Initial performance of large area neutron imager based on boron coated straws

US government plans to equip major seaports with large area neutron detectors, in an effort to intercept the smuggling of nuclear materials, have precipitated a critical shortage of ³He gas. This is strongly limiting the prospects of neutron science, whose annual demand of ³He is on the order of 20 kiloliters. Clearly, alternate neutron detection technologies that can support large sensitive areas, and have low gamma sensitivity and low cost must be developed. We propose a technology based on close-packed arrays of long aluminum tubes (straws), 4 mm in diameter, coated on the inside with a thin layer of ¹⁰B-enriched boron carbide (¹⁰B₄C). In addition to the high abundance of boron on Earth and low cost of ¹⁰B enrichment, the boron-coated straw (BCS) detector offers significant advantages over conventional ³He tubes in neutron imaging applications, including higher counting rate capability, parallax-free position decoding, lower cost, safer operation and a wide array of geometry configurations. A prototype imager with a 1 m² sensitive area has been developed and tested at ORNL´s High Flux Isotope Reactor (HFIR). The imager incorporates 22 detector modules (only 1 with enriched B₄C), each consisting of a 5×10 close-packed array of aluminum straws. Straws were 4 mm in diameter, and 100 cm long, and lined with a nominal 1 μm thick coating of B₄C. The applied potential was 1150 V and the gas mixture was Ar/CO₂ (90%/10%, continuous flow). Initial testing focused on detection efficiency over the range of neutron wavelengths present in the beam. Neutron time-of-flight data were collected and used to identify the energy of incident neutrons. A 30% efficiency was measured in the enriched module for 1.8 Å neutrons.