GBS Elektronik MCA166-USB Influence of P/Z setting on peak area and lower spectrum cutoff - Manual

Application Note
Influence of P/Z setting on Peak area and lower spectrum
cutoff with MCA166
6.4.2000
Jörg Brutscher
GBS Elektronik GmbH
Problem:
Correct P/Z setting is something which is often neglected when doing measurements with the MCA166. P/Z is
mainly thought to have influence on Peak shift and Peak broadening at higher count rates. But there are other
disadvantageous effects of incorrect P/Z setting. Wrong P/Z setting seems to be a fault often made, and the
connection of P/Z setting with increased thresholds and faulty peak areas is not apparent.
The problem is that the autothreshold circuit and also the PUR (Pile up rejector) use the reverse polarity of the
shaping amplifier output as a reference for noise measurement. If there is a severe underpolezero adjustment,
then the autothreshhold circuit gets the signal that there is increased noise in the system and the lower level
threshold is increased.
These weird false P/Z setting effects get especially significant:
-If the interest is in low energy peaks, but there are also high energy peaks present.
- at long shaping times
-with high count rates
-with high amplification factors.
What is P/Z (Pole Zero) adjustment?
The shaping amplifier converts the
voltage step function signals delivered by
the detector preamplifiers to gaussian
shaped pulses, which amplitude can be
easily
measured by the ADC.
Unfortunately, the gaussian pulse returns
exactly to baseline only for a single
preamp signal decay time constant. As
this time constant differs with different
preamplifiers, there will be always some
overshoot or undershoot on the shaping
amplifier output signal (see Fig. 1). The
over- or undershoot can now cause a close
following pulse to be measured with an
incorrect amplitude.
To adjust this over- or undershoot to zero,
the P/Z setting is introduced.
P/Z can be set by choosing PZC
adjustment in the menu setup-amplifier.
The P/Z adjustment of the MCA 166 can
compensate preamp signal decay time
constants from 40 µs to infinite.
0 20 40 60 80
Time (µs)
Overpolezero
Underpolezero
Correct pole zero setting
MCA 166 shaping amplifier test output
with different P/Z settings.
shaping time 2µs, amplification 150,
2" Crismatec detector with Am241 source.
Fig 1: Signal measured at the MCA 166 shaping amplifier test
output. Wrong P/Z setting distort the amplitude measurement of
close following pulses