Scattered neutron background in thermal neutron detectors

Abstract

Inelastic neutron scattering instruments require very low background; therefore the proper shielding for suppressing the scattered neutron background, both from elastic and inelastic scattering is essential. The detailed understanding of the background scattering sources is required for effective suppression. The Multi-Grid thermal neutron detector is an Ar/CO 2 gas filled detector with a 10 B 4 C neutron converter coated on aluminium substrates. It is a large-area detector design that will equip inelastic neutron spectrometers at the European Spallation Source (ESS). To this end a parameterised Geant4 model is built for the Multi-Grid detector. This is the first time thermal neutron scattering background sources have been modelled in a detailed simulation of detector response. The model is validated via comparison with measured data of prototypes installed on the IN6 instrument at ILL and on the CNCS instrument at SNS. The effect of scattering originating in detector components is smaller than effects originating elsewhere. IntroductionInelastic neutron scattering is a very powerful technique for exploring atomic and molecular motion, as well as magnetic and crystal field excitations [1]. Timeof-Flight (ToF) spectrometers allow a broad phase space to be measured in a single setting; this is typically achieved with a large area detector array [2]. In typical state-of-the-art neutron instruments [2][3][4][5][6][7][8], this detector array can be 10-50 m 2 . One of the main performance criteria of these spectrometers is typically defined by the Signal-to-Background Ratio (SBR), therefore understanding and enhancing the latter is important for the instrument optimisation. In particular, scattered neutrons have a significant contribution to the SBR. The estimation of the SBR is done currently on a series of prescriptions based on observations of historical instrument installation.As a consequence of the recent restructuring of the 3 He market [9], a need for cost effective 3 He-replacing detector solutions is raised [10], especially for inelastic neutron scattering instruments, where large area detectors with high SBR are required. A potent new solution for this type of instruments is the Multi-Grid detector [11; 12], which will be used for the three Time-of-Flight chopper spectrometers at ESS [13][14][15][16]. The Multi-Grid design was invented at the Institut Laue-Langevin (ILL) [17; 18], and the detector now is jointly developed by the ILL and the ESS within the CRISP [19] and BrightnESS [20] projects.The Multi-Grid detector is an Ar/CO 2 -filled proportional chamber with a solid boron-carbide ( 10 B 4 C) neutron converter, enriched in 10 B [21][22][23]. The basic unit of the Multi-Grid detector is the grid, an aluminium frame; thin aluminium lamellas, coated on their both sides with boron-carbide, the so called blades are placed in this frame, parallel with each other and the entrance window of the grid, dividing the grid into cells. In the detector the grids are structured into columns, and this way the cells ...

Investigation of neutron scattering in the Multi-Blade detector with Geant4 simulations

“…This item is an integral component of the detector, separates the counting gas from the detector environment and assures its operation in vacuum or atmospheric conditions. Similar studies have been performed with other detector types [48]. The values selected represent typical thicknesses used in neutron instruments, in addition to extreme values.…”

Detector rates for the Small Angle Neutron Scattering instruments at the European Spallation Source

“…With the combination of the two software packages it is possible on one hand to take into account the correlations of neutron properties, like TOF, energy, spatial and momentum vectors, and on the other hand to reliably evaluate the detector performance. The Geant4 validation of the latter is addressed elsewhere [20] and is not discussed in this study. At a pulsed source the peak rate could be more than an order of magnitude higher than the time-averaged one.…”

Event-Mode Neutron Imaging Using the TPX3Cam - Breaking the Boundaries of Conventional Neutron Imaging Techniques

“…More speci cally, g. 2A shows that independent of the intensity of the neutron eventbased data for bright or dark regions on the detector area (localized higher or lower neutron ux), the measured ux drops to the same value of ~30 neutrons/cm 2 /s at t > 30 ms and can be attributed to a constant background. This background is typical for neutron ToF measurements [22] due to the experimental cave made from concrete lling up with essentially a "neutron gas", which to a good approximation can be assumed constant throughout the entire wavelength range. The photon eventbased data in contrast shows a signi cantly different "stationary" value at t > 30 ms for bright and dark detector regions, resulting from the uorescent afterglow effect that is depended on the localized exposure of the scintillator by neutrons, inducing the consequently emitted photons.…”