Large area x-ray detectors based on phosphors coupled to flat panel amorphous silicon diode technology offer significant
advances for cargo radiologic imaging. Flat panel area detectors provide large object coverage offering high throughput
inspections to meet the high flow rate of container commerce. These detectors provide excellent spatial resolution when
needed, and enhanced SNR through low noise electronics. If the resolution is reduced through pixel binning, further
advances in SNR are achievable. Extended exposure imaging and frame averaging enables improved x-ray penetration
of ultra-thick objects, or "select-your-own" contrast sensitivity at a rate many times faster than LDAs. The areal
coverage of flat panel technology provides inherent volumetric imaging with the appropriate scanning methods. Flat
panel area detectors have flexible designs in terms of electronic control, scintillator selection, pixel pitch, and frame
rates. Their cost is becoming more competitive as production ramps up for the healthcare, nondestructive testing (NDT),
and homeland protection industries. Typically used medical and industrial polycrystalline phosphor materials such as
Gd2O2S:Tb (GOS) can be applied to megavolt applications if the phosphor layer is sufficiently thick to enhance x-ray
absorption, and if a metal radiator is used to augment the quantum detection efficiency and reduce x-ray scatter.
Phosphor layers ranging from 0.2-mm to 1-mm can be "sandwiched" between amorphous silicon flat panel diode arrays
and metal radiators. Metal plates consisting of W, Pb or Cu, with thicknesses ranging from 0.25-mm to well over 1-mm
can be used by covering the entire area of the phosphor plate. In some combinations of high density metal and phosphor
layers, the metal plate provides an intensification of 25% in signal due to electron emission from the plate and
subsequent excitation within the phosphor material. This further improves the SNR of the system.
KEYWORDS: Inspection, Explosives, Explosives detection, Signal detection, X-rays, Radiography, Weapons of mass destruction, Carbon, Algorithm development
A Pulsed Fast Neutron Analysis (PFNA) system for the inspection of cargoes and trucks has been designed to be relocatable. The modular packaging of the component subsystems allows it to be installed and relocated relatively quickly to meet shifting threats. A variety of deployment scenarios have been considered in the design. The material specific detection capabilities of the PFNA system have been extended beyond drugs and explosives to the detection of chemical weapons and special nuclear materials. An existing fixed-site PFNA system has been used to measure signals from a simulated chemical weapon (Sarin) concealed in a variety of cargoes; algorithms for the detection and location of the Sarin have been developed. A microsecond 'macro-pulsing,' added to the injector portion of the PFNA neutron production module, and added neutron detectors, allow measurement of delayed fission neutrons from concealed special nuclear materials (SNM). Measurements carried out with an enriched uranium sample demonstrated that the concealed SNM can be detected in cargoes. This paper will give an overview of the development program and review measurement results.
The drug problem in the U.S. is serious and efforts to fight it are constrained by the lack of adequate means to curb the inflow of smuggled narcotics into the country through cargo containers. Also, events such as the disastrous explosion in Oklahoma City, the IRA bombing in London, and the bombing of the U.S. military residence in Dharan make the development of new tools for the detection of explosives and drugs in vehicles imperative. Thermal neutron analysis (TNA) technology, developed for the detection of explosives in suitcases, and detection of landmines and unexploded ordnance is presently being applied to the nonintrusive detection of significant amounts of explosives and drugs concealed in cars, trucks and large cargo containers. TNA technology is based on the analysis of characteristic gamma rays emitted following thermal neutron capture. A TNA system can be used in a variety of operational scenarios, such as inspection before an unloaded cargo container from a spit is moved to temporary storage, inspection of trucks unloaded from a ferry, or inspection of vehicles parked close to Federal building or military bases. This paper will discuss the detection process and operational scenarios, and will present results from recent simulations and measurements.
We present an analysis of the use of thermal neutron analysis (TNA) to confirm the presence of energetic materials in unexploded ordnance detection. Our analysis is based on the performance of a prototype built by ANCORE and tested at Socorro, NM, and at Yuma, AZ, as part of the Environmental Security Technology Certification Program (ESTCP). From that data, we were able to develop a semi-empirical model for the expected signal strength as a function of the target's nitrogen content and depth. We found that the dependence on depth differs greatly between the two sites. We expect this simple model to be useful in future assessments of the feasibility of this approach. We also determine the Pd/PFA performance of the system at the two sites and found it to correspond to a signal-to-noise ratio of order unity. We estimate that an increase in signal-to-noise of roughly three will be necessary to extent the applicability of this technology in unexploded ordnance detection. Such improvements may be possible if the NaI detectors currently employed are replaced with high purity germanium (HPGe) detectors.
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