Nate and Don:

I don't think we need a lot of fancy Monte Carlo to estimate the residual
field we can tolerate.  In fact, I think a calculator and some common sense
will suffice (at least up to a factor of 2 or so).  Here is my attempt:

To estimate what we can tolerate, remember that, in addition to
straight-through beam particles at momenta up to ~120 MeV/c, the alignment
studies will include "straight" positron tracks from Michel decays.  The
latter will be a far more stringent constraint, especially since we want
them to be "straight" even at relatively low energy.  So, for a figure of
merit, consider a positron with a pT of 10 MeV/c.  At 100 G, this will have
a radius of curvature of 334 cm.  If I did my arithmetic correctly, such a
positron, if it has a total energy of 50 MeV, will deflect 1.6 mm over a
longitudinal distance of 50 cm.  A 14 MeV positron with the same pT would
deflect 5 times this much.

Clearly this is too much to tolerate.  In fact, these calculations indicate
that even 10 G is probably too much to treat as precisely straight, given
the chamber resolution, but at least it's in the ballpark.

My conclusion:  Nate's 2 G "reach and grab" number isn't crazy. (!?!)  We
might be able to tolerate twice that, but not five times.  Thus, my own
original reach and grab guess -- 10 G -- was too high.

Carl

+-----------------------------------------------------------------+
| Carl A. Gagliardi                                               |
| Cyclotron Institute              Phone:   (979)  845-1411       |
| Texas A&M University             FAX:     (979)  845-1899       |
| College Station, Texas  77843    E-mail:  cggroup@comp.tamu.edu |
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