<HTML> <HEAD> <TITLE>voxellized phantom attenuation (no) problem</TITLE> </HEAD> <BODY> Hi Ahmet, Here is what I did.  I used standard Gate code (not your stepping action).  I defined a spherical shell detector, large enough to contain the phantom and I “attached” a phantomSD to the voxel phantom (uncompressed).  I collected “singles” in a root file and I looked at the distribution of scattered/unscattered photons using the field “comptonPhantom”.  Here’s my results with 5000 photons:                                         %unscattered All Bone phantom:                   3.1 All water phantom:                  7.1 MCAT phantom:                       9.0 All Lung phantom:                  48.3 These results make sense to me.  I did have one problem though: since the phantom has an even number of voxels in each direction, the source at (0,0,0) is at the corner of 8 voxels, and the GEANT4 navigator couldn’t determine in which voxels the particle was.  I moved the source by 1 micron in each direction.  You may want to try this approach (spherical shell detector) instead of using GateSteppingAction. Richard -- Richard Taschereau, Ph.D. Crump Institute for Molecular Imaging University of California Los Angeles 700 Westwood Plaza, A222 Los Angeles, CA 90095-1770 Phone: (310) 825-0697 Fax:     (310) 206-8975 email: RTaschereau@mednet.ucla.edu ---------------------------------------------------------- IMPORTANT WARNING: This email (and any attachments) is only intended for the use of the person or entity to which it is addressed, and may contain information that is privileged and confidential. You, the recipient, are obligated to maintain it in a safe, secure and confidential manner. Unauthorized redisclosure or failure to maintain confidentiality may subject you to federal and state penalties. If you are not the recipient, please immediately notify us by return email, and delete this message from your computer. ---------------------------------------------------------- </BODY> </HTML>