The US and the Russian Federation are currently engaged in negotiating or implementing several nuclear arms and nuclear material control agreements. These involve placing nuclear material in specially designed containers within controlled facilities. Some of the agreements require the removal of nuclear components from stockpile weapons. These components are placed in steel containers that are then sealed and tagged. Current strategies for monitoring the agreements involve taking neutron and gamma radiation measurements of components in their containers to monitor the presence, mass, and composition of plutonium or highly enriched uranium, as well as other attributes that indicate the use of the material in a weapon. If accurate enough to be useful, these measurements will yield data containing information about the design of the weapon being monitored. In each case, the design data are considered sensitive by one or both parties to the agreement. To prevent the disclosure of this information in a bilateral or trilateral inspection scenario, so-called information barriers have evolved. These barriers combine hardware, software, and procedural safeguards to contain the sensitive data within a protected volume, presenting to the inspector only the processed results needed for verification. Interlocks and volatile memory guard against disclosure in case of failure. Implementing these safeguards requires innovation in radiation measurement instruments and data security. Demonstrating their reliability requires independent testing to uncover any flaws in design. This study discusses the general problem and gives a proposed solution for a high resolution gamma ray detection system. It uses historical examples to illustrate the evolution of other successful systems.
A field-portable instrument has been developed for the purpose of classifying and identifying chemical agents within munitions by measurement of the linear attenuation property of the agent. For most chemical agents of interest, the gamma-ray linear attenuation coefficient is sufficient to separate the agents by class (nerve, blister, and blood). In addition, many chemical agents of a particular class are separable by the attenuation coefficient. Complications in gamma ray transmission measurements arise due to the packaging of the chemical agents in thick-walled steel containers of various configurations, corrosion, and changes in the state of the material from liquid to solid or gas. Identification of chemical agents within a container and without imaging is contingent upon sampling a region of the container that has a homogeneous distribution of the agent. The instrument allows for several degrees of freedom to accommodate multiple data acquisition protocols, including tomographic imaging. A variety of algorithms have been investigated including single-ray transmission to complete 2D-computed tomography using a collimated isotopic source. Recent results indicate that gamma ray measurements can provide identification of chemical agents in reasonable time frames. This paper will describe the system, data acquisition and processing, and present results from laboratory and field studies.
The Research Institute of Pulse Techniques, in collaboration with the Proliferation Prevention and Arms Control Program at Lawrence Livermore National Laboratory, has constructed a gamma-ray camera for use in arms control agreements such as Mutual Reciprocal Inspections and Warhead Dismantlement Transparency. The camera is designed to have high efficiency (in order to reduce inspection times), moderate resolution (to decrease the intrusiveness of the measurements), and sturdy construction (to allow operation in the types of conditions that might be met during shipment and use at various forward weapons sites). The imaging element consists of a honeycomb or soda-straw lead collimator and a 312-mm- diameter Nal(Tl) scintillator viewed by an array of phototubes. Software was developed to display 2- and 3-D views of the data and to analyze shape and peak areas. The first model was tuned for plutonium radiation in the 375- to 415-keV energy range. Images from various arrays of point sources were obtained and will be presented.
Monitoring nuclear materials that is dangerously radioactive, remotely located, or difficult to access is a challenging task. The necessary research required to develop a system capable of remotely monitoring radioactive materials has been undertaken at Radiation Monitoring Devices, Inc. We report on a system utilizing a spectroscopic gamma-ray imager for real-time observation of sensitive nuclear materials over the Internet or dedicated networks. Research at RMD has produced a spectroscopic gamma-ray imager centered on a position-sensitive photomultiplier tube coupled to scintillation crystal and a coded aperture. A gamma-ray intensity pattern from the detector is stored and processed by a portable computer workstation and then mathematically corrected to yield the original radiation-source image. The pseudo-color, radiation-source image is overlaid on a co-registered video picture of the same area captured by a high-resolution charge-coupled device. The combined image is displayed as an accurate map of gamma-ray sources in the physical environment. Recent developments involve instrument control and data transmission through computer networks. Alarm triggers based on changes in the video image, the radiation image, the energy spectrum are under development. Work to remotely control alarm sensitivity and type, as well as the image update frequency, has also been examined.
We are currently developing a high-energy (10 - 15 MeV) neutron imaging system for use in NDE applications. Our goal is to develop an imaging system capable of detecting cubic- mm-scale voids or other structural defects in heavily- shielded low-Z materials within thick sealed objects. The system will be relatively compact (suitable for use in a small laboratory) and capable of acquiring tomographic image data sets. The design of a prototype imaging detector and multi-axis staging system will be discussed and selected results from recent imaging experiments will be presented. The development of an intense, accelerator-driven neutron source suitable for use with the imaging system will also be discussed.
We have investigated the utility of employing a short (<4 ns) pulsed laser with wavelength tunable between 600 - 950 nm as a tool for studying and characterizing CdZnTe detectors. By using a single mode optical fiber and simple optics, we can focus the beam to a spot size of less than 10 micrometers and generate the number of the excess carriers equivalent to a several MeV gamma-ray either at the surface or deep inside the sample. The advantages of this technique over use of a collimated X-ray or alpha particle source are strong induced signal, precise pointing, and triggering capability. As examples of using this technique, we present the results of measurements of the drift velocity, electron lifetime, and electric field line distribution inside CZT pixel detectors.
Two methods are presented for obtaining excellent spectroscopic performance and good photo-peak efficiencies for CdZnTe coarsely segmented pad and strip detectors. The `correlation method' utilizes the correlation between the signals in the planar and segmented electrodes. This correlation provides an indication of the point of interaction. Application of a correction which is based on this correlation generates spectra with a resolution of 1.4% FWHM at 356 keV for pad detectors. Furthermore, for strip detectors this method yields improved spectra and an increase of more than 35% in photo-peak efficiencies at 140.5 keV. The `compensation method' involves tuning the degree of electron trapping to compensate for the almost completely trapped holes. Contrary to planar detectors, for segmented detectors the compensation condition yields induced charge signals which are 80% of the maximum signal. Using this method we obtained a spectrum with energy resolution of 3.5% FWHM at 122 keV and a peak-to-valley ratio of approximately 100:1.
Preliminary results from Gamma ray experiment installed on a micro-satellite, Techsat 1, are reported. The experiment is based on CdZnTe detectors coupled to custom designed CMOS electronics, which includes low noise charge sensitive preamplifiers, pulse shaping amplifiers and sampling circuits. It was realized as a mile stone towards a micro- satellite mounted Gamma ray space telescope. The experiment is a stand-alone spectroscopy system that measures the radiation inside the micro-satellite and transmits the spectra to ground station via the main satellite computer. The radiation level inside micro-satellites is expected to be significantly lower compared to that inside large satellites. Additional goal of the experiment is to test the CdZnTe detectors and the front-end electronics, implemented in a standard CMOS process, under space radiation environment. In particular, the degradation in performance will be monitored. The Techsat 1 micro-satellite has been designed and constructed at Technion-Israel Institute of Technology. The satellite is approximately 50 X 50 X 50 cm-3 cube with a total weight of about 50 kilograms. It was successfully launched in July 1998 to a 820 km orbit.
A prototype Compton camera using ambient-temperature semiconductor detectors is developed for gamma ray spectroscopic imaging. Two camera configurations are evaluated, one using an intrinsic silicon detector for the front plane detector and the other using a CdZnTe detector for the front plane. Both configurations use a large-volume coplanar grid CdZnTe detector for the back plane. The effect of detector noise, energy resolution, and timing resolution on camera performance is described. Technical issues underlying the development of Compton cameras for spectroscopic imaging are presented and imaging of radioactive sources is demonstrated.
The Spallation Neutron Source (SNS) under construction at the oak Ridge National Laboratory will be the most important new neutron scattering facility in the United States. Neutron scattering instruments for the SNS will require large area detectors with fast response (< 1 microsecond), high efficiency over a wide range of neutron energies (0.1 to 10 eV), and low gamma ray sensitivity. We are currently developing area neutron detectors based on a combination of 6LiF/ZnS scintillator screens coupled to a wavelength- shifting fiber optic readout array. A 25 X 25-cm prototype detector is currently under development. Initial tests at the High Flux Isotope Reactor have demonstrated good imaging properties coupled with very low gamma ray sensitivity. In addition, we have developed a multi-layer scintillator/fiber detector to replace existing He-3 gas detector tubes for higher speed operation. This detector has demonstrated a neutron detection efficiency of over 75% at a neutron energy of 0.056 eV or about twice thermal. The response time of this detector is approximately 1 microsecond. Details of the design and test results of both detectors will be presented.
High-energy X rays provide the capability for examining the interior of large, complex objects, measuring densities and dimensions, finding flaws, and detecting contraband. Although various types of X-ray imaging systems have been in use for some time, recent developments have greatly extended the envelope of capabilities. Two ARACOR X-ray vision systems will be discussed that offer new and advanced capabilities for contraband detection and nondestructive evaluation. The Eagle is a new mobile, transportable, high- efficiency X-ray imaging system designed for inspection cargo and detecting drugs, explosives and weapons at seaports, airports and border crossings. ARACOR's line of industrial computed tomography systems provide quantitative 3D X-ray images for such applications as the inspection of Minuteman and Peacekeeper solid rocket motors, the safety and security of nuclear weapons, and metrology and failure studies of automobile components and castings. Newly developed software enables the accurate reverse engineering of complex parts to form CAD descriptions and the direct input of image data into rapid prototyping systems for the production of replacement parts.
Experiments have been conducted in Sarov, Russia, with a dynamic radiographic system designed to establish the volume of an imploding 14-mm-diameter tungsten cylinder. Images were formed using a 65-MeV gamma source, lutetium oxyorthosilicate doped with cerium (LSO:Ce) radiation-to- light converter, and a fiber optic imaging bundle. Three radiographs were recorded in the course of approximately 2 microseconds using an electronic streak camera with intensified CCD readout. Significant improvements in system performance were achieved over lens coupling of components by the introduction of coherent fiber optics.
An overview of the radiographic capabilities with emphasis on electronic image detection and processing at the Los Alamos National Laboratory is presented. Fixed facilities and portable x-ray sources and imaging systems make up the Los Alamos capability. Examples of imaging with large area amorphous silicon imaging panels, a portable computed tomography system, high speed x-ray imaging applications and equipment, and small area, high resolution imagers are given. Radiographic simulation and reverse engineering from radiographic images to computer aided design files and solid models is also presented.
This paper examines the potential role of thermal neutron analysis (TNA) in unexploded ordnance (UXO) and mine detection. The results of two different field tests for two different TNA systems are presented. The two tests differed greatly in intent and scope: one test focused on the application of TNA to UXO remediation, where the targets of interest ranged from 105-mm projectiles containing about 610 g of nitrogen down to 20-mm rounds containing less than 5 g of nitrogen; the second test focused on the application of TNA to antitank landmine detection, where the targets ranged in size from 1 to 3 kg. The first test clearly demonstrated a proof-of-concept, but the performance fell short of the values needed to make the system operationally useful. Hardware upgrades are suggested to improve performance, with an emphasis on HPGe detectors to eliminate the problems introduced by the Si29 background. The performance of the second TNA system in the second test also fell short of what would be desired in an operational system, although all results from that test have an uncertainty associated with them because of the small sample size involved. The hardware upgrades discussed for the first system would also be applicable to the second system.
We have previously reported on a nuclear technique, the Nitrogen Camera, that has produced images of elemental nitrogen in concentrations and with surface densities typical of buried plastic anti-personnel mines. Since then we have been developing (1) enabling technologies, with most important of which is a mobile 70 MeV light source; (2) computer simulations to determine detection sensitivities with burial depth, explosives content, detector configurations, etc; and (3) deployment practices for both humanitarian and military field operations. Here we review the technique and summarize the state of the implementation.
The implementation of a backscattered x-ray landmine detection system has been demonstrated in laboratories at both Sandia National Laboratories (SNL) and the University of Florida (UF). The next step was to evaluate the modality by assembling a system for fieldwork and to evaluate the systems performance with real laboratories. To assess the system's response to a variety of objects, buried simulated plastic and metal antitank landmines, surface simulated plastic antipersonnel landmines, and surface metal fragments were used as targets for the field test. The location of the test site was an unprepared field at SNL. The tests conducted using real landmines were held at UF using various burial depths. The field tests yielded the same levels of discrimination between soil and landmines that had been detected in laboratory experiments. The tests on the real landmines showed that the simulated landmines were a good approximation. The real landmines also contained internal features that would allow not only the detection of the landmines, but also the identification of them.
A thermal neutron analysis (TNA) detector was developed as a confirmatory sensor for the Canadian ILDP multisensor teleoperated vehicle mounted land mine detector system. ILDP is the only multisensor mine detection system with a confirmation sensor which can reduce false alarms to acceptable levels. The TNA has been experimentally proven to be capable of detection of anti-tank and large anti- personnel mines in acceptable short time periods in its intended role. It has performed well, in extreme climates, in Canadian and U.S. stand-alone tests and in U.S. tests of the complete ILDP system. Current research is aimed at developing a version which is ready for production and field.
The FRIS/PINS hybrid integrates the LLNL-developed Field Radionuclide Identification System (FRIS) with the INEEL- developed Portable Isotopic Neutron Spectroscopy (PINS) chemical assay system to yield a combined general radioisotope, special nuclear material (SNM), and chemical weapons/explosives (CWE) detection and identification system. The PINS system uses a neutron source and a high- purity germanium (gamma) -ray detector. The FRIS system uses an electromechanically cooled germanium detector and its own analysis software to detect and identify SNM and other radioisotopes. The FRIS/PINS combined system also uses the electromechanically-cooled germanium detector. There is no other currently available integrated technology that can combine an active neutron interrogation and analysis capability for CWE with a passive radioisotope measurement and identification capability for SNM.
A neutron generator-based on-line coal analysis system has been developed, capable of measuring the major and minor chemical elements contained in coal. The system utilizes nuclear reactions produced from fast and thermal neutrons, as well as from neutron activation of isotopes with half- lives of seconds or minutes. Characteristic gamma rays detected with BGO (bismuth germanate) detectors are used for the identification of the various chemical elements. A key feature of the analyzer is its ability to analyze automatically three distinct gamma-ray spectra, and produce the elemental content of coal as it moves through a coal chute. A prototype analyzer has been built, able to analyze several tons/hour of coal. The main features of the analyzer are self-calibration independent of the coal seam, better accuracy in the determination of elements such as carbon, oxygen, and sodium, and diminished radiation risk.
A nuclear logging tool has been developed that determines the moisture content of subsurface earth formations by measuring the gamma rays produced by thermal neutron capture in hydrogen. The tool employs a 252Cf fast neutron source and a hyperpure germanium gamma-ray detector. The tool has demonstrated excellent sensitivity to changes in formation moisture content when used in air-filled boreholes cased with steel. The tool is also sensitive to other elements that produce neutron capture gamma rays, such as silicon, calcium, aluminum, sodium, chlorine, chromium, cadmium and mercury. Extensive computer modeling of the tool has been done to aid its design and in the interpretation of logging data taken under a variety of conditions. The logging tool has been calibrated for its moisture and chlorine response in a set of physical models and is now in use logging boreholes at the U.S. Department of Energy Hanford Site.
Pulsed Fast/Thermal Neutron Analysis (PFTNA) is being employed in such diverse applications as: on-line coal analysis, detection of improvised explosive devices (IEDs), detection of contraband drugs, characterization of unexploded ordnance, and detection of landmines. In this work, the current research in the utilization of PFTNA in detection of drugs and IEDs will be discussed. Man-portable PFTNA systems have been built and currently are undergoing field trials. These systems can be inserted in confined spaces such as the boiler of a ship or into a tanker truck filled with liquid. The PFTNA system provides information on the major and minor chemical elements which compose the interrogated object. By measuring the elemental content or ratios of various elements, this system can differentiate between innocuous materials and materials such as drugs and IEDs. In laboratory trials, the PFTNA system can measure the carbon to oxygen ratio to an accuracy of 15% within a 5 minute time period. In all cases, hidden drugs and IEDs are identified through the measurement of the elemental content of the object, and the comparison of expected and measured elemental ratios.
We have performed radiation detection measurements to explore the feasibility of radioisotopic analysis and detection of a neutron source in a marine environment. We determined the maximum range in seawater through which complex (gamma) -ray emitting materials could be accurately assayed for isotopic content. Additionally we used the gamma rays from neutron capture on chlorine to detect a neutron source. Results from our experiments have been used to determine the greatest distance at which the presence of a neutron emitter can be confirmed. The measurements used an electromechanically-cooled high-purity germanium detector system in both laboratory and fielded seawater conditions. The laboratory experiments used a variety of sources in an arrangement where both the source and detector were surrounded by seawater. The field experiments were performed underwater with the detector in a sealed container that was separate from the source.
Mercuric Iodide is a preferred candidate material for truly room-temperature radiation detectors because of its large electronic bandgap (2.1 eV) and the high atomic number of its constituent elements, which results in a high photopeak efficiency. The spectroscopic performance of the detectors is determined by the electronic transport properties of the material which depends on the purity and the structural homogeneity of the single crystals from which the detectors are fabricated. Recent advances in purification and crystal growth have made it possible to fabricate routinely large, stable gamma ray and X-ray detectors and counters.
In recent years silicon detectors have nearly replaced Geiger-Muller tubes in personal dosemeters for gamma and X- rays. Several tentatives have been undertaken to extend these dosemeters to fast neutrons, but problems arose since in the generally present mixed fields it appeared difficult to separate the effect of both kind of radiations. In this paper, a method for fast neutron monitoring is investigated. It results from a computer simulation analysis of fast neutrons and gamma-ray interactions in silicon detectors. For neutron energies between 0.75 and 15 MeV, it proposes a real time personal dosemeter, with a response accuracy better than 30% in a mixed neutron and gamma-ray field.
A sealed, D-T, pulsed neutron generator is used for the in vivo measurement of body carbon and oxygen by neutron inelastic scattering. The generator is operated at 10 KHz, at a neutron output of about 2 X 107 n/s/4(pi) . Gamma ray spectra are collected with two B4Ge3O12 crystal detectors. The measurements are used to measure fat and lean content and distribution in the body, with minimal radiation exposure (0.08 mSv). When combined with other measurements (such as total body potassium), this whole body scanning device provides us with the `quality of lean mass', a measurable outcome of treatments designed to improve nutritional status and function. The method is used in studies of human nutrition and for assessing the efficacy of new anti-obesity and anti-cachexia pharmaceuticals.
There is a large variety of mining and manufacturing operations where process monitoring and control can benefit from on-site analysis of both chemical and mineralogic constituents. CHEMIN is a CCD-based instrument capable of both X-ray fluorescence (XRF; chemical) and X-ray diffraction (XRD; mineralogic) analysis. Monitoring and control with an instrument like CHEMIN can be applied to feedstocks, intermediate materials, and final products to optimize production. Examples include control of cement feedstock, of ore for smelting, and of minerals that pose inhalation hazards in the workplace. The combined XRD/XRF capability of CHEMIN can be used wherever a desired commodity is associated with unwanted constituents that may be similar in chemistry or structure but not both (e.g., Ca in both gypsum and feldspar, where only the gypsum is desired to make wallboard). In the mining industry, CHEMIN can determine mineral abundances on the spot and enable more economical mining by providing the means to assay when is being mined, quickly and frequently, at minimal cost. In manufacturing, CHEMIN could be used to spot-check the chemical composition and crystalline makeup of a product at any stage of production. Analysis by CHEMIN can be used as feedback in manufacturing processes where rates of heating, process temperature, mixture of feedstocks, and other variables must be adjusted in real time to correct structure and/or chemistry of the product (e.g., prevention of periclase and alkali sulfate coproduction in cement manufacture).
We are developing a CdZnTe pixel detector with a custom low- noise analog VLSI readout for use in the High-Energy Focusing Telescope balloon experiment, as well as for future space astronomy applications. The goal of the program is to achieve good energy resolution (< 1 keV FWHM at 60 keV) and low threshold in a sensor with approximately 500 micrometers pixels. We have fabricated several prototype detector assemblies with 2 mm thick, 680 by 650 micrometers pitch CdZnTe pixel sensors indium bump bonded a VLSI readout chip developed at Caltech. Each readout circuit in the 8 X 8 prototype is matched to the detector pixel size, and contains a preamplifier, shaping amplifiers, and a peak stretcher/discriminator. In the first 8 X 8 prototype, we have demonstrated the low-noise preamplifier by routing the output signals off-chip for shaping and pulse-height analysis. Pulse height spectra obtained using a 241Am source, collimated to illuminate a single pixel, show excellent energy resolution of 1.1 keV FWHM for the 60 keV line at room temperature. Line profiles are approximately Gaussian and dominated by electronic noise, however a small low energy tail is evident for the 60 keV line. We obtained slightly improved resolution of 0.9 keV FWHM at 60 keV by cooling the detector to 5 degree(s)C, near the expected balloon- flight operating temperature. Pulse height spectra obtained with the collimated source positioned between pixels show the effect of signal sharing for events occurring near the boundary. We are able to model the observed spectra using a Monte-Carlo simulation that includes the effects of photon interaction, charge transport and diffusion, pixel and collimator geometry, and electronic noise. By using the model to simulate the detector response to uncollimated radiation (including the effect of finite trigger threshold for reconstruction of the total energy of multi-pixel events), we find the energy resolution to be degraded by only 10% for full-face illumination, compared to the collimated case. The small value of the degradation is due directly to the low readout noise and amplifier threshold.