Neutron Activation Background in the NvDEx Experiment

TL;DR

This study uses Geant4 simulations to evaluate neutron backgrounds in the NνDEx-100 experiment and finds effective shielding strategies to reduce neutron-induced events below natural radioactivity levels.

Contribution

It provides the first detailed simulation-based analysis of neutron backgrounds and optimized shielding configurations for the NνDEx-100 neutrinoless double beta decay experiment.

Findings

A 60 cm HDPE shield reduces neutron background to 0.24 events/year.

Placing HDPE between lead and steel allows for a 20 cm shield to achieve 0.15 events/year.

Neutron background can be suppressed below natural radioactivity levels with optimized shielding.

Abstract

An extremely low-background environment is a crucial requirement for any neutrinoless double beta decay experiment. Neutrons are very difficult to stop, because they can pass through the shields and activate nuclei in the detector, even inside the fiducial volume itself. Using Geant4 simulations we have studied the neutron background for N$\nu$DEx-100 and the most efficient way to reduce it. Using a 60 cm thick external HDPE shield the neutron background can be reduced down to $0.24\pm 0.06$ events/year, lower than the background rate due to natural radioactivity (0.42 events/year), which was used as a benchmark for these calculations. The amount of shielding material needed can be significantly reduced by placing HDPE in the empty space between the lead shield and the steel vessel; in this way, it is sufficient to add 20 cm external HDPE shield to reduce the neutron background down to $0.15\pm0.05$ events/year.