Fast neutron background characterization of the future Ricochet experiment at the ILL research nuclear reactor

TL;DR

This paper characterizes fast neutron backgrounds at the Ricochet experiment site using $^3$He counters, compares measurements with simulations, and estimates neutron backgrounds to optimize shielding for detecting coherent elastic neutrino-nucleus scattering.

Contribution

It provides the first detailed neutron background characterization for Ricochet using $^3$He counters and validates simulations to improve background estimations.

Findings

Neutron background levels are quantified at the site.

Simulations agree with measured neutron fluxes.

Estimated backgrounds inform shielding design for the experiment.

Abstract

The future Ricochet experiment aims at searching for new physics in the electroweak sector by providing a high precision measurement of the Coherent Elastic Neutrino-Nucleus Scattering (CENNS) process down to the sub-100 eV nuclear recoil energy range. The experiment will deploy a kg-scale low-energy-threshold detector array combining Ge and Zn target crystals 8.8 meters away from the 58 MW research nuclear reactor core of the Institut Laue Langevin (ILL) in Grenoble, France. Currently, the Ricochet collaboration is characterizing the backgrounds at its future experimental site in order to optimize the experiment's shielding design. The most threatening background component, which cannot be actively rejected by particle identification, consists of keV-scale neutron-induced nuclear recoils. These initial fast neutrons are generated by the reactor core and surrounding experiments (reactogenics), and by the cosmic rays producing primary neutrons and muon-induced neutrons in the surrounding materials. In this paper, we present the Ricochet neutron background characterization using $^3$He proportional counters which exhibit a high sensitivity to thermal, epithermal and fast neutrons. We compare these measurements to the Ricochet Geant4 simulations to validate our reactogenic and cosmogenic neutron background estimations. Eventually, we present our estimated neutron background for the future Ricochet experiment and the resulting CENNS detection significance.