Some thoughts on alignment of the detector with respect to the magnetic field.

For the technique which I am proposing, we need:
 
     (1) a chamber map of (x,y) coordinates which has been fixed by other
          measurements (optical, beam, ...);

     (2) a beam of positrons (say 50 MeV/c) or pions with a rather uniform
          coverage in phase space.

The proposal is to do a two step check.


1st step

With the magnetic field off, align the beam to the detector axis.  This can
be done by looking at the distribution of straight through rays and getting the

angles in the distribution to be centered on 0 degrees.  Presumably we will be
close enough in our initial alignment that slight shifts in the beam steering
would 
allow us to fix this.  

2nd step

Now turn on the B-field.  If the detector and field are aligned, we should get
spirals in the detector with helix axis centered along 0 degrees throughout the

track.  A misalignment between the field and detector would cause the the axis
of the helix to precess away from 0 degrees as the particles spiral through. 
If
we have a uniform coverage in phase space, it would probably suffice to simply
verify that that centroids in the distributions for each detector plane stay
fixed with
field on versus field off.  

Note for the case where we have uniform coverage, it is not crucial to align
the beam
with the detector since the centroids of the distributions should remain the
same 
with the field on or off.  Of course the distributions themselves will look
much
different in the two situations!

I have not looked at effects of multiple scattering but I think that they
should provide
smearing and not affect the alignment procedure.  This can be checked with
Monte
Carlo of course.  

Robert Tribble			                           NOTE: new area code in effect!
Cyclotron Institute/Physics Department    Phone: 979-845-1411 
Texas A&M University                             FAX:    979-845-1899 
College Station, TX 77845                        email:  tribble@comp.tamu.edu

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