Anthony J. Peurrung

Radiation detector materials: An overview

Due to events of the past two decades, there has been new and increased usage of radiation-detection technologies for applications in homeland security, nonproliferation, and national defense. As a result, there has been renewed realization of the materials limitations of these technologies and greater demand for the development of next-generation radiation-detection materials. This review describes the current state of radiation-detection material science, with particular emphasis on national security needs and the goal of identifying the challenges and opportunities that this area represents for the materials-science community. Radiation-detector materials physics is reviewed, which sets the stage for performance metrics that determine the relative merit of existing and new materials. Semiconductors and scintillators represent the two primary classes of radiation detector materials that are of interest. The state-of-the-art and limitations for each of these materials classes are presented, along with possible avenues of research. Novel materials that could overcome the need for single crystals will also be discussed. Finally, new methods of material discovery and development are put forward, the goal being to provide more predictive guidance and faster screening of candidate materials and thus, ultimately, the faster development of superior radiation-detection materials.

Detection of moving radioactive sources using sensor networks

A variety of recent applications have led to a great interest in the development and application of sensor networks with the goal of providing more effective detection of moving radioactive sources. This paper endeavors to analyze and evaluate the costs and benefits associated with the use of a network of radiation detectors for applications involving the detection of a moving radioactive source. This analysis is restricted to the one-dimensional case, i.e., to the case where the moving source is constrained to move along a single path. It is found that the relative advantage resulting from sensor dispersal depends upon the goals, objectives, and constraints of the measurement scenario. The dispersal of sensors into a network may be advisable or required for operational reasons, but from a statistical perspective does not directly lead to improved performance in terms of detection efficiency and false detection rate.

Recent Developments in Neutron Detection

Developments in neutron detection technology during the past three years are reviewed with special emphasis on those technologies with known or possible application to safety, security, or industrial development. The technologies discussed include track detectors, image plates, activation detectors, bubble detectors, proton recoil instruments, time-of-flight instruments, moderating detectors, proportional counters, semiconductor detectors, and scintillation detectors. The review concentrates on advances in detection technology rather than on the use of existing methods for applications. For the purposes of this review, neutron spectrometry is considered to be a subset of the general area of neutron detection.

The photon haystack and emerging radiation detection technology

The resources devoted to interdicting special nuclear materials have increased considerably over the last several years in step with growing efforts to counter nuclear proliferation and nuclear terrorism. This changing landscape has led to a large amount of research and development that aims to improve the effectiveness of technology now deployed worldwide. Interdicting special nuclear materials is most commonly addressed by detecting and characterizing emitted gamma rays, but modest signature emissions can be obscured by attenuating material and must be differentiated from large and highly variable environmental background emissions. It is a daunting technical challenge to identify special nuclear materials via gamma-ray detection, but a host of new detection technologies is now emerging. This challenge motivates our review of special nuclear material signatures, the physics of detection approaches, emerging technologies, and performance metrics. The use of benchmark gamma-ray sources aids our discussion.

Neutron-gamma discrimination in plastic scintillators

Direct detection of fast neutrons prior to moderation offers increased performance at lower cost for future neutron detection technologies. Neutron detection by proton recoil in plastic scintillators could provide this capability if efficient techniques for discrimination against gamma events were available. We describe two possible approaches to neutron/gamma discrimination; one based on digital pulse processing to differentiate pulse types and the other based on low density scintillators to lengthen the time interval between multiple interactions.

Electron-Hole Pairs Created by Photons and Intrinsic Properties in Detector Materials

A Monte Carlo (MC) code has been developed to simulate the interaction of gamma-rays with semiconductors and scintillators, and the subsequent energy partitioning of fast electrons. The results provide insights on the processes involved in the electron-hole pair yield and intrinsic variance through simulations of full electron energy cascades. The MC code has been applied to simulate the production of electron-hole pairs and to evaluate intrinsic resolution in a number of semiconductors. In addition, the MC code is also able to consider the spatial distribution of electron-hole pairs induced by photons and electrons in detector materials, and has been employed to obtain details of the spatial distribution of electron-hole pairs in Ge, as a benchmark case. The preliminary results show that the distribution of electron-hole pairs exhibit some important features; (a) the density of electron-hole pairs along the main electron track is very high and (b) most electron-hole pairs produced by interband transitions are distributed at the periphery of the cascade volume. The spatial distribution and density of thermalized electron-hole pairs along the primary and secondary tracks are important for large scale simulations of electron-hole pair transport.

Detection and Location of Gamma-Ray Sources With a Modulating Coded Mask

The detection of high-energy γ-ray sources is vitally important to national security for numerous reasons, particularly nuclear materials smuggling interdiction and threat detection. This article presents two methods of detecting and locating a concealed nuclear γ-ray source by analyzing detector data of emissions that have been modulated with a coded mask. The advantages of each method, derived from a simulation study and experimental data, are discussed. Energetic γ-rays readily penetrate moderate amounts of shielding material and can be detected at distances of many meters. Coded masks are spatial configurations of shielding material (e.g., small squares formed from plates of lead or tungsten) placed in front of a detector array to modulate the radiation distribution. A coded mask system provides improved detection through an increased signal-to-noise ratio. In a search scenario it is impossible to obtain a comparison background run without the presence of a potential concealed source. The developed analysis methods simultaneously estimate background and source emissions and thus provide the capability to detect and locate a concealed high-energy radiological source in near real time. An accurate source location estimate is critically important to expedite the investigation of a high-probability γ-ray source. The experimental examples presented use a proof-of-concept coded mask system of a 4 × 4 array of NaI detectors directed at a γ-ray source in a field-of-view roughly 4 m wide × 3 m high (approximately the size of the side panel of a small freight truck). Test results demonstrate that the correct location of a radiologic source could be determined in as little as 100 seconds when the source was 6 m from the detector.

Materials Science for Nuclear Detection

The increasing importance of nuclear detection technology has led to a variety of research efforts that seek to accelerate the discovery and development of useful new radiation detection materials. These efforts aim to improve our understanding of how these materials perform, develop formalized discovery tools, and enable rapid and effective performance characterization. We provide an overview of these efforts along with an introduction to the history, physics, and taxonomy of radiation detection materials.

Induced temporal signatures for point-source detection

Detection of radioactive point sources is inherently divided into two regimes encompassing stationary and moving detectors. The two cases differ in their treatment of background radiation and its influence on detection sensitivity. Stationary detectors are limited by the statistical fluctuation of the background, while moving detectors may be subjected to widely and irregularly varying background radiation as a result of geographical and environmental variation. This significant systematic variation, in conjunction with the statistical variation of the background, requires a very conservative threshold in order to yield the same false-positive rate as the stationary detection case. This manuscript discusses a novel detector geometry that induces a unique time-encoded signature (TES) when exposed to point sources. The identification of temporal signatures for point sources using TES has been demonstrated and compared with the canonical method. This work demonstrates that temporal signatures mitigate systematic background variation and thus increase point-source detection in a moving detector system.

On the long-range detection of radioactivity using electromagnetic radiation

Abstract A series of recent publications (Tech. Phys. Lett. 19 (1993) 184; Atom. Ener. 80 (1996) 47; Atom. Ener. 80 (1996) 135; Physics–Uspekhi 168 (1998) 515; Izvestiya 28 (1992) 262; Geomagn. Aeron. 34 (1994) 229; J. Atmos. Solar-Terres. Phys. 59 (1997) 961; Stud. Geophs. Geod. 42 (1998) 197; Adv. Space Res. 20 (1997) 2173) has provided experimental evidence that radiation fields can be detected well beyond the 10–100-m limit that holds for conventional (direct) approaches for detecting radiation. The techniques that are claimed to provide this capability use remote electromagnetic interrogation to record changes atmospheric electrostatic parameters arising from elevated radiation levels. This paper examines the physics that underlies these proposed new approaches for detecting radiation. If found to be viable for applications, the proposed techniques would be highly significant as they directly address a variety of problems in national security and environmental monitoring.