Disappearing Hits in Geant



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When switching to the split geometry, hits no longer appeared in the Pisa output. Since the geometry hierarchy changed completely, and lots of new volumes were introduced, this was a very difficult bug to find.

Starting with the old, working, software, I slowly introduced a few of the new volumes. I simplified the geometry by only using SCM1 (the South-East cage), and only station 4. Note there are hits in the disk (as well as in the barrel).

Here is the definition of the SE cage mother volume: 

  par(1) = 45.   !50.    ! copied from 'dspec SICG'
  par(2) = 270.    !260.
  par(3) = 3
  par(4) = -37.95
  par(5) = 3.4
  par(6) = 19.6
  par(7) = -24.25
  par(8) = 3.4
  par(9) = 19.6
  par(10) = -18.1
  par(11) = 3.4
  par(12) = 12.3
  CALL GSVOLU( 'SCM1', 'PCON', sili_med_coldair, PAR, 12, IVOL1)
  call gspos ( 'SCM1', 1, 'SICG', 0.,0.,0., irotnull, 'MANY')



  

      

  

    
      Next I swapped out the hardwired PCON parameter numbers with the 
      values calculated from the PHNX.PAR namelist values, which should
      be exactly the same.
      
 
      No hits! what is different? The volume SCM1 is a PCON, 
      and the in one case the z-positions of the polycone are defined in 
      the positive direction (-37, -24, -18), and in the non-hit case 
      the direction is negative (-18, -24, -37).
      

      This is not a documented GEANT3 feature/bug.
    
 


  

      







  

     After fixing this, and several other items, we now see GEANT hits in all FVTX layers
     

  

     A fix was required to get these hits into the root output.


In the VTX/FVTX software, LAYERs 1-4 are allocated to the VTX barrel,
and 5-20 are used by the FVTX to refer to half-disks as shown here: 


          SCM2           West          SCM4
          +-------------\    /------------+
          | si06 08 10   \  /   16 18 20  |
          |            12    14           |
  South   +---------------  --------------+   North
          |            11    13           |
          |   05 07 09   /  \   15 17 19  |
          +-------------/    \------------+
          SCM1           East          SCM3




SCM1-4 are the geant volume names for the cages.
    		 







    
Hubert van Hecke



Last modified: Tue Jan  4 17:25:23 MST 2011