Timothy Shaw

A whole-system approach to x-ray spectroscopy in cargo inspection systems

The bremsstrahlung x-ray spectrum used in high-energy, high-intensity x-ray cargo inspection systems is attenuated and modified by the materials in the cargo in a Z-dependent way. Therefore, spectroscopy of the detected x rays yields information about the Z of the x-rayed cargo material. It has previously been shown that such ZSpectroscopy (Z-SPEC) is possible under certain circumstances. A statistical approach, Z-SCAN (Z-determination by Statistical Count-rate ANalysis), has also been shown to be effective, and it can be used either by itself or in conjunction with Z-SPEC when the x-ray count rate is too high for individual x-ray spectroscopy. Both techniques require fast x-ray detectors and fast digitization electronics. It is desirable (and possible) to combine all techniques, including x-ray imaging of the cargo, in a single detector array, to reduce costs, weight, and overall complexity. In this paper, we take a whole-system approach to x-ray spectroscopy in x-ray cargo inspection systems, and show h...

Implementation of Noise Spectroscopy Using Biased Large-Area Photodiodes

The bremsstrahlung X-ray spectrum produced by X-ray sources used in cargo inspection systems is attenuated and modified by materials in the cargo in a Z-dependent way and then detected in a radiographic detector array. We have previously shown that it is possible to obtain spectral information indirectly by analyzing statistical fluctuations (the “noise”) in radiographic data, in a technique we call Noise Spectroscopy (NS) or Z-SCAN (Z-determination by Statistical Count-rate ANalysis). The technique is especially effective when the detector array consists of fast scintillation detectors and waveform digitization electronics, measuring both the waveform mean and variance during each X-ray pulse. This previous work, however, used photo-multiplier tubes, which are not especially suitable for a practical implementation. Here we describe the results of R&D performed to produce an effective version of the technique that lends itself to implementation in a single detector array that can be used both for radiography and for NS, with the same imaging spatial resolution as in conventional systems. We characterize the performance of biased photodiodes in combination with trans-impedance preamplifiers, read out with commercially available waveform digitizers. We report on experiments performed with a 16-channel detector array in a test beam, with test samples of different atomic number Z. We also report the implementation of an NS algorithm in an FPGA. In combination, we show that a practical implementation of Noise Spectroscopy in cargo inspection systems is feasible.

Thermal Neutron Die-Away Studies in a 14 MeV Neutron-Based Active Interrogation System

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.

Improvement of SNM Detection Performance by Fusion of Data From Multiple Inspection Systems

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.

Fissile Material Detection by Differential Die Away Analysis

Detection and interdiction of Special Nuclear Material (SNM) in transportation is one of the most critical security issues facing the United States. Active inspection by inducing fission in fissile nuclear materials, such as 235U and 239Pu, provides several strong and unique signatures that make the detection of concealed nuclear materials technically very feasible. Differential Die‐Away Analysis (DDAA) is a very efficient, active neutron‐based technique that uses the abundant prompt fission neutrons signature. It benefits from high penetrability of the probing and signature neutrons, high fission cross section, high detection sensitivity, ease of deployment and relatively low cost. DDAA can use any neutron source or energy as long as it can be suitably pulsed. The neutron generator produces pulses of neutrons that are directed into a cargo. As each pulse passes through the cargo, the neutrons are thermalized and absorbed. If SNM is present, the thermalized neutrons create a new source of (fission) neutrons with a distinctive time profile. An efficient laboratory system was designed, fabricated and tested under a US Government DHS DNDO contract. It was shown that a small uranium sample can be detected in a large variety of cargo types and configurations within practical measurement times using commercial compact (d,T) sources. Using stronger sources and wider detector distribution will further cut inspection time. The system can validate or clear alarms from a primary inspection system such as an automated x‐ray system.Detection and interdiction of Special Nuclear Material (SNM) in transportation is one of the most critical security issues facing the United States. Active inspection by inducing fission in fissile nuclear materials, such as 235U and 239Pu, provides several strong and unique signatures that make the detection of concealed nuclear materials technically very feasible. Differential Die‐Away Analysis (DDAA) is a very efficient, active neutron‐based technique that uses the abundant prompt fission neutrons signature. It benefits from high penetrability of the probing and signature neutrons, high fission cross section, high detection sensitivity, ease of deployment and relatively low cost. DDAA can use any neutron source or energy as long as it can be suitably pulsed. The neutron generator produces pulses of neutrons that are directed into a cargo. As each pulse passes through the cargo, the neutrons are thermalized and absorbed. If SNM is present, the thermalized neutrons create a new source of (fission) neutro...

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...

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:...