[neutron-mc] Flux and Intensity

Kim Lefmann kim.lefmann at risoe.dk
Mon Feb 16 11:44:39 CET 2004

Dear Timur,

Notice that the units of flux is "n/(sec cm^2 AA)" Hence, the number of
neutrons you detect will depend on the wavelength window. Further, the
source uses "targeting" into a square area, e.g. the opening of a guide.
So your beam monitor should have dimensions so that it is within the
target beam, otherwise you will get "wrong" results. Try e.g. to set the
target distance and size equal that of the monitor.

I do not know if this solves your problem, otherwise please send me your
instrument file.

all the best,


Kim Lefmann
Senior Scientist
Materials Research Department
Building 227
Risoe National Laboratory

Phone: +45 46 77 47 26
Fax:   +45 46 77 57 58

On Fri, 13 Feb 2004, Timur Elzhov wrote:

> Dear McStas experts!
> I have a spectra (a histogram) from the real source and want to build
> my own source component based on that histogram. I looked through the
> existing components like 'Source_Maxwell' and did it. One question
> remained: what the normalizing multiplier should be chosen? At first I tested
> the following instrument: 'Source_flux' with radius = 0.01m/sqrt(PI)
> (so square = 1cm^2), and with target = 1e-4m (so very close to the source).
> Plus Monitor with square of (0.01x0.01)m^2 was placed at 1e-4m from the source
> too. Initial flux was 1e+13. But sum of Intensities from each Monitor bin
> was 1.9e+14! But I'd like to get exactly the same flux on the monitor! :)
> Where am I wrong?
> Energy range on the source as well as on Monitor was the same, of course.
> Thanks a lot.
> --
> WBR,
> Timur
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> neutron-mc at neutron.risoe.dk
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