GBS Elektronik MCA 166 - Manual (Page 27)

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Choice
Allows to set the preset choices. Possible are: Real time (just the elapsed time, sec.), live
time (the elapsed time subtracted by the MCA dead time, in sec., default), Integral (the
total number of counts in a ROI), Peak Area (peak integral subtracted by background, see
appendix 2 for the formula) and None for an infinite measurement. For integral and peak
area a ROI must be set. Use the space bar to toggle between the choices.
Default is live time.
Value:
Use this field to input a preset value - this can either be set as the number of seconds or
the number of counts. The maximum number of preset seconds is 65536, the maximum
number of preset counts is 10.000.000 . Default is 300s or counts.
ROI
Allows to select one of the ROIs which is used as preset for the integral counts or the
peak area in that ROI.
3.3.2.4 MCA setup
Allows to setup the acquire functions like MCA channels, Threshold, Upper and lower
Level discriminators (LLD, ULD) and ADC input.
Channels
Allows to set the number of MCA channels. Use the space bar to toggle between
128...4096 channels. Note that the last 128 channels of 4096 can not be used and remain
empty (and correspondingly the last 64 of 2048, etc.). Default is 512 channels. For a
HPGe detector, the choice will be in most cases 4096 channels, whereas for a CdZnTe
detector 1024 channels are enough. For a NaI 256 channels are sufficient.
Threshold
Allows to set a threshold value between 2% and 60% of the current MCA scale
. Default is
2%. This analogue threshold cuts the pulses before they reach the ADC. Setting the
threshold high discriminates unwanted pulses and reduces the dead time of the ADC.
Do visual threshold adjustment
Allows to adjust the threshold using visual control of the spectrum.
LLD, ULD
LLD allows to set the lower level discriminator of the ADC. All channels below the LLD
value remain empty. The ADC dead time, however, is not reduced. The value for LLD
must be < ULD. Default is 0.
ULD allows to set the upper level discriminator (ULD) of the ADC. All channels above the
ULD value remain empty. The ADC dead time is not reduced. The value for ULD must be
> LLD. Default is the highest possible channel.
It is more recommended to use Threshold than LLD to remove noise and low energy
components of the spectrum.
Do visual LLD/ULD adjustment