Boron-coated straw detectors: A novel approach for helium-3 neutron detector replacement

US efforts to equip key points of entry with large area neutron detectors to intercept Special Nuclear Materials (SNM) have been undermined by a critical shortage of ³He gas. Yearly demand for ³He in US security applications alone is roughly 22,000 liters, exceeding current world supply. Neutron science, safeguards, defense, and other applications that depend on ³He detectors have been severely limited as a result. Alternative neutron detection technologies with large sensitive areas, low gamma sensitivity, and low cost are needed to ensure the long term viability of US detection and interdiction capabilities. We propose a technology based on closely-packed arrays of long, 4 mm diameter, aluminum or copper tubes (straws), internally coated with a thin layer of ¹⁰B-enriched boron carbide, as a ready replacement for ³He in a variety of detection applications. The high abundance of boron on Earth and low ¹⁰B enrichment cost give boron-coated straw (BCS) technology key advantages over ³He detectors. We review three BCS detector configurations including: a large-area neutron-imaging panel, a long range monitor, and a detector module for portal monitors. The imaging panel has 1100 aluminum straws, each 1 m long and 4 mm in diameter. It offers a sensitive area of 1 m², 3D spatial resolution of 7×4×4 mm³, and can sustain count rates up to 200,000 cps for each of its 22 readout channels without significant loss in resolution. The long-range monitor has 1100 copper straws of similar dimensions and has been subjected to rigorous environmental testing. Finally, the portal monitor design adopts the outer dimensions of currently deployed ³He-based designs, but takes advantage of the small straw diameter to achieve more uniform distribution of neutron converter throughout the moderating material.