Search for neutrino oscillations at a fission reactor
The energy spectrum of neutrinos from a fission reactor was studied with the aim of gaining information on neutrino oscillations. The well-shielded detector was set up at a fixed position of 8.76 m from the pointlike core of the Laue-Langevin reactor in an antineutrino flux of 9.8×10^11 cm^-2 s^-1. The target protons in the reaction ν̅ep→e+n were provided by liquid-scintillator counters (total volume of 377 l) which also served as positron detectors. The product neutrons moderated in the scintillator were detected by 3He wire chambers. A coincidence signature was required between the prompt positron and the delayed neutron events. The positron energy resolution was 18% full width at half maximum at 0.91 MeV. The signal-to-background ratio was better than 1: 1 between 2 and 6 MeV positron energy. At a counting rate of 1.58 counts per hour, 4890±180 neutrino-induced events were detected. The shape of the measured positron spectrum was analyzed in terms of the parameters Δ2 and sin22θ for two-neutrino oscillations. The experimental data are consistent with no oscillations. An upper limit of 0.15 eV2 (90% C.L.) for the mass-squared differences Δ2 of the neutrinos was obtained, assuming maximum mixing of the two-neutrino states. The ratio of the measured to the expected integral yield of positrons assuming no osciliations was determined to be ∫Yexp/∫Yth=0.955±0.035(statistical)±0.110(systematic).
©1981 The American Physical Society Received 7 April 1981 The authors wish to thank the ILL for its generous hospitality and support. C. Bouton, C. Barnoux, and B. Guerre Chaley have participated in the construction of the detector. W. Mampe's help in planning the experiment is appreciated. Stimulating discussions with B. Davis, P. Ramond, F. Reines, K. Schreckenbach, H. Sobel, and V. Zacek are gratefully acknowledged. The continuous advice and help by P. Vogel has been invaluable. H. Fritzsch and P. Minkowski, who sparked these investigations during their visit to Caltech in 1975, have provided us with much encouragement. Part of this work constitutes the Ph.D. thesis of H. Kwon. Financial support was provided by the U.S. Department of Energy through Contract No. DE-AC-03-76ER00063, the Alfred P. Sloan Foundation, the Bundesministerium für Forschung and Technologie of the Federal Republic of Germany, and by the Institut National de Physique Nucléaire et Physique de Particules of France. F. Boehm is grateful for an award from the Humboldt Foundation. A.A. Hahn has profited from a cooperative NSF-CNRS grant.