Non-member submission from ["M.E. Hagen" <m.e.hagen at phys.keele.ac.uk>]

[Kim Lefmann]

>From m.e.hagen at phys.keele.ac.uk Tue May 15 02:31:53 2001
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Date: Mon, 14 May 2001 23:42:47 +0100 (BST)
From: "M.E. Hagen" <m.e.hagen at phys.keele.ac.uk>
Subject: Some infor on Fermi chopper routine
To: neutron-mc at risoe.dk
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Stuart,

The curved Fermi chopper routine that Andrew wrote for McStas is, I t=
hink, a version of the Fermi chopper routine that I wrote for PRMON i=
n FORTRAN-90. Andrew and I shared an office at KEK for a while and wo=
rked on simulating a curved Fermi chopper for MARI. I'll send you dir=
ectly a copy of the FORTRAN-90 code which also includes a routine for=
 a "straight" Fermi chopper which may (or may not) help. Also I can o=
utline the "theory" behind the curved Fermi chopper routine which may=
 help with understanding the McStas code.

The routine works by transforming to the frame of reference of the cu=
rved chopper not the lab frame. In the chopper frame the curved blade=
s remain fixed in position of course but the neutron now follows a /c=
urved/ path. Assuming that the Fermi chopper is a cylinder shape then=
 in the lab frame one can calculate the entrance and exit positions a=
nd times at which the neutron intersects the chopper body. One then t=
ransforms those positions into the chopper frame, the transform invol=
ves the frequency and phase of the chopper.

The slats (channels) of the chopper are assumed to be on constant rad=
ii from an origin. One computes the radial distances of the entrance =
position and exit position of the neutron and checks if they correspo=
nd to the same slat. If they don't the neutron is rejected. If they d=
o one must then check that the neutron always stays within the slat a=
s it passes through the chopper. The algebra we worked out showed tha=
t if one plots the radial distance of the neutron along its flight pa=
th (in the chopper frame) from entrance to exit as a function of time=
 (or polar angle) it can only have a maximum of one turning point. Th=
us first one checks the derivatives of the radius at the entrance and=
 exit points, if they are the same the neutron stayed within the slat=
 and is accepted. If they are different then one must bisect down to =
the position where the derivative is zero. If at any time the radial =
distance of the neutron wanders outside the inner and outer radii of =
the slat the neutron is rejected. If it doesn't the neut

If the neutron makes it through one just uses the exit position and t=
ime in the lab frame.=20

Andrew was only interested in MARI and so only produced a curved chop=
per routine I believe, the straight chopper can be simplified slightl=
y (see FORTRAN-90 code).

Hope that is some help, e-mail me if you want to know more,

cheers,

Mark