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×${10}^{11}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{s}}^{-1\mathrm{}}$. The target protons in the reaction ${\overline{\nu }}_{e}p\to e^{+}n$ 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±180 neutrino-induced events were detected. The shape of the measured positron spectrum was analyzed in terms of the parameters ${\Delta }^2$ and ${sin}^{2}2\theta$ 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 ${\Delta }^2$ 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 $\frac{\int Y_{\exp }}{\int Y_{\mathrm{th}}}=0.955\mathrm{}\pm 0.035\left(\mathrm{statistical}\right)\pm 0.110\left(\mathrm{systematic}\right)$.