VITESS Tool Generate Mirror Files
This tool generates reflectivity files that are used in the modules 'guide'
and 'bender'. Two options are available: a linear decrease of reflectivity with
scattering angle, or a description using an additional quadratic term. In both
cases, an output filename has to be given. If no path is included in that
filename, the generated reflectivity file will be written to the parameter directory.
Quadratic Swiss Neutronics description
If this option is chosen, the only additional input parameter necessary is the m-value of
the supermirror coating. The reflectivity is the calculated as
R = 1/2 R0 (1 - tanh((Q - m Qc,Ni)/W)
(1 - a(Q - Qc) + b(Q - Qc)2)
where R0=0.99 and W, a, b and m2 are linear functions of m:
W = -0.0002*m + 0.0022
a = 0.1204*m + 5.0944 if m>3, else a=m
b = -7.6251*m + 68.1137 if m>3, else b=0
m2 = 0.9853*m + 0.1978
The parameters used here are the results of fits to experimental data
(H. Jacobsen). The reflectivity curve gained for m=4 is shown by the black line in figure 1.
Linear description
If quadratic Swiss Neutronic description is answered with no, the
parameters m, R0, Rm, Qc and W (s. table) have
to be given in addition to the filename.
For W=0 the reflectivity curve consists of linear functions and a sharp cut-off
at Q = m Qc,Ni. The red line in figure 1 shows an example for m=4.
For W > 0 and Q > Qc, the reflectivity R is calculated as
R = 1/2 R0 (1 - tanh((Q - m Qc,Ni)/W)
(1 - a(Q - Qc))
using
a = (Rm - R0) /
(m Qc,Ni - Qc)
The product m Qc,Ni must be greater or equal Qc, because m is meant to extent the Q range
to values greater than Qc.
An example for m=4 and W=0.003 is shown by the green line in figure 1.
Parameter
Unit |
Description |
typical values |
R0 | reflectivity for 0 <= Q <=Qc | 0.99 |
m | factor of supermirror | 1 ... 4 |
Qc [Ang-1] | crit. momentum transfer |
0.0217 |
Rm | reflectivity for Q = m Qc(Ni) for a sharp cut-off (W=0) | 0.6 .... 0.95 |
W [Ang-1] | width of the cut-off range W=0 causes a sharp cut-off |
0 ... 0.004 |
mirror file | name of the file into which the reflectivity curve is written | - |
Figure 1: Reflectivity curves
obtained with the tool for m=4 using different options.
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