GBS Elektronik MCA-527 - Manual (Page 32)

MCA527
3.2.7 Shaping Time
The shaping time defines the length of the spectroscopical filter, or how many values
before and after the voltage step are averaged to evaluate the pulse height. The shaping
time is half the rise or integration time. So, for a shaping time of 1µs 20 values before and
20 values after voltage rise are averaged. Depending on the detector and its noise
spectrum, very different values can be optimum for best resolution. A long shaping time
eliminates a lot of high frequency noise, but is more sensitive for low frequency noise.
For a coaxial HPGe detector the optimum is typical something like 4µs...6µs, for a CZT
the best may be 0.7µs and for a NaI 0.2µs. The best shaping time to set depends also on
expected count rate. For higher count rates a lower shaping time is useful, as pile-up
probability and necessary processing power decreases with decreasing shaping time.
Default value for shaping time is 1µs.
For MCA527L only shaping times up to 2µs are available.
32
Figure 12: Effects of PUR setting on spectrum, here a Cs137 spectrum taken with 11kcps.
Everything right of the prominent Cs137 peak in the middle of the spectrum is
due to pile-up. The lowest pile-up component in the spectrum is achieved with
the 1, -2, 1 trigger filter. The -1, 0, 1 filter is not as good in suppressing pile-up,
but still much better than pile-up switched off.
0 200 400 600 800 1000 1200 1400
1
10
100
1000
10000
100000
-1, 0, 1 Cs137, 11kcps,
PUR off
-1, 0, 1 Cs137, 11kcps
1, -2, 1 Cs137, 11kcps
Energy [keV]
C
ou
nt
s