Long range active detection of HEU based on thermal neutron multiplication

We report on the results of measurements of proton irradiation on a series of targets at Brookhaven National Laboratory's (BNL) Alternate Gradient Synchrotron Facility (AGS), in collaboration with Los Alamos National Laboratory (LANL) and Sandia National Laboratories (SNL). We examined the prompt radiation environment in the tunnel for the Defense Threat Reduction Agency (DTRA) sponsored series (E-972), which investigated the penetration of air and subsequent target interaction of 4 GeV proton pulses. Measurements were made by means of an organic scintillator with a 500 MHz bandwidth system. We found that irradiation of a depleted uranium (DU) target resulted in a large gamma-ray signal in the 100-500 mu s time region after the proton flash when DU was surrounded by polyethylene, but little signal was generated if it was surrounded by boron-loaded polyethylene. Subsequent Monte Carlo (MCNPX) calculations indicated that the source of the signal was consistent with thermal neutron capture in DU. The MCNPX calculations also indicated that if one were to perform the same experiment with highly enriched uranium (HEU) target there would be a distinctive fast neutron yield in this 100-500 mu s time region from thermal neutron-induced fission. The fast neutrons can be recorded by the same direct current system and differentiated from gamma-ray pulses in organic scintillator by pulse shape discrimination. Published by Elsevier B.V.