GBS Elektronik MCA166-USB Influence of P/Z setting on peak area and lower spectrum cutoff - Manual (Page 2)

Experiment
An Experiment was carried out to
point out the effects of wrong P/Z
setting. The conditions were chosen
such that the effects show very
drastically:
A 18 kBq Am241 source was
attached directly to a 2" NaI detector.
The detector has already an internal
alpha source, which produces pulses
corresponding in height to 3 MeV.
The effective amplification is 150. To
get the effects worse, a shaping time
of 2 µs is chosen. The count rate was
about 7500 cps, the dead time about
14%.
Now a series of spectra with the same
setup, but different P/Z and PUR
setting was measured. (Fig. 2 + 3)
The correct P/Z setting obtained for
this detector with the MCA program
visual adjustment is 2170. Please
note that this value may be different for each individual detector and preamplifier.
A too high P/Z setting leads to some peak
broadening. A too low P/Z setting
increases progressively the lower level
threshold; in extreme cases this inhibits
any useful spectrum measurement at all.
Conclusions
-Correct P/Z adjustment is very important
to make good quantitative measurements.
-The PUR setting effect is just a symptom
but not the cause of the problem.
-Underpolezero has more severe effects
than overpolezero.
Fig. 2: spectra measured with the same setup but different P/Z settings,
PUR on. The peak is the 60 keV Am241 peak.
1600 1800 2000 2200 2400
Pole/Zero Setting
0
20000
40000
60000
80000
100000
P
e
a
k
A
re
a
0
128
256
384
L
o
w
e
r
C
u
to
ff
c
h
a
n
n
e
l
Correct P/Z
Peak position
2% full scale
With PUR
Without PUR
Lower Cuttoff and Peak Area dependence on P/Z setting
Setup:
Crismatec 2" detector with Am241 source,
Amplification 150, 512 channels total;
Peak in channel 165, dead time 14%
2µs shaping time, ca. 7000 cps
Fig. 3: Effect of P/Z setting on Peak area and lower cutoff
threshold. Just before a peak is completely cut off, the PUR setting
gains adverse influence on peak area.
GBS -Elektronik GmbH, Bautzner Landstr. 22,
01454 Großerkmannsdorf / b. Dresden
Tel.: (0351) 217007-0 Fax: (0351) 217007-21
homepage: http://www.gbs-elektronik.de/
email: [email protected]