With Masatoshi Koshiba
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IPMU Interview with Masatoshi Koshiba 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.