Hi all, I dug more to find the reason of the strange behaviour in energy spectra and found those results : - I first installed Gate3.0.0, 3.1.0, 3.1.1 and 3.1.2 on both 32 and 64 bits architectures,and both cluster and single PC. - I ran PET and SPECT benchmark on all configurations and compared both energy spectra and results from benchmark.C with root : everything was identical between all different config. - Then I ran the single_layer.mac simulation on all config and obtained the same difference in energy spectra as Irene and Jasmine reported. Then I found that the change in energy spectra occurs between Gate3.0.0 and 3.1.0. Results from 3.1.0, 3.1.1 and 3.1.2 were the same. (see attached images) - I realised then that between Gate3.0.0 and 3.1.0, the associated Geant4 versions are 8.0.p01 and 8.1.p02. And between these two different Geant4 versions one of the main changes was the upgrade of the most important data package G4EMLOW, that move from 3.0 to 4.0, and that includes major changes. - I decided then to run the single_layer.mac simulation with Gate3.1.0, Geant4.8.1.p02 and to change the G4EMLOW4.0 package with G4EMLOW3.0 package from the older version. Then the energy spectra obtained was more similar than this obtained with Gate3.0.0, but still different (see energy_3.1.0.EMLOW3.0.jpg image). So I think this little difference is due to other main changes between Geant4.8.0.p01 and Geant4.8.1.p02, it is statistical I think. - The change in behaviour do not come from any digitizer module (adder, readout, any blurring ...), they are still the same since Gate3.0.0 and even before, I compared all files and algorithms. - For me it is not possible too to have such differences only due to gcc version or stdlib version. But here I ask myself a question : MJ, you said you get results with 3.1.2 similar to 3.0.0 but what G4 data packages do you use ?? Then if you use the good packages version, I do not understand anything anymore ... Let me know your thoughts about that. - I will now try to reproduce the problem in intersector coincidences with the Gate coincidence sorter to see where this problem come from and to see if it is relative to the energy spectra problem too. But now, it is time to go on holidays :-) Dig, dig, dig and dig more as say MJ ! Cheers, Simon PS : sorry but I send the images in different mails due the size limitation.