Improved manufacturing and performance of the dual-sided microstructured semiconductor neutron detector (DS-MSND)

Abstract

Microstructured semiconductor neutron detectors (MSNDs) have shown to be a viable candidate for 3He detector replacements offering low cost, minimal power consumption, and high intrinsic thermal-neutron detection efficiency. MSNDs are vertically operated pνn-diodes with microfeatures etched into the semiconductor substrate that are subsequently backfilled with neutron conversion material. Charged particles emitted after a neutron is absorbed within an microfeature can interact in the adjacent semiconductor substrate, and those interactions can then be measured. Commercially produced MSNDs have an intrinsic thermal-neutron detection efficiency of approximately 30%. The dual-sided microstructured semiconductor neutron detector (DS-MSND) is a pνp-type diode, which implements microstructures on the back-side of a MSND that complement the front-side microstructures and eliminate neutron free streaming paths. The intrinsic thermal-neutron detection efficiency of DS-MSNDs was previously limited to less than 55%. The major limiting factor in detection efficiency of DS-MSNDs was determined to be the 6LiF powder packing fraction within the DS-MSND trenches. The packing fraction was previously assumed to be greater than 90%; however, recent measurements show the actual packing fraction was approximately 30%. MCNP6 simulations were performed with the updated packing fraction and showed good agreement with the detection efficiencies measured with the previous generation of detectors. A new backfilling method was developed that utilizes a mixture of two 6LiF powders with different powder particle size distributions. In the new method the powder is pressed into the DS-MSND trenches with a roller instead of using the centrifugal backfill method, which could remove previously backfilled material during the back-side trench filling centrifuge process. The new backfill method has improved the attainable 6LiF packing fraction to 55%. The new 6LiF backfilling method coupled with an improved wet etching process have yielded DS-MSNDs with intrinsic thermal-neutron detection efficiencies as high as 69.2 ± 0.8%, which matched well with updated MCNP6 simulations.