Re: fcal test in hall-b

["Daniel S. Carman" <carman@jlab.org>]

Elke,

Your question is a good one and we have been discussing this in
the group for some time.  I think that we feel that the 200 micron
resolution goal is going to be extremely demanding from the standpoint
of design and reconstruction.  At this point, using our small-scale
prototype, we have direct measurements of all of the main contributions 
to the FDC resolution, with one notable exception.  How will the
performance of the chambers be degraded when they are in a 2.2T magnetic
field?  We already know that this contribution to the resolution
will be the dominant one (by far).  What we do not know, because there is
very little direct information in the literature, is how much the
resolution will be degraded as a function of field strength.  We
have seen some published information (in a cathode chamber with an
electrode structure quite different from ours) that the chamber
resolution degraded by a factor of 8 at 2T.  Certainly if this was
the case, I think we would have to go back to the design drawing
board and reconsider what we are trying to build.

So the bottom line is that we are extremely concerned about the
impact of the field and we want to know as soon as possible if our
current design plans will work.  We do not want to go through the
design and construction of a full-scale chamber system if we are
ultimately going to have to come up with an entirely new concept
or radically change our approach.  In short, the resolution degradation
of the FDCs in the magnetic field is the single biggest unanswered
question as we move forward.

With this said, we feel strongly that we want to address this question
before too much more serious design work proceeds.  The amount of
manpower to set up the test is minimal, and we have an opportunity
to really ensure that our design is sound.  It may set us back a month
or so to spend the time to acquire and analyze the data after we take it,
but this seems to be a much better option than proceeding with a design
that we cannot guarantee will meet the tight specifications required
from this tracking system.  Our deadline for completing the design is
Oct 2008.  We would be much more comfortable getting this question out
of the way in 2007 than allowing ourselves to go into 2008 with this
unknown hanging over us.

Anyway, let me know what you think.


				Regards,

					Daniel


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*                                                                    *
* Dr. Daniel S. Carman                   e-mail : carman@jlab.org    *  
* Staff Scientist                        office : (757)-269-5586     *
* Jefferson Laboratory                   web: www.jlab.org/~carman   *  
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