[mcstas-users] Virtual source with fixed-m divergence

[Peter Kjær Willendrup]

Dear Thierry,

The first thing that comes to mind for me is the MCPL_output (which can store an event list at a given point in the instrument) and MCPL_input (which canvas such a file as a source). The weakness of this approach is that the files can become big, and are intrinsically limited in statistics, the strong point on the other hand is that all possible correlations between neutron parameters are preserved.

MCPL_input has the option to “smear” the events by MC (when repeating the events), i.e. in the form of a “sphere of confusion”, slightly tuning energy, position and divergence, all at the discretion of the user...

Other than that, one can envision to use any of the simpler analytical sources in McStas, possibly enriched with further adjustments in an EXTEND block (e.g. modify intensity or divergence based on position etc.)

I am also thinking of machine-learning approaches to synthesize a “semi-analytical” source from the content of an MCPL file, but this is so far not implemented in practice.

Best
Peter

Hent Outlook til iOS<https://aka.ms/o0ukef>
________________________________
Fra: mcstas-users <mcstas-users-bounces at mcstas.org> på vegne af Thierry Bigault <bigault at ill.fr>
Sendt: onsdag, marts 10, 2021 4:53 PM
Til: mcstas-users at mcstas.org
Emne: [mcstas-users] Virtual source with fixed-m divergence

Dear McStas,

Here is my problem:
I would like to simulate the exit of a cold neutron guide with a source. The aim is to concentrate on the instrument optics, without having to calculate all the guide from the reactor.
A simplified and quite general description of a guide exit is a beam with a fixed "m-value" divergence: for a given wavelength, the width of the angular divergence (either box-type, Gaussian or whatever) is essentially proportional to the m-value of the guide coating and to the wavelength.
If the guide has different supermirror coatings on horizontal and on vertical faces, this leads to different angular widths.
I assume the beam characteristics does not depend on the position at the guide exit in this approach.
My question is : what is the simplest way to simulate such a secondary (or virtual) source in McStas ? I only found components where the angular width is geometrically fixed (in degrees), therefore wavelength-independent.
Ideally I would also like to include a non-flat wavelength intensity distribution, but this can probably be done by adding a Filter_gen after the source.
I am thinking of making several calculations at different wavelengths, but if anyone knows a all-in-one solution I am interested.

Thierry




--
[Logo ILL]<https://www.ill.eu/>
Dr. Thierry BIGAULT
Research Engineer
Multlayer Neutron Optics
Institut Max von Laue - Paul Langevin (ILL)
71, avenue des Martyrs - CS 20156
38042 Grenoble cedex 9 - France
+33 (0)4 76 20 76 95
bigault at ill.eu<mailto:bigault at ill.eu>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman2.mcstas.org/pipermail/mcstas-users/attachments/20210310/85a3c586/attachment.html>