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Five things you need to know about using high-purity germanium detectors for homeland security
Gary W. Phillips
High-purity germanium (HPGe) detectors are the recognized gold standard for detection and identification of characteristic gamma rays from nuclear or radiological sources. No other detector comes close to matching their extremely high resolution and sensitivity. However, until recently, their use outside the laboratory has been limited. A number of challenges had to be overcome before they could be deployed routinely in the field. These included the need for cryogenic cooling, the size and weight of the portable HPGe systems, the need for rugged packaging for extreme environments and the need for expert interpretation of the data.
Today, rugged, low-power, lightweight and highly reliable HPGe systems are available off-the-shelf. Ruggedized mechanical cooling systems have eliminated the need for liquid nitrogen. Thanks to improvements in data processing hardware and software, the task of analysis has been transferred to the instrument. Built-in software, in most cases, can automatically analyze and interpret the data, without expert assistance, and can alert the operator if a threat exists.
The following are five things you need to know about the use of HPGe detector systems for homeland security applications in the field:
( 1 ) This is not your father’s germanium detector
High purity germanium detectors (HPGe) are not just laboratory instruments anymore. Today, rugged self-contained HPGe detector systems can go anywhere in the field to search for and identify suspect sources, such as weapons-grade plutonium or radiological dispersal devices (dirty bombs.) A built-in computer automatically sorts the signals into a gamma-ray spectrum, analyzes it for known nuclides and displays the results. All this is now packaged into a small-held unit.
These units have been deployed in a variety of challenging environments, including desert and artic conditions. Reliable, low-power Stirling-cycle mechanical coolers are built into the units. This eliminates the need for liquid nitrogen (LN) and the associated logistical difficulties with providing LN for worldwide deployments. This also eliminates the safety hazards associated with handling LN.
The units can operate on external AC or DC power to cool down the detector to operating temperature, while at the same time charging the batteries. The detector systems can then be unplugged from the external power source and operated independently for greater than three hours. They typically weigh between 7 and 12 kg, depending on detector size and optional neutron detectors. With the addition of two small three-pound batteries, which can be attached to a belt, the systems can now operate continuously in the field for more than 20 hours.
Gone are the days when HPGe detectors were anchored to the laboratory by large liquid nitrogen (LN) dewars to provide cooling. Gone also are rack-mounted electronic modules to provide power and process the detector signals, and a separate computer to sort the signals into a spectrum and analyze the results.
( 2 ) It’s not rocket science
It doesn’t take a highly trained analyst to interpret the data in the field. Portable HPGe detector systems are designed for non-technical users. Built-in software can analyze the gamma-ray spectrum reliably and compare the results to a catalog of characteristic peak energies, in order to identify the nuclides in the source. Nuclides found are displayed on the screen and alarms can be set for specific sources. There is a single button to push for ID and a single button to search for special nuclear material (SNM.) Typically, a complete user training regimen can be conducted in one morning or afternoon.
( 3 ) Resolution matters
It is often said that “size matters.” However, it is really “energy resolution” that matters for source detection and identification.
Resolution matters for detection of weak gamma ray peaks above the natural gamma-ray background. Resolution also matters for reducing the probability of false alarms. There are several gamma-rays characteristic of weapons-grade plutonium in the energy range from about 330 to 450 keV. However, other common sources also have gamma rays in this energy region and can result in false alarms when using low-resolution detectors. These include iodine-131, used for treating thyroid cancer, and barium-133, a common calibration source. The resolution of HPGe systems is needed to distinguish between these sources and special nuclear material (SNM), such as plutonium.
In addition, resolution matters for detection of a weak source of interest in the presence of a much stronger source. Iodine and barium sources can be used to mask the plutonium gamma rays, when viewed with a low-resolution detector.
( 4 ) HPGe systems can search for and identify weak sources
HPGe detectors can be used to search for specific gamma rays from SNM or from a source used in a radiological dispersal device (RDD) or “dirty bomb.” Shielding can reduce the signal from these sources, making them difficult to detect with a low-resolution detector. With an HPGe detector, it takes only a few counts above background to identify a peak from a source of interest, since the peak counts occur in one of two channels while the background is spread out over the entire spectrum.