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

A novel approach of using thin epitaxial silicon PIN detectors for thermal neutron measurements with reduced $\gamma$ sensitivity 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–$25\mu m$ compared to a detector of thickness of $300\mu m$. Epitaxial PIN silicon detectors with the thickness of $10\mu m$, $15\mu m$ and $25\mu m$ were 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 33 $\mu$Ci, 662 keV, ¹³⁷Cs source. Considering the count rates, compared to a $300\mu m$ thick detector, the gamma sensitivity of the $10\mu m$, $15\mu m$ and $25\mu m$ thick 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 of $10\mu m$, $25\mu m$ and $300\mu m$ respectively. The thermal neutron response of thin detectors with $10\mu m$ and $25\mu m$ thickness showed significant reduction in the gamma sensitivity compared to that observed for the $300\mu m$ thick detector. Considering the total count rate obtained for thermal neutrons with a ¹⁰B converter film, the count rate without the converter layer were about 4%, 7% and 36% for detectors with thicknesses of $10\mu m$, $25\mu m$ and $300\mu m$ respectively. The detector with $10\mu m$ thickness showed negligible gamma sensitivity of 4 cps, but higher electronic noise and reduced pulse heights. The detector with $25\mu m$ thickness demonstrated the best performance with respect to electronic noise, thermal neutron response and gamma sensitivity.