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Photon Detector Categories Operation in Magnetic Fields If it is necessary to operate a germanium detector in a high magnetic field (~ several hundred millitesla) there is danger that even with a good vacuum a Penning discharge may cause surface leakage current, which will make the detector inoperable.ORTEC can, on request, prevent such an occurrence by providing a modified detector mount which includes an insulator between the endcap wall and the detector outer contact sitting at high voltage. Peak-to-Compton RatioThe peak-to-Compton ratio, also measured in accordance with ANSI/IEEE 325–1996, is the key indicator of a detector’s ability to distinguish low-energy peaks in the presence of high-energy sources. The peak-to-Compton ratio is one of the most important and yet most often overlooked — sometimes even unspecified — measures of detector performance. The Compton plateau results from Compton interactions in the detector in which the resulting photon, reduced in energy, escapes from the sensitive volume of the detector. The peak-to-Compton ratio is obtained by dividing the height of the 1.33-MeV peak by the average Compton plateau between 1.040 and 1.096 MeV. Again, the typical measured peak-to-Compton ratio for ORTEC detectors is substantially better than the warranted specifications. For a given value of the relative efficiency, higher peak-to-Compton values are achieved with better values of energy resolution. [Note: For two HPGe detector elements having the same diameter and length, the product of resolution (at 1.33 MeV) times the peak-to-Compton ratio is a constant; therefore, if one detector has 10% better resolution, it will have a 10% higher peak-to-Compton ratio.] In cases where two peaks have nearly identical energies (and the smaller peak is on the low-energy side of the larger peak), near-perfect Gaussian peak shape is essential to quantify the smaller peak’s net area. As demands for reduced MDAs become more pervasive, excellent peak shape is increasingly important. Even when the most sophisticated software is employed to deconvolute interferences, the precision of the result and the MDA is limited by the extent of the interference of the peaks with each other.The ratios FW.1M/FWHM (FW.1M = Full Width at One-Tenth Maximum) and FW.02M/FWHM (FW.02M = Full Width at One-Fiftieth Maximum) are excellent means of describing this shape. The theoretical Gaussian peak has a FW.1M/FWHM ratio of 1.83 and an FW.02M/FWHM ratio of 2.38. Most ORTEC detectors have peak shapes close to these theoretical numbers. 22-keV Peak/88-keV Peak AreaThis specification quantifies the thinness of the entrance window in GAMMA-X detectors. The natural ratio of gamma rays from the 22-keV and 88-keV lines of a 109Cd source is ~21:1. A GAMMA-X detector typically displays a ratio >20:1. For comparison, the ratio for a GEM (p-type) detector is ~1:100. |
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