Search for neutrino oscillations at a fission reactor

Abstract

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\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\ensuremath{-}2}$ ${\mathrm{s}}^{\ensuremath{-}1}$. The target protons in the reaction ${\overline{\ensuremath{\nu}}}_{e}p\ensuremath{\rightarrow}{e}^{+}n$ were provided by liquid-scintillator counters (total volume of 377 1) which also served as positron detectors. The product neutrons moderated in the scintillator were detected by $^{3}\mathrm{He}$ 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\ifmmode\pm\else\textpm\fi{}180 neutrino-induced events were detected. The shape of the measured positron spectrum was analyzed in terms of the parameters ${\ensuremath{\Delta}}^{2}$ and ${sin}^{2}2\ensuremath{\theta}$ for two-neutrino oscillations. The experimental data are consistent with no oscillations. An upper limit of 0.15 ${\mathrm{eV}}^{2}$ (90% C.L.) for the mass-squared differences ${\ensuremath{\Delta}}^{2}$ 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 $\frac{\ensuremath{\int}{Y}_{\mathrm{exp}}}{\ensuremath{\int}{Y}_{\mathrm{th}}}=0.955\ifmmode\pm\else\textpm\fi{}0.035(\mathrm{statistical})\ifmmode\pm\else\textpm\fi{}0.110(\mathrm{systematic})$.