Particle background characterization and prediction for the NUCLEUS reactor CE$ν$NS experiment

TL;DR

This work develops and validates a Geant4-based background model for the NUCLEUS reactor CEνNS experiment at the Chooz VNS, combining site measurements with detailed shielding optimization to predict sub-keV backgrounds. It shows that a multi-layer shielding strategy plus muon and cryogenic vetoes can suppress particle backgrounds by >$2$ orders of magnitude, leaving cosmic-ray–induced neutrons as the dominant residual source. In the 10–100 eV CEνNS region, the predicted residual background is around $\sim$250 d$^{-1}$kg$^{-1}$keV$^{-1}$ in CaWO$_4$, yielding a signal-to-background ratio of order unity and meeting the experimental requirements. The study also identifies key uncertainties (absolute neutron flux normalization and unmeasured material radioactivity) and outlines a plan to refine the model with commissioning data and targeted R&D on low-energy backgrounds.

Abstract

NUCLEUS is a cryogenic detection experiment which aims to measure Coherent Elastic Neutrino-Nucleus Scattering (CE$ν$NS) and to search for new physics at the Chooz nuclear power plant in France. This article reports on the prediction of particle-induced backgrounds, especially focusing on the sub-keV energy range, which is a poorly known region where most of the CE$ν$NS signal from reactor antineutrinos is expected. Together with measurements of the environmental background radiations at the experimental site, extensive Monte Carlo simulations based on the Geant4 package were run both to optimize the experimental setup for background reduction and to estimate the residual rates arising from different contributions such as cosmic ray-induced radiations, environmental gammas and material radioactivity. The NUCLEUS experimental setup is predicted to achieve a total rejection power of more than two orders of magnitude, leaving a residual background component which is strongly dominated by cosmic ray-induced neutrons. In the CE$ν$NS signal region of interest between 10 and 100 eV, a total particle background rate of $\sim$ 250 d$^{-1}$kg$^{-1}$keV$^{-1}$ is expected in the CaWO$_4$ target detectors. This corresponds to a signal-to-background ratio $\gtrsim$ 1, and therefore meets the required specifications in terms of particle background rejection for the detection of reactor antineutrinos through CE$ν$NS.

Figures (12)

• Figure 1: Simplified schematic view of NUCLEUS at the VNS, breaking down the main components of the experiment.
• Figure 2: Projections of the VNS overburden as a function of the zenith and azimuth angles in units of meters of water equivalent (), as obtained from a measurement campaign using the cosmic wheel detector (left) and from a Geant4 simulation (right). See text for further details.
• Figure 3: Best fit model of the BS-w-Pb sphere acquired data at the surface (left) and in the VNS (right) at the Chooz nuclear power plant. The bottom plots show the residuals of the fit and the $\pm$ 10 % band.
• Figure 4: Reduction of the cosmic ray-induced neutron flux at the VNS. The left panel compares the Gordon spectrum measured on ground Gordon2004 to the energy spectrum of neutrons reaching the NUCLEUS setup location as predicted by Geant4. The right panel displays the differential flux attenuation obtained from the ratio of these two spectra (pink solid line) and a cumulative attenuation obtained from integrating the spectra above a given energy (green dashed line). The blue square and the red triangle show the reduction factor of >10 MeV neutrons measured with the Bonner Sphere (BS) setup (see section \ref{['subsubsec:BS_measurement_at_vns']}) and the >10 MeV neutron cumulative attenuation as predicted by a Geant4 simulation of the Bonner Spheres setup, respectively. The gray areas on each panel indicate the >10 MeV energy range from which neutrons are most likely to induce a nuclear recoil in the CE$\nu$NS RoI. See text for further details.
• Figure 5: Gamma-ray ambiance spectra measured in the VNS (green line) and in two other NUCLEUS laboratories, the respective measuring time is stated in the legend.