Thomas Martin Miller

Systematic Assessment of Neutron and Gamma Backgrounds Relevant to Operational Modeling and Detection Technology Implementation

This report summarizes the findings of a two year effort to systematically assess neutron and gamma backgrounds relevant to operational modeling and detection technology implementation. The first year effort focused on reviewing the origins of background sources and their impact on measured rates in operational scenarios of interest. The second year has focused on the assessment of detector and algorithm performance as they pertain to operational requirements against the various background sources and background levels.

Foreword: 18th Topical Meeting of the ANS Radiation Protection and Shielding Division (RPSD 2014)

The 18th Topical Meeting of the American Nuclear Society’s Radiation Protection and Shielding Division (RPSD 2014) was held in Knoxville, Tennessee, on September 14–18, 2014. In total 106 technical papers were presented at RPSD 2014. This special issue of Nuclear Technology contains a sampling of the best papers from RPSD 2014, which have been expanded to full-length, peer-reviewed papers. The technical program of RPSD 2014 was broken into four broad tracks—Health Physics, Medical Physics, Radiation Transport Methods and Nuclear Data, and Shielding and Radiation Protection Applications—and the papers in this special issue represent submissions from each of these tracks. These papers cover topics from radiation protection in space and measuring secondary particle production from charged particles seen in the space environment to Monte Carlo simulations of background radiation and analog simulations of neutron detectors. Two topics that have recently formed a unique synergy, and which were well represented at RPSD 2014, are hybrid radiation transport methods and radiation transport analyses of fusion facilities. Hybrid radiation transport methods are pushing the boundaries of deep-penetration shielding analyses with detailed Monte Carlo simulations, which are needed for large facilities like ITER. This synergy is clearly illustrated by a few of the papers in this special issue. On behalf of all the RPSD 2014 organizers—Larry Townsend, General Chair; Irina Popova, Assistant General Chair; and Ahmad Ibrahim, Publications Chair—we would like to thank everyone that attended RPSD 2014 and made it such a success. We would also like to particularly thank the members of the Technical Program Committee who helped review summaries submitted to the meeting and full-length papers for this special issue of Nuclear Technology.

Computational Assessment of Naturally Occurring Neutron and Photon Background Radiation Produced by Extraterrestrial Sources

Abstract A computational assessment of the variation in terrestrial neutron and photon background from extraterrestrial sources is presented. The motivation of this assessment is to evaluate the practicality of developing a tool or database to estimate background in real time (or near–real time) during an experimental measurement or to even predict the background for future measurements. The extraterrestrial source focused on during this assessment is naturally occurring galactic cosmic rays (GCRs). The MCNP6 transport code was used to perform the computational assessment. However, the GCR source available in MCNP6 was not used. Rather, models developed and maintained by NASA were used to generate the GCR sources. The largest variation in both neutron and photon background spectra was found to be caused by changes in elevation on Earths surface, which can be as large as an order of magnitude. All other perturbations produced background variations on the order of a factor of 3 or less. The most interesting finding was that ~80% and 50% of terrestrial background neutrons and photons, respectively, are generated by interactions in Earths surface and other naturally occurring and man-made objects near a detector of particles from extraterrestrial sources and their progeny created in Earths atmosphere. This assessment shows that it will be difficult to estimate the terrestrial background from extraterrestrial sources without a good understanding of a detectors surroundings. Therefore, estimating or predicting background during a measurement environment like a mobile random search will be difficult.

Hybrid Monte Carlo/Deterministic Methods for Accelerating Active Interrogation Modeling

Abstract The potential for smuggling special nuclear material (SNM) into the United States is a major concern to homeland security, so federal agencies are investigating a variety of preventive measures, including detection and interdiction of SNM during transport. One approach for SNM detection, called active interrogation, uses a radiation source, such as a beam of neutrons or photons, to scan cargo containers and detect the products of induced fissions. In realistic cargo transport scenarios, the process of inducing and detecting fissions in SNM is difficult due to the presence of various and potentially thick materials between the radiation source and the SNM and the practical limitations on radiation source strength and detection capabilities. Therefore, computer simulations are being used, along with experimental measurements, in efforts to design effective active interrogation detection systems. The computer simulations primarily consist of simulating radiation transport from the source to the detector region(s). Although the Monte Carlo method is predominantly used for these simulations, difficulties persist related to calculating statistically meaningful detector responses in practical computing times, thereby limiting their usefulness for design and evaluation of practical active interrogation systems. In previous work, the benefits of hybrid methods that use the results of approximate deterministic transport calculations to accelerate high-fidelity Monte Carlo simulations have been demonstrated for source-detector-type problems. In this work, hybrid methods are applied and evaluated for three example active interrogation problems. Additionally, a new approach is presented that uses multiple goal-based importance functions depending on a particle’s relevance to the ultimate goal of the simulation. Results from the examples demonstrate that the application of hybrid methods to active interrogation simulations dramatically increases their calculational efficiency.

Monte Carlo Simulation for LINAC Standoff Interrogation of Nuclear Material

The development of new techniques for the interrogation of shielded nuclear materials relies on the use of Monte Carlo codes to accurately simulate the entire system, including the interrogation source, the fissile target and the detection environment. The objective of this modeling effort is to develop analysis tools and methods-based on a relevant scenario-which may be applied to the design of future systems for active interrogation at a standoff. For the specific scenario considered here, the analysis will focus on providing the information needed to determine the type and optimum position of the detectors. This report describes the results of simulations for a detection system employing gamma rays to interrogate fissile and nonfissile targets. The simulations were performed using specialized versions of the codes MCNPX and MCNP-PoliMi. Both prompt neutron and gamma ray and delayed neutron fluxes have been mapped in three dimensions. The time dependence of the prompt neutrons in the system has also been characterized For this particular scenario, the flux maps generated with the Monte Carlo model indicate that the detectors should be placed approximately 50 cm behind the exit of the accelerator, 40 cm away from the vehicle, and 150 cm above the ground. This position minimizes the number of neutrons coming from the accelerator structure and also receives the maximum flux of prompt neutrons coming from the source. The lead shielding around the accelerator minimizes the gamma-ray background from the accelerator in this area. The number of delayed neutrons emitted from the target is approximately seven orders of magnitude less than the prompt neutrons emitted from the system. Therefore, in order to possibly detect the delayed neutrons, the detectors should be active only after all prompt neutrons have scattered out of the system. Preliminary results have shown this time to be greater than 5 ?s after the accelerator pulse. This type of system is illustrative of a host of real-world scenarios of interest to nonproliferation and homeland security. Due to the multistep procedure of the MCNPX/MCNP-PoliMi code system, the analysis of somewhat modular - meaning that changing details such as the detector type, position, or surroundings does not require a re-calculation of the source-target interactions. This feature allows for efficient parametric analysis of numerous system parameters without recomputing the constant source-target behavior. Such efficient analysis mechanisms could prove invaluable in the design and future deployment of an active interrogation detection system.