GBS Elektronik MCA-527 User 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 (<1µ s) rise and a slow (around 50µ s) decay.

Positive preamplifier signal, also the amplitude is acceptable.

Check if such signals are present. If the pulses point downwards, set input polarity to negative.

Negative preamplifier signal. In this case, input polarity should be set to negative, and also it is possible to
increase coarse gain by another step.

Preamplifier signal too large, it exceeds the oscilloscope window and therefore the ADC measurement range.
Reduce coarse gain.

If the amplitude seem quite small, increase coarse gain. If the signal exceeds the oscilloscope window, and not
only in exceptional cases, then decrease coarse gain.
If signal amplitude allows it, coarse gain settings of 2 and 5 should be avoided as they are attenuating the signal
in reality. Especially for HPGe the resolution will then be less than optimum.

Special cases:
If no signal can be seen, and the oscilloscope window tells “Waiting for trigger”, then change Trigger mode to
“free run”.

No signal present. Check if high voltage is set (polarity?), HV cable connected, signal cable connected,
preamplifier supply connected. In case of a very insensitive detector, put a source close to it. Check if there are
preamplifier supply currents. Check detector.

Signal is permanently out of range, here much to positive. Try smaller coarse gain. Significant offset can be a
sign of a degrading detector with leakage currents. Up to 100mV may be acceptable. Very high offsets may be a
sign of a defective preamplifier or detector, also a warm HPGe would exhibit such a behavior.

Without any signal or even with extreme DC Offset, it is useless to try a setup, rather there should be
troubleshooting on the detector.

Signal rise time, Flattop setting

Next setting to be adjusted is the Flat top setting, which corresponds to the rise of the preamplifier signal. The
purpose of this is to exclude the rise part of the signal from evaluation as this would only add error to the
amplitude evaluation.