Proof-of-Concept Assessment of a Photofission-Based Interrogation System for the Detection of Shielded Nuclear Material

Abstract

A photonuclear interrogation method was experimentally assessed for the detection of shielded nuclear materials. Proof-of-Concept assessment was performed at the Los Alamos National Laboratory (LANL) TA-18 facility and used the INEEL VARITRON electron accelerator. Experiments were performed to assess and characterize the delayed neutron emission responses for different nuclear materials with various shield configurations using three "nominal" electron beam energies; 8-, 10-, and 11-MeV. With the exception of highly enriched uranium (HEU), the nuclear materials assessed represent material types commonly encountered in commerce. The specific nuclear materials studied include a solid 4.8-kg HEU sphere, a 5-kg multiple -object, depleted uranium (DU) [uranium with about 0.2% enrichment with U-235] target, and two 11-kg thorium disks. The shield materials selected include polyethylene, boratedpolyethylene, and lead. Experimental results, supported with numerical predictions, have shown that the photonuclear interrogation technique is quite capable of detecting shielded nuclear material via the direct measurement of the photofission-induced delayed neutron emissions. To identify or discriminate between nuclear material types (i.e., depleted uranium, HEU, and thorium), a ratio of delayed neutron counts at two different beam energies is utilized. This latter method, referred to as the dual-beam energy ratio Figure-

High Repetition Rate, LINAC-based Nuclear Resonance Fluorescence FY 2009 Final Report

“…The first consideration was how the net counts from the baseline (i.e., fully background subtracted) were obtained. The net baseline counts were calculated with C net = C Gr -C PBkg -C ABase , (12) where C Gr is the gross counts (i.e. the raw data), C PBkg is the appropriately scaled passive background, and C ABase is the active baseline from the exponential fit.…”

“…The time-dependence of delayed neutron emission has been demonstrated for isotope identification, while other photonuclear inspection techniques have been utilized to detect 100 kg of explosives contained in an automobile [8,10,11]. The Pulsed Photonuclear Assessment technology has a demonstrated ability of detecting shielded nuclear material at distances exceeding 100 m, with outdoor testing rapidly extending this standoff capability [5,[12][13][14].…”

Radiation Detection for Active Interrogation of HEU

“…10. 15 The count rate between pulses is from: (1) prompt neutrons generated from induced photo fissions in the nuclear material, (2) delayed neutrons from induced photo fission by the source that leak to the detectors, and (3) prompt neutrons from fissions induced by the delayed neutrons that do not leak from the system. The ratio of the two components to the signal depends on the fissile mass since the ratio of leakage to fission depends on mass.…”

“…4) Photofission from 4-14 MeV discrete gamma rays from a 1-2 MeV proton accelerator 9) Pulsed photonuclear assessment with a 10-20 Mev photon (bremstralung from a LINAC) induced fission 33,34 with delayed neutron and delayed gamma detector. 10) Nuclear Resonance fluorescence induced by an 8 MeV LINAC 35 and detection of gamma rays with high purity Ge detectors.…”

Radiation Detection from Fission

“…8. 15 The count rate between pulses is from: (1) prompt neutrons generated from induced photo fissions in the nuclear material, (2) delayed neutrons from induced photo fission by the source that leak to the detectors, and (3) prompt neutrons from fissions induced by the delayed neutrons that do not leak from the system. The ratio of the two components to the signal depends on the fissile mass since the ratio of leakage to fission depends on mass.…”