Pile Up - Pyramid D100 User Manual

PSI System Controls and Diagnostics
300
250
200
Differential
Discriminator (slope)
150
100
50
0
-50
Figure 16. Detail of the ADC conversions at the start of a pulse

11.2.6 Pile Up

If two pulses are within Pile Up Time of each other, the first is counted and analyzed, the second
is not. Typical values for sodium iodide are from 0.2 usec to 2.0 usec. You can set pile up time
equal to shaping time if you wish.
Figure 17 illustrates how two pulses with varying degrees of overlap mare handled. The cyan
bar represents the shaping time, and the yellow bar is the pile up time.
Both pulses
counted and analyzed
Figure 17. Pile up rejection conditions.
The second pulse in figure 17 is still able to extend the deadtime. For example if Pile Up = 3
usec, Shaping Time = 2 µsec, and if the first pulse is at t = 0, the second at t = 2.5 µsec, and a
third arrives at at t = 5 µsec, then only the first pulse is energy analyzed and assigned to a region
of interest. All three pulses are counted, however. In other words, the D100 implements a
paralyzable deadtime model.
11.2.7 Decay Time
Although pulses with gross overlap are identified and rejected for analysis by the pile up
parameter, there can still be errors in the integration if a pulse starts on the extended tail of a
prior pulse. For a given scintillator type, the pulses decay at a specific rate. By knowing this
rate, we can subtract that decay away, and therefore reduce the interference between pulses.
D100 User Manual
Pulse
Discriminator
Both pulses
counted, only first
pulse analyzed
D100_UM_081027
30
25
20
15
10
5
0
-5
Neither pulse
counted or analyzed
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