[neutron-mc] Propagation in neutron guide

[Emmanuel Farhi]

Hi again Timur,

If we consider that the source is far away from the guide entry, and its
dimensions are realistic, then the neutron wave can be considered as a spherical
wave. In fact, if the source emission is quasi-isotropic, then the portion of
the flux that reaches the guide entry is the solid angle/4Pi times the total
neutron flux coming out of the source. This is where the r^2 factor comes into
account, and it stands for the fact that the total flux on the neutron wave
spherical surface is constant as the radius increases.

In guides, the neutron wave is planar, and the total flux is directly
transmited. Thus there is no distance dependency, except for absorption.

Emmanuel.

Timur Elzhov wrote:

> Dear McStas experts,
>
> I noticed, that in "source" components the neutron weight is
> multiplicated by 1/r^2 factor. That's clear -- that's reverse
> square law for isotropic radiation. But I do not see this multiplier
> on neutron propagation in the guides. I looked through macro PROP_Z0
> as well as PROP_DT(), but found nothing about 1/r^2.
>
> Why?
>
> Regards,
> Timur V. Elzhov
>
> _______________________________________________
> neutron-mc mailing list
> neutron-mc at risoe.dk
> http://neutron.risoe.dk/mailman/listinfo/neutron-mc

--
What's up Doc ?
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Emmanuel FARHI, http://www.ill.fr/Computing/people/Farhi  \|/ ____ \|/
CS-Group ILL4/156, Institut Laue-Langevin (ILL) Grenoble  ~@-/ oO \-@~
6 rue J. Horowitz, BP 156, 38042 Grenoble Cedex 9,France  /_( \__/ )_\
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