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 <table width=100%> <tr valign=top> <td> <td>Angela M K Foudray <afoudray@stanford.edu> Sent by: gate-users-bounces@lphe1pet1.epfl.ch 09/02/2004 02:58 PM Please respond to GATE feedback and helpline for Users <td>                To:        GATE feedback and helpline for Users <gate-users@lphe1pet1.epfl.ch>         cc:                 Subject:        [gate-users]         Energy Spectrum Woes - Dual Head and 4 Head Designs & Number of        Heads</table> <tt>I forgot to mention why I used the subject I did for the last post - the energy spectrum problems at the high energy (700 keV) that I mentioned is most apparent in the dual head design, goes slightly away in the 4 head design, and is not apparent at all in a ring design of 20+ "heads".  In other words, the severity of this anomalous high energy peak seems to be a function of the number of heads in the system.  (In case you're wondering, we have /gate/digitizer/Coincidences/minSectorDifference 1) I'm attaching spectra (lower counts than the previous spectra I sent) for 4,5,6 heads and a ring design. I'll also send you the .mac file for the dual head ("dhci_sen10cm.mac"). You'll notice, even for the ring, the spectra are not similar in the low energy region ("sasens_ring_np60_oe.mac")... Ange _______________________________________________ gate-users mailing list gate-users@lphe1pet1.epfl.ch http://lphe1pet1.epfl.ch/mailman/listinfo/gate-users </tt>