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 B4C 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.
Highlights
Due to the positive impact on the efficiency and SBR in the low wavelength region, the application of long blade coating is recommended for the CSPEC instrument, if it can be done at a moderate cost, and sufficient mechanical properties, and should be considered for any instrument with respect to the costs and other requirements
The intrinsic scattered neutron background is determined for the Multi-Grid detector module of the CSPEC instrument at European Spallation Source (ESS)
That the efficiency can be increased by 8–19%, and the SBR can be increased by 8–14% in the 0.4–4.0 Å wavelength region with the application of 1 μm 10B4C coating on the long blades
Summary
The Multi-Grid detector is an Ar/CO2-filled proportional chamber, consists of the so-called ‘grids’: aluminium frames divided into 2.5 × 2.5 × 1 cm cells by 0.5 mm thick aluminium ‘blades’, coated on both sides with enriched 10B4C, 97% enriched in 10B [36,37,38]. The impact of the long blade coating is indicated in the blue curve (in the ‘reference’ detector), especially in figure 4b: detection peaks and valleys appear on the inner and outer side of the long blades, respectively The cause of this phenomenon is that due to the point source, the neutrons reach the inner side of the long blades in a high incident angle, resulting in a high absorption probability in the inner coating layer (‘detection peak’). These observations indicate that the proper detector shielding has greater significance for thermal neutrons than for cold neutrons
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