Fermi Chopper

[Andrew Garrett]

  The following is a routine to simulate a fermi chopper.  The math is
set up to work in the accelerated frame of reference in which the
chopper is at rest.  By working in this frame, the computer can quickly
reject a large fraction of the incoming neutrons.  Only those neutrons
that have a reasonable likelihood of passing through the chopper take up
processing time.

  If a neutron deviates by less that 1 slit width in its passage through
the chopper, its probability is adjusted according to its actual
deviation.  This allows for a greater number of neutrons to pass through
the chopper than absorbing those that hit a blade.

  The blades separating the slits are currently held to be infinitely
thin and perfectly absorbing.

  I have tested this routine for choppers whose curvature is optimized
for 50, 100, 200, and 400 meV, and have compared the resolution to
calculated values taken mostly from Adv. Colloid Interface Sci, 71-72
(1997) 209-232.  Transmission values were obtained from an unpublished
calculation, with calculated values comparing favorably at energies
below the optimized energies, but reduced at higher energies.  Our
current belief is that the theoretical values are incorrect, due to an
inappropriate small angle approximation, and that the values obtained
from this routine are better.

  I would appreciate hearing about any other test results obtained from
this routine.

  Thank you,

  Andrew Garrett


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