HPGe Detector Terminology for Detailed Comparison

         Interested in High Purity Germanium Detectors ? Here are a few benchmarks to consider:

         Energy resolution defines the detector’s ability to produce narrow peaks in the spectrum. Energy resolution is affected by detector geometry and electronic noise. Resolution is generally specified at 1.33 MeV and 122 KeV for most coaxial detectors and 5.9 KeV and 122 KeV for most semi-planar and X-ray detectors. Resolution is typically measured as the width at the full width at half maximum for the given energy. In all cases the smaller the number specified for energy resolution is best and gives the user a better probability of separating peaks for easier isotopic identification.

         There are three “types” of detector efficiencies used when describing germanium detectors. The most common has to do with the size of a coaxial detector. In fact this is a specific measurement and is more correctly called “relative efficiency”. It is described in IEEE standards and means the counts in the 1332 KeV peak of cobalt 60 detected from a point source placed 25 cm from the center of the flat face of the crystal relative to the number of counts that would be detected by a 3” by 3” NaI detector with the energy source, source to detector distance and count time.

         Sometimes the terms absolute efficiency and intrinsic efficiency are used. These terms are relevant to any photon detector. Absolute efficiency is the measure of photons detected from a specific source vs. the number of photons emitted from that source. Intrinsic efficiency is the ratio of counts in a given photopeak vs. the number of those photons that impact the crystal. Higher efficiency means greater sensitivity. The benefits of higher efficiency include shorter count times and better peak to background ratios at higher energies. The downside may include count rates that are so high that they cause high dead times and higher backgrounds interfering with delectability of weak lower energy photons.

   Peak to Compton Ratio  
         This is another specification that has to do only with a coaxial detector and cobalt 60. The P/C ratio is the number of counts in the peak channel of a 1332 KeV vs. the integral of the counts in the spectrum ranging from 1040 KeV to 1096 KeV. The P/C ratio is interesting because it is a detector characteristic that takes both efficiency and energy resolution into account. A higher efficiency detector will have more 1332 KeV counts in the photopeak. With better resolution, the photopeak will be narrower forcing the peak channel counts to increase.

    Peak Shape  

         This is typically more important for higher energies. As energy increases, the crystal charge collection is an increasingly more important factor and the electric field within a large crystal is never 100% uniform.. Photopeaks displayed on an MCA are ideally Gaussian, but that is not always attainable. A ration of the tenth maximum to the half maximum is going to vary from the ideal with increasing detector size. Peak asymmetry will be displayed as an increasing number of counts skewing the lower energy side of the photopeak. Germanium coaxial detectors are often specified to have tenth maximum to half maximum ratios ranging from 1.9 to 2.0. Sometimes the full width fiftieth maximum to full width half maximum is of interest and generally is specified as less than 2.8/1. once again, peak shape is almost always specified at 1332 KeV.

         To compare suppliers, be sure to review the datasheets from Princeton Gamma Tech here: http://www.pgt.com/Nuclear/NucLit.html

         All Princeton Gamma Tech germanium detectors are available with appropriate signal processing electronics and software. PGT can provide with the complete spectroscopy package in any application that uses germanium detectors. More information is available under the headings of particular detector types on this web-site. You can call PGT directly at 609-924-7310 or contact your PGT representative in your area for further information.

         Thank you for the consideration.

Rich Varall  x306
Princeton Gamma Tech Instruments