of a faculty club where they had luncheons at a round table in the lunchroom that held about eight people. So people used to go there, mostly at the same time. Tolman was one of these. I used to sit in once in a while at that table and hear him talk, very interesting. Hubble, and people like that. That's how I got a concept of Tolman.
Maybe that's not the best term, but I think he was a very practical and critical fellow. He wouldn't go for any wild ideas.
Somehow he must have classified in 1948 that the neutrino was in that category of wild ideas.
Perhaps so. I'm just a little surprised to hear that he went along with the possibility of the nonconservation of energy.
I'm not sure he believed that, but perhaps he was feeling desperate about the neutrino in such a way that he would believe anything else, maybe something he didn't feel strongly about. In that same review article of 1948 you were talking about how the neutrino hypothesis had such great usefulness, but that one should not forget that it was a great mystery that would not clear up until it was caught at some distance from the emitting nucleus. This seems to imply that you felt that there was a missing physical proof at that time.
Maybe that's the sentence that got Fred Reines started.
He was looking for that missing proof?
Had you known Reines before this time? He picked this sentence out of the published version and said, "ah, this is something that poses a challenge?"
Yes, he has told me a couple times that the reason he did it was a result of reading my paper. It gave him the idea for a good experiment. Maybe he was just being complimentary. But I've seen him a couple of times when he's said it.
He obviously carried that idea through quite successfully. Has anyone else mentioned that their work followed from your paper? Davis, or anyone else?
Not that I recall. I think Davis's experiment was pretty much like my bag of salt experiment, but I have no idea what he got from me.
When you heard about the confirmation of the neutrino in 1956 by Reines and Cowan, how did you feel at that time about it?
A fine piece of work, a great piece of work.
You had no doubts; you were expecting it?
I didn't have any guesses whether his experiment would pan out, but it did, and that was fine.
May I ask you a bit more about your bag of salt experiment? It's quite an interesting one, since it was a forerunner of larger experiments. Where did your idea originate? Why didn't you follow up on the experiment, maybe try different radioactive sources?
I think I dreamed up the idea that you would need some reaction where the product of the neutrino reaction would be separable from the mass of the material. That's what Davis's experiment did. In fact, argon can be separated from large amounts of carbon tetrachloride. You need a large mass of something to absorb the neutrinos, and the product has to be something which can be separated in minute amounts from the mass, from the bulk of stuff. I think he used carbon tetrachloride. The product is argon, a noble gas, where you can separate very small amounts of argon from a large mass of carbon tetrachloride.
The argon was radioactive?
That's a fairly obvious requirement. I must have thought -it up and decided to use salt, because it had chlorine. I could draw a few conclusions from that. Didn't I conclude something about the earth absorbing neutrinos going through the Earth? I got into a little argument over that.
I remember that in your 1948 review article you did discuss the cross section for the neutrino based on the result of the neutrino going through the earth.
I think a geologist named Benfield questioned my conclusions Even if neutrinos were absorbed in the earth, there wouldn't be enough time for the heat to come up to the surface in the time of the age of the earth, which seemed very surprising. But he could calculate that, so he wrote another letter in Physical Review.
Is that a valid conclusion; is his argument valid?
I guess so, he was the geophysicist and knew about heat conduction. I didn't know much about heat conduction.
Let me go back to the 1930s. I have a question on your 1936 experiment on the Compton effect. Robert Shankland did an experiment in that year where he reintroduced or raised the conservation of energy issue.
He thought he proved that the Compton effect didn't work.
What was your reaction to Shankland's results?
Well, we had a cloud chamber, a pretty good one at that time, and we had some gamma-ray sources, little capsules of thorium and things like that, that make some gamma rays. When we saw that article by Shankland, we hopped to it. We thought maybe it's not so, that somebody ought to see whether it's right or not. It was a fairly straightforward thing to set up with the tools we happened to have available. So that's why we did that.
What do you remember of your reactions to Dirac's comments about Shankland's results, where he indicated that quantum electrodynamics might be greatly in error and even suggested most of it might need to be thrown out?
That wasn't my department. Some other people tried to check Shankland though.
That's right. Bothe and Geiger, and even Shankland himself. They all came to the same conclusion that you did, even Shankland did so a year later in a repeat of his experiment.
My impression is that not many people intuitively believed that Shankland was correct in his experiments almost right from the beginning. Is that a correct sort of intuition as far as you recollect?
Well, when just one experiment turns up something that disproves a lot of things we believe in, the first thing to do is check the experiment.
Spoken as a true experimentalist. Most of your neutrino work, all of it, except for your review paper, you finished up by 1939, then you went on to other things. Why did you leave that sort of work dealing with the neutrino in particular?
We did those recoil experiments in the cloud chamber counting the droplets and massaged that for what it was worth. I guess I didn't know anything next to do in that line, so that was the end of the experiments. It might have been also that Halpern left about that time, he was a research fellow who had worked with me for a few years and left. That may have been one of the reasons we quit these experiments. I just didn't see anything very smart to do next. Kind of wound up that experiment.
Of course, after the war, when nuclear reactors were available that's when Reines and Cowan realized that here was a concentrated source of neutrinos. You were off on other work then, and didn't feel like going back into that area?
I guess so, I wouldn't say I didn't feel like going back. I didn't have an interesting idea for a next experiment on neutrinos.
That's the way it often is; you just reach the end of a research program not only in physics but in history and other things.
It seems to me that you've pumped me about dry on the neutrino.
I have been pretty particular about neutrinos, I know, but the experiments you did are quite fascinating. You went through a very logical sequence in your investigations that are very interesting.
There were other quite interesting and clever experiments in the early days with tabletop experiments. The one by Leipunsky was clever.
Going back to your graduate studies - what were you learning about beta decay and the neutrino when you were a graduate student as opposed to what you taught your students at Michigan. What was the difference?
About neutrinos, about beta decay, about the whole idea of the conservation laws, or why beta decay was special.
I don't think I taught any courses, which involved that.
No courses in nuclear physics.
I did teach a course in nuclear physics for a few years, which included beta decay. I'm sure I said all those things; they were just a part of teaching nuclear physics.
Did you mention alternatives to the neutrino idea, that conservation of energy might not be valid?
I don't think I described all those experiments I talked about in that 1948 paper in my class, but I'm sure I talked about the branching in the natural radioactive series and how the branching came back together, and the energy was still the same that was lost in both branches. That's part of nuclear physics.
Roger and I thank you again for meeting with us, and we hope you enjoy your stay in Minneapolis.