Performance study of thin epitaxial silicon PIN detectors for thermal neutron measurements with reduced γ sensitivity

摘要

A novel approach of using thin epitaxial silicon PIN detectors for thermal neutron measurements with reducedγsensitivity has been presented. Monte Carlo simulations showed that there is a significant reduction in the gamma sensitivity for thin detectors with the thickness of 10–25μmcompared to a detector of thickness of300μm. Epitaxial PIN silicon detectors with the thickness of10μm,15μmand25μmwere fabricated using a custom process. The detectors exhibited low leakage currents of a few nano-amperes. The gamma sensitivity of the detectors was experimentally studied using a 33μCi, 662 keV,137Cs source. Considering the count rates, compared to a300μmthick detector, the gamma sensitivity of the10μm,15μmand25μmthick detectors was reduced by factors of 1874, 187 and 18 respectively. The detector performance for thermal neutrons was subsequently investigated with a thermal neutron beam using an enriched 10B film as a neutron converter layer. The thermal neutron spectra for all three detectors exhibited three distinct regions corresponding to the 4He and 7Li charge products released in the 10B-n reaction. With a 10B converter, the count rates were 1466 cps, 3170 cps and 2980 cps for the detectors of thicknesses of10μm,25μmand300μmrespectively. The thermal neutron response of thin detectors with10μmand25μmthickness showed significant reduction in the gamma sensitivity compared to that observed for the300μmthick detector. Considering the total count rate obtained for thermal neutrons with a10B converter film, the count rate without the converter layer were about 4%, 7% and 36% for detectors with thicknesses of10μm,25μmand300μmrespectively. The detector with10μmthickness showed negligible gamma sensitivity of 4 cps, but higher electronic noise and reduced pulse heights. The detector with25μmthickness demonstrated the best performance with respect to electronic noise, thermal neutron response and gamma sensitivity.