Answer to Question #4904 Submitted to "Ask the Experts"

Why is the relative efficiency of a high-purity germanium detector calculated with respect to 3" x 3" NaI(Tl) detectors only and why not with the other size NaI(Tl) detectors?

Perhaps the major reason that germanium detector efficiencies are specified relative to a 3" x 3" NaI (sodium iodide) detector is simply historical. For many years preceding the introduction of germanium detectors, the "standard" detector used in the laboratory for gamma spectral analysis was the 3" x 3" NaI scintillation detector. When the germanium detectors became available, people interested in gamma spectrometry were naturally interested in how the efficiencies of these new semiconductor detectors compared to the NaI detectors that were so familiar to them. Thus arose the convention of comparing efficiencies of the new detectors to the earlier ones. Efficiencies for the germanium detectors are usually specified as a percentage of the 3" x 3" NaI detector photopeak efficiency at a specified energy, most often the higher photon energy of cobalt-60 energy (1.33 MeV).

A second reason for presenting germanium efficiencies relative to NaI is that germanium detectors are available in different geometries, such as planar detectors, coaxial detectors, and others, all of which have different efficiencies even when their volumes are the same. Using the efficiencies relative to NaI may reduce some uncertainties or confusion surrounding such differences. I hope this answers your question.
 
George Chabot, PhD, CHP