SPECIAL NUCLEAR MATERIAL DETECTION
Detection of Neutron Emitting Nuclear Materials
Since 9-11, there has been an increased awareness that enemies of the US and its allies will not hesitate to consider all available means to inflict damage on national interests, both domestic and abroad. There is a very worrisome potential for asymmetric attacks using terrorist tactics with nuclear materials and weapons. All attempts must be made to thwart such attacks through careful intelligence, together with effective, economical and widely deployed sensors to detect special nuclear materials (SNM) such as uranium and plutonium.
The danger of nuclear materials proliferation and unauthorized movement simply cannot be underestimated or overstated. Those who have been charged with responsibilities in the area of special nuclear materials monitoring and control, bear a very heavy, almost impossibly challenging burden. They must have the most advanced and effective detection tools at their disposal to help them with their task, and should leave no stone unturned in the quest to identify such tools.
Indeed, the 'SNM detection problem' is enormously challenging, due not only to logistical issues where networks of fixed detector systems can be circumvented, but just as importantly due to the laws of physics, where natural background radiation and time and distance factors attenuating the radiation signal, severely limit detectability of the desired signal.
NOVA is working hard to provide solutions to this very difficult problem. Small, hand-held neutron detectors would service a broad range of first responders and government personnel in identifying the presence of clandestine nuclear materials. Moreover, direct integration of such small detector modules into moving vehicles is the preferred solution, allowing sufficient time and flexibility to record statistically significant data - as opposed to a networks of fixed detectors, which is increasingly being seen as an ineffective approach.
The currently favored neutron detectors are based on pressurized gaseous 3He tubes. Such systems are very expensive and of limited supply for widespread application due partly because it is artificially produced. Furthermore, pressurized gas systems may present an explosion hazard unacceptable in certain situations.
In a parallel approach to neutron imaging, NOVA is developing and prototyping sensitive neutron detectors based on the solid-state 10B-doping concept for use in compact, low power, and covert systems to detect the secondary charged particles emitted from these materials. NOVA's extensive expertise in neutron detection techniques, based on microchannel plate (MCP) technology, has lead to revised and improved concepts of nuclear materials detection.
We are now extending the capabilities of neutron detector technology with a solid-state technique well beyond that of the standard 3He tube. The detectors will be smaller in size, superior in efficiency, and promise to be less expensive. Furthermore, the harder the neutron spectrum (the higher the average neutron energy), the more favorable the performance compared to gaseous 3He tube detectors.
The neutron emission level from the source of interest is obviously dependent upon the amount of source material present, the distance from the source to the detector, as well as the presence of scattered neutrons from thermalization by air and soil, surrounding materials and other factors. From a practical standpoint, the neutron flux striking any nominal detector area is expected to be extremely low and to be buried in the background or noise. For this reason the intrinsic noise level of the detector element and packaging materials must be kept extremely low, and any emitted gamma component must be rejected or removed.
Sources of Background in the Detector
Traditional MCP detectors, when properly constructed, have a intrinsic background of ~1 count/cm2/sec, a level similar to that of interesting sources. Removal or radioactive contaminants in the MCP glass, particularly 40K, can reduce the background levels to ~0.01 counts/cm2/sec. The detector can be operated with very stable voltage sources such as small batteries and can be sealed into an ultra-low vacuum to prevent ion-induced signals. Precautions are taken to ensure that construction materials are free of noise-inducing radionuclides. When the detectors are degassed and sealed, the only significant sources of noise are external to the detector. These sources include a number of environmental radiation sources and types.
The primary remaining source of background is common to all detectors used above ground level. Cosmic ray-induced particles that offer the greatest noise at sea-level include muons and neutrons. The muon flux is nominally 0.016 events/cm2/sec and the neutron flux is slightly less.
NOVA's specially constructed MCPs have exhibited extremely low background noise. This is due to judicious care in raw materials selection and process control.
Neutron-Gamma Discrimination Requirements
Environmental gamma-ray sources will be detected with MCP detectors, as current MCP detectors have a gamma quantum detection efficiency of 1-3% for the energy range of relevance for most environmental gamma rays (> 100 keV). A modest thickness of shielding (1-2 mm) will effectively shield the device from most of these gamma rays. This gamma sensitivity is primarily due to the photoelectron production from interactions with the high-Z materials used in the traditional lead-based MCP glasses. To a lesser extent, Auger and Compton electron production also will contribute.
In two currently funded programs, NOVA is evaluating low-Z detector materials to effectively remove or reduce the gamma-induced pulse. The gamma ray would experience minimal interactions that release energetic free electrons along its path in the active volume of the detector. The gamma ray detection efficiency (noise level) could be expected to drop significantly due to the reduced secondary product yield, stopping power of the detector structure and reduced structural mass density.
However, further reductions in gamma sensitivity are required, and electronic rejection of gamma and muon pulses is currently being actively investigated at NOVA.
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