GBS Elektronik MCA-527 - Manual (Page 17)

Hardware
High Purity Germanium (HPGe) Detectors
A HPGe detector is basically a huge cooled high voltage germanium diode with a big
radiation sensitive depletion region. In first order, the output does not depend on HV
setting as long as the voltage is above a certain voltage and the detector is fully depleted.
Below that value, the resolution degrades, the sensitivity decreases and the signals get
smaller.
This is important when dealing with detectors whose HV rating (e. g. 4500V) is above the
3600V the MCA527 can supply
3 . If the 3600V is still above the depletion point, it is no
problem to use this detector with lower voltage. However, it is not possible to estimate the
depletion voltage from the detector nominal voltage; this has to be found out
experimentally.
3 In conjunction with a HV3600 high voltage module
17
Figure 5: Typical dependence of FWHM, Centroid position and counting rate on HV
setting. Peaks at 59 keV and 661 keV are used for evaluation. The nominal
rating of this detector is 2500V, the depletion point is at 1250V.
0 500 1000 1500 2000 2500
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0,5
0,6
0,7
0,8
0,9
1,0
1,1
1,2
FWHM 661 [eV]
FWHM 59 [eV]
Centroid 661 [rel]
Centroid 59 [rel]
Area 661 [rel]
Area 59 [rel]
Detector Bias Supply [V]
F
W
H
M
[e
V
]
re
l