Kenneth E. Sale

Physics-Based Detection of Radioactive Contraband: A Sequential Bayesian Approach

The timely and accurate detection of nuclear contraband is an extremely important problem of national security. The development of a prototype sequential Bayesian processor that incorporates the underlying physics of ¿-ray emissions and the measurement of photon energies and their interarrival times that offers a physics-based approach to attack this challenging problem is described. A basic radionuclide representation in terms of its ¿-ray energies along with photon interarrival times is used to extract the physics information available from the uncertain measurements. It is shown that not only does this approach lead to a physics-based structure that can be used to develop an effective threat detection technique, but also motivates the implementation of this approach using advanced sequential Monte Carlo processors or particle filters to extract the required information. The resulting processor is applied to experimental data to demonstrate its feasibility.

Threat Detection of Radioactive Contraband Incorporating Compton Scattering Physics: A Model-Based Processing Approach

The detection of radioactive contraband is a critical problem in maintaining national security for any country. Gamma-ray emissions from threat materials challenge both detection and measurement technologies significantly. The development of a sequential, model-based Bayesian processor that captures both the underlying transport physics of gamma-ray emissions including Compton scattering and the measurement of photon energies offers a physics-based approach to attack this challenging problem. The inclusion of a basic radionuclide representation of absorbed/scattered photons at a given energy along with interarrival times is used to extract the physics information available from noisy measurements. It is shown that this representation leads to an “extended” physics-based structure that can be used to develop an effective sequential detection technique. The resulting model-based processor is applied to data obtained from a controlled experiment in order to assess its feasibility.

Bayesian Processing for the Detection of Radioactive Contraband from Uncertain Measurements

With the increase in terrorist activities throughout the world, the need to develop techniques capable of detecting radioactive contraband in a timely manner is a critical requirement. The development of Bayesian processors for the detection of contraband stems from the fact that the posterior distribution is clearly multimodal eliminating the usual Gaussian-based processors. The development of a sequential bootstrap processor for this problem is discussed and shown how it is capable of providing an enhanced signal for eventual detection.

A Bayesian sequential processor approach to spectroscopic portal system decisions

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.

Neutron-damaged GaAs detectors for use in a Compton spectrometer

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.

Applications of the Monte Carlo radiation transport toolkit at LLNL

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.

Signal predictions for a proposed fast neutron interrogation method

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.

Realistic modeling of radiation transmission inspection systems

We have applied Monte Carlo particle transport methods to assess a proposed neutron transmission inspection system for checked luggage. The geometry of the system and the time, energy and angle dependence of the source have been modeled in detail. A pulsed deuteron beam incident on a thick Be target generates a neutron pulse with a very broad energy spectrum which is detected after passage through the luggage item by a plastic scintillator detector operating in current mode (as opposed to pulse counting mode). The neutron transmission as a function of time information is used to infer the densities of hydrogen, carbon, oxygen and nitrogen in the volume sampled. The measured elemental densities can be compared to signatures for explosives or other contraband. By using such computational modeling it is possible to optimize many aspects of the design of an inspection system without costly and time consuming prototyping experiments or to determine that a proposed scheme will not work. The methods applied here can be used to evaluate neutron or photon schemes based on transmission, scattering or reaction techniques.<<ETX>>

Wide-range, permanent-magnet Compton spectrometer

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.

Fiber optic array for continuous energy coverage in a gamma-ray spectrometer

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.