New cooling methods for HPGE detectors and associated electronics

Abstract

Despite the on-going development of room temperature semiconductors for use as gamma-ray detectors, the only material which can provide a solution to the combined requirements of stability, high-energy resolution and high-detection efficiency (at useful energies) is still germanium (HPGe). These properties of HPGe gamma-ray detectors make them invaluable in meeting the demands of the newly emergent and increasingly important applications relating to homeland security and the interdiction of smuggled nuclear material. However, HPGe detectors require cooling to cryogenic temperatures (<120 K) to operate as gamma-ray detectors. Traditionally, this cooling has been accomplished with liquid nitrogen (LN2). The use of LN2 as a coolant is, at best, inconvenient. Maintenance, operating cost, availability at remote locations, and the hazardous nature of the material all combine to limit the practicality of a LN2-cooled device, no matter how desirable it might be from other standpoints. Mechanical methods of achieving cryogenic temperatures have existed for many years. The first mechanically-cooled HPGe systems appeared commercially in the early 1980s.1 These systems had high cost, high power requirements, degraded system performance, were bulky in size, and unreliable. Other developments have produced prototype versions of portable (or transportable) mechanically-cooled HPGe systems. More recent advances in mechanical cooling technologies have the potential to make HPGe detectors easily adaptable to a wide variety of applications including battery-operated, truly man-portable systems for use in inspection, unattended monitoring, and Homeland Security. The major problems of mechanical coolers are degraded performance due to vibration and power consumption. The systems described here have reduced both of these to useable limits. The vibration or microphonic noise created in real-world systems is significantly reduced by optimizing the digital filter technology in the signal processing electronics associated with such detectors. Data presented here show reliability and performance results of the mechanically-cooled systems. These results show the improvements gained through the use of the optimally-matched digital filters.