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Photon Detector Assembly
Detector Element
The detector element establishes the energy range and efficiency for
useful x- or gamma-ray spectroscopy (Fig.12).
The principles of operation of the
Germanium and Si(Li) detectors described in
this section are described in Reference 2.
As shown in Fig. 10, the detector element is
cooled from the outer or the front contact to which the HV is applied. The thermal
connection between the cooling rod and the detector mounting cup is made with aluminum
oxide, which is both a good thermal conductor and a good electrical insulator. The minimum
distance between the endcap (which is at room temperature and ground potential) and the
detector cup (which must be maintained at 85–100 K and to which HV is applied) is
determined by field emission and thermal emissivity considerations.
The detector element contact at ground potential is connected to the
input stage of the "front-end" electronics (see Fig. 13).The Detector Shape Advantage
All ORTEC coaxial
Germanium detectors are shaped as true right-circular
cylinders with a deep hole (central contact) and "bulletized" front
"corners." This precise machining of the crystals results in detectors with
superior peak shape and timing characteristics.
Table 1. Some
Basic Properties of Silicon and Germanium.* |
| |
Si |
Ge |
| Atomic number |
14 |
32 |
| Density (300 K); g·cm–3 |
2.33 |
5.33 |
| Atoms; cm–3 |
4.96 X 1022 |
4.41 X 1022 |
| Dielectric constant |
12 |
16 |
| Forbidden energy gap (300 K); eV |
1.115
|
0.665 |
| Forbidden energy gap (0 K); eV |
1.165 |
0.746 |
| Electron mobility (300 K); cm2·V–1·s–1 |
1350 |
3900 |
| Hole mobility (300 K); cm2·V–1·s–1 |
480 |
1900 |
| Electron mobility (77 K); cm2·V–1·s–1 |
2.1 X 104 |
3.6 X 104 |
| Hole mobility (77 K); cm2·V–1·s–1 |
1.1 X 104 |
4.2 X 104 |
| Carrier saturation velocity; cm·s–1 (300 K) |
8.2 X 106 |
5.9 X 106 |
| Carrier saturation velocity; cm·s–1 (77 K) |
107 |
9.6 X 106 |
| Energy per hole-electron pair (300 K); eV |
3.62 |
(not applicable)
|
| Energy per hole-electron pair (77 K); eV |
3.76
|
2.96 |
| *All ORTEC semiconductor photon detectors are made of either
germanium or lithium-drifted silicon. |
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