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Photon Detector Categories Relative Efficiency (at 1.33 MeV)
For historical reasons, the relative detection efficiency of coaxial
germanium detectors is defined at 1.33 MeV relative to that of a standard 3-in.-diameter,
3-in.-long NaI(Tl) scintillator. The measurement is performed by the method that is
described in the IEEE Standard Test Procedures for Germanium Detectors for Ionizing
Radiation (ANSI/IEEE 325–1996) and in the equivalent IEC standard. A National
Institute of Standards 60Co source with known intensity is positioned 25 cm
from the endcap face, and a fixed-time count is taken for the 1.33-MeV peak. The absolute
efficiency is the ratio of the number of counts in the photopeak divided by the number of
gamma rays emitted from the source during the same period of time. This absolute
efficiency is then divided by 1.2 X 10–3, which is the absolute efficiency
at 1.33 MeV of a standard 3-in. by 3-in. NaI(TI) crystal 25 cm from the source. The ratio
of these measurements is the basis for the relative efficiency specification of the
germanium detector.Relative efficiency, while giving a general indicator of
detector performance, can be highly misleading in regards to specific
geometries (e.g., filter paper or Marinelli beakers). For this reason, ORTEC
offers the PROFILE Series GEM detectors with warranted crystal dimensions. See the
PROFILE
Series data sheet,
PROFILE Series Selection
Guide, and References 19 and 20.
The Efficiency AdvantageMany ORTEC coaxial germanium detectors have a measured relative
efficiency substantially higher than the warranted value.
The PROFILE AdvantagePROFILE Series GEM detectors offer warranted crystal dimensions, greatly
increasing detection limit predictability.
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