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GAMMA-X
Germanium (HPGe) Coaxial Detectors
Figure 8 compares ORTEC’s GAMMA-X and GEM detector elements. The GAMMA-X detector element depicted is different from that of the GEM detector because the former’s starting material is n-type germanium. The GAMMA-X detector is the only Ge spectrometer designed for both gamma- and x-ray spectroscopy with high precision and efficiency for both. This point can be illustrated by comparing the GAMMA-X detector with a LEPS and with an HPGe coaxial detector (Fig. 9). The GAMMA-X detector offers a combination of the performance of the LEPS at low energies and a coaxial detector at high energies.High- and Low-Energy Performance of the GAMMA-X Detector The high-energy performance of a GAMMA-X detector is defined by its relative efficiency, resolution, and peak-to-Compton ratio at 60Co.The low-energy performance of this detector is defined by its resolution at 5.9 keV, its active surface area, and the detector window thickness. The thickness of the entrance contact of the GAMMA-X detector is described by the ratio of the areas of two peaks of a readily available source. The peaks chosen are those of the 88-keV gamma rays from the 109Cd and of the 22.16-keV Ag K x rays from the same source. The warranted window attenuation ratio
is 20. Obviously, the ability to see and measure the resolution accurately at 5.9 keV speaks eloquently of the thinness of the entrance window. Figures 10–12 show a comparison of the low-energy performance of a GEM HPGe coaxial detector (Fig. 10), a 5-cm active area, 10-mm-deep LEPS (Fig. 11), and a GAMMA-X detector (Fig. 12).In the GEM coaxial detector the thick (~700 µm) lithium-diffused outer contact completely absorbs the Ag K x rays of the 109Cd source (Fig. 10). Only the 88-keV gamma-ray line is visible. In the GAMMA-X detector, the entrance window of the detector element itself is 0.3 µm thick. The Ag K x rays are perfectly visible, with excellent peak-to-valley ratios (Fig. 12). The very low-energy Ge escape peaks in Fig. 12 are totally missing in Fig. 10. Figure 11 shows the spectrum as obtained with an HPGe detector expressly designed for work at energies below 100 keV: a 5-cm active area, 10-mm-deep LEPS. The spectra of Figs. 11 and 12 are quite similar. Beryllium WindowDetectors supplied with 2-3/4-in.-diam endcaps (10 to ~35%) are supplied with 2-in.-diam Be windows; those supplied in 3-1/4-in.-diam endcaps (~30 to 65%) are supplied with 2-1/2-in.-diam Be windows. These windows are 0.020 in. thick and have a transmission coefficient of ~95% at 5.9 keV. (Low-background carbon fiber windows are optional. See Figure 22 for transmission characteristics of the Be and carbon fiber windows.) Detectors in 3-3/4-in.-diam. endcaps (~60 to 100%) receive 3.3-in.-diam. Be windows which are 0.030 in. thick. Guaranteed Performance at 5.9 keVTo achieve good energy resolution at 5.9 keV, the technology of this state-of-the-art detector must be well understood by the manufacturer. Resolution specifications stated only at 14 or 22 keV can be misleading and may be indicative of having failed to master the technology. High-Voltage Shutdown and High-Rate IndicatorGAMMA-X detectors have high-voltage shutdown and high-rate indicator protection features. If the LN2 supply is exhausted and the detector begins to warm while high-voltage bias is applied (when using the Model 659 Bias Supply), the high voltage automatically shuts off, thus protecting the FET from damage. This is accomplished with a temperature sensor (located on the mount behind the detector) that shuts down the high voltage before the molecular sieve can outgas and cause a dangerous high-voltage arc. Using the high-leakage current of a warming detector to shut down the high voltage can result in FET and detector damage.Neutron Damage Resistance In the GEM detector, in which the outer contact is positively biased, hole collection dominates the charge collection process; in the GAMMA-X detector, electron collection is the dominant process.Fast neutrons generate hole-trapping centers; that is, negatively charged defects that trap holes but not electrons. Therefore, the GAMMA-X detector, in which the hole collection process is of secondary importance, is basically less sensitive to radiation damage than coaxial Ge devices in which the hole collection process is of primary importance. These theoretical considerations have been experimentally confirmed.2Figure 13, a plot of the 1.33-MeV FWHM resolution as a function of fast neutron fluence for both a GAMMA-X and a GEM detector of the same efficiency, shows that the GAMMA-X detector is far more resistant to fast neutron radiation damage.2 As noted, the detector temperature affects its radiation damage resistance to fast neutrons. It should be noted that once severe radiation damage has occurred, the "longest mileage" is obtained by avoiding cycling the detector to room temperature.3 This is true for either p- or n-type Ge detectors. However, for slightly damaged GAMMA-X detectors (~0.1 keV degradation), cycling, or even leaving the detector warm for an extended period, will have no unfavorable effect.4GAMMA-X detectors should be maintained at a temperature as close to 77 K as possible to minimize the extent of radiation damage. Therefore a streamline cryostat, with one less thermal connection, is a better choice than a PopTop for this purpose. Customer-Neutron-Damage-Repairable DetectorsRepair of neutron-damaged GAMMA-X detectors can be performed at any of our worldwide repair facilities, or by you in your own laboratory. Contact us for information about our Customer-Neutron-Damage-Repairable GAMMA-X detectors. Options of Interest
GAMMA-X Germanium (HPGe) Coaxial Detector* (Non-PopTop or PopTop)
†FWHM = Full Width at Half Maximum; FW.1M = Full Width at One-Tenth Maximum; FW.02M = Full Width at One-Fiftieth Maximum; total system resolution for a source at 1000 counts/s measured in accordance with ANSI/IEEE Std. 325-1996, using ORTEC standard electronics. **Typical Value. Specification is in eV for efficiencies <60% and thereafter in keV.NOTE: For those familiar with HPGe detector specifications, you will notice that ORTEC now offers ONLY "first category" detector specifications. Recent process improvements now make this possible. 2R.H. Pehl, N.W. Madden, J.H. Elliott, T.W. Raudorf, R.C. Trammell, and L.S. Darken, Jr., "Radiation Damage Resistance of Reverse Electrode Ge Coaxial Detectors," IEEE Trans. Nucl. Sci. NS-26, N1, 321–23 (1979). 3H.W. Kraner, R.H. Pehl, and E.E. Haller, "Fast Neutron Radiation Damage of High-Purity Germanium Detectors," IEEE Trans. Nucl. Sci. NS-22, N1, 149 (1975). 4T.W. Raudorf, R.C. Trammell, and Sanford Wagner, "Performance of Reverse Electrode HPGe Coaxial Detectors After Light Damage by Fast Neutrons," IEEE Trans, Nucl. Sci. NS-31, N1, 253 (1984). |
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