MCA527
Most important when applying HV to a HPGe is that the detector must be cooled down
properly and must not become warm while HV is on. As this is a mistake which easily
destroys expensive HPGe detectors, most HPGe have a HV inhibit out which becomes
active if the detector gets warm. The HV inhibit input for the MCA527 is Pin 5 of the
D-SUB9 preamplifier power connector (see chapter 2.2.12, Preamplifier Power Supply
Connector). As some HPGe have the HV inhibit out on a separate BNC cable, a
corresponding adapter is optionally available.
Table 1:
HV-Inhibit Signal
HV-Inhibit mode
4
Detector inhibit
output voltage,
inhibit active
Inhibit voltage,
detector cold
MCA527 condition
to switch HV off
off
-
BSI, DFG, Canberra 0V
+12V
V
PIN5
< 0.5V
Ortec
+5V
0V
V
PIN5
> 0.5V
The voltage with connector open at the HV inhibit input of MCA527 is around 2.2V. So for
HV inhibit to work properly with Canberra detectors, it must be made sure that the inhibit
is really connected. For Ortec detectors, unconnected HV is similar to detector warm, so
there is no problem. This behavior is opposite to MCA166, which interprets open input as
0V.
Room Temperature Semiconductor Detectors
Room temperature semiconductor detectors (CZT) behave somehow similar to HPGe,
Centroid position depends rather weak on HV setting. Choosing a too low voltage effects
that charge loss processes become more dominant and therefore the peaks more
asymmetric. With a too high voltage, leakage currents become a problem which cause
additional noise and therefore peak broadening. Best is to stay with the rated voltage.
2.2.9 Exchanging the High Voltage Module
Switching from positive HV to negative HV can only be done manually by exchanging the
HV module. Proceed similar as described in chapter 2.2.5 Exchanging Batteries. When
inserting the HV module be careful to insert all pins properly and not to bend or break off
pins.
2.2.10 Functions of the D-SUB9 connector
Different functions are available on the MCA527 D-SUB9 connector which are useful with
various detectors. Figure 6 shows the block diagram. The MCA527OEM has not
implemented this functions and the MCA527L only supports the ±12V supply. The pin
diagram of the D-SUB9 connector is shown in Table 3.
Traditionally this port is used to provide the supply voltages for the detectors preamplifier.
The MCA527 delivers ±12V and ±24V while the MCA527L delivers only ±12V. Each
voltage can be loaded with up to 60mA. Furthermore the MCA527 has some additional
features. Some HPGe detectors provides a HV inhibit signal which indicates that the
4 see Windows DLL description
18
