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PSI System Controls and Diagnostics
11.1.2 Data processing stages
Pile up usec
Derivative discrim
Peak width min, max
Background
correction
Figure 15. Processing stages in the D100 pulse processing
The incoming stream of ADC samples is first corrected for any background resulting from any
inherent offset of the photomultplier and the input amplifiers, and then for the tails of prior
pulses, based upon a standard model of the pulse shape. The width detector looks for the sudden
increase in slope that means a pulse is starting. As the pulse passes through its peak and falls
again, its width at half maximum is calculated and compared with a minimum and maximum
value. If the result of these three discriminations is positive, then the integration is accepted and
continues until the specified shaping time is reached. Otherwise the pulse is rejected. It is not
assigned to a region of interest, because any attempt to gain energy information would have been
too compromised by the overlap. However it is counted, as a rejected pulse.
The D100 counts the number of pulses accepted and pulses which were rejected for not meeting
the width requirements or for violating the shaping time or pile up time constraints. Pulses
which are too small to exceed the pulse or derivative discriminator are not counted, however, as
they are generally due to noise. By monitoring the counters for accepted and rejected pulses you
can optimize the resolution verses efficiency tradeoff as you tune the pulse processing
parameters.
There is a parallel data path to a current accumulator function. This accumulates and averages
all ADC conversions over a specified time period. It provides an independent, somewhat crude,
but robust indication of the input count rate. Its main function is to give an indication of detector
saturation.
D100 User Manual
Pile-up
detection
Rise &
Width
detector
Integration
Decay time %
Shaping time
Pulse discrim
D100_UM_081027
Validation
Validation
(Rejected
from ROI)
(Rejected
completely)
ROI
assignment
Scaler
Current
accumulator
Current denom
Current time usec
Page 31 of 46
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