Evaluation of Lithium Gadolinium Borate Capture-Gated Spectrometer Neutron Efficiencies

Abstract

Accurate determination of neutron dose equivalent requires knowledge of the neutron energy distribution. Existing neutron spectrometers, such as Bonner sphere sets, are typically bulky and require long acquisition times. Hence, a handheld or portable system that could perform area monitoring with acceptable accuracy would be of interest. Such a device will require a detector that is efficient, and possesses gamma-neutron discrimination capabilities. An organic scintillator that contains lithium, gadolinium, and boron, all three of which possess isotopes with large neutron capture cross-sections for highly exothermic reactions, has been recently developed by MSI/Photogenics. This combination of materials provides for the detection of fast neutrons by proton recoil which when used in conjunction with a slow neutron capture gate allows total energy information to be obtained. The system also responds to gamma-rays but n-gamma discrimination techniques allow it to be applied in mixed fields. Photogenics recently completed the full demonstration of a Lithium Gadolinium Borate (LGB) neutron spectrometer's performance under a Department of Homeland Security Domestic Nuclear Detection Office (DNDO) grant. A potential application suggested for this technology is in the identification of shielded fissile materials. A 2" times 2" (50.8 mm times 50.8 mm) sample of composite scintillator, consisting of lithium gadolinium borate crystals in a plastic scintillator matrix, produced by Photogenics has been tested for this purpose. The Tests consist of verifying the n-gamma discrimination and measurements of both capture and capture gated efficiencies using <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">252</sup> Cf and Am-Li neutron sources of various activities under a variety of gamma shielding and neutron attenuation geometries. The results, for this small test sample, showed that in addition to its rudimentary spectroscopic capability, the Photogenics LGB detector has a overall neutron detection efficiency that is attractive for handheld applications.