Material Innovations, Inc. (Knoxville, TN) garnered U.S. Patent 7,919,758 for an improved neutron detector that has a volume of neutron moderating material and a plurality of individual neutron sensing elements dispersed at selected locations throughout the moderator, and are particularly arranged so that some of the detecting elements are closer to the surface of the moderator assembly and others are more deeply embedded.
The arrangement captures some thermalized neutrons that might otherwise be scattered away from a single, centrally located detector element. Different geometrical arrangements may be used while preserving its fundamental characteristics. Different types of neutron sensing elements may be used, which may operate on any of a number of physical principles to perform the function of sensing a neutron, either by a capture or a scattering reaction, and converting that reaction to a detectable signal. High detection efficiency, an ability to acquire spectral information, and directional sensitivity may be obtained.
This arrangement makes more efficient use of the moderator by capturing some thermalized neutrons that might otherwise be scattered away from a single, centrally located detector element. It further allows more detailed neutron energy information to be collected for spectroscopic analysis and estimation of the direction of travel of the detected neutrons and the direction to the neutron source. The dispersion of the neutron detecting elements throughout the moderator according to the principles disclosed herein also has the effect of minimizing time jitter (time delay) between the time at which a neutron enters the apparatus and the time at which the neutron is detected by the neutron detecting elements.
This is a very significant advantage when using the device in conjunction with active interrogation and when using data analysis techniques such as fission chain neutron identification in which the time distribution of neutron arrival at the device is very important. Because the device can obtain information regarding neutron energy and directionality simultaneously with a high neutron detection efficiency, and can be further designed to detect neutrons with high time accuracy, data analysis and operating software can be provided that yields much more detailed and useful information than a traditional neutron detector that only yields a raw neutron count rate.
Neutron detection is used in national security (e.g. protection against nuclear terrorism), scientific research (e.g. neutron scattering for materials research), health physics (e.g. monitoring and control of personnel exposure at nuclear power plants), and other application areas. Neutron detector requirements vary according to the application area and specific intended use and can range from simple counting to detecting the presence of a neutron source and providing information about its identity and location. In general, most neutron detectors do not perform in an optimal way for their intended use and the performance of most neutron detectors is well below that of theoretical limits. An example of this is the neutron detectors used in radiation portal monitors.
Ideally, one would want to detect 100% of the neutrons emitted by a neutron source present in the object being scanned (e.g. a vehicle or cargo container) as this would maximize the likelihood of the portal monitor determining that the source was present. For neutron detection, most portal monitors use a neutron detector that consists of one or more 3He proportional counters embedded in a blanket of neutron moderator (e.g. high-density polyethylene, or HDPE).
For most current systems, a fast neutron (e.g. energy between 100 keV and 20 MeV) entering the surface of the device orthogonally has a probability of being captured and detected in the 3He counter of between 15 and 20%. Not only would one want to know whether or not a source is present, ideally one would also like to know what type of source it is (e.g. potentially threatening or not), how big it is, where it is, etc.