sPHENIX tracking discussion

[John Haggerty <haggerty AT bnl.gov>]

When we put together the sPHENIX schedule a year ago, one issue that 
came up was magnetic field mapping.  The problem for the schedule is 
that one can't really do a meaningful field map until the solenoid is 
inserted in the Outer HCAL which serves as the flux return, and as we 
plan to cool down the solenoid with the RHIC cryogenic plant, one 
couldn't really do a field map until the detector is almost complete, at 
which time one imagines we would be shit hot to put the tracking system 
inside and start trying to run the experiment.

I haven't spent the time to analyze it through thoroughly, but I suspect 
that we may not need to do a dedicated field mapping campaign.  I wonder 
if we could get away with building some number of field probes into the 
tracking detectors and relying on calculation of the field from those 
probes.  I imagined some number of probes (8?) at the outer radius if 
the TPC at either end in z, and perhaps additional probes at the inner 
radius, or attached to the vertex tracker.  They could be surveyed in 
place during construction.  With, say, 32 measurements of the field, do 
we have enough information about the field?

Since PHENIX was built, a number of small 3D Hall probes have become 
available; for example these:

> http://www.senis.ch/magnetometer/hall-probes/integrated-hall-probes

or some very nifty ones that Achim came across:

> http://www.metrolab.com/products/magvector-mv2/

I have also been in touch with a group at NSLS II that have developed a 
calibration device for Hall sensors:

> http://accelconf.web.cern.ch/accelconf/pac2013/papers/thpac14.pdf

which is apparently still functional at BNL which we could uses.

There even seems to me to be some advantages to doing the "mapping" as 
we start up, since one can directly measure the field as the detector is 
installed, and in the case of a TPC, one could be measuring the magnetic 
field of the TPC and aligning B with E during the detector commissioning.

Thoughts?

--
John Haggerty
email: haggerty AT bnl.gov
cell: 631 741 3358