McStas

[Kristian Nielsen]

> Date: Sat, 14 Aug 1999 04:27:32 +0200 (CEST)
>From john at rrdjazz.nist.gov Sat Aug 14 04:27:31 1999

> I have just seen the article in Neutron News about McStas. Please add me to
> your mailing list.

You have been added with the address "john.copley at nist.gov". Another
good source of information on McStas is the World Wide Web page:

    http://neutron.risoe.dk/mcstas/

> As a member of the Neutron Optics Advisory Committee to the SNS project I am
> interested in knowing more about the extent to which a standard has been
> established for neutron ray-tracing calculations.  Who should I contact at
> Oak Ridge and at Los Alamos to find out about their projects?  I'm guessing
> Lee Robertson and Phil Seeger?

I just returned from a visit at Argonne National Laboratory. I talked to
Ken Herwig from Argonne, he and Lee Robertson are the ones to contact
about the standard. Phil Seeger is in charge of the MCRUN/MCLIB program,
which as far as I know does not use the new standard at this point.

At Argonne, I talked to several Scientists designing instruments for the
SNS. I started three simulation projects based on McStas. One is a guide
design for a reflectometer by Frank Klose and John Ankner. Another is a
setup for time-focusing with off-cut analyser crystals by J. Carpenter.
A third is an investigation of background contribution from small-angle
scattering off Soller collimators.

My colleague Kim Lefmann told me that there are some people at NIST
already using the McStas program. I CC'ed this mail to him, perhaps he
will send you details.

> You may or may not know that I and collaborators wrote a Monte Carlo program
> to calculate multiple scattering effects, MSCAT.  It is almost an antique
> but perhaps some of the concepts could be incorporated in your program.

Actually, Ken Herwig mentioned the program to me during my visit, and I
was planning to take a look at it. Do you have any pointers to
references or downloads for the program that could serve as a starting
point?

> Meantime can I ask you whether you can simulate a neutron optical filter
> with coatings that have a rather unusual (to my mind) refelctivity as a 
> function of Q?  Roughly speaking R(Q) is near 100% up to about 0.027 A-1.
> Thereafter it drops rpaidly but it has a broad peak at about 0.04 A-1
> whose peak reflectivity is about 30%!

This question was actually raised also in March when I was visiting the
ILL in Grenoble, France. We have not simulated such a filter yet.
However, I believe it would be quite simple to add, by modifying the
current simulation of supermirrors. The supermirror code contains a
simple expression for the reflectivity as a function of Q; simply
replacing this with an expression for the reflectivity curve for the
filter should be sufficient. If you are interested, I would be happy to
assist with this.

Please contact me when you are back from your vacation, and we can
follow up on this.

 - Kristian.

--
Kristian Nielsen        kristian.nielsen at risoe.dk
Risø National Laboratory
Condensed Matter Physics and Chemistry Department
Tel. +45 4677 5515   Fax +45 4677 4790