Hi Paola, comptonPhantom is the variable containing the number of Compton interactions that occured in every volume declared as phantomSD. And comptonCrystal is the one containing the number of Compton interactions that occured in every volume declared as crystalSD. And no matter when (before or after which one...). Hope it helps, Simon <div>2008/3/27, Paola Solevi <<a href="mailto:Paola.Solevi@mib.infn.it">Paola.Solevi@mib.infn.it</a>>:<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> Dear Gate users, in my simulation set-up I have an Aluminum box that contains the crystal matrix. Since I'm interested in studying the effect of the Al box on the detected signal, I set the Al box as PhantomSD. Is is true that the "comptonPhantom" flag gives the number of Compton intractions experienced by the photon _before_ reaching the CrystalSD? What happens if I have a first Compton in the crystal, a second Compton in the Aluminium box and again a third interaction in the crystal? Because what I found is that if I set the Aluminium box as made of Air, the sensitivity (number of detected events with comptonPhantom==0) is higher than the one of the housing made of Al. And this can occur if comptonPhantom!=0 also when I have a first hit in the crystal. Thank you for your help, best regards         Paola _______________________________________________ Gate-users mailing list <a href="mailto:Gate-users@lists.healthgrid.org">Gate-users@lists.healthgrid.org</a> <a href="http://lists.healthgrid.org/mailman/listinfo/gate-users">http://lists.healthgrid.org/mailman/listinfo/gate-users</a> </blockquote></div>