In the field of CBRNE (chemical, biological, radiological, nuclear and explosives) emergency response, detection of radiological sources is paramount to the safety of both civilians and first responders.
While critics have disregarded cadmium zinc telluride (CZT) as a viable option for radiation detection in the past, at H3D, CZT is different. Crystals are either 15 or 10 mm thick, thicker than CZT used in other systems, providing much higher efficiency. These thicknesses are not as large as NaI(Tl) or even the largest HPGe, but with stopping power 50%greater than NaI(Tl) scintillators per volume, 15 mm of CZT is enough to catch about half of 662-keV gamma rays. If more efficiency is required, H3D has larger volume and higher efficiency systems.
H3D has something for almost any application: imaging spectrometers, permanent-mount spectrometers, hand-held directional RIIDs, and custom systems. For CBRNE applications, their detectors can be used for dirty-bomb detection, emergency-response efforts, hot-source walkup, and other forms of security. H3D systems are lightweight, compact, and feature
industry-leading resolution and efficiency to identify and localize gamma-ray sources with a single measurement.
Radiation Imaging
When a gamma ray interacts with H3D’s position-sensitive CZT, it will occur as either Compton scattering, pair production, or photoelectric absorption. Each interaction results in a scattered electron, which creates a signal for the detector to record. The recorded signal provides the energy and location of the interacting gamma ray. This detection method is combined with mathematical techniques to develop radiation images. Figure 1 shows an identical measurement imaged with the two different imaging techniques used at H3D, Compton imaging (right) and coded-aperture imaging (left).
Compton imaging is a technique based on Compton scattering to determine the position of a radiological source. When a gamma ray undergoes Compton scattering followed by photoelectric absorption in an H3D CZT detector, the 3D position and energy of the photon is recorded. Using the energy, 3D position, and Compton-scatteringformula, the angle of scatter between the incident and scattered photon can be determined. The angle of scatter for each
incident photon is used to create a “ring” of potential source locations. As more photons are collected, the “rings” begin to overlap and eventually converge at the direction of the source. This technique works best for gamma rays with energies greater than about 250 keV, as these photons are the most likely to undergo Compton scattering and then photoelectric absorption. At energies below 250 keV, different imaging methods must be used because thephotons will tend to interact via photoelectric absorption only.
Coded-aperture imaging (CAI) is a technique that is able to image gamma rays of energies that may be missed with Compton imaging. CAI images gamma rays that interact only a single time in the CZT crystal via photoelectric absorption. By using a perforated mask made of thin pieces of tungsten (a common gamma-ray shield), a shadow of the source will be recorded on the detector. If a gamma-ray interaction occurs in the detector, the gamma ray must have come through one of the holes in the mask. Over time the possible source locations will converge to show the correct source location. Coded-aperture imaging can be used for gamma rays up to about 1 MeV, but athigher energies the tungsten mask will no longer provide effective shielding. Coded-aperture imaging has a limitedfield of view, indicated by the red outline seen in Figure 1.
H420 spotlight
The H3D H420 is a high-efficiency imaging spectrometer with the ability to image both low- and high-energygamma rays using both Compton and coded-aperture imaging. The H420 is perfect for CBRNE applications with the ability to identify, localize, and quantify gamma-ray sources.
The H420 is easy to use and highly portable with spectroscopic performance competitive with cryogenically cooled detectors, at under 9 lbs., and able to start up in 90 seconds. With the option for less than 0.8% FWHM energyresolution at 662 keV, the H420 can easily differentiate and identify sources. The H420 has many possible applications, and Idaho National Laboratory has proven it an effective tool in the CBRNE space.
Idaho National Laboratory
Idaho National Laboratory (INL) is the hub of nuclear energy research and development in the United States. INL assists work at the Department of Energy in energy, national security, and other areas. The relationship between H3D and INL was born out of INL’s research in the CBRNE space, specifically their work in understandingemergency response to dirty bombs and radiological dispersal devices (RDDs). INL works to support the efforts of the National Nuclear Security Administration (NNSA) nuclear incident response. Specifically, INL focuses on the mission areas of Search and Consequence Management. Here’s where H3D comes in: INL does exclusive research on RDD monitoring and detonations for nuclear/radiological incident response using an H420 system.
In the response to a nuclear/radiological incident, emergency responders must be extremely cautious in monitoring radiation levels surrounding the package. H3D equipment helps the responders evaluate the scenario both before and after a dirty-bomb explosion. H3D’s imaging detectors help search for and identify the RDD pre-detonation.Post-detonation, H3D equipment helps identify the most appropriate re-entry path for cleanup efforts, as well as observe the movement of the radioactive plume.
Search
Searching and identifying an RDD is another realm of experimental research done at INL with the use of H3Dequipment. Sources hidden in different locations can easily be localized in short periods using the H420.
Plume Mapping
INL has used the H420 in order to map the plume deposition emerging from the detonation of an RDD. Mapping plumes from dirty-bomb fallout is extremely important in consequence management and incident response inorder to protect individuals that may be in the path of the plume. The H420 was used to record measurements during a recent INL RDD experiment. To start, the detector was pointed straight down the plume, with ground zero positioned behind the detector. The following three images are of real data taken with an H420 after this detonation.
The optical image on the H420 captures ~180° of view, so radiation images will show the image from the optical camera on the left, representing the area to the left, front, and right of the system. The right side of the combined optical radiation image shows behind the detector. The first figure shows the radiation image of ground zero positioned behind the detector. In the
second figure the H420 was moved farther down the plume, where the intensity of the plume and ground zero were similar. The final image is pointed at ground zero, which was so much stronger than the plume that it is no longer shown. Information like this is extremely useful in responding to RDD detonations, and INL is the exclusive center for research and training regarding these scenarios.
Conclusion
H3D is changing the way radiation detectors are used in CBRNE emergency response. Through the help of INL, their imaging detectors have proven effective for the search, identification, and mapping of RDD detonations and dirty-bomb response. Their cutting-edge CZT imaging spectrometers have set a new standard for monitoring RDDs.
