Suppression of intrinsic neutron background in the Multi-Grid detector

Abstract

One of the key requirements for neutron scattering instruments is the Signal-to-Background ratio (SBR). This is as well a design driving requirement for many instruments at the European Spallation Source (ESS), which aspires to be the brightest neutron source of the world. The SBR can be effectively improved with background reduction. The Multi-Grid, a large-area thermal neutron detector with a solid boron carbide converter, is a novel solution for chopper spectrometers. This detector will be installed for the three prospective chopper spectrometers at the ESS. As the Multi-Grid detector is a large area detector with a complex structure, its intrinsic background and its suppression via advanced shielding design should be investigated in its complexity, as it cannot be naively calculated. The intrinsic scattered neutron background and its effect on the SBR is determined via a detailed Monte Carlo simulation for the Multi-Grid detector module, designed for the CSPEC instrument at the ESS. The impact of the detector vessel and the neutron entrance window on scattering is determined, revealing the importance of an optimised internal detector shielding. The background-reducing capacity of common shielding geometries, like side-shielding and end-shielding is determined by using perfect absorber as shielding material, and common shielding materials, like B$_{4}$C and Cd are also tested. On the basis of the comparison of the effectiveness of the different shielding topologies and materials, recommendations are given for a combined shielding of the Multi-Grid detector module, optimised for increased SBR.