GBS Elektronik MCA-527 - Manual (Page 65)

Troubleshooting
5. The threshold of the spectrum seems to be much higher than expected.
● If the lower cutoff rises by itself, this is most probable caused by the auto
threshold circuit which is responsible for detecting the noise level and adjusting
the threshold to it. Check for excessive noise in the system.
6. The high energy part of the spectrum is reduced or even missing.
● This may be a problem with high count rates and high coarse gain settings.
Please check the preamplifier signal with the oscilloscope mode: Is the base
line correct at 10% (or 90% for negative signals)? Does the signal fit into the
oscilloscope window or does it overflow very often? In such cases, reduce
coarse gain and readjust offset if necessary.
● Check if the behavior changes if the pile up rejector is switched off. Check the
signal from the preamplifier. If the preamplifier signal rise time is slower than
500ns, then there is the danger that regular pulses (especially the large ones)
are misinterpreted as pile-up. The same may happen if the preamplifier signal
exhibits overshoot or ringing. Try to use another preamplifier or just switch the
pile-up rejector off.
● Similar effects can be seen if detector cables longer than 50m are used.
7. Dead time shown is very high although the count rate is low.
● Count rate may be extremely high so that the MCA is overloaded.
● Preamplifier signal is out of range most of the time. Check with oscilloscope
window, reduce coarse gain, readjust offset.
● Electronic noise in the system.
8. Too high background rate with BICRON detectors
● Set Fast Threshold to 600 (You have to edit a setup file with a text editor for
this) for gain > 50 (Edit setup file)
9. Everything should be okay, but the MCA gets no signals or just measures strange
spectra
● wrong input polarity. Check input polarity, just toggle it for a test.
● The amplification is severely wrong set.
● wrong input mode. Check also ADC input mode
10. Very bad resolution in a HPGe spectrum
● Check settings: Pole zero, shaping time, flat top.
● Detector worn out or defective. Change and regenerate detector.
● Problems with the high voltage module. Try another high voltage module.
● Check cables. Keep them away from switch mode power supplies and other
possible sources of electromagnetic noise.
● In case of low frequency interference: Try BLR setting =1 or LF reject.
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