Table of Contents
Advertisement
PSI System Controls and Diagnostics
The decay time correction works by assuming that every pulse is sitting on the tail of a prior
pulse which is decaying exponentially. The decay time parameter is the fraction of that signal
which is the fraction to which it falls in following ADC conversions, expressed as a percentage,
ie
DecayTime
=
S
n
100
This can be related to the time constant of the exponential decay because we know the time
interval between successive conversions occurring at 80 MHz is 12.5 nsec. Thus
= e
DecayTime
where τ is the time constant in nsec. For example, if τ is 250 nsec, then Decay Time should be
0.951, or 95.1%.
Figure 18 shows how the effect of the tail of a preceding pulse is handled by this process. The
overlap is exaggerated for clarity, and the pulses are normalized to unit height. In practice
pulses with this amount of overlap would be discriminated out by the Pile Up test. Figure 19
shows how the correct integrated area of the second pulse is recovered by the subtraction.
1.6
1.4
First point of
assumed decay tail
1.2
1
0.8
0.6
0.4
0.2
0
0
20
Figure 18. Overlapping pulses showing assumed continuation of the first pulse under the
second.
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
20
Figure 19. Effect of overlapping prior pulse on integrated area of second pulse removed by
subtracting the assumed decay tail.
D100 User Manual
Sn+1
S
−
1
n
τ
−
12
5 .
40
60
40
60
D100_UM_081027
Assumed tail
80
100
80
100
120
140
120
140
Page 34 of 46
Advertisement
Table of Contents
