N2C1  Instrumentation for Homeland and National Security, Active Interrogation of SNM

Tuesday, Nov. 3  14:00-16:00  Town and Country

Session Chair:  Clair Sullivan, University of Illinois-Nuclear, Plasma, and Radiological Eng, United States; Dave Petersen, Defense Threat Reduction Agency, United States

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(14:00) N2C1-1, Differentiating HEU Assemblies with Similar Mass, Multiplication, and/or Reflectors Using Active Neutron Interrogation

D. L. Chichester, S. M. Watson, J. T. Johnsons

Idaho National Laboratory, Idaho Falls, Idaho, USA
Active neutron interrogation can be used to verify the presence or absence of special nuclear material (SNM) within sealed containers. Under some circumstances it can also be used to characterize SNM within sealed containers. An experimental scoping study has been performed to assess some operational capabilities for using active neutron interrogation to inspect containers holding highly-enriched uranium (HEU) with the goal of differentiating these assemblies with respect to the mass of HEU, the multiplication level of the assembly, and the near-field reflector environment surrounding the HEU. Two HEU mass values were studied in the effort: 5.49 kg and 14.35 kg, the materials were in the form of right circular cylinders. Multiplication ranges considered in this effort ranged from 1.75 to 8.90. Reflectors considered in this effort include 5 cm thick shells of polyethylene and tungsten, as well as cases with no reflector. The combined analysis of die-away neutron and inter-pulse ß-delayed neutrons was found to be a powerful pair of signatures for discriminating between variations in these three parameters. Using these two signatures it appears likely that deviations in either the HEU mass, geometry, or reflector could be positively detected

(14:20) N2C1-2, Fast-Neutron Elastic-Scatter Imaging for Material Characterization

M. A. Blackston, P. A. Hausladen

Oak Ridge National Laboratory, Oak Ridge, TN, USA
Fast-neutron associated-particle imaging (API) using neutrons created via the deuterium-tritium (DT) fusion reaction is a powerful technique for imaging the internal geometry of target items, even if they contain large amounts of shielding. Although traditional neutron transmission radiography and tomography done using API give detailed information about the geometry, they provide no information about the material content of items under inspection. However, a considerable amount of material-specific information is available in the neutrons that elastically scatter in the target but are still detected. Because the API technique provides knowledge of the time and initial direction of 14 MeV neutrons produced in the DT reaction, the angular deviation and time of flight of detected neutrons can be used to identify elastically scattered neutrons and to infer information about materials along the initial neutron path. The most distinctive elastic-scatter signal exists for materials with low mass numbers (A), so initial development has focused on producing low-A images. This paper describes the elastic-scatter imaging approach; describes the API system, which uses an electronic DT neutron generator and highly pixelated plastic scintillator neutron detectors; describes analysis and image reconstruction approaches with a focus on elastic scatters from low-A nuclides; and presents the first elastic-scatter images from measurements.

(14:40) N2C1-3, Neutron and Gamma Ray Coincidence Measurements of a High-Multiplication, Subcritical Assembly of Weapons-Grade Plutonium

J. M. Mueller, J. Mattingly

Dept. of Nuclear Engineering, North Carolina State University, Raleigh, NC, USA
Measurements of neutrons and gamma rays emitted from subcritical, but highly multiplying, assemblies of plutonium and uranium were performed at the Device Assembly Facility located at the Nevada Nuclear Security Site. These measurements are the first ever conducted by an academic institution of these sources. The plutonium assembly is approximately 4.5 kg of 94% 239Pu weapons-grade plutonium metal, and the uranium assembly is approximately 13.7 kg of 93% enriched uranium metal. Data were collected using two EJ-309 liquid scintillators, two EJ-299 plastic scintillators, one CeBr3 detector, and one NaI detector. Each pulse from the detectors was digitized using a CAEN digitizer, and the data acquisition software was modified to reduce dead time associated with the high throughput from significant trigger rates. These measurements enable novel comparisons of pulse height spectra and correlations, such as neutron-neutron angular correlations or time-correlated pulse height analyses, with Monte Carlo simulations such as MCNP-PoliMi.

(15:00) N2C1-4, Characterization of Neutron Contribution in a Monoenergetic Gamma Ray Radiography System for Detection of Nuclear Materials

J. M. Rahon, A. Danagoulian, Z. S. Hartwig, R. C. Lanza, H. Lee, T. D. MacDonald, B. E. O'Day, B. Osmanov, J. R. Vavrek

Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA
Detecting clandestine cross-border shipping of shielded special nuclear materials (SNM) is presently a difficult task requiring fissile material confirmation and localization. Highly enriched uranium lacks a strong neutron or gamma natural signature, increasing its difficulty to be passively detected. A monoenergetic gamma transmission radiography system is a strong candidate to provide adequate materials discrimination while maintaining low dose levels in the event of stowaways or sensitive cargo. Transmission ratios of multiple monoenergetic gamma lines resulting from the 11B(d,n?)12C reaction can be employed to detect shielded high-Z materials. Inherent in this nuclear reaction is the production of fast neutrons which may enable neutron radiography and further characterization of the effective-Z of the cargo. These primary neutrons downscatter into a wide range of energies, which are joined by photonuclear neutrons produced from the 15.1 MeV 12C de-excitation and delayed neutrons from induced fissions. While primary and secondary neutron production could be used in additional SNM scanning techniques, the neutrons also induce secondary gammas via capture and inelastic scattering reactions. Careful characterization of the neutron-induced gamma spectrum is critical to accurately and consistently analyze the monoenergetic peak ratios in gamma radiography. We present neutron radiography techniques to ascertain low-Z container content, as well as a characterization of the secondary neutron background which may enable the use of active interrogation techniques via the detection of direct or delayed photofission neutrons.

(15:20) N2C1-5, Monoenergetic Gamma Ray Radiography System for Detection of Shielded Nuclear Materials

T. D. MacDonald, A. Danagoulian, Z. S. Hartwig, R. C. Lanza, H. Y. Lee, B. E. O'Day, B. Osmanov, J. M. Rahon, J. R. Vavrek

Nuclear Sciences and Engineering, Massachusetts Institute of Technology, Boston, MA, USA
The field of nuclear security addresses the risks posed by the smuggling of nuclear weapons and seeks to prevent an act of nuclear terrorism. A primary anti-smuggling efforts is to detect special nuclear materials (SNM, e.g. U, Pu, Th) or a nuclear device hidden and shielded amongst benign commercial cargo. This is a particularly challenging for shielded highly enriched uranium (HEU) which has an inherently weak natural radioactive signature. We describe the development of a monoenergetic ~MeV gamma transmission radiographic imaging system for the detection of shielded SNM. The large difference in pair production cross-sections between low-Z and high-Z materials can be exploited to independently determined cargo density and effective-Z to infer the presence of SNM. Monoenergetic gammas (4.4 and 15.1 MeV) were produced using the 11B(d,n?)12C reaction, and their transmission through cargo is measured with an array of NaI detectors. Initial experimental results demonstrate the proof of concept of simultaneous density and effective-Z imaging for the detection of shielded SNM. The data from the detector array clearly demonstrates dependence of the detected spectroscopic signal on the Z of the test cargoes with the ability to reliably distinguish between high- and medium-Z materials. An additional advantage of this method is the dose rate is orders of magnitude lower than conventional dual energy electron bremsstrahlung sources. This effort was a part of a collaboration with PSU and the Georgia Tech which are pursuing advanced detector concepts that can be integrated into a fieldable active interrogation system. Preliminary experimental measurements and simulations demonstrate the feasibility of using dual-monoenergetic gamma radiography transmission to determine the areal density and effective atomic number of interrogated cargo, which would allow to detect shielded SNM/HEU. Preliminary results and data will be presented, and the overall technique will be discussed.

(15:40) N2C1-6, MCNP Simulation of Discrete Gamma-Ray Spectra for PGNAA Applications

E. H. Seabury, C. J. Wharton, A. J. Caffrey

Idaho National Laboratory, Idaho Falls, ID, U.S.A
Prompt Gamma Neutron Activation Analysis (PGNAA) systems, such as Idaho National Laboratory’s PINS system [1], are used by the U.S. Army to identify chemical warfare materiel (CWM) and explosives in recovered munitions. The wide variety of fill chemicals and assessment conditions for these munitions makes Monte Carlo modeling of the gamma spectra attractive in order to assess the impact of these variations on identification algorithms as have recent improvements to the prompt gamma production in the nuclear data libraries for low-Z chemical elements. The availability of High Performance Computing (HPC) systems makes these simulations more rapid than laboratory measurements, and can be very useful provided the codes and associated data libraries accurately reproduce the laboratory measurements. We present here a comparison of simulation results using the MCNP6 code [2] and the ENDF/B-VII libraries with laboratory measurements using two different neutron sources, a californium-252 source and a deuterium-deuterium (DD) neutron generator