[NOBUGS] New McStas release in the workings, fixes issue with ESS moderator bispectral moderator

Peter Kjær Willendrup pkwi at fysik.dtu.dk
Tue Feb 25 09:28:08 GMT 2014

Hi all,

This is just to let you know that the McStas team is working hard toward a new McStas release, McStas 2.1.

Only one bit is missing: An updated ESS moderator component with:

* Possibility for “pancake” spectra
* Simplified interface with less parameters, brilliance is defined via a simple string like “2001”, “TDR” or “2014”
* Possibility for either
- MCNPX-like geometry with cylindrical cold moderator, thermal wings
- Simple flat, mono-spectral source
* Fixes the bispectral weighting issue mentioned below

A tentative list of changes wrt. McStas 2.0 is available as an attachment to this email. The release should be available within apx. 1 week.

Now for the bispectral weighting issue:

In the currently available McStas bispectral sources, i.e.
- The “standard” McStas 2.0 bispectral source
- The “updated” ESS source for 1.12c and 2.0 use, (inside http://www.mcstas.org/download/share/ESS_source_20130502.zip)

a bug was found, affecting bispectral instruments ONLY,  where  0 < cold_frac < 1.

The “standard”, simple 1.12c mono-spectral source is NOT affected.

When assigning statistics to the cold part of the spectrum via the cold_frac input parameter, a re-weighting of the spectrum was missing, i.e.:

if (cold) {
   p/=cold_frac; // Divide by cold_frac to renormalise cold moderator intensity
 } else {
   p/=(1-cold_frac); // Divide by (1-cold_frac) to renormalise thermal moderator intensity

* In the case of a cold_frac of 50%, this means that existing results have been UNDERestimated by a ‘clean’ factor of 2.

* As you can see  from the “equations", a cold_frac = 1 (i.e. cold neutrons only) or cold_frac = 0 (i.e. thermal neutrons only), these cases are NOT affected by the mis-weighting.

* For non-trivial choices of cold_frac, the re-weighting is not as simple.

We advice you to install McStas 2.1 to fix the issue. This release should be available within apx. 1 week.

If you have any questions or concerns regarding this or other issues with McStas, feel free to write me an email.

Best regards, and sorry for any inconvenience this may have caused!

Peter Willendrup

Peter Kjær Willendrup
Senior Research Engineer, Special Consultant

DTU Physics

[cid:79e8d7e5-955b-41f0-9f75-406d70cbe04f at win.dtu.dk]

Technical University of Denmark

[cid:976f2279-8e5e-4bfc-9555-276b77fd9d22 at win.dtu.dk]

Department of Physics

Building 307

DK-2800 Kongens Lyngby

Direct +45 2125 4612

Mobil +45 2125 4612
Fax +45 4593 2399

pkwi at fysik.dtu.dk<mailto:pkwi at fysik.dtu.dk>

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