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Photon Detector Categories Efficiency as a Function of Energy
 As shown in Fig. 14 (Refs. 3–5),
the absolute efficiency of
Germanium coaxial detectors varies with energy. The ratio of the
number of counts in the full-energy photopeak to the total number of gamma rays emitted
from a source is known as the absolute full-energy photopeak efficiency. This includes the
effect of the solid angle subtended by the detector, and thus the source-to-detector
distance. This absolute detection efficiency is a function of energy. For a gamma-ray or
x-ray to be detected, the photon must transfer part or all of its energy by one of three
interaction modes: photoelectric effect, Compton scattering, or pair production. For a
count to occur within a nuclide’s full-energy photopeak, all of the photon’s
energy must be deposited in the detector’s active volume, either as a single
photoelectric interaction or as a multiple event. At 1.33 MeV, ~80% of the full-energy
counts start with a Compton interaction.At gamma-ray and x-ray energies up to ~40 keV, the relationship of
efficiency to energy is dominated by the attenuation of these photons by materials outside
the detector and by any dead layers on the detector periphery. For this reason, the GEM
(p-type) and GAMMA-X (n-type) detectors have different responses.
In GAMMA-X detectors, the 0.3-µm boron ion-implanted contact and thin
beryllium front window allow photons of energy down to 3 keV to enter the active volume of
the detector. Except for the anomaly at the 11-keV germanium absorption edge, virtually
all photons up to 200 keV are detected. Above that energy, the efficiency falls off with
the total absorption cross section of Ge, which is dominated by the fall-off in the
photoelectric cross section (see Fig. 4
in "Review of the Physics of Semiconductor Detectors").Due to the 700-µm-thick Li-diffused outer contact of the GEM
detector, it experiences a fall-off of efficiency below ~100 keV, with
almost all photons below 40 keV being absorbed in the outer dead layer.
At higher
 energies the relationship between efficiency and energy is
dominated by the average path length in the active volume of the
detector. The efficiency decreases with increasing energy because the
probability that the photon will interact within the detector also
decreases with energy. Because it is primarily the detector volume (and
somewhat the detector dimensions) that determines this average path
length, both GEM and GAMMA-X detectors have the same efficiency at high
energies (Refs. 3, 4, and 5).
A useful presentation is in Figure 15 (after Vano*), which demonstrates
there is little relationship between the relative efficiency at 1.33 MeV and the relative
efficiency at other energies. See Section B.1.3 for further discussion.*Nucl. Instrum. Methods, 23 (1975) 573–4. |
