Hi Assen, Reading the Gate Reference Manual I've figured out that volumeID is an array of numbers given to each volume in the simulation. Using these numbers one can tell where the current hit occured. For example for a PET scanner, volumeID[1] holds the ID of the rsector for the current hit, volumeID[2] holds the ID of the module for the current hit and so on. See this document for more information: <a href="http://opengatecollaboration.healthgrid.org/GATEusers/documentation/REFERENCE_MANUAL/html_v1.0/classGateVSystem.html" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)"> http://opengatecollaboration.healthgrid.org/GATEusers/documentation/REFERENCE_MANUAL/html_v1.0/classGateVSystem.html </a> The problem is that when this array is written to the .root file, some kind of compression is applied to it (i suppose) and the numbers change their values. And that's why reading the ROOT file, those numbers seemed meaningless to me. Does anyone know if there is a way to decompress those values so I can get the real volumeID information for the hits? Maybe there is a function in ROOT that does the work. All the best, Georgi <div>On 4/15/07, <a href="mailto:kirova@mskcc.org" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">kirova@mskcc.org </a> <<a href="mailto:kirova@mskcc.org" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)"> kirova@mskcc.org</a>> wrote:<blockquote class="gmail_quote" style="margin-top: 0; margin-right: 0; margin-bottom: 0; margin-left: 0; margin-left: 0.80ex; border-left-color: #cccccc; border-left-width: 1px; border-left-style: solid; padding-left: 1ex"> Hi Georgi, As far as I remember volumeID is a  number given to each of the volumes simulated in the phantom or scanner. I think it should be one number per volume. Not sure why you see 10.  Probably there are 10 volumes total in your geometry, but someone else from the list may help more. All the best, Assen -----Original Message----- From: <a href="mailto:gate-users-bounces@lists.healthgrid.org" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)"> gate-users-bounces@lists.healthgrid.org</a> on behalf of George Gerganov Sent: Sat 4/14/2007 2:43 PM To: <a href="mailto:gate-users@lists.healthgrid.org" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)"> gate-users@lists.healthgrid.org</a> Subject: [Gate-users] What is the meaning of "volumeID" ? Hello everyone, The Hits tree in the ROOT-output files generated by GATE contains a branch called "volumeID". Each leaf is an array of 10 integers. Does anyone know what is the meaning of each of these integers? Thanks in advance, Georgi      =====================================================================      Please note that this e-mail and any files transmitted with it may be      privileged, confidential, and protected from disclosure under      applicable law. If the reader of this message is not the intended      recipient, or an employee or agent responsible for delivering this      message to the intended recipient, you are hereby notified that any      reading, dissemination, distribution, copying, or other use of this      communication or any of its attachments is strictly prohibited.  If      you have received this communication in error, please notify the      sender immediately by replying to this message and deleting this      message, any attachments, and all copies and backups from your      computer. </blockquote></div>