A Coincidence Counter Design Based Corrugated Boron-­-Coated Straws

Previous studies [1] have reported that boron-coated straw (BCS) detectors can marginally replace 3He tubes in compact coincidence counters, like the High-Level Neutron Coincidence Counter (HLNCC-II). Here we explore thicker boron coatings in this counter leading to small decrease in efficiency but a large decrease in die away time that markedly improves the e2/t figure of merit (FOM) index. As a result the BCS-HLNCC-II can achieve significantly better performance than the 3He system through use of 800 round straws with a coating thickness of 1.75 µm. In a previous study of a larger counter [2], the Active Well Coincidence Counter (AWCC), we found that even with thicker round straw coating the BCS coincidence counter fell slightly short of the 3He tube device operating at 4 atm. In previous studies we have found that we did not fully appreciate the role of reduced die-away produced by thicker coatings. In this work we explore application of a new corrugated straw design to the AWCC. This geometry increases wall area by a factor of 1.4 compared with the round tube geometry and therefore allows much more 10B to be packed in the detector without loss of moderator which occurs by increasing round diameter. The performance of this new star-shaped detector which is being utilized currently in homeland security detectors composed of many such tubes bundled together is studied in both simulations and experiments with laboratory prototypes. Specifically, the electric field within the straw is mapped, and the corresponding electron drift time is calculated. Results show that generated signals are sufficiently fast for proper counting of neutron events, and for excellent gamma rejection. Furthermore, two BCS-based AWCC counter designs are presented, that employ 1400 corrugated BCS detectors, and maintain the overall dimensions of the 3He-based design. The FOM for a 6-point corrugation BCSAWCC is shown to be 21% higher than the 3He-AWCC commercial product, while a hypothetical 12-point design achieves 77% higher FOM compared with 3He-AWCC.