GBS Elektronik MCA-527 How to do an optimum setup - Manual

How to do an optimum setup for the MCA527
24.9.2013
Jörg Brutscher
GBS Elektronik GmbH
This paper describes how to do a setup for a MCA527 with an arbitrary detector for an application, with a focus
on IAEA and Euratom applications.
Goal of the adjustments are best spectral resolution, but also high throughput, a good dynamic range, and exact
the window of photon energies which is needed for the applications.
Compared with its predecessor, the MCA166, the MCA527 offers additional possibilities to optimize and fine
adjust the measurements. This variety of possible settings however may confuse the average user. So this shall
be a step by step guide how to adjust the settings. For this, it is assumed that the MCA527 and the corresponding
detector are connected and radioactive source similar to those occurring in the application are available.
Also it is assumed that when connecting to the computer, the MCA527 is reset to its default values.
High voltage
Most gamma detectors need some kind of high voltage to operate. Setting this is easy in most cases, as value and
polarity of the high voltage is indicated on the detector. If polarity indicated on the detector and offered by the
MCA don’t match (blue for negative, red
for positive) the HV module within the
MCA has to be exchanged. So just go to
Setup-High voltage and enter the high
voltage indicated on the detector.
Exceptions:
-For scintillation detectors with
photomultiplier (e.g. NaI, LaBr, ect.) the
HV may be varied around 10-20% to
change the amplitude of the preamplifier
signal, if this is necessary.
-For HPGe detector the high voltage
setting is in many cases uncritical, as in
most cases there is a large plateau, where
resolution, efficiency, and preamplifier
signal amplitude hardly changes with
voltage. Selecting a HV value different
from the one indicated on the detector is
an option mostly if the HV value on the
detector is higher than the MCA527 can
deliver (e.g. 4000V). In this case it is
recommended to measure the
dependence of resolution and efficiency
on high voltage and check if the chosen
value is still within the plateau.
For a HPGe detector, check that the HV inhibit signal is set to the correct mode and that the signal is connected.
Signal presence, polarity, coarse gain
If HV is set, the next thing is to check the preamplifier signal. This is done best with the oscilloscope mode, goto
Setup-Amplifier-Oscilloscope.
Expected are pulses with a fast (<1µs) rise and a slow (around 50µs) decay.