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จากนักเรียนท้ายแถวสู่นักฟิสิกส์รางวัลโนเบล (4)

DeVorkin:
Right.
Koshiba:
And then I said to Beppo Occhialinni, this place is deep enough, and since it is salt mine, if you make a pool-size hole in the ground and pour water in, it will make the saturated salt water; after a few days all the dirt will settle to the bottom and you get very clear, transparent salt water. Since it is saturated, there will be no bacteria or no plant growth. It will stay very clean and transparent for years. What would we observe if we could install a large number of phototubes facing downward? You know that cosmic ray muons do come down to that depth, but the Cherenkov light they produce would be pointing downward. So if you had photomultipliers on the surface facing downward, you wouldn't see those downward-going muons.
DeVorkin:
The light is directed downward.
Koshiba:
Yes.
DeVorkin:
You wouldn't see it from the back of the envelope?
Koshiba:
Well, you would observe some scattered light. Also, at that time, the photomultiplier was very small, expensive, and not very dependable.
DeVorkin:
This was in the early 1960s. Right.
Koshiba:
So, we couldn't do it, but when I received this call from Sugawara since the most favored mode of proton decay by this SU(5) in question was into positron and pi-zero [0], and pi-zero immediately decays into two gammas, and those gammas and positrons produce an electromagnetic shower in the water. What you observe is back-to-back generated showers. Therefore, if you had a photosensitive device on the inner surface, you would observe this Cherenkov light immediately, and there will be no question about the nature of the event. So within about one hour after the phone call from Sugawara I took a piece of paper, drew the detector and photomultiplier installation, and since I had another appointment on the symposium day, I sent my assistant to present this idea to the KEK symposium.
DeVorkin:
At the KEK, where Sugawara would be.
Koshiba:
Yes. Sugawara was presiding over the symposium. By the way, the assistant is now a full professor at the Tokyo Institute of Technology. There were competing proposals from other groups in Japan, but finally our proposal was approved, and we immediately started. That was in December '78. The next month, in January, I received a copy of the IMB proposal.
DeVorkin:
I see.
Koshiba:
Then I had to start thinking, very seriously, since they were planning a much bigger detector. And they had much more money.
DeVorkin:
Did your money come from KEK?
Koshiba:
No, it came through the ministry of education.
DeVorkin:
But was this a proposal approved by KEK or what?
Koshiba:
Not by KEK, but through the University of Tokyo, faculty of science.
DeVorkin:
Okay. Did you have to do any politicking or pushing?
Koshiba:
Well, the most difficult part was to get a higher ranking in the faculty of science budget process. There are projects which have been proposed for years occupying the first rank, the second, and so forth. If I had to wait at the bottom of the line, it would easily take ten years. So I proposed to the committee of the faculty of science to put my proposal outside the regular ranking. There's a condition that even if my project was approved, it wouldn't affect the acceptance of the usual list. I wouldn't disturb the other people.
DeVorkin:
So you are asking to make the pie bigger rather than to get the first piece of the piece.
Koshiba:
Yes, that's right.
DeVorkin:
I see. Who did you have to convince to do that?
Koshiba:
Well, first the faculty of science committee members and the dean of the faculty of science.
DeVorkin:
And this was knowing that IMB was going to be a competitor?
Koshiba:
Yes.
DeVorkin:
Did that help?
Koshiba:
Well, those people were not very much interested in IMB. They were only interested in whether their list would be affected by the addition of my proposal. And luckily I could convince Mombusho, the Ministry of Education, that all I needed was something on the order of 200 million yen — which in the exchange rate of those days was about $600,000. I knew that IMB had $3½ million. So I had to think very seriously, how could we compete in the proton decay work with IMB? Okay? If our aim was only to detect a proton decaying into a positron and a π0, we didn't have a chance, because they had a much bigger volume and there was no difficulty, even for IMB people, to identify the decay into a positron and a π0. Then I thought, well, SU(5) is not the only candidate for the grand unified theory. There are other possibilities too. What would be the useful result for identifying the future direction beyond the standard theory? I thought, not only the detection of π0e+, if we could also observe other decay modes like K+ anti-neutrino, or mu+ + π0, and so forth. And if we could measure the branching ratios into those various decay modes, that would constrain the type of grand unified theory very much. Then, in order to detect such other possible decay modes, your detector has to be very sensitive in the following sense: we can observe only the Cherenkov light, the secondary particle produced in the water. So ideally if all the inner surface is covered by photocathodes, that would be ideal. So how can we approach that limit within the limited amount of funding? That was the reason why I came up with the idea of developing very large phototubes. Then, using the same number of channels, like one thousand channels, this big phototube will give very much more sensitivity than the smaller phototube.
DeVorkin:
Now a lot must have happened in Japanese electronics between 1960 and 1978 to allow you to think in these ways. Is this correct?
Koshiba:
Well, I don't know if you know this, but this Hamamatsu photonics —
DeVorkin:
Hamamatsu, yes.
Koshiba:
— used to produced phototubes for RCA.
DeVorkin:
Oh really?
Koshiba:
Yes. In those days they just produced them according to the specifications of RCA. And they were sold in the name of RCA.
DeVorkin:
I didn't know that.
Koshiba:
But when we started the DASP and JADE experiments, we needed thousands of phototubes. So we contacted this Hamamatsu photonics outfit, and we specified what characteristics were important in this particular type of experiment, and so forth. They responded very nicely, and that was the reason why I asked this company to try producing 20-inch phototubes.
DeVorkin:
So you had previous experience and contact with Hamamatsu.
Koshiba:
Yes.
DeVorkin:
Was there also an element there that a company like Hamamatsu wanted to make its name in something that was really dramatic?
Koshiba:
Yes. Luckily, the president of this company wanted to do something different, something new. He was very aggressive in his field.
DeVorkin:
What was his name?
Koshiba:
Hiruma. A very nice man.
DeVorkin:
It's very important to understand how this technology developed to the point where you could think of such large phototubes. Can you give me some sense of how you felt the technology was ready for it? What other types of tubes did you know existed that compared with this, what you had in mind?
Koshiba:
Well, I knew that this company had experience with tubes up to something like 12-inches in diameter. We had a rather long discussion with the president and the chief of the technical division of that company in my office at Tokyo University. The chief of the technical division was quite reluctant to go beyond anything bigger than 14 inches in diameter, because that was the size he felt was safely approachable. But I insisted on 20 inches, that was the size I really wanted.
DeVorkin:
What were the technical limitations?
Koshiba:
One was to blow that big a glass tube. Making one or two is not a problem, but to make the same shape, same thickness, and the same strength in this size phototube is not. The glass tubes were not easy to make.
DeVorkin:
Standardization is very important.
Koshiba:
Yes.
DeVorkin:
Well, let me just ask this out of pure ignorance. Television tubes are large vacuum tubes. Are we talking about a tube where the tolerances on the glass, or the degree of the vacuum, is much greater than a regular television tube?
Koshiba:
One thing was of course the degree of vacuum. More important, however, is the fact that our big phototube had to be immersed in deep water, which means additional pressure. In the case of a television tube, the pressure difference is only one atmosphere.
DeVorkin:
Right.
Koshiba:
But in the case of say a 40-meter depth in water, there would be a five atmosphere difference. So the glass has to be very strong.
DeVorkin:
Was the thickness of the glass also a factor? Did it have to be —?
Koshiba:
Of course. You have to choose a correct thickness to withstand the pressure difference.
DeVorkin:
Okay. So the glass thickness was a function of the mechanical stability of the tube. There was no problem making it too thick for the nature of the experiment. No problem there, because it's visible light.
Koshiba:
Mm-hmm [affirmative]. However, if you make it too thick, several problems arise. In the case of thick glass, it's not easy to make things uniform. That's one of the problems. And another thing is that even though it is transparent to visible light, the Cherenkov light contains a short wave component too. A more important difficulty with the large size is the fact that unless you design the shape of the photocathode and the shape of dynode very carefully, the arrival time of photoelectrons from various parts of the photocathode will be quite different, making the time resolution of the tube very bad. Because at this low voltage difference, the slow electron has to travel a long distance. So if the electric field is not very well conditioned, different electrons will take different transit times, and you would lose the time resolution.
DeVorkin:
We're talking about extremely precise timing.
Koshiba:
Mm-hmm [affirmative]. In order to help design this large tube, including the dynode structure and so forth, I sent my research assistant and one graduate student to Hamamatsu photonics to help design and test the tube.
DeVorkin:
And who were they?
Koshiba:
The assistant was Atsuto Suzuki, who is now a professor at Tohoko University. He is now starting a new project at the old Kamioka Kamiokande site using large amounts of liquid scintillator.
DeVorkin:
The water or the oil?
Koshiba:
Oil. The graduate student is now a full professor at UCLA.
DeVorkin:
Oh really? Who is he?
Koshiba:
Arisaka.
DeVorkin:
Okay.
Koshiba:
He had just entered graduate school, and when I told him about the Kamiokande project, he was very interested and became enthusiastic. The first thing he did was to help design these big phototubes, was the first collaborator of mine on the Kamiokande experiment. Totsuka and others joined later.
DeVorkin:
Oh, I see. But they were still part of your group?
Koshiba:
Totsuka at that time was working on JADE in Germany.
DeVorkin:
Oh, I see, okay. And he came back to work —
Koshiba:
Mmm [affirmative].
DeVorkin:
Was that actually a general characteristic? I mean you had a number of students who were out working on different projects, but when this one, when you built Kamiokande you brought them back?
Koshiba:
Mm-hmm [affirmative]. Well, I never told them to do this or that. I explained what the possibilities were, and then let them decide.
DeVorkin:
That's good.
Koshiba:
I never forced them.
DeVorkin:
Okay. Well, the phototubes were the first big technical hurdle. When were you quite sure that you had the phototubes in hand, that they would work? How were they tested?
Koshiba:
Well, first we made a 1 meter cube water container, filled it with water, immersed the phototube in it, and used cosmic rays muons to see the response.
DeVorkin:
And so that was the method of testing.
Koshiba:
Mmm [affirmative]. And also we used the secondary beam of the KEK accelerator.
DeVorkin:
So you took them there. Okay.
Koshiba:
Yes. Specifically to check the timing characteristics.
DeVorkin:
Did anybody else get interested in your phototubes while you were testing them? Other people must have seen them, seen how big they were.
Koshiba:
Yes. For instance IMB people, some of the IMB people did show interest, but they had already committed to the 5-inch tubes.
DeVorkin:
The 5-inch tubes, yes. What contact did you have with Fred Reines at this time?
Koshiba:
I invited him to come and visit Kamioka, and he invited me to Irvine. Fred and I got along very well.
DeVorkin:
Okay, good. But you were essentially in competition.
Koshiba:
Yes.
DeVorkin:
Now there's many aspects to this project that I would like to better understand. We've talked a bit about the funding, how you did that, but there is the question of the logistics of the experiment, how you organized it. We've talked a bit about the detectors, but not about the system of collecting the data. I guess I need some word from you on what technical and logistical milestones did you have to meet to build it, what constraints were you under. You didn't have that much money, as you indicated. So the phototubes were the first big technical hurdle. What was the next one that you had to overcome?
Koshiba:
Well, the second problem I had to overcome was the fact that our funding was not quite sufficient. Since I decided to maximize photon sensitivity by developing big phototubes, I used much of my funding in this area. But, when it came to the electronics I knew that the timing electronics, what is called TDC, Time to Digital Converter, would be nice to have. I couldn't afford it. So I used only ADC, Analog to Digital Converter. In order to measure pulse height. So, because the total number of channels was about one thousand.
DeVorkin:
You had a thousand phototubes in the first instrument.
Koshiba:
Not only that, I made a very detailed cost breakdown of 20" phototubes. The company wanted to charge something like 200,000 yen per tube. But I worked it out that the cost of making one tube would be something like 120,000 yen. I gave them 130,000 yen per tube. They didn't like it, but since we were the only customer of these tubes they had to swallow that price.
DeVorkin:
This is after they had made the prototypes?
Koshiba:
Mm-hmm [affirmative]. So I saved a considerable amount there, but still I could afford only ADC, not TDC.
DeVorkin:
Getting back just for a second to the tubes themselves, I understand from Professor Suzuki that the critical factor was the ability to blow the glass. I know that you said you weren't too much involved with the glass part of it, but did you know that there were only two glass blowers, or were there only two glass blowers, who could do this work?
Koshiba:
In Japan you mean?
DeVorkin:
Or in Hamamatsu. Were they already in Hamamatsu, or did Hamamatsu have to hire them?
Koshiba:
I don't know exactly. You'd better ask Atsuto Suzuki, because he is the one directly involved.
DeVorkin:
Okay.
Koshiba:
There was another thing I worried about, namely, the the background contamination of the glass.
DeVorkin:
The glass itself having material in it that was radioactive. Yes.
Koshiba:
That I had checked by my student and assistant. I told him, "Don't forget to check this."
DeVorkin:
Okay. So you were planning out the costing. You had to make a compromise in the electronics for the TDC. What about the siting, where to put the detector? You didn't have any salt mines. So where were you going to put it?
Koshiba:
We had to dig a new cave, and for this I had to obtain additional funding.
DeVorkin:
Oh, I see.
Koshiba:
I got it through KEK.
DeVorkin:
Did they know that you didn't ask for enough money in the beginning?
Koshiba:
Initially, the minister of education did give me the instrumentation money, but not the digging money.
DeVorkin:
Oh, so that was —
Koshiba:
Later, the minister of education gave me the digging money upon the recommendation of the KEK director general.
DeVorkin:
Was the director general a friend of yours?
Koshiba:
Not the director general himself. At that time, the director general was Nishikawa, who was three years senior to me. But Sugawara, head of the theoretical division, made a strong recommendation, because we already had produced successful 20-inch phototubes.
DeVorkin:
What kind of survey did you do to find the best site?
Koshiba:
Oh, the people in the mining company were very experienced, they made a test hole, and examined the composition of the rock and also the shape of the cave had to be taken into account. We consulted with a professor in the engineering faculty who is an expert on this kind of thing. Upon his advice, we decided on the shape of the cave. Luckily the cave is located in the oldest rock formation on the entire Japan island. It is very old, very solid rock, necessitating the use of explosives, not a jack-hammer or anything.
DeVorkin:
So this is a relatively stable area, geologically speaking.
Koshiba:
Yes.
DeVorkin:
Did you consider other sites other than the Kamioka area?
Koshiba:
Yes, I did, but I picked this place. One reason was that I had experience with this company over many years — dating from my first underground experiment.
DeVorkin:
The muon experiment. Right.
Koshiba:
Of the other places I investigated, one was a copper mine, but the water inside the mine contained a lot of sulphur which is deteriorating to the iron container wall. That was one reason for rejecting this site. I also investigated an under-ocean tunnel between Hokkaido and the mainland, but it was too narrow and too shallow.
DeVorkin:
You mean there wasn't enough water.
Koshiba:
The mine I chose not only has very solid, stable rock, but also the miners there are very experienced. There can be a lot of dangerous situations in a deep mine.
DeVorkin:
Oh yes.
Koshiba:
And the company has been friendly to me from the very beginning. When I carried out my first experiment here, the company was booming, was very prosperous, and making lots of money. During the last 20 years, however, they ran into some difficulties. People downstream in the Toyama area claimed that because of the poisonous water, polluted by the mine, people there suffered from a special type of disease.
DeVorkin:
It was cadmium, wasn't it?
Koshiba:
Cadmium, yes.
DeVorkin:
Cadmium poisoning, yes.
Koshiba:
So finally the company had to pay something like $1 billion every year to those people. Combined with the fact that the world price of lead has been low, the company has been in difficulty for many years. Further, the number of employees has been reduced by a very large factor.
DeVorkin:
What is the name of the particular company here?
Koshiba:
The Kamioka Mine.
DeVorkin:
The Kamioka Mine. Okay. And they're part of Mitsui?
Koshiba:
Yes.
DeVorkin:
So the company's name is the Kamioka Mine. They've been here for years and years. Actually how long have they been here as the Kamioka Mine?
Koshiba:
I think Mitsui obtained this mine at the beginning of the 1850's or the 1860's.
DeVorkin:
So the 1860s, or the 1850s-60s. Okay. But as we discussed at lunch, there has been mining in this area for over you said —
Koshiba:
One thousand years.
DeVorkin:
Over a thousand years. Okay. So then you decided on the Kamioka Mine, but then it was a question of finding the best spot. And you also had an engineer, or a professor of engineering at —
Koshiba:
To give me advice.
DeVorkin:
Yes, to give advice, with the mining engineers. And what was the final decision? How was it finally made?
Koshiba:
Well, we found a region of very solid rock which would be stable for some time to come. We made a number of test drillings to check the properties of the rock, and then started digging.
DeVorkin:
Was the chamber you were digging bigger than anything these miners had dug up to this point?
Koshiba:
Not quite. They did have a vacant cave which was bigger in size. They used it as a place for dumping sand and rock.
DeVorkin:
Oh, the stuff they didn't use. I know, yes, there's a word for it. Tailings.
Koshiba:
I didn't know that.
DeVorkin:
Yes. I think that's it. The material left over after the minerals have been harvested. But that was not an acceptable place?
Koshiba:
No, because first thing was safety. And then what came next? As I explained, after the cave was dug, we installed phototubes layer by layer using graduate students.
DeVorkin:
Mmm. But in making the tank itself, did that require any special technology?
Koshiba:
Not very much, but we had to worry about residual radioactivity. We lined the inner side of the tank with a coating to prevent water from leaking. Originally, our aim was to search for proton decays.
DeVorkin:
Yes.
Koshiba:
For that purpose we didn't need any anti-coincidence system, because the signal was large and very typical, that is back-to-back. Yeah? So we didn't worry about the background. So we installed the tank and we laid down the phototubes on the wall and on the bottom and top and filled it with filtered water.
DeVorkin:
Oh, and just to be clear, because I think I cut you off in the middle of your statement, you put detectors on the floor first, and then your graduate students went around connecting them as you filled more and more water.
Koshiba:
Mm-hmm [affirmative]. Storing one layer after another, gradually raising the level of water.
DeVorkin:
How long did this process take?
Koshiba:
I think we started the installation operation in January, and it was completed by the end of June. About half a year operation.
DeVorkin:
That would be in 19 —?
Koshiba:
'83.
DeVorkin:
'83, right. From the chronology I have found from your records, you started observations on the 4th of July, 1983. Right?
Koshiba:
Not exactly, because the 4th of July was the time when we started pouring water.
DeVorkin:
Oh, I see.
Koshiba:
So at the beginning there was only this depth, then this depth. We gradually increased it.
DeVorkin:
So the water that you'd put in there for your graduate students to float around in, you then got rid of it?
Koshiba:
Mm-hmm [affirmative]. Because that water would have been dirty.
DeVorkin:
Did they then have to clean all of the tubes?
Koshiba:
Yes, wash them.
DeVorkin:
I have this picture that you brought the water level down as they cleaned all the tubes, leaving everything above crystal clear.
Koshiba:
Yes. Mm-hmm [affirmative].
DeVorkin:
What a job.
Koshiba:
Yes. It was a big job.
DeVorkin:
Now you also knew of Rines's IMB.
Koshiba:
Mmm [affirmative].
DeVorkin:
And you didn't know how quickly they would be in operation, or did you?
Koshiba:
Well, I didn't have detailed information, but they were about one year ahead of me, because of the time we used to develop the 20-inch phototube. So we were one year behind. This I knew.
DeVorkin:
Were you afraid of being scooped?
Koshiba:
At the very beginning when it was only a matter of finding e+ + π0, IMB has very much of an advantage over Kamioka.
DeVorkin:
That's right. You explained, that you had different capabilities. And so you knew that you would be able to make a wider —
Koshiba:
Even if they discovered the e+ + π0 decay, eventually our detector would supplement the information by observing other types of decay modes. That would be rather important.
DeVorkin:
Yes. Were you working also in the same energy range as they were?
Koshiba:
Yes, proton decay produces the same amount of energy.
DeVorkin:
Okay then, from the chronology, you started pouring water back on the 4th of July. At this point there was something that I had read in one of your reviews that indicated that you began to realize that you could detect solar neutrinos about this time. Is that —?
Koshiba:
I think all the water was back in sometime in early August, and then we started full data taking. I wanted to make the energy calibration. What you observe is the number of photons in each phototube. You have to convert this to the actual energy of the event. For this purpose, I used the cosmic ray muons stopping in the water detector, and then decaying into an electron. We know the energy spectrum of decay electrons accurately. Therefore if you observe the number of photons from decay electrons, you can calibrate the number of photons to the energy of the event. After about three months of operation, we had accumulated a considerable number of decay electrons.
DeVorkin:
This is by the end of 1983?
Koshiba:
Sometime around the end of October. We could see a beautiful distribution of decay electron energy distribution down to an energy of about 12 MeV. Below that, there is a very sharp rise due to environmental background. It may be interesting to tell you that during the three months of operation we found a very attractive event which looked like just proton decaying into mu+ and [eta]0, decaying into 2 gammas. This is a neutral particle, just like π0, only eta0 is more massive than π0.
DeVorkin:
Is that the symbol?
Koshiba:
Yes, that's the symbol.
DeVorkin:
Yes. Eta zero. Okay.
Koshiba:
Which then decays into 2 gamma rays.
DeVorkin:
Into 2 gamma rays?
Koshiba:
A photon decaying into mu+ and eta zero. We were very much excited about this event. You probably know the expression "beginner's luck." That was exactly the case. We had been searching for a similar event for more than ten years with Kamiokande and also with the Super-Kamiokande. We didn't find it.
DeVorkin:
So you don't really know —?
Koshiba:
We don't claim it was a proton decay, not only because it was the only event, but also if it were a proton decay it must have occurred inside an oxygen nucleus. The total momentum didn't exactly balance.
DeVorkin:
This is what you understand from theory.
Koshiba:
No. From the observation.
DeVorkin:
From the observation.
Koshiba:
Yes. The total momentum was not quite on a flat surface, but it was a little bit like that.
DeVorkin:
It was asymmetric.
Koshiba:
It was not on a plane. If it were a 3 body decay, it should be on the same plane.
DeVorkin:
Oh, all three tracks have to be on the same plane.
Koshiba:
Yes. Coplanar.
DeVorkin:
Coplanar. But it wasn't.
Koshiba:
It was not quite coplanar.
DeVorkin:
Indicating that something else was going on.
Koshiba:
Yes. There was something which absorbed the excess momentum. Forget about this, because we are not claiming it as a proton decay.
DeVorkin:
But it was exciting.
Koshiba:
Yes. And then I thought, if we can somehow reduce the background at the low energy end below 12 MeV, then there is a possibility of observing solar neutrinos by means of neutrino-electron scattering. Because we know that the boron component of solar neutrinos does have an energy in excess of 14 MeV.
DeVorkin:
Boron 8?
Koshiba:
The Boron 8 decay. Yes.
DeVorkin:
Were you reading the literature at this time and realized that there was a problem with detecting solar neutrinos? Did you know about Davis' work at this time?
Koshiba:
Of course.
DeVorkin:
I had to ask. [laughs] And you knew that there was a deviation from prediction.
Koshiba:
Mm-hmm [affirmative]. Only one-third is observed of the predicted value.
DeVorkin:
Right.
Koshiba:
Of course I knew that. I admire Davis' work very much. However, it all depends on how much you can reduce the background. To do this we had to install additional layers of anti-coincidence detectors.
DeVorkin:
When did you actually decide to search for solar neutrinos? Was it before you met with Alfred Mann at Utah in January of '84?
Koshiba:
Somehow we had to do it. But how. Because I had already exhausted my funding. When I thought of how to go about it, I had to install additional layers of anti-counters as a first step. Also, because those events do not give very many photons, the timing device, TDC, was necessary to reconstruct the event.
DeVorkin:
That's what you needed.
Koshiba:
Yes. But as I said, I didn't have money to buy a TDC. The trouble is the minister of education. Once an experiment is approved, it is completed as proposed. In our case, the experimentalists found new possibilities, and we wanted to change the setup. But, the minister of education wouldn't give us the money.
DeVorkin:
Why?
Koshiba:
Because a project has to be thought out thoroughly before proposing it. Changing the structure of the detector and so forth, after only three months of operation, would not be permitted.
DeVorkin:
The predictions of GUT, the grand unified theories, had originally said that proton decay should occur, and that it should have a lifetime of 10 to the 28th or 29th years.
Koshiba:
29th, yes.
DeVorkin:
But at some point, just as you were building Kamiakande, the first Kamiokande, that estimate was shifted up to 10 to the 33rd.
Koshiba:
Not quite.
DeVorkin:
When did that happen or —?
Koshiba:
Well, it was only after IMB and Kamiokande didn't find the event. The experimentalists were the ones who pushed up the lifetime.
DeVorkin:
How long had IMB been operating by the time that was the case? They were operating before you were.
Koshiba:
Mmm [affirmative].
DeVorkin:
And then you were only operating for three months.
Koshiba:
Mmm [affirmative].
DeVorkin:
Had you thought that you should have found something within the first three months?
Koshiba:
Yes. At that time, because of this particular event, I wasn't sure whether proton decay was occurring or not.
DeVorkin:
Aha. Okay.
Koshiba:
We didn't make any statement.
DeVorkin:
Oh, you didn't tell anybody.
Koshiba:
We showed the event, but we didn't claim that this was a proton decay.
DeVorkin:
Okay. But, I guess what I'm trying to clarify is this, if the experimentalists showed that there were no proton decays, that would require a certain amount of observing time to be definitive, and yet you were only observing for three months. So was that enough time?
Koshiba:
No. In order to set the low limit for the lifetime, three months operation can give a very poor lower limit, while IMB can give a better lower limit. Yeah? But when it comes to a positive result, that is finding something, it doesn't matter whether it is only three months operation or three years operation.
DeVorkin:
Right. So you had not given up on detecting proton decay?
Koshiba:
No. I haven't given up the possibility of detecting proton decay, even now. I know that the theoretical people have been pushing up the limit after we failed to find anything, but at some point the proton has to decay. This is my personal belief the lifetime is 1035 years or 1036 years.
DeVorkin:
So is it correct to say that even though you may be not too concerned about the details of the theory, you basically are working within the general framework of the theory that says protons should be decaying?
Koshiba:
Yes.
DeVorkin:
Okay. So, within that three months, you decided that there was another use for your instrument.
Koshiba:
Mm-hmm [affirmative]. I figured that I had to install additional anti-coincidence layers, and I had to obtain TDC’s, for thousand channels. As for installing the additional anti-coincidence layers, I could use graduate student labor and some scraped-up funds for an additional small number of phototubes. But when it came to TDC’s, for thousand channels, I didn't have any idea of how to get it.
DeVorkin:
Mm-hmm [affirmative]. It was a question of money. Were these things commercially available?
Koshiba:
Yes.
DeVorkin:
Yes, alright. Because I know ADCs are commercially available.
Koshiba:
And then I went to the Park City meeting in January next year.
DeVorkin:
That was January of 1984.
Koshiba:
Mmm [affirmative].
DeVorkin:
Okay. And that's in Utah.
Koshiba:
Yes.
DeVorkin:
Okay.
Koshiba:
At that time three underground experiments were ongoing. One was of course IMB. The second involved another American vintage detector by Carlo Rubbia and David Cline. I don't remember the name of that experiment.
DeVorkin:
But it's in your review paper.
Koshiba:
Cline is now at UCLA.
DeVorkin:
So there were three underwater detectors and probably representatives from all three were at Park City? You mentioned that you were also planning to present three papers at the symposium.
Koshiba:
One was to report on the preliminary results of the Kamiokande detector. I showed the 3-ring event, the 2-ring event, the 5-ring event, and so forth.
DeVorkin:
Ring event?
Koshiba:
Yes. One particle produces a Cherenkov ring.
DeVorkin:
Okay. So by then you'd had a family of events that you had seen, a series of rings.
Koshiba:
Yes.
DeVorkin:
Which said that something was happening, your detector was working. Okay.
Koshiba:
I was glad that the audiences were very much impressed by actually looking at the Cherenkov rings. I showed this expanded view which you saw on the real time display of the event.
DeVorkin:
Right. It's an equatorial projection of the cylinder plus the polar.
Koshiba:
That's right. Exactly the same display.
DeVorkin:
You didn't have the ability in real time, but you had the ability to do it in analysis.
Koshiba:
Mm-hmm [affirmative].
DeVorkin:
Yes. Of the three underwater detectors, was yours the only one that could really do imaging like that?
Koshiba:
I think so, yes.
DeVorkin:
Or at least with a resolution and show the rings?
Koshiba:
There is an IMB report which didn't show this actual ring image and so forth. They were just talking about the statistically analyzed upper limit and lower limit and so forth. And when I showed this actual event display, people were very much interested and started shouting that IMB should show a similar event display. And they of course had to take some time to produce it to show their version of the event. But unfortunately, because their sensitivity is only one-sixteenth of the Kamioka experiment, what you saw was scattered crosses.
DeVorkin:
Oh, their resolution was only one-sixteenth. Not sensitivity, but resolution.
Koshiba:
Well, not resolution. Resolution is one fourth, sensitivity is one-sixteenth.
DeVorkin:
So you have 16 times the resolution of the original IMB?
Koshiba:
No.
DeVorkin:
Okay. Do I have that wrong?
Koshiba:
The resolution would be the accuracy.
DeVorkin:
Spatial resolution.
Koshiba:
Not only the spatial, but also the amount of energy. The energy estimation would be proportional to the square root of the total number of photon events. Right?
DeVorkin:
Yes.
Koshiba:
In this sense we were observing 16 times more photons as compared to IMB. Therefore, our accuracy was four times greater — the square root of 1 over 16, or 1 over 4. In my second paper, I wanted to get my American collaborators to work on the problem of solar neutrino detection, and possibly bring TDC’s to the Kamiokande site.
DeVorkin:
Yes.
Koshiba:
Unfortunately, however, I had a very bad case of the flu.
DeVorkin:
Flu.
Koshiba:
Yes. My throat was swollen, it hurt like hell, and I couldn't speak.
DeVorkin:
Oh. So you had laryngitis.
Koshiba:
I didn't know what it was. But I talked to Al Mann because after I had shown him the Cherenkov ring patterns, he complimented me on my work. So I said to him, "Are you interested in trying to observe solar neutrinos by this type of detector and through electron scattering?" He said, "yes", and and then we started planning on how we could go into it together.
DeVorkin:
But with your flu —
Koshiba:
I could speak only with a low voice when we were talking —there were only two of us — but not in a manner to make a speech.
DeVorkin:
So what happened?
Koshiba:
So I made a transparency, three transparencies for each topic, and I asked Al Mann to present them for me. One was the feasibility study of solar neutrinos.
DeVorkin:
You had already written that out.
Koshiba:
Mmm [affirmative], with transparency paper. I had written the transparency and gave it to Al Mann to show it to the audience.
DeVorkin:
Okay.
Koshiba:
What I wanted to clarify was that there was a definite possibility of observing solar neutrinos with the scattering of electrons in water. In this way we can not only observe the total flux, get directional information, and also obtain energy information. That means we can make astrophysical observations by neutrinos. I said a few minutes ago that I have great respect for Davis' work.
DeVorkin:
For Davis' work. Yes.
Koshiba:
However, I wouldn't call it a breakthrough in neutrino astrophysics observation, because with his method you don't have time information, directional information, nor energy information.
DeVorkin:
Did you make that statement at Park City?
Koshiba:
Mmm [affirmative].
DeVorkin:
How did people react to it?
Koshiba:
Well, I'll tell you. To observe solar neutrinos by this method, I described how we needed to install additional anti-counter layers, and also to improve the event reconstruction accuracy we needed one thousand channels of TDC. Many people were interested; however, Al Mann was the one who actually was very eager to try out this possibility. For instance, Dave Cline and the IMB people were all very much involved in their own experiments. Another paper I asked Al to present for me was the possibility of a new U.S.-Japan collaboration — a Super-Kamiokande.
DeVorkin:
Even at that time.
Koshiba:
Uh-huh [affirmative]. And I considered a nickname for this. I called it JACK, J-A-C-K, which means Japan-America Collaboration at Kamioka. I figured it to cost about $100 million, but nobody showed interest.
DeVorkin:
What was your argument for the value of building the Super-K at that time?
Koshiba:
Well, first of all the possibility of observing solar neutrinos through electron scattering was very good. This was actually opening up a new field of astrophysics, neutrino astrophysics. So I felt that this would justify $100 million. That's what I thought. But others didn't think so.
DeVorkin:
Were there people like John Bahcall at the meeting?
Koshiba:
I don't remember.
DeVorkin:
I was just wondering who first —
Koshiba:
The organizer of the conference was David Cline.
DeVorkin:
Okay. But —
Koshiba:
David Cline promised to produce a proceeding, but he never did.
DeVorkin:
So this was never published.
Koshiba:
That's right.
DeVorkin:
Do you still have the manuscripts that you gave?
Koshiba:
I have the transparencies somewhere.
DeVorkin:
Good. I hope that your papers are being preserved.
Koshiba:
Yes, but it's a handwritten transparency.
DeVorkin:
That's okay. Those things are very important. But as well as your letters and your correspondence are the different records that help future historians understand your career. Have you deposited your papers at the University of Tokyo or anywhere?
Koshiba:
No. Because I believed that David Cline would produce the proceedings.
DeVorkin:
No, I'm talking about all the papers produced throughout your entire career. Not published papers, but your private correspondence, letters, and things like that.
Koshiba:
Well, I don't accumulate such papers. From time to time I throw them away.
DeVorkin:
Oh, you do? Oh, that's too bad. Because that's the basic data for historians.
Koshiba:
I'm sorry. But I am not very good in keeping things.
DeVorkin:
Mmm. But what about when you have grants from the minister of education, do they keep records, do you keep records of your proposals to them, and reports to them, things like that?
Koshiba:
Aahhhh, the minister of education should have kept the proposals somewhere.
DeVorkin:
Okay. Well, we'll talk about this later. I don't want to derail you. So you gave these three talks at Park City, and from there Alfred Mann became very interested.
Koshiba:
Mm-hmm [affirmative].


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