5.3.2. Collimator components

Neutron guides, benders, soller collimators etc. are represented in ray-tracing routines of RESTRAX by a single component schematically sketched bellow.

The corresponding line in the configuration file is:

n-guide (use,dist,length,hor1,hor2,ver1,ver2,ro,gh,gv,refh,refv):
0  10.  6300.  2.5  2.5  5.  15.  2.4E-4   1  1  1  1

for the guide section or

1st collimator (dist,length,hor1,hor2,ver1,ver2,ro,gh,gv,refh,refv):
236.  534.  8.05  5.  9.05  11.  0.  0.  0 .  1  1

for the collimator sections.

These two differ only by the first integer in the line for n-guide, which indicates the presence of the guide section (if use=0, then the guide segment is ignored). The meaning of other items is defined in the figure and table below.

Definition of a collimator in RESTRAX

dist  distance of the entrance opening from the preceding component
lengthL collimator length
hor1,hor2w1,  w2   widths of the entrance and exit window
ver1,ver2h1,   h2 heights of the entrance and exit window
ro1/R curvature of the collimator [m-1] (1/R < 0 in the figure)
gh,gvmh, mv critical angles of the horizontal and vertical reflecting mirrors (*)
refh,refvrh,rv reflectivities corresponding to mh, mv
 D thickness of the lamellas (fixed to 0.2 mm in this version)

(*)  mh, mv are expressed in the units of the critical angle of nat. Ni.

The number of lamellas is calculated to fit the collimator size and divergence angle (ALF1 .. ALF4).
Special cases:
ALFn≥500  … no lamellas are considered.
ALFn=0  … the n-th collimator is ignored.

Lookup tables for guide reflectivity

Whenever mh or mv differs from zero, a file with a corresponding lookup table for the mirror reflectivity is searched. The naming convention is mirrorx.x, where x.x is the critical angle specified as  mh or mv.

The directory search order is the same as for the other configuration files, i.e. first the current directory, then the other directories with *.cfg and *.res files defined upon startup and finally the installation directory. If the corresponding mirrorx.x file is not found, a step-function is taken with r(theta)=rh (or rv) for theta lower than the critical angle for the given neutron wavelength.

Example:

The file mirror4.1 contains a table for the supermirror reflectivity, which will be used any time when 4.1 (or 4.14, 4.06 etc.) is specified as the critical angle for a collimator. Its 3 columns contain the angle of incidence theta [Ni nat.] and r(theta) for the two spin states (e.g. polarizing guide can be simulated in this way.
Caution! This table must contain one row as a header and up to 128 rows with a constant step in theta. All following lines are ignored, i.e. the reflectivity is cut to 0 for theta>theta(last line).