Motivation
As reported at the Collaboration meeting in February 2003,
the comparison between the experimental data on photon conversions in the
STAR detector, when compared to the Monte Carlo results, indicated
the presence of "extra material" in the real detector, unaccounted for
in the then current GEANT model. This resulted from initial studies by
Ian Johnson, further continued by other collaboration members.
It is highly desirable to rectify
this situation for a variety of physics reasons.
Approach
The direct deconvolution of the "extra material" map from such
studies, although tempting, is not feasible due to large systematic
errors which become obvious when one takes a closer look at the problem.
It was decided that instead we use these data as indicators of where
to look in the physical detector (complete with all sort od technical
documentations, pictured etc), in order to add the rigth materials
in the right locations. This was done by myself, M.Potekhin, with the
expert help from David Lynn. The following elements were identified, that
were previously omitted from the model:
- G10 printed circuit boards on each end of each ladder
- Copper cables (high and low voltage) feeding the ladders
- Plastic tubes feeding the coolant from the manifold
to the SVT structure, and water contained therein
- The outer shielding which features the support "cage" made of
carbon composite tubes
- The main carbon support struts which were originally described
as made of Beryllium and had incorrect cross section
All of this has been added or corrected in the new version of the
SVT geometry description file, svttgeo2.g. This is only
activated in the version y2003b of the STAR geometry. As a reminder,
y2003a is a version which contains mainly bug corrections, whereas
y2003b has the aforementioned SVT improvements, plus additions in
endcap calorimeter (to be done), Photon Multiplicity Detector,
the FPD and other changes as well.
Results
There have been simluations run with y2003a ("old" SVT) and
y2003b ("new" SVT) geometries. As a rough metrics, plots were
produced of the radiation length accrued by geantinos propagating
in the detector, as a function of rapidity. It should be noted that
this is more of a demonstration of principle and the physics
implications may or may not be significant depending on the
nature of study in question (the effects might be felt outside the
apparent rapidity range covered by the added material)
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