To investigate the possibility of neutrino oscillations, the positron kinetic energy spectrum was measured using the inverse beta decay of the neutron (ν̄e + p →e+ + n) as a detector reaction. The experiment was carried out at the fission reactor of the Institut Laue Langevin at Grenoble, France. The well shielded detector was set up at 8.76 meters from the point-like reactor core in an antineutrino flux of 9.8 x 1011 cm-2 sec-1. The target protons were provided by a liquid scintillator (total volume of 377 liters) which also served as a positron detector. The product neutrons were moderated in the scintillator and detected by 3He wire counters via the 3He (n,p)3H reaction. A delayed coincidence was required between the prompt positron and the delayed neutron events. The positron energy resolution was 18% FWHM at 1 MeV. The signal to background ratio was better than one to one between 2 MeV and 6 MeV positron energy. With a counting rate of 1.56 counts per hour, 2919 ± 131 neutrino induced events were detected. The shape of the measured positron spectrum was analyzed in terms of parameters for neutrino oscillations. The experimental data are consistent with no oscillations. An upper limit of 0.2 eV2 (90% c.l.) for the mass squared differences of the neutrinos was achieved, assuming maximum mixing of two neutrino states. The ratio of the measured to the expected integral yield of positrons was determined to be ∫Yexp/∫Yth = 0.89 ± 0.04 (statistical) ± 0.13 (systematic).
