Hi, Angela Foudray, I checked your files and I think you don't need to worry to much about the energy spectrum problem. We know that there are two singles in each coincidence data, let's call them s0 and s1. When Gate output the coincidences data, the program should have some algorithm which decides which one should be output first and which one should be the second. We don't know what's that algorithm is, but we are sure it's not based on the index of the rsector. In this case we can not say that s0 and s1 should have the same distribution function. So the difference of the two energy spectrum plot from the output coincidences file doesn't mean anything to us. What is meaningful for us is that the spectrum of each sector (or module, crystal,...) should be the same. You can do the statistics of the spectrum of the two sectors by yourself from the ASCII coincidence data, and you will see the two spectrum are indeed identical. You can check these four spectrum. Figures of coincidences1.gif and coincidences2.gif are plot with the coincidences data. Sector1 and 2 are plot according to the sector number. 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