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How to Choose the Right
Photon Detector Dead Layers, Windows and Absorption Now you need to consider the gamma-ray range of energies to be analyzed. All materials will absorb gamma rays. The materials between the emitting nuclide and the crystal can absorb (or attenuate) the gamma-ray flux. The absorption processes are a function of energy and described by the exponential attenuation equation below: I = I0e–µ(E) X (2) Where I0 is the unattenuated gamma-ray flux, I is the flux after passing through the material and µ is the linear attenuation coefficient of the absorber and x is the thickness. This relationship determines both how deep a detector needs to be to stop the incident gamma rays and the reduction in efficiency due to the window thickness and crystal dead layer thickness. The exponential function in the equation means there is no absolute cutoff length for absorption or stopping power, so that a thin planar detector will have reduced, but not zero efficiency at high energy and a thick contact coaxial detector will have reduced and not zero efficiency at low energy. The optimum choice of detector is a tradeoff of all measurement parameters.
Figure 9 compares absolute efficiency of two detectors, one P-type (GEM) and one N-type (GMX). The crystals are of very similar diameter, but the GEM is 14 mm deeper than the GMX. As you look at the efficiency above about 150 keV, there is little difference in efficiency. The efficiency curves are diverging slightly with increasing energy because the deeper GEM crystal will stop more gamma rays. Below 150 keV, the GMX has higher efficiency and below 100 keV, the difference increases rapidly as you go down in energy. This is because the dead layer of the GEM (~600 microns) is much larger than that of the GMX (~0.3 microns). Any gamma rays stopped in the dead layer do not produce an output. At 60 keV (241Am), the GMX has about 1.7 times the absolute efficiency of the GEM and a proportionately better detection limit for 241Am (Eq. 1). This does not mean the GEM cannot measure 241Am, it simply means that it is not as good as the GMX. The GMX however would cost significantly more, and for the measurement of higher energy gamma rays, for example, 137Cs at 661 keV, is no better. Don’t forget the GEM will have superior resolution and p/C, because it is a P-type and has bigger dimensions. So the GEM will have better MDA at the higher energies.
Detector TYPE "Rules of Thumb"
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