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5. The threshold of the spectrum seems to be much higher than expected.
● The input polarity may be wrong. Does the threshold depend on radioactive
source and count rate? Try some other input polarity.
● P/Z may not be set correctly. Please check if P/Z is correctly adjusted.
● 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. This auto threshold circuit uses the negative part of the
amplifier output pulses to estimate noise. If now pole zero is severely
maladjusted with strong undershoot, the undershoot is misinterpreted as noise
and the threshold is increased, especially at medium and high count rates.
● Please check the detector preamplifier waveform. The decay half time of the
signal should be optimum 40 us, but never shorter than 25 us. If it is shorter (as
experienced with some NaI's) then it is not possible to adjust P/Z correctly, the
time constant of the preamp has to be adjusted instead.
6. The high energy part of the spectrum is reduced or even missing
● 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. 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.
● There may be some detectors with excessive electronic noise. Try higher fast
and slow threshold (You have to edit a setup file with a text editor for this).
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. Strange error messages
● MCA opened recently, flat cables loose? Connect flat cable again .
11. Very bad resolution in a HPGe spectrum
● 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.
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