We are currently trying to debug the TPC geometry (tpcegeo3.g aka the TPCe04 option). The issue is that the material in the TPCe04 geometry Comparison of Victor's AgSTAR TPC geometry and Yuri's Root/TGeo TPC geometry on which it was based.  We suspect that the problem is in the TSAS and TSA1 volumes.  These volumes describe the aluminium support structure of the TPC endcap.

 

Figure 1.1 -- Number of radiation lengths vs eta for TPC envelope.

 

 

Figure 1.2 -- Number of radiation lengths vs eta for TPC envelope without the TSAS and TSA1 volumes present.

 

 

Figure 1.3 -- Difference in radiation lengths between the above geometries.

 

The problem is that the TSAS and TSA1 volumes contain significant amounts of material.  For example, 1.8 cm thick aluminium ribs which present much more material than the 2% of a radiation length shown above. 

 

 

In order to investigate the origin of this bug, I commented out all other volumes in the TPC geometry and only create the TSAS and TSA1 blocks.

Figure 2.1 below shows the material distribution of just these two blocks. 

 

Figure 2.1 -- Number of radiation lengths vs eta for just the TSAS and TSA1 volumes  (i.e. this is what figure 3 should look like).

 

 

Conclusion:  There is something in the tpc geometry which is interfering with the tpc support structure (TSAS and TSA1 volumes).  

 

(1) We can confirm that the TSAS/1 volumes are present, and 

(2) Rule out a large mistake in the material / medium definitions, and

(3) Begin to suspect that another volume is overlapping TSAS/1 (despite the fact that root doesn't identify it).

 

 

In order to determine which volume is at fault, I will systematically go through the list of volumes at the same level in the geometry tree as TSAS and TSA1, turning those volumes back on until I see the material distribution in figure 2.1 change.

 

The most likely culprit is the TPGV (TPC gas volume), as it is unlikely that  the other volumes (inner / outer field cage and membrane) would overlap the entire endcap of the TPC.  So...

 

 

Figure 3.1 -- Number of radiation lengths vs eta for the TSAS and TSA1 volumes with TPGV added .  The number of radiation lengths seen in the TSAS and TSA1 volumes dramatically decreases.  Bing-o.

 

 

Figure 3.2 -- Same as figure 3.1, but I zoom in on the y-axis.  Compare this to figure 1.3.  What we see here is the TPC gas distribution plus an additional contribution consistent with the material distribution seen in figure 1.3.