High resolution energy dispersive spectroscopy with high purity germanium detectors and digital pulse processing

Abstract

Energy dispersive spectroscopy (EDS) has traditionally been performed with analytical instrumentation systems comprised of cryogenically cooled lithium-drifted silicon (Si(Li)) X-ray spectrometers and analog amplification electronics. Improved X-ray microanalysis systems have been developed and are commercially available that utilize high purity germanium (HPGe) detectors and digital pulse processing. HPGe offers several distinct advantages over lithium-drifted silicon for the detection and quantitative measurement of X rays in the 100 eV to 100 keV energy range: superior detection efficiency for X rays above 20 keV, increased sensitivity for the detection of low energy photons in an environment of high energy radiation, and superior energy resolution at all X-ray energies. Optimized semiconductor processing techniques developed at Princeton Gamma-Tech, Inc. (PGT) have resulted in HPGe spectrometers with exceptionally thin entrance windows that do not exhibit incomplete charge collection, peak distortions or peak shifts. Since the replacement of lithium-drifted Ge with HPGe in 1974, PGT HPGe detectors can be thermally cycled or stored at room temperature for long periods without degradation. In addition, digital amplification and shaping electronics have been developed which were introduced in the spring of 1993. Compared to traditional amplification and shaping with analog electronics, digital signal processing with adaptive shaping provides enhanced low energy sensitivity, improved pile-up rejection of photons below 1 keV, and superior throughput versus resolution. Adaptive pulse shaping utilizing asymmetric weighting functions allows the processing time for each pulse in the data stream to be optimized to obtain superior resolutions at high count rates and low dead times. The X-ray microanalysis system described offers the best combination of resolution and throughput of any commercially available EDS system. >