Online monitoring of the Osiris reactor with the Nucifer neutrino detector

Originally designed as a new nuclear reactor monitoring device, the Nucifer detector has successfully detected its first neutrinos. We provide the second-shortest baseline measurement of the reactor neutrino flux. The detection of electron antineutrinos emitted in the decay chains of the fission products, combined with reactor core simulations, provides a new tool to assess both the thermal power and the fissile content of the whole nuclear core and could be used by the International Agency for Atomic Energy to enhance the safeguards of civil nuclear reactors. Deployed at only 7.2 m away from the compact Osiris research reactor core (70 MW) operating at the Saclay research center of the French Alternative Energies and Atomic Energy Commission, the experiment also exhibits a well-suited configuration to search for a new short baseline oscillation. We report the first results of the Nucifer experiment, describing the performances of the $\ensuremath{\sim}0.85\text{ }\text{ }{\mathrm{m}}^{3}$ detector remotely operating at a shallow depth equivalent to $\ensuremath{\sim}12\text{ }\text{ }\mathrm{m}$ of water and under intense background radiation conditions. Based on 145 (106) days of data with the reactor on (off), leading to the detection of an estimated $40760{\overline{\ensuremath{\nu}}}_{e}$, the mean number of detected antineutrinos is $281\ifmmode\pm\else\textpm\fi{}7(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}18(\mathrm{syst}){\overline{\ensuremath{\nu}}}_{e}/\mathrm{day}$, in agreement with the prediction of $277\ifmmode\pm\else\textpm\fi{}23{\overline{\ensuremath{\nu}}}_{e}/\mathrm{day}$. Because of the large background, no conclusive results on the existence of light sterile neutrinos could be derived, however. As a first societal application we quantify how antineutrinos could be used for the Plutonium Management and Disposition Agreement.