Starsim implements a general stepping routine aggstep, which is responsible for tracking particles through the detector geometries, storing the HITs created in any sensitive detectors, and passing control to any user-defined digitization routines.  At present, only charged particles which deposit energy in a given detector volume will register a HIT.

aggstep.age:
0075 | * - - - - - - - - - - - - - - - sensitive - - - - - - - - - - - - -:
0076 |  AdEstep+=DeStep;  Astep+=Step;
0077 |  If (Charge=0 & dEstep=0)  Go To :done:
0078 |  If (isvol<=0)             Go To :done:
0079 |  Iret=0;

Shower Shape Based on Cerenkov Photon indicate that cherenkov photons (charge=0) have a significant effect on the shower-shape for photons and electrons in the FMS lead-glass towers. In order to enable these studies we need to pass cherenkov photons through the cone in aggstep.  We propose the following change...

aggstep.age:
0075 | * - - - - - - - - - - - - - - - sensitive - - - - - - - - - - - - -:
0076 |  AdEstep+=DeStep;  Astep+=Step;
---- |  IF IPART .neq. 50 { """Allow cherenkov photons to be passed to user routines""" 
0077 |     If (Charge=0 & dEstep=0 )  Go To :done:
---- |  }
0078 |  If (isvol<=0)             Go To :done:
0079 |  Iret=0;

ITRTYP == 7 indicates a particle which is a cherenkov photon.

FMS meeting 20100620 - changes to FPD geometry, as well as additional user-routines to handle the cherenkov photons, are also required.  These have been discussed within the FMS group, and no objections raised.  The changes to the geometry code do not add material or change the structure of the detector.  They only instruct starsim to append additional information to the FMS hits.  We can therefore make these changes without effecting existing simulation data sets.