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Photon Detector Categories Attenuation Effects
An example of attenuation effects in external materials is shown in the
table below of the percentage of photons transmitted through 1 mm of aluminum, a
material commonly used in detector endcaps. The relationship describing this attenuation
is:N = N0e-µx
where N is the number of remaining photons in the beam of original
intensity N0 after traversing distance x, and µ is the absorption coefficient
for aluminum.
Table 3. Percentage of Photons
Transmitted, as a Function of Energy,
through 1 mm of Aluminum.
|
|
Energy (keV) |
% Transmitted |
|
3 |
2 X 10–32 |
|
5 |
1 X 10–6 |
|
10 |
7 |
|
20 |
50 |
|
30 |
90 |
|
50 |
96 |
|
80 |
98 |
|
100 |
98.3 |
|
400 |
99.1 |
|
1000 |
99.4 |
Another example is the percentage of photons transmitted through 0.7 mm
of germanium, which is the typical thickness of the outer contact of a GEM (p-type)
detector.
|
Table 4. Percentage of Photons Transmitted, as a Function
of Energy,
through 0.7 mm of Germanium. |
|
Energy (keV) |
% Transmitted |
|
20 |
0 |
|
30 |
0.2 |
|
40 |
7 |
|
50 |
24 |
|
60 |
42 |
|
80 |
67 |
|
100 |
79 |
A practical example of the effects of detector dead layers on low-energy
spectra is shown in Fig. 16.Figure 16.
109Cd Spectrum
Observed with: (a) a 10% Relative Efficiency GEM Detector; (b) a 5-cm2
Active Area,
10-mm Active Depth Germanium (HPGe) LEPS Detector; (c) a 10% Relative Efficiency GAMMA-X Detector.
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