|Year : 2018 | Volume
| Issue : 4 | Page : 163-164
Criteria-based selection of high-purity germanium detectors: Focus on environmental applications
Editor, RPE; Ex. Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
|Date of Web Publication||6-Feb-2019|
D D Rao
Editor, RPE; Ex. Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Rao D D. Criteria-based selection of high-purity germanium detectors: Focus on environmental applications. Radiat Prot Environ 2018;41:163-4
|How to cite this URL:|
Rao D D. Criteria-based selection of high-purity germanium detectors: Focus on environmental applications. Radiat Prot Environ [serial online] 2018 [cited 2019 Apr 20];41:163-4. Available from: http://www.rpe.org.in/text.asp?2018/41/4/163/251674
High-purity germanium (HPGe) detectors are the most popular detectors for the detection, identification, and quantification of gamma-emitting radionuclides in various samples. As on today and also in the foreseeable future, these detectors offer the highest energy resolution among the gamma radiation-detecting materials. Another popular detector, namely the NaI(Tl) though offers higher detection efficiency, suffers in energy resolution, which is ultimately compensated by the superior energy-resolving power of HPGe detectors.
HPGe detectors are characterized by several parameters such as relative efficiency (RE), full width at half maximum (FWHM), full width at tenth maximum (FWTM), full width at fiftieth maximum (FWFM), peak-to-Compton ratio (PCR), coaxial type, and planar type for choosing a suitable detector. The energy resolution (FWHM or FWFM or FWTM) does not vary greatly among HPGe detectors of different types and different volumes, capping the maximum variation within about 20% at a given energy for different detectors. Typical FWHM value for HPGe detectors will be around 2 keV at 1332 keV gamma energy of 60 Co radionuclide. In comparison, the NaI(Tl) detectors have about 50 keV FWHM at 662 keV gamma energy of 137 Cs radionuclide. Typical PCRs are about 60:1 for HPGe detectors.
HPGe detectors are available in several geometries to suit different applications of radioactivity measurements or detection. For instance, planar (thin) detectors are suitable for low-energy (<100 keV) detection applications. In these applications, the detector area will be the deciding factor, depending on the source geometry. Co-axial-type detectors with Al canning are generally suitable for environmental radioactivity measurements. Well-type HPGe detectors are also available, where high efficiency/sensitivity is the requirement, particularly for mono energetic gamma ray detection.
The efficiency of HPGe detectors is generally quoted by the manufacturer as RE in comparison with a standard NaI(Tl) detector. It is specified as X% RE at 1332 keV 60 Co gamma energy, when a point source of 60 Co is kept at a distance of 25 cm from the face of the detector. The RE is defined as the ratio of absolute efficiency of HPGe detector to that of 3″ × 3″ NaI(Tl) detector for a point source of 60 Co kept at 25 cm distance from the surface of the detector. The absolute efficiency of standard 3″ × 3″ NaI(Tl) detector is 1.2 × 10−3 for 60 Co point source at 1332 keV. Therefore, by measuring the absolute efficiency of a given HPGe detector with known point source standard kept at 25 cm distance, one can estimate the RE. The RE is 50%, if the HPGe detector gives 0.6 × 10−3 absolute efficiency.
For environmental radioactivity measurements, where the detection of low-level natural/anthropogenic radionuclides is of importance, co-axial HPGe detectors are the preferred choice with either an Al canning or carbon fiber window. These detectors offer most common detector parameters such as FWHM and PCRs within about 20% of their average values. However, major variations lie in the RE, which vary in the range of 10% to ~200%. Obviously, at the first thought, when low-level detections are of concern, one tends to choose a detector of the highest RE or a one close to the highest, as all the other parameters do not vary greatly.
It must be noted that the REs are quoted for 1332 keV of 60 Co energy, and it will not be guaranteed for lower or higher energies. For instance, a HPGe detector of 150% RE means it will have ~1.5 times the absolute efficiency of a 3″ × 3″ NaI(Tl) detector at 1332 keV for a point source, and it will be lower for say at 662 keV gamma energy of 137 Cs, where it could be only ~1.3 times for a given geometry. If the geometry changes from a point source to any other geometry, the changes are likely to be more pronounced. Therefore, for lower energies, the difference in absolute efficiencies for a given measurement geometry, between a 100% and 150% RE HPGe detector, will be marginal. Thus, if the interest is from low-to-high-energy gamma radionuclide determination, a compromise is needed at RE choice. A 50% RE HPGe detector serves the needs of low-level measurements appropriately. The disadvantage of high-efficiency detectors (≥80%) is that, in general, they tend to have lower usable life compared to low RE detectors in addition to appreciable cost difference (escalation) involved for high RE detectors. They show multiple problems of poor resolution, repeated sweating. and high noise over a period of use. This is a general personal observation of this author over a long period of working with various types of HPGe detectors.
A relative performance of absolute efficiency of two detectors is given both for comparison of 50% and 150% RE HPGe detectors for 100-mL cylindrical bottle geometry for 241 Am (60 keV),133 Ba (356 keV),137 Cs (662 keV), and 60 Co (1332 keV).
60 keV : 50% RE (1.9%) : 150%
RE (2.0%) : Ratio of 150% to 50% (1.0%)
356 keV : 50% RE (2.6%) : 150%
RE (5.7%) : Ratio of 150% to 50% (2.2%)
662 keV : 50% RE (1.6%) : 150%
RE (4.1%) : Ratio of 150% to 50% (2.6%)
1332 keV : 50% RE (1.1%) : 150%
RE (3.2%) : Ratio of 150% to 50% (2.9%).
The ratios indicate that, for low energies, the higher RE detectors do not offer much advantage as the thickness/volume required for complete absorption of photon energy is adequate in both detectors. For environmental measurements, a good/standard lead shielding arrangement around the detector gives more or less similar background spectra for both 50% and 100% RE detectors.
A standard gamma analysis software with de-convolution fitting feature of multiplets would be adequate for radioactivity determination. Other optional features such as attenuation correction mechanisms, extensive radionuclide library, and coincidence correction techniques will be an added advantage, though not a must for low-level environmental measurements.
Therefore, the recommendation is a 50% RE HPGe detector either with Al canning or a carbon fiber window and with good tested lead shielding including graded lining would be adequate for environmental radioactivity measurements. For effluent's monitoring and routine testing of samples, lower RE such as around 30% would be sufficient for radionuclide quantification.