Hand-held Neutron Detector
Department of Homeland Security
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Small Business Information
Radiation Monitoring Devices, Inc.
44 Hunt Street, Watertown, MA, -
Socially and Economically Disadvantaged:
AbstractProliferation of the weapons of mass destruction such as nuclear weapons is a serious threat. Prevention of their spread has reached a state of heightened urgency in recent years. One of the ways to passively determine the presence of nuclear weapons is to detect and identify characteristic signatures of highly enriched uranium and weapons grade plutonium. Neutrons and gamma rays are two signatures of these materials. Gamma ray detection techniques are useful because the presence of gamma rays of specific energies can confirm the presence of a particular isotope. This technique however, has one significant limitation. In the presence of a dense surrounding material such as lead, gamma ray attenuation can be significant. This can mask the gamma ray signatures of these special nuclear materials (SNM). Neutrons, on the other hand, easily penetrate dense and high atomic number materials. For heterogeneous or dense materials such as samples of metals, oxides, and nuclear waste, gamma ray attenuation is too high to permit accurate correction of the measured signal. Under these circumstances, passive assay techniques based on neutron detection are preferable. When detected, neutrons directly indicate the presence of spontaneously fissioning isotopes (plutonium and californium) and induced fissions (uranium). Therefore, neutron detection is an important component of the overall detection techniques used in identifying SNM. In radioisotope identification devices to date, the neutron detection was readily achieved using He-3 tubes. Unfortunately, in recent years the quantity of this gas is becoming limited, therefore, new solutions are required for an efficient detection system that would allow neutron detection with an ability to discriminate gamma ray events from neutron events. Gamma discrimination is critical because gamma rays are common background in neutron detection environment during SNM monitoring. In this project we propose a handheld thermal neutron detector based on a Cs2LiYCl6:Ce (CLYC) scintillator [Combes, van Loef], which is an ideal candidate for the task [Bessiere, Glodo 08, Glodo 09]. CLYC offers (1) efficient thermal neutron detection (higher per-volume than He-3); (2) excellent separation between gamma and neutron particles (better than 10-6); and (3) gamma-ray energy resolution as good as 4% at 662 keV for dual mode (neutron and gamma) detectors. The last property is very fortunate, since the majority of current handheld thermal neutron detectors include a separate gamma detector. In most cases, in addition to neutron counts the detection system should provide information about the dose rate and / or simple isotope characterization based on four categories – NORM (Natural occurring radiation materials), SNM (Special nuclear materials), Medical and Industrial Radio-nuclides. The good gamma ray energy resolution of CLYC should guarantee an accurate energy compensated dose rate and reliable characterization of gamma ray radiation. In the last couple of years, CLYC manufacturing has progressed and 1 and 2 inch crystals are being routinely grown at RMD for internal and government purposes [Higgins]. Crystals with diameter as large as 3 inch have been grown. Moreover, the CLYC technology is currently being transferred by RMD to a commercial setting (Hilger), where full scale manufacturing of these crystals will take place.The final goal of this effort is to develop a handheld thermal neutron detector utilizing CLYC scintillators. In Phase I of the project we provided strong foundations for achieving this goal. We have shown that CLYC works well with silicon photomultipliers (MPPC from Hamamatsu). A CLYC/MPPC system provides a very compact device due to small size of this light detector.Built detectors showed clear neutron peaks (7% energy resolution), were capable of pulse shape discrimination, and could easily provide dose equivalent information for gamma ray radiation. Such combination works even if the crystal is in a form of a 1 in right cylinder, although pillar type geometry was found to be optimal from the efficiency point of view. Due to their optimal surface to volume ratio, our pillar detectors provided twice as many counts as a high pressure He-3 tube per volume unit. The objective of the Phase II effort is to design and construct prototypes of a compact neutron detector based on the CLYC scintillator. The Phase II work will be based on the Phase I experiments and results. It will focus on developing the detector and instrumentation technology to achieve the project goal of designing and prototyping a compact handheld neutron detector. The main areas of research and development will include (1) detector module optimization, such as detector form factor, light readout, and interface; (2) study of the detector signal shape and PSD performance as a function of the temperature and temperature stabilization of the system; (3) electronic module design and prototype development. Our goal will be to develop and realize a concept that can be expanded into multi-component systems, e.g. backpack implementation. In this project we will collaborate with Dr. Sara Pozzi at the University of Michigan. She will assist with the modeling and optimization of the neutron and gamma ray response of our detectors. The optimization will include detector and moderator dimensions.
* information listed above is at the time of submission.