The development of faster more reliable techniques to detect radioactive contraband in a portal type scenario
is an extremely important problem especially in this era of constant terrorist threats. Towards this goal the
development of a model-based, Bayesian sequential data processor for the detection problem is discussed. In the
sequential processor each datum (detector energy deposit and pulse arrival time) is used to update the posterior
probability distribution over the space of model parameters. The nature of the sequential processor approach
is that a detection is produced as soon as it is statistically justified by the data rather than waiting for a fixed
counting interval before any analysis is performed. In this paper the Bayesian model-based approach, physics
and signal processing models and decision functions are discussed along with the first results of our research.
Modern Monte Carlo radiation transport codes can be applied to model most applications of radiation, from optical to TeV photons, from thermal neutrons to heavy ions. Simulations can include any desired level of detail in three-dimensional geometries using the right level of detail in the reaction physics. The technology areas to which we have applied these codes include medical applications, defense, safety and security programs, nuclear safeguards and industrial and research system design and control. The main reason such applications are interesting is that by using these tools substantial savings of time and effort (i.e. money) can be realized. In addition it is possible to separate out and investigate computationally effects which can not be isolated and studied in experiments. In model calculations, just as in real life, one must take care in order to get the correct answer to the right question. Advancing computing technology allows extensions of Monte Carlo applications in two directions. First, as computers become more powerful more problems can be accurately modeled. Second, as computing power becomes cheaper Monte Carlo methods become accessible more widely. An overview of the set of Monte Carlo radiation transport tools in use a LLNL will be presented along with a few examples of applications and future directions.
We have applied the Monte Carlo radiation transport code COG to assess the utility of a proposed explosives detection scheme based on neutron transmission. In this scheme a pulsed neutron beam is generated by an approximately seven MeV deuteron beam incident on a thick Be target. A scintillation detector operating in the current mode measures the neutrons transmitted through the object as a function of time. The flight time of unscattered neutrons from the source to the detector is simply related to the neutron energy. This information along with neutron cross-section excitation functions is used to infer the densities of H, C, N, and O in the volume sampled. The code we have chosen to use enables us to create very detailed and realistic models of the geometrical configuration of the system, the neutron source, and of the detector response. By calculating the signals that will be observed for several configurations and compositions of interrogated objects we can investigate and begin to understand how a system that could actually be fielded will perform. Using this modeling capability, many aspects of the design of a system can be optimized early on with substantial savings in time and cost and with improvements in performance. We will present our signal predictions for simple single element test cases and for explosive compositions. From these studies it is clear that the interpretation of the signals from such an explosives identification system will pose a substantial challenge.
A wide variety of nonintrusive inspection systems have been proposed in the past several years for the detection of hidden contraband in airline luggage and shipping containers. The majority of these proposed techniques depend on the interaction of radiation with matter to produce a signature specific to the contraband of interest, whether drugs or explosives. L-Division of the LLNL has developed a unique expertise in the combined numerical and experimental modeling of these types of systems. Based on our experience, we are convinced that detailed numerical modeling provides a much more accurate estimate of the actual performance of complex experiments than simple analytical modeling. Furthermore, the construction of detailed numerical prototypes allows experimenters to explore the entire region of parameter space available to them before committing their ideas to hardware. This sort of systematic analysis has often lead to improved experimental designs and reductions in fielding costs. L-Division has developed an extensive suite of computer codes to model proposed experiments and possible background interactions. These codes allow us to simulate complex radiation sources, model 3D system geometries with `real world' complexity, specify detailed elemental distributions, and predict the response of almost any type of detector. In this work, we will present several examples illustrating the use of these codes in modeling experimental systems at LLNL and discuss their potential usefulness in evaluating nonintrusive inspection systems.
Optical fibers are being used to obtain full coverage over a range of energies in a multi- channel, time-resolved gamma ray spectrometer. Gamma rays are incident upon a beryllium foil 60 cm from the entrance port of a Sm-Co magnet. Compton electrons from the foil are focussed according to their energy onto quartz optical fibers arrayed in close-packed configuration behind a low-Z vacuum window at the focal plane. Cerenkov radiation produced inside each of the fibers propagates down the fiber which is brought out of the magnet. Fibers are grouped into preselected energy bins corresponding to streak record channel assignments. The light from the fibers in an energy bin are combined into one signal and then transmitted to a streak camera with a specified number of channels. This unique optical fiber array serves both as the detector and as a means to define energy bins of our choosing for a streak camera recording system.
KEYWORDS: Sensors, Gallium arsenide, Electrons, Signal detection, Spectroscopy, Electric field sensors, Gamma radiation, Electrodes, Collimators, Electron beams
Detectors made of GaAs are being studied for use on the focal plane of a Compton spectrometer which measures 1 MeV to 25 MeV gamma rays with high energy resolution (1% or 100 keV, whichever is greater) and 200 ps time resolution. The detectors are GaAs chips (Cr-doped or un-doped) that have been neutron-damaged to improve the time response. The detectors are used to measure fast transient signals in the current mode. The properties of various GaAs detector configurations are being studied by bombarding sample detectors with short pulses of 4 MeV to 16 MeV electrons at the Linac Facility at EG&G Energy Measurements, Inc., Santa Barbara, California Operations. Measurements of detector sensitivity and impulse response versus detector bias, thickness, and electron beam energy and intensity have been performed and are presented.
We are using a Sm-Co based permanent magnet spectrometer to analyze Compton electrons ejected from a Be converter foil that is illuminated by a gamma-ray beam. The distance along the focal plane at which a mono-energetic electron beam entering the spectrometer will cross the focal plane is proportional to the square root of the electron momentum. This design achieves a very broad range of energies that are analyzed (from momentum of about 1 MeV/c up to 30 MeV/c) while maintaining good energy resolution (the electron momentum resolution is the larger of 0.1 MeV/c or 1% of the momentum). In addition this design has a fairly large acceptance. By restricting the angular acceptance of the spectrometer for the Compton electrons, incoming gamma ray energies and ejected electron momenta are simply related. The electron optical properties of this spectrometer are discussed as well as some aspects of the overall system design and testing.
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