Microfabrication of a gadolinium-derived solid-state sensor for thermal neutrons

Modelling Gd-diamond and Gd-SiC neutron detectors

A custom Monte Carlo (MC) computer model was developed to simulate thermal neutron absorption in, and subsequent photon and electron emission from, natural Gd with a view to using the material as a neutron conversion layer for neutron detectors. The MC code also modelled photon and electron detection with two dissimilar detectors: a thick (500 μm) single crystal diamond detector; and a thin (5.15 μm) commercial off the shelf (COTS) 4H-SiC photodiode detector. The detectors' quantum detection efficiencies (QE) for hard X-rays and γ-rays were relatively low in comparison to their QE for electrons, thus making it possible to collect electron spectra from the Gd layer neutron conversion products which were not overwhelmed by photon emissions from the Gd. The MC code was utilised to determine the optimal thickness of Gd for the efficient detection of a thermal neutron flux. These radiation hard and spectroscopic detectors paired with natural Gd could find utility as robust and compact thermal neutron detectors for nuclear science and engineering, space science, and other applications.

Analysis of a neutron-induced conversion electron spectrum of gadolinium

A 100-nm-thick gadolinium layer deposited on a pixelated silicon sensor was activated in a neutron field to measure the internal conversion electron (ICE) spectrum generated by neutron capture products of ¹⁵⁵Gd and ¹⁵⁷Gd. The experiment was performed at the ISIS neutron and muon facility, using a bespoke version of the HEXITEC spectroscopic imaging camera. Signals originating from internal conversion electrons, Auger electrons, x rays and gamma rays up to 150 keV were identified. The ICE spectrum has an energy resolution of 1.8-1.9 keV at 72 keV and shows peaks from the K, L, M, N+ ICEs of the 79.51 keV and 88.967 keV 2⁺-0⁺ gamma transitions from the first excited states in ¹⁵⁸Gd and ¹⁵⁶Gd, respectively, as well as the K ICEs of the 4⁺-2⁺ transitions at 181.931 keV and 199.213 keV from the respective second excited states. Spectrum analysis was performed using a convolution of a Gaussian with exponential functions at the low and high energy side as the peak shaping function. Relative ICE intensities were derived from the fitted peak areas and compared with internal conversion coefficient (ICC) values calculated from the BrIcc database. Relative to the dominant L shell contribution, the K ICE intensity conforms to BrIcc and the M, N, O+ ICE intensities are somewhat higher than expected.