Direction sensitive neutron detectors in near field measurements

Abstract

Contrary to situations in which a reactor is compromised, in emergency response situations generally the count rates are low, and neutrons may be scattered around in inhomogeneous intervening materials; in fact, both situations are typical for field operations, including emergency response missions. It is also true that neutron sensors are most efficient for the lowest energy neutrons, so it is not as easy to detect and count energetic neutrons. Most of the emergency response neutron detectors are offshoots of nuclear device diagnostics tools and special nuclear materials characterization equipment, because that is what is available commercially. These instruments mostly are laboratory equipment, and not field-deployable gear suited for mobile teams. Our goal is to design and prototype fielddeployable, ruggedized, lightweight, efficient neutron detectors.Experimental work performed in the area of neutron detector development at the Remote Sensing LaboratoryAndrews Operations (RSL-AO) sponsored by the U.S. Department of Energy, National Nuclear Security Administration (NNSA) in recent months is presented in this article. Ten pressurized 3 He tubes with an inch of annular polyethylene moderator around them are arranged to make an isosceles triangle with three fold symmetry. MCNPX simulation code has been used to obtain the angular response function of the detector array using a bare 252 Cf source. The neutron counts profile shows possibility of determining the angle of a near -field neutron source with adequate angular resolution for emergency response.The count distributions among the tubes lend themselves to detect the angular position of the neutron source with a good degree of certainty. A fundamental goal in Homeland Security and Defense Application work (HS&DA) and nuclear non-proliferation is to develop sensors that can determine the location of a possibly mobile neutron source. One common approach has been to design sensor arrays that give location dependent, composite signatures. Different configurations of sensors, such as different geometries and materials, resolve signatures differently, and for several commonly-used configurations we will develop empirical mathematical models based on sensor simulations that will resolve neutron source direction at high resolution and that will accurately detect and locate multiple sources. Initial results have shown success, and in this project we will fully develop the empirical models approach to be implemented by in-field sensors to improve detection success, efficiency, and spatial resolution and by designers to optimize future detectors for source localization.