Long Range Neutron Detection: A Progress Report

The detection of neutron sources horn a considerable distance constitutes a problem that must be treated separately from the bulk of other neutron-detection applications. This report analyzes this problem, describes a number of possible approaches, and describes the design and construction of a square-meter detection system using the approach of moderator-free directional neutron detection. Although experimental results are not the focus of this report a few preliminary results are offered in the last section. Both theoretical and preliminary experimental results confirm that usefi.d detection of neutron sources for national-security applications is relatively easy at a distance of 50 meters, yet becomes somewhat challenging from a distance of 100 meters. The square-meter detection system designed for this effort was intended to be, in decreasing order of priority, optimally capable of neutron-source detection at 100 meters, lightweight and easy to use, and low in cost. Thus, the majority of design decisions were driven by the need to maximize sensitivity for remote source detection. Several surprises resulted from this design effort. First, we discovered that%, rather than cadmium or gadolinium, must be used as a shielding material. Second, we discovered that a relatively open collimator is best for remote detection. These and other design decisions are described in detail in the third section of this report. The final detector weighs roughly 45 kg and inco~orates hardware with a cost of roughly $1OOK. Of course, lighter or cheaper detection systems could be designed with some reduction in sensitivity. As designed, our l-square-meter moderator-free detection system is expected to be superior to conventional moderate-and-capture detection for some applications.