High-Purity Germanium Technology for Gamma-Ray and X-Ray Spectroscopy
- PDF / 1,626,493 Bytes
- 11 Pages / 414.72 x 648 pts Page_size
- 93 Downloads / 134 Views
HIGH-PURITY GERMANIUM TECHNOLOGY FOR GAMMA-RAY AND X-RAY SPECTROSCOPY L.S. DARKEN AND C. E. COX Oxford Instruments Inc., 601 Oak Ridge Turnpike, Oak Ridge, TN 37831-2650 USA ABSTRACT High-purity germanium (HPGe) for gamma-ray spectroscopy is a mature technology that continues to evolve. Detector size is continually increasing, allowing efficient detection of higher energy gamma rays and improving the count rate and minimum detectable activity for lower energy gamma rays. For low-energy X rays, entrance window thicknesses have been reduced to where they are comparable to those in Si(Li) detectors. While some limits to HPGe technology are set by intrinsic properties, the frontiers have historically been determined by the level of control over extrinsic properties. The point defects responsible for hole trapping are considered in terms of the "standard level" model for hole capture. This model originates in the observation that the magnitude and temperature dependence of the cross section for hole capture at many acceptors in germanium is exactly that obtained if all incident s-wave holes were captured. That is, the capture rate is apparently limited by the arrival rate of holes that can make an angular-momentum-conserving transition to a s ground state. This model can also be generalized to other materials, where it may serve as an upper limit for direct capture into the ground state for either electrons or holes. The capture cross section for standard levels GS.L. is given by g m0
6.40 x 10-12 cm 2
Mcm*
T
S.L. = ___
___
___
where g is the degeneracy of the ground state of the center after capture, divided by the degeneracy before capture. Mc is the number of equivalent extrema in the band structure for the carrier being captured, m. is the electronic mass, m* is the effective mass, and T is the temperature in degrees Kelvin. I. INTRODUCTION High-purity germanium (HPGe) is not a room-temperature detector technology, but it is widely used for gamma-ray spectroscopy and is also emerging in x-ray applications. Thus it serves as a standard of comparison for other materials. In addition, many issues, such as charge collection, are common to all semiconductor diode technologies. In this paper detector technology and hole trapping in HPGe will be reviewed. The primary material requirements for HPGe are size and purity. HPGe gamma-ray detectors are single crystals typically 200-500 cm3 in volume and require a net electrically active impurity concentration, NA-ND, less than 1 to 2 x 1010 cm 3 in order to achieve full depletion. High-purity germanium single crystals meeting these criteria are produced by extension of techniques developed in the 1950s to obtain single crystal germanium for transistors. Electronic-grade germanium (NA-ND = 1013 cm"3) is further purified by zone refining, and single crystals are grown by the Teal-Little' modification of the Czochralski technique. The liquid-solid segregation coefficients of columns 3 and 5 impurities in germanium are all much smaller than unity with the exception of boron w
Data Loading...
