[Halld-tagger] Field mapping diagram and fringe field

[Daniel Sober]

Dear Tim et al.,
Here's an updated version of the map request in the form of a diagram in 
mapper coordinates.  I think it reflects most of what you reported earlier.
The main thing I would like to add is to assure that we make some 
provision for measuring the *fringe field at the exit flange*, which is 
about 6 cm from the nominal focal plane.  This field will be of order 10 
gauss, and only a crude hand measurement is needed, but it may be 
important in adding another significant figure to the energy 
calibration.  We also need to know whether the magnetic shielding 
planned for the fixed-array PMTs in this region is adequate.
If you have not made other arrangements for measuring this fringe field, 
I would propose marking off distances along the exit flange and making a 
measurement with a hand-held gaussmeter at intervals of about 50 cm.  
The origin of coordinates could be referred to the magnet yoke.
Dan
-- 
/Daniel Sober
Professor
Physics Department
The Catholic University of America
Washington, DC 20064
Phone: (202) 319-5856, -5315
E-mail: sober at cua.edu/
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