With Masatoshi Koshiba

Home , Atsuto Suzuki, Masatoshi Koshiba, Takaaki Kajita, Yoji Totsuka

Interviewer: Kunio Inoue

Making every effort to win and Culture) the IMB project the competition in the US suddenly emerged. Inoue: Professor Koshiba, It had a much larger budget, you started neutrino research much larger water volume, using Kamiokande, from and PMTs (photomultiplier which subsequently the tubes) of comparable photon Super-Kamiokande and sensitivity to ours. I thought KamLAND, and then K2K and that we would lose the T2K experiments evolved. I competition, and spending think that Japan now takes tax payers’ money on an a leading role in the world in experiment which we were the field of neutrino research. bound to lose would not be I would like to hear about justified. My conclusion, after how you started the neutrino careful thinking, was that I research using Kamiokande, would not even try to request and how Kamiokande more money, but rather was able to achieve such improve the sensitivity of each developments. PMT by orders of magnitude. Koshiba: These are difficult I approached the president questions, and not easy to of Hamamatsu Television (now answer. Kamiokande was Hamamatsu Photonics). I had initially designed for the known him for the previous 10 purpose of searching for years or so because my group proton decays. But when our was developing new PMTs for proposal was about to be an electron-positron collider accepted by the Monbusho experiment in Germany. (Ministry of Education, Science, Through this experience, I knew that the company would be willing to become Masatoshi Koshiba is Honorary Professor Emeritus of the involved in new developments. University of Tokyo. He was I tried to talk him into the awarded the 2002 Nobel Prize in Physics jointly with idea of developing a large Raymond Davis Jr for pioneering . 20” diameter PMT. He was contributions to astrophysics, in particular, the detection of hesitant at first. The chief cosmic neutrinos. Among many engineer accompanying him other distinguished awards he received, particularly noteworthy was opposed to the idea, was the 1997 Order of Cultural saying“, That’s out of the Merit conferred by The Emperor of Japan in person. question, sir!”

16 IPMU News No. 2 June 2008 It took more than three Our experiment would hours to persuade him to go have been like buying a ahead. I assigned Teruhiro lottery using tax payer's Suda, Atsuto Suzuki, and money if it had only searched Katsushi Arisaka［★1］, from for proton decay, as I said my group for the project. earlier. So I thought, since we Subsequent development could see these low-energy went well and we got good electrons cleanly, we should PMTs. It was the spring of try to reduce the background 1983. by orders of We immediately began magnitude installation of those PMTs, and try to see and more or less completed electrons below the work by August. Then 10 MeV cleanly. If we we ﬁlled the water tank and could achieve that, we would started to take data from be able to measure electrons September. We found the scattered by solar neutrinos. data to be of unexpectedly This way we ought to have good quality. In other words, been able to make real-time we could see a beautiful observations of the arrival energy spectrum of decay time, incoming direction, and electrons from muons which energy spectrum of solar entered the 3,000-ton water neutrinos. tank and stopped. We could clearly see the spectrum down Giving chances to capable to as low as 12 MeV. Below young people that, however, the background But, after a simple increased drastically and calculation, it was obvious buried the signal completely. that we could have collected at most only one event per week, even if we had ★1: At that time the late Professor 1,000-ton ﬁducial mass. Teruhiro Suda, Kobe University, was an associate professor at ICRR, the University of Tokyo; Professor Atsuto Suzuki, now Director General Interview of KEK, was an assistant professor Kunio Inoue is the Director at the University of Tokyo; and of the Research Center for Professor Katsushi Arisaka, UCLA, Neutrino Science, Tohoku was a graduate student of the University, and a principal University of Tokyo. investigator of IPMU.

17 The question was, could we experiments were low-profile money, but it was not realistic PMTs with high sensitivity. further reduce the background work, but had to continue to request additional funding I continued to explain that of several events per second. for many years. Therefore, it from the Monbusho. We we wanted to pursue the This required purification of was important to attract the had to look around for other possibility of an astronomical the water. So I appointed local people’s attention to sources of money. observation of solar neutrinos Atsuto Suzuki to take charge the project in order for it to Also at that time, there by observing electrons. My of a project to produce“ the operate smoothly. So, I took were quite a few objections to first proposal was to invite cleanest water in the world”. “Kamioka” as the first part of the upgrade plan amongst the collaborators who could bring I was pleasantly surprised the experiment name hoping collaborators, although the TDCs, associated electronics, when he, completely new to that people would feel closer group was still small. and some other resources. this business, made numerous to the project. Then, I added Inoue: Were the objections Mann［★2］ and his group from investigations and succeeded “NDE” as an acronym for from within the group? University of Pennsylvania in producing the cleanest “Nucleon Decay Experiment”. Koshiba: Yes, from within responded to this proposal water in the world in less than As you know, since the group. If we wanted to and said they wanted to take a year. Kamiokande had been detect solar neutrinos, we had part. I am often invited to lecture producing physics results, to install a 4π anti-counter, as I stressed that this was just at business executives’ mostly concerning neutrinos, I told you. To do this, we had to show feasibility because meetings in Japan. I mention people worldwide tend to remove the PMTs in the even if we succeeded that you should boldly appoint to think“ NDE” stands bottom part of the detector, in making astrophysical capable people to responsible for“ Neutrino Detection which we had worked so observations of solar positions, even if you think Experiment”. I don’t mind. hard to install, and had to re- neutrinos with Kamiokande, they may be too young. Such Either way is OK with me. install them at a higher level. the facility was not big people are sure to make great We had to waste a lot of time enough. To go beyond the progress. I tell them that I have Toward solar neutrino and lose a significant part of feasibility experiment and seen several such cases in the observations the fiducial mass. This meant into real observation of solar Kamiokande experiment, and Anyway, we needed more a loss of the fiducial mass for neutrinos with this method, a one was the case of Atsuto money. In order to detect a proton decay search, so the considerably bigger detector Suzuki. solar neutrinos using this objections were rather strong. would be needed. So my One of the key elements for method, we had to select one But I made a decision to second proposal was to invite the success of Kamiokande event or so per week out of go ahead as a group leader. collaborators to build Super- was that I seldom said“ do the background. We had to Now I had to look around Kamiokande, containing that” or“ do this” to the reduce the background by for the sources for TDCs and 50,000-tons of water. young members of my orders of magnitude. the funds for installing the group. I tried to create a To do this, we first had to anti-counter. About three Supernova neutrinos situation where each member install an anti-counter having months later, in January detected! understood what they had to 4π solid-angle coverage 1984, an international Inoue: I’m impressed that you do. I think this is an important and capable of surrounding conference on baryon number had a master plan from very point for making progress in a the entire detector. The non-conservation, called early on. project. other thing we needed was “ICOBAN”, was held in Park Koshiba: Yes, it was January Inoue: Do you think a secret upgraded electronics. At that City, Utah. I attended this 1984. But, there was no key to Kamiokande's success time, lack of money meant conference and presented response to the second was giving chances to young that we had only digitizers the preliminary results from proposal. I made these two people and letting them to record signal pulse height Kamiokande, and at the same proposals in the first week having big responsibilities? (ADCs). We had to have time presented two proposals. Koshiba: I think so. The other digitizers capable of recording First I explained that we thing was choosing the name signal timing (TDCs) to make could make a clean detection ★2: Alfred Mann, now Professor Emeritus at University of “Kamiokande.” Earlier proton more precise measurements. of electrons down to as low Pennsylvania, was a professor there decay and solar neutrino These upgrades required as 10 MeV because of new at the time.

18 IPMU News No. 2 June 2008 Inside Kamiokande. The detector was located 1,000m underground in the Kamioka Mine. The cylindrical detector tank, 15.6m in diameter and 16m in height, contained 3,000 tons of pure water. The interactions of elementary particles inside the detector were observed by 948 photomultipliers with a diameter of 50cm, the largest in the world. Kamiokande’s observations ceased in 1995 as the construction of Super-Kamiokande approached completion. Later, KamLAND was constructed at the former Kamiokande’s site. (©ICRR)

of January 1984. Mann and elements, so there is plenty of for some period, or further saying that a supernova had his group came to Japan in radioactive radon gas in the improving the ﬁltering system occurred in the southern February and visited Kamioka. mine tunnel. Its concentration developed by Atsuto Suzuki. sky. They wanted to know if They made it clear that they is roughly ten times higher We reached a point where we had seen a signal of the wanted to work together. compared to other tunnels. we reduced the background neutrino pulses that should They would provide 1,000 Radon gas easily dissolves in to a manageable level for have been produced when channels of TDCs and a new water. Since we had to supply starting the solar neutrino the supernova explosion computer. We managed to get cold underground water to observation in January 1987. was triggered. I called the some money to build the 4π prevent a temperature rise, In early January, the radon Kamiokande site and told anti-counter with the bottom radon caused more and more peak ﬁnally disappeared, the person on duty to send PMTs re-installed at an upper background. and we were set to embark the magnetic tapes to the level, and to further purify the In order to cope with this on the neutrino data-taking. University of Tokyo right away. Interview water. problem, we tried several Within two months I began to When we analyzed the tapes As you know, the Kamioka methods, such as storing receive frequent phone calls in our computer, we found the mine produced heavy the water in a buffer tank from many different places, signal.

19 Inoue: That is a fascinating to detecting solar neutrinos group, and Teruhiro Suda and was the least developed story. Did you have a plan using the method I just Jiro Arafune were participating branch of particle physics to detect solar neutrinos described. The pamphlet also in many workshops at ICRR experiments over the past from the beginning of the mentioned the possibility of ［★3］. few decades. The main reason Kamiokande-I proton decay detecting neutrino oscillation. is that among elementary experiment? Inoue: I see. You had Future of particles, neutrinos are the Koshiba: You probably don’t tremendous foresight. neutrino physics only ones that do not carry know this. I was determined Koshiba: Well, I was having Inoue: It’s interesting to hear an electric charge. Particles to show at the Utah a variety of discussions with that discussion was taking with an electric charge are conference in January 1984 people around me those place in those places. After easy to detect. But we have to that we could detect solar days. So these ideas may not your pioneering work on let those without an electric neutrinos using this method. necessarily have been my neutrino astronomy, Super- charge interact in matter and Also, in the budget request for own. But new possibilities Kamiokande measured solar convert into charged particles 1985, which was submitted appear when we really think neutrinos with impressively in order to observe them. This in spring 1984, I attached a hard about what we can do high precision, and KamLAND is a difficult process. small pamphlet explaining with what we have. detected geoneutrinos. It was only recently that that a certain number of Inoue: With whom did you Although in a slightly different the neutrino mass, which was neutrinos could be detected if have these discussions? Your direction, we now have believed to be zero, was found a supernova explosion were to collaborators or people from neutrino experiments in the not to be zero, as you well occur in our galaxy, in addition wider fields? South Pole and undersea. know. I think neutrino physics Koshiba: I retired from the What’s your opinion about has entered a new era, but ★3: Yoji Totsuka, Honorary Professor University of Tokyo in March close ties with other fields there are still many things we Emeritus of the University of Tokyo, was an associate professor at that 1987. In those days, namely in neutrino research. I guess don’t know about neutrinos. time and Jiro Arafune Professor , , between 1984 and 1987, Yoji there are merits in it. We don’t know θ among Emeritus of ICRR, the University of 13 Tokyo, was a professor. Totsuka had already joined my Koshiba: Neutrino research the oscillation parameters. We

Left: Inside KamLAND. The inner surface of the spherical 18-m diameter detector tank is fitted with 1,897 giant photomultipliers. This photo shows the upper part of the detector, viewed from the bottom. At the center of the tank is located a 13-m diameter transparent balloon filled with 1,000 tons of liquid scintillator. (It was not yet installed when this photo was taken.) (©RCNS) Right: Inside Super-Kamiokande. The inner surface of the cylindrical detector tank, which contains 50,000 tons of pure water, is fitted with 11,146 photomultipliers measuring 50cm in diameter. (©ICRR)

20 IPMU News No. 2 June 2008 don’t know the CP violation “more or less correct.” I asked Koshiba: It will be exciting if studied from the Super- parameter, either. We might him again to study it more one could catch even a hint of Kamiokande data, and make find out more regarding why seriously. relic neutrinos. them public. How about doing only matter remained after Professors Takeda and Inoue: Oh, you mean 1.9K this by the end of the year? the Big Bang and all of the Arafune have been working relic neutrinos originating Inoue: Do you have anything anti-matter disappeared, by hard on this problem from the Big Bang. you want to see from knowing these parameters. recently, and finalized their Koshiba: Yes. But it will be KamLAND? I want to stress that we must results into an article. This very, very difficult. So people Koshiba: I think Atsuto Suzuki have a clear understanding phenomenon is not as simple should not start haphazardly, has done an excellent job in of neutrino oscillation in as electromagnetic waves, or they may waste a few nurturing that facility. I often order to understand the because three different decades. mention, when asked to give observed results in neutrino neutrinos oscillate among I want to add one thing in lectures, that it was KamLAND astrophysics. This is clear from them. So compared with the particular, which I said recently that made it possible to catch the case of solar neutrinos. equations corresponding to to Takaaki Kajita, who has anti-electron neutrinos from As you know, COBE the electromagnetic wave just become the Director of the isotopes inside the earth. If observed microwaves in passing through matter, they ICRR［★5］. Super-Kamiokande one can utilize this technique the universe and obtained become diffusion equations started full operation with and continue measurements information about the that are an order of magnitude all PMTs installed. You can for a few years with several universe of 370,000 years more difficult. It is indeed a now continue data-taking of the similar detectors across after the Big Bang. The same very difficult problem. They for quite some time. You the world, one can conduct thing can be done with the think, however, they have should continue further solar earth tomography, with all background neutrinos if we made enough progress so neutrino observation seriously. the data of the places and the can ever detect them. they will publish their article Besides that, the data from quantities of the materials. But how to detect them is as an ICEPP preprint. Super-Kamiokande will play This would be a revolutionary really a difficult problem. For an essentially important role occurrence in the field of the past several years I have Super-Kamiokande should should a supernova explosion geophysics. been approaching theorists continue reliable data-taking occur in our galaxy. There is I think it is important to whom I respect with an idea Inoue: I have a final question. no other detector in the world carry on this project as a that neutrinos with such low IPMU is pursuing projects of comparable capability. worldwide project. I try to energy would go through at Super-Kamiokande and Inoue: I agree with you. convince audiences about this total reflection. I have asked KamLAND. Super-Kamiokande Koshiba: So it is very whenever I give lectures. For various people about this will try to measure θ13 and the important. You must continue example, people in Hawaii possibility, which is still based CP violation parameter, which reliable data-taking so that want to do it, and I hear the on my own intuition. I first you mentioned earlier, in T2K. you can be ready for a SNO group is also interested. asked Yoichiro Nambu, who It is also preparing for the supernova explosion to occur There is a movement in sent me a two-page response, detection of relic supernova at any time. This is one point. Europe as well. I did not saying“ it might be correct”. neutrinos (neutrinos from The other point which I seriously think about the But his two-page note past supernova explosions) want to stress is not to forget possibility of anti-electron was too difficult for me to by adding gadolinium to the the very original aim of the neutrino detection. Atsuto understand, so I wrote to water. KamLAND will try to experiment, proton decay. Suzuki brought this idea to Gyo Takeda in Sendai asking detect neutrinos from the Super-Kamiokande can pin reality by building KamLAND. for an explanation［★4］. solar CNO cycle. Also, it aims down the signals, not only That was a very good thing. He responded, saying that to observe double beta decay those predicted by Glashow’s Inoue: Thank you for your Yoichiro Nambu’s answer was by adding xenon to liquid SU(5) theory, but those from time today. scintillator. In future, what do several other decay processes. Interview ★4: Yoichiro Nambu is Distinguished ★5: Takaaki Kajita is the Director Service Professor Emeritus of the you want to see from these You should pin down the of the Institute for Cosmic Ray University of Chicago. Gyo Takeda two experiments or from lower limits of partial life Research, the University of Tokyo is Professor Emeritus of Tohoku (since April 2008). He is also a University. neutrino research in general? for all possible decay modes principal investigator of IPMU.