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PROFILE SERIES GEM Germanium (HPGe) Detector It has been recently shown1,2 that specifications of an HPGe detector simply in terms of its relative efficiency, defined according to the familiar IEEE standard3, does not predict very well how the detector will perform in the actual "real world" measurement situation. In these "real world" applications, the samples are Marinelli beakers, filter papers or bottles. These geometries are very different from a point source at 25 cm from the end cap, as defined by the standard. This means that in almost every case, specifying a detector requirement in terms of the IEEE "point source" relative efficiency is at best imprecise, but at worst may be totally misleading. The IEEE relative efficiency measurement does not distinguish among the different germanium crystal sizes and shapes that are made. Germanium crystals can be made with diameter equal to length ("square"), diameter larger than length ("over-square"), or diameter less than length ("under square"). The efficiency is affected by both the front surface area or solid angle (diameter) and the stopping power (length). Three detectors with very different shapes (over, under, and square) could all have the same relative efficiency at 1332 keV, but would have very different efficiencies for other samples. This is well illustrated by Figure 1, which represents three detector geometries, over-square, square, and under-square, all of which have the same relative or absolute efficiency for a point source at a distance (source A), but which very clearly have significantly different absolute efficiency for the counting of the thin disk source B (e.g., filter paper) on endcaps, where the diameter of B is greater than the smallest diameter detector, but less than the other two larger diameter detectors.Consider the Figures 2, 3 and 4, which show data which was taken from the study in reference 2. Three very different detectors, whose "vital statistics" are given in Table 1.
A: A point source, For the case of the 5 cm filter, in Fig. 3., the crystals are all as large, or larger than the source itself. Once again, the smaller detector loses out rapidly because of length, but this time whereas up to about 100 keV for the point source the small detector has similar efficiency (it is "black" to radiation of 100 keV and below), the efficiency ratio (large to small) for the 5 cm filter at 170 keV is 1.66 because of the solid angle. The filter and small crystal have the same diameter, whereas the other detectors are much larger than the filter; the extra crystal area gives a substantial efficiency advantage. However the medium and larger detectors have almost identical efficiencies at about 170 keV. At 1.33 MeV, this time, the larger detector has an efficiency advantage of 1.41 and 4.22 respectively, over the medium and small detectors.For the case of the 10 cm filter, in Fig. 4, NO crystals are bigger than the filter. Here the smaller detector loses out dramatically. The large detector at 170 keV is 2.5 times as efficient and at 1.33 MeV the ratio is 6.15! Compared to the medium detector at 170 keV and 1.33 MeV, the large detector is more efficient by a factor of 1.13 and 1.52 respectively. The extra diameter between medium and large is 15%, and because the detector diameters are similar to the sample diameter, the 15% diameter increase results in a 13% improvement in low-energy efficiency. For the same reason at 1.33 MeV, the extra diameter now results in an extra 1.52/1.41 or 8% in absolute efficiency.Thus we can say that if the crystal is equal to or smaller than the diameter of the filter, the efficiency is seriously compromised, but if the crystal diameter is already significantly larger than that of the filter, gains in efficiency due to extra crystal diameter will be slight. It is interesting to look at these efficiency gains in the context of the volume ratio of Germanium. The volume of material used, largest to smallest, is almost 11. The detector cost ratios are likely to be of this order also.Making the best choice of detector for YOUR samples is clearly a tradeoff among several variables, some are: absolute counting efficiency in the geometry in question, energy resolution, and lastly cost. The new ORTEC PROFILE series GEM detectors allow you to make an informed choice. A simple guideline on choosing an optimum detector is trying to choose a detector which best suits the minimum geometrical requirements for the sample, within budgetary constraints. For example:For counting filters, bottles, and Petri dishes on endcap, choose a detector whose diameter is AT LEAST 20% greater than the diameter of the sample. After that decision, the detector can be chosen as a crystal as long as the budget will allow, within reason, thereby increasing high-energy efficiency. If high energy is not of interest, a shorter detector may be lower cost and have an unexpected bonus in low-energy resolution. The PROFILE F-Series give the maximum diameter for a given relative efficiency (price); the M-Series are deeper versions of the same diameter and thus have superior high energy efficiency. For Marinelli beakers, the detector should be chosen such that it "fills as much of the well of the beaker with germanium" as is possible. PROFILE series detectors are built to have the maximum practical crystal diameter for a given endcap size. This meets the requirement that the gap between the beaker contents and the crystal be as small as possible. The M-Series crystals have a length of about 110% of their diameter, which is optimum for a Marinelli beaker with conventional well proportions.PROFILE series GEM detectors remove the guess work and add back the science into HPGE detector selection for counting laboratory samples.
*For extended sources; including filters, bottles, and Petri
dishes, the detector diameter should be ³1.2 x sample diameter
for best results. 1 . Keyser, R.M., Twomey, T.R., Sangsingkeow. Advances in HPGe detectors for Real-World Applications ORTEC 1999 2. Keyser, R.M., Twomey, T.R. and Sangsingkeow, P., "Matching Ge Detector Element Geometry to Sample Size and Shape: one does not fit all!", Proceedings of the 1998 Winter Meeting of the ANS, Nov 19983. ANSI/IEEE Std 325-1996, IEEE Standard Test Procedures for Germanium Gamma-Ray Detectors.
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