ALTERNATIVE NEUTRON DETECTORS TO ELIMINATE THE USE OF 3He FOR HOMELAND SECURITY, NONPROLIFERATION, AND OTHER APPLICATIONS

The He supply crisis and the associated shortage of He-based detectors is currently the highest priority for the global radiation detection communities - including homeland security, nuclear nonproliferation, defense, and the neutron-detection-based research community. 10B and 6Li, via the 10B(n,a)7Li and 6Li(n,t)a reactions, are the most likely alternative neutron converters for use in thermal neutron detectors. A phenomenon known as the “wall effect” is, however, of specific importance in the replacement of a gas neutron converter like 3He by a solid converter in a conventional proportional counter design. Simply put, the “wall effect” is the loss of a portion of the signal amplitude and the detection efficiency due to the stopping of the charged particles produced in the solid 6Li or 10B thermal neutron conversion reactions by the detector wall itself. Any detector where the solid neutron converter simply coats or lines the detector chamber wall will suffer from this effect. Our work seeks to essentially eliminate the wall effect by overcoming the materials science challenges associated with the fabrication of thin neutron converter plates where the charged particles produced by the converter material can escape and enter the gas chamber from both faces of the converter foil.. We have successfully tested a 6Li- metal-lined neutron detector that was built as a proof-of-concept device to show that, in fact, a neutron detector could be fabricated that utilized metallic 6Li in a gas-based detector that employed a neutron converter other than 3He. This detector was tested with a moderated Am-Li neutron source producing approximately 100 n/cm2/s, a 2-inch-thick Pb shield, and a Cd shield. We now report on our ongoing work on advanced detector designs that have the potential for higher detection efficiencies, better gamma-ray/neutron discrimination, and improved robustness through the application of a more efficient geometry that will permit the use of a more stable, non-metallic form of 6Li as well as B-containing compounds.