hi all, In my simulation I noticed a strange result that I can not figure it out. My setup is that: using cylindricalPET system with 30 rsetors, one point source at the center. The radiation activity is very high, thus there are lots of random coincidences (accidental coincidences) in the totoal coincidences. Now I do the statistics on these random coincidences according to the sector index, then I found that the coincidences between 2 opposite rsectors (e.g. No.0 and 15, or No.5 and 20) are much more than the coincidences between other 2 sectors ( No. 0 and 14, or No. 0 and 16 ...) Usually the difference can be 50% or 100% depend on the thickness of the detector. Is anyone else noticed that? Can you have a try in your computer to find out whether this is a bug of GATE or there is somthing wrong with my simulation? Thank you very much. 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