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 the 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 those 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