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<font size="-1"><font face="Verdana">Dear McStas users,<br>
<br>
First, as I am trying to use some basic components, I think that
this statement from the McStas component manual (2.6), about the
use of the analytical reflectivity calculation for mirror and
guide devices, is wrong:<br>
<br>
"It is important to notice that when m < 1, the reflectivity
remains constant at R = R0 up to q = Qc, and not m.Qc. <br>
This means that m < 1 parameters behave like m = 1
materials."<br>
<br>
If you try the attached file instrument, you will that as
expected the cut-off wavelength of a simple mirror with 1deg
incidence angle, for a collimated beam, depends on the m-value
of the surface:<br>
- 10 Ang for m=1<br>
- 5 Ang for m=2<br>
- 20 Ang for m=0.5<br>
- 30 Ang for m=0.333<br>
The two last results show that when m<1, the reflectivity
falls to 0 after m.Qc, and not after Qc. This is in
contradiction with the documentation, but in agreement with what
the user would expect (at least me...).<br>
I tested here with Mirror.comp, but I observed the same
behaviour with Guide_gravity.comp, and as I understand all guide
components use the same routine for calculating the
reflectivity.<br>
I may have done something wrong, as </font></font><font
size="-1"><font face="Verdana"><font size="-1"><font
face="Verdana">I am not experienced with McStas</font></font>.<br>
<br>
Second, I think that the analytical empirical formula for
supermirror reflectivity sometimes behaves in a strange way:<br>
For a given value of m (e.g. m=2), if one changes Qc to
something different from the default value, the actual
supermirror cut-off also changes.<br>
This is not what I would expect. The general convention is that
the "m" value is defined as Qmax(supermirror)/Qc(natural
Nickel), therefore it should not depend on another parameter
than m.<br>
On the other hand, one can imagine having a material different
from nickel on the supermirror surface, which would have a
different Qc (for total reflection).<br>
Therefore it may be useful to change Qc and m, but these should
be completely independent parameters.<br>
For these reasons, I would suggest to replace the formula used
in the reflectivity calculation by this one:<br>
<br>
R= 1/2*R0*(1 - tanh[(Q-m.QcNi)/W]) (1-alpha* (Q - Qc)) <br>
<br>
It is the same as today, except that the parameter Qc in the
tanh function is replaced by the constant QcNi(=0.0217). This
makes the supermirror cutoff independent from the Qc parameter,
which determines the range of total reflection but nothing else.<br>
As I understand one can always use a file to model the
reflectivity of guide faces, but I think it it also nice to have
simple analytical formula with a clear and predictable
behaviour.<br>
<br>
<br>
Thierry Bigault<br>
<br>
<br>
<br>
<br>
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