GBS Elektronik MCA-166 Influence of High voltage on HPGe detector resolution - Manual

Application Note
Influence of High voltage on HPGe detector resolution
17.6.2001
Jörg Brutscher
GBS Elektronik GmbH
Problem:
The MCA166 can deliver voltages up to 3kV. But larger HPGe detectors are specified for voltages up to 5kV
and cannot be operated from the MCA166 with their specified voltage.
This report shall show the dependence of detector parameters on high voltage and point out what happens if a
detector is operated below its specified voltage.
Experiment
The voltage dependency was evaluated for a Hex#136 detector from Eurisys. This one was specified for 4000V
bias and a warranted resolution of <2.4 keV at 1333 keV and 4 µs shaping time. The relative efficiency for this
detector was given with 58%.
The detector was operated at voltages between 500V and 4000V, at first using Co60 with 8% Dead Time
(4000cps) and second with Ra226 and 29% Dead Time (11000 cps)
0 1 00 0 2 0 00 3 0 00 40 00
B ia s V olta g e (V )
0
2
4
6
8
1 0
F
W
H
M
(
k
e
V
)
P h oton E n e rg y
43 ke V
60 9 ke V
13 33 k eV
Fig. 1: Dependence of resolution on bias.
This detector shows a certain range (down to 3kV) where the detector operates stable and parameters depend
only very little on bias. Below this, detector performance decreases almost linear.
Connected with an increase of FWHM is always connected a decrease of amplitude and a decrease of detector
efficiency.
Efficiency decrease means that the crystas is not fully depleted any more. Only depleted regions of the crystal are
sensitive for radiation. The surface region of the crystal seems to be depleted also on lower voltages, as the
efficiency for low energy photons, which penetrate only the surface, keeps quite high also for lower voltages.
As the signal amplitude decreases with decreasing bias, this mean that the charge collection efficiency drops.
This is quite connected with the decreasing resolution