Neutron capture-induced nuclear recoils as background for CE$\nu$NS~measurements at reactors

TL;DR

This paper analyzes how neutron capture-induced nuclear recoils from ambient neutrons at reactor sites can mimic CEνNS signals in silicon and germanium detectors, emphasizing the need for neutron mitigation strategies to improve measurement sensitivity.

Contribution

It quantifies the impact of neutron capture backgrounds on CEνNS measurements at reactors and establishes neutron flux thresholds for effective detection.

Findings

Neutron flux must be below ~7×10⁻⁴ n/cm²s at 10m from a 1MW reactor for 5σ CEνNS detection.

Silicon detectors require neutron fluxes nearly ten times lower than germanium detectors for similar sensitivity.

Active veto systems can effectively reduce neutron capture-induced backgrounds.

Abstract

Nuclear reactors represent a promising neutrino source for CE$\nu$NS (coherent-elastic neutrino-nucleus scattering) searches. However, reactor sites also come with high ambient neutron flux. Neutron capture-induced nuclear recoils can create a spectrum that strongly overlaps the CE$\nu$NS signal for recoils $\lesssim$\,100\,eV for nuclear reactor measurements in silicon or germanium detectors. This background can be particularly critical for low-power research reactors providing a moderate neutrino flux. In this work we quantify the impact of this background and show that, for a measurement 10\,m from a 1\,MW reactor, the effective thermal neutron flux should be kept below $\sim$~7$\times$~10$^{-4}$\,n/cm$^2$s so that the CE$\nu$NS events can be measured at least at a 5$\sigma$ level with germanium detectors in 100~kg\,yr exposure time. This flux corresponds to 60\% of the sea-level flux but needs to be achieved in a nominally high-flux (reactor) environment. Improved detector resolution can help the measurements, but the thermal flux is the key parameter for the sensitivity of the experiment. For silicon detectors, the constraint is even stronger and thermal neutron fluxes must be near an order of magnitude lower. This constraint highlights the need of an effective thermal neutron mitigation strategy for future low threshold CE$\nu$NS searches. In particular, the neutron capture-induced background can be efficiently reduced by active veto systems tagging the deexcitation gamma following the capture.