Search

The ability to classify cargos is a critical step in developing cargo dependent algorithms to improve the detection of threats like explosive or nuclear materials concealed in cargo. In a conventional pulsed neutron based active interrogation system, fast neutrons (e.g., 14 MeV) are thermalized as they travel through a moderating cargo. Once they are thermalized, they can be absorbed by the surrounding cargo. The rate at which they are absorbed (the inverse of the thermal neutron die-away) depends on the composition of the moderating cargo, especially for large cargo (e.g., pallet size). If an assessment of thermal neutron die-away times in cargos can be done externally, improved threat (e.g., explosives or SNM) detection can be achieved via different algorithms for different types of cargos. This paper shows that this can be done by measuring the time dependence of capture gamma ray production in the cargo. Index Terms--Active interrogation, cargo identification, neutron capture gamma rays, pulsed neutron techniques, SNM detection, thermal neutron die-away.

Data fusion has the potential to improve the performance of multiple systems over algorithms involving only Boolean combinations of decisions from the separate systems. To achieve this improvement, it is essential to utilize the complementary aspects of the systems involved. In this paper, we demonstrate that data fusion of x-ray radiography and neutron capture gamma ray spectroscopy provide complementary data to differential die away analysis, and thereby a substantial improvement in both detection performance and throughput. Index Terms--Contraband detection, data fusion, radiation detectors, SNM detection.

Detection and interdiction of nuclear materials in all forms of transport is one of the most critical security issues facing the United States and the rest of the civilized world. Naturally emitted gamma rays by these materials, while abundant and detectable when unshielded, are low in energy and readily shielded. X-ray radiography is useful in detecting the possible presence of shielding material. Positive detection of concealed nuclear materials requires methods which unequivocally detect specific attributes of the materials. These methods typically involve active interrogation by penetrating radiation of neutrons, photons or other particles. Fortunately, nuclear materials, probed by various types of radiation, yield very unique and often strong signatures. Paramount among them are the detectable fission signatures, namely prompt neutrons and gamma rays, and delayed neutrons gamma rays. Other useful signatures are the nuclear states excited by neutrons, via inelastic scattering, or photons, via nuclear resonance fluorescence and absorption. The signatures are very different in magnitude, level of specificity, ease of excitation and detection, signal to background ratios, etc. For example, delayed neutrons are very unique to the fission process, but are scarce, have low energy, and hence are easily absorbed. Delayed gamma rays are more abundant but 'featureless' and have a higher background from natural sources and more importantly, from activation due to the interrogation sources. The prompt fission signatures need to be measured in the presence of the much higher levels of probing radiation. This requires taking special measures to look for the signatures, sometimes leading to a significant sensitivity loss or a complete inability to detect them. Characteristic gamma rays induced in nuclear materials reflecting their nuclear structure, while rather unique, require very high intensity of interrogation radiation and very high resolution in energy and/or time. The trade off of signatures, their means of stimulation, and methods of detection, will be reviewed. Index Terms--Delayed neutrons and gamma rays, neutron fission and photofission, nuclear fission detection, prompt neutron and gamma rays, SNM detection.

The neutron slowing down time based inspection system extends the well-established differential die-away analysis (DDAA) to higher neutron energies and earlier fission neutron detection times. Simulation tools, mainly MCNPX, validated previously by experiments are used to further extend the domain of DDAA, which relies solely on thermal neutron-induced fissions, into the high-sub-MeV neutrons. Detailed time–energy correlated behavior of slowing-down neutrons, following a narrow pulse of 14-MeV neutrons, is calculated for ideal moderators like polyethylene and beryllium as well as for generic hydrogenous and metallic cargo materials. The time evolution and energy spectra of neutrons escaping the cargo-containing special nuclear materials (SNMs) (such as 233U, 235U, 239Pu, and 241Pu) are shown to be very different and easily separable from the same quantities when an SNM is absent. The key to the technique is the “vanishing time”: the time when high-energy source neutrons are scattered down to energies below those of fission neutrons. If fissile material is present, high-energy neutrons (>1 MeV) reappear solely due to fissions induced by the slowed down source neutrons, with energy below 1 MeV. The fission neutrons, which escape cargo absorption, are then detected by a direct fast neutron detector. The detectors can be, in principle, organic or 4He recoil-based scintillators, which are inherently insensitive to neutrons with lower energy than the fission neutrons. In addition, a successful implementation of this SNM inspection technique requires an intense medium-to-high duty factor narrow-pulse fast neutron generator, e.g., (d, T) or (d, D), and fast two-parameter (energy and time) data digitizers. [ABSTRACT FROM PUBLISHER]

A study is presented on the detection of illicit special nuclear materials (SNM) in cargo containers using a conceptual neutron-based inspection system with xenon-doped liquefied argon (LAr(Xe)) scintillation detectors for coincidence gamma-ray detection. For robustness, the system is envisioned to exploit all fission signatures, namely both prompt and delayed neutron and gamma emissions from fission reactions induced in SNM. However, this paper focuses exclusively on the analysis of the prompt gamma ray emissions. The inspection system probes a container using neutrons produced either by (d, D) or (d, T) in pulsed form or from an associated particle neutron generator to exploit the associated particle imaging (API) technique, thereby achieving background reduction and imaging. Simulated signal and background estimates were obtained in MCNPX (2.7) for a 2 kg sphere of enriched uranium positioned at the center of a 1\ m\times 1\ m\times 1\ m container, which is filled uniformly with wood or iron cargos at 0.1 g/cc or 0.4 g/cc. Detection time estimates are reported assuming probabilities of detection of 95% and false alarm of 0.5%. [ABSTRACT FROM PUBLISHER]

An adaptation of the differential die away (DDA) technique, which enhances the detection of fissile materials, has been developed and demonstrated in the laboratory. The differential time-of-flight (DTOF) technique applies to situations where special nuclear material (SNM) is located some distance (up to a few meters) away, is accessible from only one side, and is either bare or embedded in an environment that is slightly neutron moderating, and where the more conventional DDA pulsed neutron source technique is less effective. The technique is based on the fission reaction of thermal neutrons with fissile material, if present. The thermal neutrons are furnished by the moderation of neutrons from an electronic neutron generator (ENG). Fast neutrons are generated in relatively narrow pulses of tens to hundreds of microseconds. They are slowed down in a suitable moderator surrounding the source. The time of flight (TOF) of these neutrons, before arrival at the SNM, spreads over orders of magnitude due to the wide spread in their velocities, which lead to a time spread of the neutron-induced fission in the SNM. Neutron signatures resulting from the fissions, e.g., prompt fast neutrons (as well as few delayed neutrons) can then be detected by fast neutron detectors well after the original source neutrons have died away. [ABSTRACT FROM PUBLISHER]

Rapiscan Laboratories and Helicon Thin Film Systems have designed and developed a detector for the replacement of ^3\He-based detector modules. Our neutron detector consists of a thinly coated ^10\B flat-panel ionization chamber that can be deployed as a direct drop-in replacement for current RPM ^3\He detectors. The uniqueness of our approach in providing a large-area detector is the simplicity of construction, scalability of the unit cell detector, ease of adaptability to a variety of applications, and low cost. In the initial development, a smaller area, multi-layer prototype ^nat\B4{\C}-based ionization chamber was built and tested, demonstrating neutron detection capability and gamma-ray insensitivity. In the subsequent stage of detector development, enriched ^10\B8{\C} films were robustly deposited, passed temperature cycling testing, and were tested in the prototype chamber for neutron sensitivity. In addition to the development of the boron-coated substrates, a complete signal processing circuit, which includes a preamplifier, amplifier, and discriminator, was fabricated on a circuit board and tested within the prototype chamber. Currently, a portable detector has been tested. [ABSTRACT FROM PUBLISHER]

Liquefied noble gas (LNG) detectors have already been successfully employed in areas of fundamental particle physics research due to features such as their high energy resolution, fast response times, excellent discrimination between neutron and gamma-ray interactions, and relatively low cost. Such detectors are also attractive for nonintrusive inspection for the presence of special nuclear material (SNM) in large-scale objects such as cargo containers and trucks. An effective method of interrogation involves pulsing the object being interrogated with neutrons, which induces fission in the SNM. The fission reaction promptly releases gamma rays and neutrons. This reaction can be distinguished from background through the coincidence measurement of these particles striking multiple detectors. Rapiscan Laboratories, Yale University Physics Department, and Adelphi Technology have constructed two 18-L liquid argon prototype detectors to investigate the suitability of LNG detectors in performing this form of interrogation. The pulse shape, energy resolution, time resolution, detector efficiency, and the effects of doping with xenon were measured. [ABSTRACT FROM PUBLISHER]

MCNPX simulations have been used to guide the development of a portable inspection system for narcotics, explosives, and special nuclear material (SNM) detection. The system seeks to address these threats to national security by utilizing a high-yield, compact neutron source to actively interrogate the threats and produce characteristic signatures that can then be detected by radiation detectors. The portability of the system enables rapid deployment and proximity to threats concealed in small spaces. Both dD and dT electronic neutron generators (ENG) were used to interrogate ammonium nitrate fuel oil (ANFO) and cocaine hydrochloride, and the detector response of NaI, CsI, and \LaBr3 were compared. The effect of tungsten shielding on the neutron flux in the gamma ray detectors was investigated, while carbon, beryllium, and polyethylene ENG moderator materials were optimized by determining the reaction rate density in the threats. In order to benchmark the modeling results, experimental measurements are compared with MCNPX simulations. In addition, the efficiency and die-away time of a portable differential die-away analysis (DDAA) detector using ^3\He proportional counters for SNM detection has been determined. [ABSTRACT FROM PUBLISHER]

Abstract: Differential Die-Away Analysis (DDAA) is a sensitive technique for detecting the presence of fissile materials such as 235U and 239Pu. In DDAA, a neutron generator produces repetitive pulses of neutrons that are directed into a cargo being inspected. As each pulse passes through the cargo, the neutrons are thermalized and absorbed. The thermalization process is very rapid and the population of the source and epithermal neutrons decays away within microseconds. The population of thermal neutrons, however, decays much slower with the diffusion decay time of the inspected medium (thermal neutron “die away” time), on the order of hundreds of microseconds. If Special Nuclear Material (SNM) is present, the thermalized neutrons from the source will cause fissions that produce a new source of neutrons. These fast fission neutrons decay with a time very similar to that of the thermal neutron die-away of the surrounding cargo. The sensitivity of DDAA for a given source of neutrons critically depends on optimizing the fast/epithermal neutron detection system. Optimization involves both time response and detection efficiency. Optimized detectors were able to detect fissile material throughout pallets of cargo. [Copyright &y& Elsevier]