[gate-users] about random coincidences

yuxuan.zhang at di.mdacc.tmc.edu yuxuan.zhang at di.mdacc.tmc.edu
Mon Sep 27 16:29:43 CEST 2004

Hi, all,

Several weeks ago, I reported an phenomenon about the strange random
coincidence distribution among the detectors with a point source placed 
in the center of the detectors. Recently I found out why it happened.
The problem is caused by the sequence of the deadtime and the energy-cut 
macros in the
digitizer segment. In GATE Users Guide and all the benchmark or demo 
files, the
definitions of deadtime are always placed after the energy-cut. However, I
think this might not be the real case. We know one detector should be dead
for a certain time (deadtime) after receiving one event, no matter how 
much energy has
been deposited by this event. So even the events with a very small or very 
large energy should
make the detector dead for a while. But if the deadtime is defined after 
energy-cut, those events with an energy that is outside the energy window 
will be cut
and can not reach the module of deadtime in the digitizer chain. Therefore 
detectors can still detect another event immediately. In this case, the 
coincidences rates
will become larger than the real one.

Best regards,

Yuxuan Zhang

Dr. Yuxuan ZHANG 
Dept. Experimental Diagnostic Imaging
Univ. Texas, MD Anderson Cancer Center
1515 Holcombe Blvd, Unit 217
Houston, TX 77030-4095

Tel: +1-713-745-1671 
Fax: +1-713-745-1672
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