Thermal Neutron Detection Efficiency of a Gadolinium-Based Silicon Pixel Sensor

Abstract

This thesis is a "proof-of-principle" study which aims to assess the feasibility of ALPIDE as a neutron imaging sensor when coupled with a natural-Gd converter foil. The neutron detection efficiency of a Gd-based ALPIDE scheme was calculated based on experimentally collected and simulated data. With the Monte Carlo simulation toolkit Geant4, the Gd-based ALPIDE scheme was simulated in a thermal-neutron flux. Numerous simulations were performed where the scheme's and neutron source's features were slightly modified to study parameters of the neutron detection efficiency. Achieved with a back-irradiated thinned-down ALPIDE exposed to a thermal-neutron beam, the highest simulated detection efficiency was 18%. The lowest simulated detection efficiency was 5%, and achieved with a standard ALPIDE chip exposed to an isotropic thermal-neutron flux. Experimental data was collected in an approximately isotropic mixed-energy neutron and gamma flux, which included thermal neutrons. The thermal neutrons were obtained by thermalizing (with plastic blocks) the fast-neutron flux of a F-18 producing PET (Positron Emission Tomography) cyclotron. The Neutron Survey Meter A480 performed control measurements of the thermal-neutron (and fast-neutron) flux (count). Two methods were used to calculate the neutron detection efficiency of the Gd-based ALPIDE setup. The experimentally obtained neutron detection efficiency of the Gd-based ALPIDE was (3.19±0.01)% and (1.50±0.05)%. The "proof-of-principle" study confirmed neutron detection possible with the Gd-based ALPIDE.