Automatic Ballistic Deficit Correction
In order to optimize performance with detectors of different sizes and varying charge
collection times, the 9660 and 9660A includes Automatic Ballistic Deficit Correction
(BDC). Ballistic deficit occurs when the signal from a detector is passed through a fil-
ter whose shaping time is too short. When this happens, the filter is unable to com-
pletely process all of the charge collected by the associated detector/preamplifier. This
can cause a “deficit” in the pulse height value which does not accurately represent the
energy of the event. Because the collection time of a detector can vary from one pulse
to the next, ballistic deficit may lead to loss of resolution and distortion of the MCA
energy peak shape. The effect becomes more pronounced with large detectors and
high energies.
Low energy tailing often indicates the presence of ballistic deficit. For traditional ana-
log signal processing, users are forced to manually inspect the peak shape of the MCA
energy spectrum and optimize the shaping time selection. For detectors that exhibit
ballistic deficit, the shaping time is often increased to improve resolution, but at the
expense of throughput. The Trapezoidal shaping function employed in the 9660 allows
independent adjustment of the Rise/Fall time and Flat Top. The Rise/Fall time sets the
noise filtering characteristics and the Flat Top adds sufficient time for the charge to be
collected and integrated. As a result, the ballistic deficit effects can be minimized by
adjusting the Flat Top time without burdening the Rise/Fall time. This results in a
overall shorter processing time and higher throughput compared to Gaussian shaping
and traditional analog signal processing.
Chapter 6 - Setup and Operation
44
Model 9660-9660A ICB Digital Signal Processor
Figure 23 Overcompensated Pole/Zero
Scope:
Vertical: 50 mV/div
Horizontal: 10
µ
s/div
Source:
60
Co
1.33 MeV peak at 6 V amplitude
Count rate: 2kcps
Shaping: 5.6
µ
s rise time
0.8
µ
s flat top
