Michael J. King

Differential Time of Flight Technique for the Detection of Special Nuclear Materials

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. The fast neutron detectors must be insensitive to the numerous source-related thermal neutrons and are a critical component of the technique, since they can differentiate, by time as well as energy, between the fission signatures and the source-related background. The choice of source moderator is also important to the success of the technique. Two moderators were designed and studied: one made of polyethylene and the other made of beryllium (Be). The latter is superior delivering a higher flux and longer neutron die-away times. The feasibility of the DTOF technique was demonstrated by detecting a sample of 350 g 235U (in the form of 19.9% enriched uranium) at a range of distances and under a variety of conditions employing a commercial (d, T) pulsed neutron generator and the two moderators.

MCNP Simulation Benchmarks for a Portable Inspection System for Narcotics, Explosives, and Nuclear Material Detection

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 3He proportional counters for SNM detection has been determined.

Development of {sup 10}B-Based {sup 3}He Replacement Neutron Detectors

Radiation portal monitors (RPM) are currently deployed at United States border crossings to passively inspect vehicles and persons for any emission of neutrons and/or gamma rays, which may indicate the presence of unshielded nuclear materials. The RPM module contains an organic scintillator with 3He proportional counters to detect gamma rays and thermalized neutrons, respectively. The supply of 3He is rapidly dwindling, requiring alternative detectors to provide the same function and performance. Our alternative approach is one consisting of a thinly‐coated 10B flat‐panel ionization chamber neutron detector that can be deployed as a direct drop‐in replacement for current RPM 3He detectors. The uniqueness of our approach in providing a large‐area detector is in the simplicity of construction, scalability of the unit cell detector, ease of adaptability to a variety of applications and low cost. Currently, Rapiscan Laboratories and Helicon Thin Film Systems have designed and developed an operational 100 cm2 ...

Combined Photoneutron And X Ray Interrogation Of Containers For Nuclear Materials

Effective cargo inspection systems for nuclear material detection require good penetration by the interrogating radiation, generation of a sufficient number of fissions, and strong and penetrating detection signatures. Inspection systems need also to be sensitive over a wide range of cargo types and densities encountered in daily commerce. Thus they need to be effective with highly hydrogenous cargo, where neutron attenuation is a major limitation, as well as with dense metallic cargo, where x‐ray penetration is low. A system that interrogates cargo with both neutrons and x‐rays can, in principle, achieve high performance over the widest range of cargos. Moreover, utilizing strong prompt‐neutron (∼3 per fission) and delayed‐gamma ray (∼7 per fission) signatures further strengthens the detection sensitivity across all cargo types. The complementary nature of x‐rays and neutrons, used as both probing radiation and detection signatures, alleviates the need to employ exceedingly strong sources, which would ot...

Simulation Of A Photofission‐Based Cargo Interrogation System

A comprehensive model has been developed to characterize and optimize the detection of Bremsstrahlung x‐ray induced fission signatures from nuclear materials hidden in cargo containers. An effective active interrogation system should not only induce a large number of fission events but also efficiently detect their signatures. The proposed scanning system utilizes a 9‐MV commercially available linear accelerator and the detection of strong fission signals i.e. delayed gamma rays and prompt neutrons. Because the scanning system is complex and the cargo containers are large and often highly attenuating, the simulation method segments the model into several physical steps, representing each change of radiation particle. Each approximation is carried‐out separately, resulting in a major reduction in computational time and a significant improvement in tally statistics. The model investigates the effect on the fission rate and detection rate by various cargo types, densities and distributions. Hydrogenous and m...

Pulse Shape Discrimination Algorithms, Figures of Merit, and Gamma-Rejection for Liquid and Solid Scintillators

Pulse shape discrimination (PSD) in scintillators is useful to distinguish between signals from neutrons and gamma rays. A standard algorithm is to divide the integral of the tail end of the pulse by the total pulse integral, which itself yields the deposited energy. When the PSD ratio is plotted against the energy, two bands emerge for scintillators that respond differently to neutrons and gammas. Often, a figure of merit (FOM) is defined as the distance between the two bands in a particular energy range, divided by the sum of the full-widths at half-maximum of the PSD ratio bands. A high FOM is usually interpreted as a good indicator of how well a certain scintillator device can distinguish between neutrons and gammas. This, however, ignores the actual shape of the two bands, which may not have a Gaussian shape in the direction of the PSD ratio axis, especially in the presence of pulse pileup. Thus, the FOM may not say much about the gamma rejection capability of the detector, and can therefore be a little misleading. Here, we describe the results of research and development performed on a stilbene detector borrowed from the Lawrence Livermore National Laboratory, but the algorithms described were also successfully used on a number of plastic and liquid scintillation detectors. The standard algorithm described above was compared with a newly developed wavelet-based algorithm that is also used to reject pileup, as well as a pulse shape fitting approach which is shown to be capable of significantly improving the gamma-rejection ratio.

Intense Photoneutron Sources For Nuclear Material Detection

Intense neutron sources are essential for cargo inspection for a broad range of threats from explosives, to contraband, to nuclear materials and especially SNM (Special Nuclear Materials). To be effective over a wide range of cargo materials, in particular for hydrogenous cargo such as food, and to offer practical inspection times, the neutron source must be very strong, typically >1010 neutrons per second. Unfortunately there are currently no reasonably compact and economical neutron generators with the required intensities. The insufficiency and inadequacy of intense neutron sources are especially conspicuous in the ≤2.5 MeV range (low voltage (d,D) generator). This energy range is needed if the strong signature of prompt fission neutrons (≈3 per fission) is to be detected and discerned from the numerous source neutrons. The photonuclear reactions of x‐rays from commercial linacs in appropriate converters can provide ample intensities of neutrons. These converters have a very low (γ,n) energy threshold:...

Neutron Slowing Down Time Based Inspection Method

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.

Liquefied Noble Gas Detectors for Detection of Nuclear Materials

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.

Development of a Portable

Rapiscan Laboratories and Helicon Thin Film Systems have designed and developed a detector for the replacement of 3He-based detector modules. Our neutron detector consists of a thinly coated 10B flat-panel ionization chamber that can be deployed as a direct drop-in replacement for current RPM 3He 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 natB4C-based ionization chamber was built and tested, demonstrating neutron detection capability and gamma-ray insensitivity. In the subsequent stage of detector development, enriched 10B8C 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.