MCA527
2.2.8 Applying High Voltage
Most gamma detectors require some high voltage (HV) supply. The MCA527 is able to
provide this. However, mistakes with the high voltage can seriously damage detectors,
therefore some caution is recommended.
●
Before applying HV make sure that detector is properly connected.
●
Check that the polarity of HV module inserted is correct for connected detector.
●
Do apply the correct voltage for the detector.
●
Do not plug or unplug HV cables while HV is on.
●
In case of a HPGe detector, make sure that the detector is fully cooled down for a
few hours.
There are also some specific properties for different detector families which are discussed
below.
Scintillation Detectors as NaI, LaBr, etc.
For scintillation detectors, the HV is needed for the operation of the photomultiplier. The
photomultiplier acts as some kind of amplifier whose amplification is strongly dependent
on the high voltage setting. Therefore, it is necessary that this voltage is very stable.
However, here the HV can also be used to adjust the gain and optimize performance. A
rule of thumb is that the preamplifier output signals should be in the order of 0.5V…1V. If
HV is too high, non-linearities of the photomultiplier may become significant in the
spectrum, whereas with too low HV signals become small and noise may become
significant and degrade the resolution. Scintillation detector manufacturers tend to give
slightly higher than optimum values for high voltage in their data sheets.
16
Figure 4: Dependence of peak position and resolution on high voltage, using a NaI
detector
300
400
500
600
700
800
10
100
1000
6
6,5
7
7,5
Centroid [Ch]
FWHM [%]
Photomultiplier Voltage [V]
C
en
tr
oi
d
[C
h]
F
W
H
M
[%
]
