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Super-Kamiokande
Nakahata
Institute forM. Cosmic Ray Research, University of Tokyo
Abstract Super- Kamiokande project is performed for the studies of solar neutrinos, nucleon decay, supernova neutrinos, and atmospheric neutrinos with high sensitivities. Among these physics goals, study of solar neutrinos is briefly described. Super-Kamiokande is a 50000 ton water Cherenkov detector which detects solar neutrinos with v-e 8B scattering. The fiducial volume of the detector (22000 ton is 30 times as large as that of the present Kamiokande-11. With high statistics and improved detector resolutions, Super-Kamiokande will observe the energy spectrum and the) time variation of solar 8B neutrinos quite precisely. 606
Introduction
The deficitof solar neutrinos, which was presented by the Cl radiochemical experiment 37 by Davis et aU1l, is one of the most serious problem in astrophysics and elementary particle physics. Recently, this problem, so called "solar neutrino problem", was confirmed by the Kamiokande-II experiment using v-e scattering.f2l Various possible solutions of the solar neutrino problem are proposed, ranging from errors in the input para.meters for the calculations, to particle-physics solutions, such as neutrino oscillations, neutrino magnetic moments, or exotic massive particles in the core of the sun. The currently running solar neutrino experiments, Cl 37 experiment and Kamiokande-II, are not so sensitive as to select the real solution out of them.
One of the physics goals of Super-Kamiokande is to observe 8B solar neutrinos with high sensitivity for the solution of the solar neutrino problem. In this paper, Super-Kamiokande project is briefly described concentrating on solar neutrinos. Details of the Super-Kamiokande are described in ref. [3] and [4].
Super-Karniokande Detector
Super-Kamiokande is a very large water Cherenkov detector with a total mass of 50000 ton. It will be located 1000 m underground in the Kamioka mine in Japan. Figure 1 shows the schematic view of the detector. The inner detector is viewed by 11200 photomultiplier tubes(PMT) of 20 inch diameter placed uniformly on the whole inner surface, corresponding to 40 3 photo-coverage. The total mass of water inside the PMT surface (sensitive volume) is 32000 ton. Anti-counter layers of about 3 m thickness of water completely cover the sensitive volume.
Fig.I. The Super-Kamiokande detector. 607
Table I. Comparison of detector parameters between Kamiokande-II and Super-Kamiokande.
Parameters Kamiokande-II Super-Kamiokande Dimension 16mhx19m 41mh x 39m¢ Total mass 4500 ton 50000 ton Fiducial mass 680 ton 22000 ton Number of PMT (inner detector) 948 11200 Coverage of photosensitive area 20 3 40 3 Energy resolution 20 3 (at 10 MeV) 14 3 (at 10 MeV) Vertex pos. resolution 1.1 m (for 10 MeV e) 0.5 m (for 10 MeV e) Analysis threshold 7.5 MeV 5 MeV Event rate (0.46x SSM) 90 ev./year (Ee >7.5MeV) 8400 ev./year (Ee >5MeV) 2: 700 PM Ts with 20" view the anti-counter. The fiducial region for the solar neutrino measure 4> ment is the volume at least 2 m inside the PMT plane, which amounts to 22000 ton of water. This fiducial volume size is 30 times as large as that of present Kamiokande-II detector. Detector parameters of the Super-Kamiokande are compared with those of Kamiokande-II in Table I. Solar neutrinos are detected by v-e scattering in Super-Kamiokande. The merits of this detection method are ; (1) directional information of neutrinos can be observed � 28' at 10 MeV), (2) energy distribution of neutrinos can be observed, (3) real time detection of signals. (o-e The energy threshold of the detector is plan to be lowered to �5 MeV in Super-Kamiokande. With such low energy threshold, we can expect �8400 events per year, assuming the 8B solar neutrino flux is 46 3 of the expectation from the standard solar modeJ.[5] Because of this high statistics, we can discuss the time variation of solar neutrinos quite precisely, for example day /night or seasonal variation with an accuracy of ±3 3 and yearly variation with an accuracy of ±1 � 2 3. Furthermore, we can measure the energy spectrum of scattered electrons, which reflects the energy spectrum of 8B solar neutrinos, quite precisely. Using the energy spectrum, neutrino oscillations can be discussed free from any ambiguities in the standard solar model.
Background to the Solar Neutrino Measurement
Background to the solar neutrino measurement are radioactivities in the detector, external gamma rays from rocks, spallation products induced by cosmic ray muons, and etc. They are studied in quite details using the present Kamiokande-II. The expected background rates are compared between Super-Kamiokande and Kamiokande-II in Table II. Because of the improved energy and vertex position resolutions in Super-Kamiokande, backgrounds of radioactivities, external gamma rays, and spallation products are reduced by a factor of ;:::: 10. The expected signal to noise ratio is 0.8 at the energy threshold of 7.5 MeV. Note that the angular correlation to the sun is not yet taken into account in this estimation, which improves the S/N ratio by a factor of � 10. 608 Table II. Number of solar neutrino event per day per fiducial volume and estimated rates of backgrounds in Kamiokande-II and Super-Kamiokande for >'7.5MeV. Directional correlation to the sun is not taken into account in this calculation. Ee Kamiokande-II Su per-K amiokande (/day/680ton) (/day /22000ton) "B solar neutrino 0.24 7.2 Total background 4..5 �9 decay 0.9 -1.4 «: 1 External rays 1.4±0.7 �0.8 1' 214BiSpallation products 2.0±1.0 �6 U fission � 0.07 �2 reactor 0.02 0.6 Ve
Solar 180 ...... , e18F) �0.02 �o.5 S/N rat10 0.05 0.8 I v (ve
Conclusion
Super-Kamiokande is a low background solar neutrino. detector with quite high sensitivity. It can test the stability of the neutrino flux to a level of a few percent and the energy spectrum of 8B solar neutrinos can be measured precisely. Using these information the solar neutrino problem must be solved.
I would like to acknowledge the Kamiokande-II collaboi:ators who helped me talking on this subject. The Kamiokande-II collaborators include; K.S.Hirata, T.Kajita, T.Kifune, K.Kihara, K.Nakamura, S.Ohara, N.Sato, Y.Suzuki, Y.Totsuka and Y.Yaginuma (ICRR Univ. of Tokyo), M.Mori, Y.Oyama, A.Suzuki, K.Takahashi (KEK), M.Koshiba (Tokai Univ.), T.Suda, T.Tajima (Kobe Univ.), K.Miyano, H.Miyata, H.Takei, M.Yamada (Niigata Univ.), Y.Fukuda, K.Kaneyuki, Y.Nagashima, M.Takita (Osaka Univ.), T.Tanimori (Tokyo Inst. of Tech .), E.W.Beier, L.R.Feldscher, E.D.Frank, W.Frati, S.B.Kim, A.K.Mann, F.M.Newcomer, R.Van Berg, W.Zhang (Univ. of Pennsylvania).
References
[1] R. Davis, Jr., in Proc. of the Seventh Workshop on Grand Unification, ICOBAN '86, ed. by J. Arafune, World Scientific, p.237, 1987; R. Davis, Jr., in Proc. of the 13th Int. Conf. on Neutrino Physics and Astrophysics, Neutrino '88, ed by J. Schneps et al., World Scientific. [2] K. S. Hirata et al., Phys. Rev. Lett. 63, 16(1989). (3] Y. Totsuka, in Proc. of the Seventh Workshop on Grand Unification, ICOBAN '86, ed. by J. Arafune, World Scientific, p.118, 1987. (4] T. Kajita, in Proc. of the Third Workshop on Elementary-Particle Picture of the Universe, Fujiyoshida, Japan, p.163, 1989; ICR-report-185-89-2. [5] J. N. Bahcall and R. K. Ulrich, Rev. Mod. Phys. 60, 297(1988).