HPGe detectors are often specified as 20%, 40% or 100%. The figure is a comparison number defined in IEEE Std 325 – not an absolute efficiency. Here is how to read relative efficiency, resolution and peak-to-Compton ratio in a data sheet.
The measurement behind the percentage in an HPGe data sheet. Scroll to follow it.
Relative efficiency is measured in a fixed geometry: a Co-60 point source placed 25 cm from the detector.
What counts is the full-energy peak at 1,332.5 keV – the photons that leave all their energy in the detector. The share of the source’s photons at that energy that end up in the peak is the detector’s full-energy peak efficiency.
In the same geometry a 3″ × 3″ (76 × 76 mm) NaI(Tl) crystal has the value 1.2 × 10⁻³. Relative efficiency is the HPGe detector’s full-energy peak efficiency divided by that value – the definition in IEEE Std 325.
A 40% detector therefore has about 0.40 × 1.2 × 10⁻³ ≈ 4.8 × 10⁻⁴ in exactly that geometry. The figure can exceed 100% – ORTEC’s GEM series covers 10–150%. It compares detectors; the efficiency for your sample is calibrated for the actual geometry.
Relative efficiency compares the detector’s full-energy peak efficiency at 1,332.5 keV (Co-60) with that of a 3″ × 3″ (76 × 76 mm) NaI(Tl) crystal. Both refer to a point source 25 cm from the detector, and the value used for the NaI crystal is 1.2 × 10⁻³. A 40% detector therefore has a full-energy peak efficiency of about 0.40 × 1.2 × 10⁻³ ≈ 4.8 × 10⁻⁴ in exactly that geometry.
The figure compares detectors with each other – it is not the efficiency for your sample, and it can exceed 100%: ORTEC’s GEM series covers 10–150%. For quantitative measurements the efficiency is calibrated for the actual geometry, such as a Marinelli beaker or a filter, using calibration sources or computational models.
For HPGe, resolution is stated as FWHM at 122 keV (Co-57) and 1,332 keV (Co-60). The peak-to-Compton ratio is the height of the 1,332.5 keV peak divided by the average level of the Compton continuum between 1,040 and 1,096 keV. A higher value makes it easier to see weak peaks that sit on the Compton background from stronger, higher-energy lines.
ORTEC’s GEM40 (p-type) and GMX40 (n-type) have the same relative efficiency – at least 40% – but are built for different tasks:
If low-energy lines are to be measured – for example Am-241 at 59.5 keV or Pb-210 at 46.5 keV – GMX is the natural choice. For measurements above about 40 keV, the GEM40 has tighter warranted resolution figures at the same efficiency.
A detector with higher relative efficiency reaches the same detection limit (MDA) in a shorter counting time, but it costs more. Background also affects the detection limit, so shielding and low-background materials can matter as much as crystal size. Tell us about the samples, nuclides and detection limit requirements and we will help you compare the options – contact us.
Unfamiliar term? Look it up in the glossary.
HPGe detectors for gamma spectroscopy
HPGe gamma detector
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