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

J L Lacy,T D Lyons,R Nguyen,Liang Sun Liang Sun,A Athanasiades,C S Martin

doi:10.1109/nssmic.2010.5874082

Abstract

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 3 He gas. This is strongly limiting the prospects of neutron science, whose annual demand of 3 He 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 10 B-enriched boron carbide ( 10 B 4 C). In addition to the high abundance of boron on Earth and low cost of 10 B enrichment, the boron-coated straw (BCS) detector offers significant advantages over conventional 3 He 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 m 2 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 B 4 C), 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 B 4 C. The applied potential was 1150 V and the gas mixture was Ar/CO 2 (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.

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