Answer to Question #4904 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
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).
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