Intrinsic noise of a superheated droplet detector for neutron background measurements in massively shielded facilities

¹ Centro de Ciências e Tecnologias Nucleares, Instituto Superior Técnico, Universidade de Lisboa, 2695-066 Bobadela, Portugal
² Laboratoire Souterrain à Bas Bruit, University of Nice, University of Avignon, Centre National de la Reserche Scientifique, Aix-Marseille University, Observatoire de la Côte d’Azur, 84400 Rustrel, France
³ Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal

^* Corresponding author: anafer@ctn.tecnico.ulisboa.pt

Published online: 25 September 2017

Abstract

Superheated droplet detectors are a promising technique to the measurement of low-intensity neutron fields, as detectors can be rendered insensitive to minimum ionizing radiations. We report on the intrinsic neutron-induced signal of C₂ClF₅ devices fabricated by our group that originate from neutron- and alpha-emitting impurities in the detector constituents. The neutron background was calculated via Monte Carlo simulations using the MCNPX-PoliMi code in order to extract the recoil distributions following neutron interaction with the atoms of the superheated liquid. Various nuclear techniques were employed to characterise the detector materials with respect to source isotopes (²³⁸U, ²³²Th and ¹⁴⁷Sm) for the normalisation of the simulations and also light elements (B, Li) having high (α, n) neutron production yields. We derived a background signal of ~10^-3 cts/day in a 1 liter detector of 1-3 wt.% C₂ClF₅, corresponding to a detection limit in the order of 10^-8 n cm^-2s^-1. Direct measurements in a massively shielded underground facility for dark matter search have confirmed this result. With the borosilicate detector containers found to be the dominant background source in current detectors, possibilities for further noise reduction by ~2 orders of magnitude based on selected container materials are discussed.