Error in Calculation of absolute Flux (Source_flux) ???

[Kristian Nielsen]

> Look at the following simple example: It is just a straight guide with a
> monitor at the beginning and at the end:

> COMPONENT part1 = Guide2(

>         h1 = 0.1,

>         h2 = 0.01,


I think you have a typo here (or else I did not understand you
properly). The guide entrance is 10cm high, but the exit is 1cm.

> Let's compare the results with the same instrument, but now the guide is
> focusing (exit height is now 8cm instead of 10cm):

> COMPONENT part1 = Guide2(

>         h1 = 0.1,

>         h2 = 0.008,

The exit height is in fact 0.8 cm! I think this probably accounts for
some of your confusion. Fortunately, errors like this will be easy to
detect with the new graphics display program that has already been
finished and will be in the next release (let me know if you want a
pre-release copy to play with).

Anyway, I think that some of your points can still be discussed.

> McStas:     Transmission (%)   Flux at Monitor2 (units ?)
> 
> straight     3.43                3.42E+08
> focusing     2.87                2.87E+08
> 
> Transmission = Flux at Monitor2 / Flux at Monitor1  
> Flux at Monitor1 = 9.977E+09

Hm. I am starting to think that I may have mislead you about fluxes in
McStas. The ONLY place the notion of flux is used in McStas is in the
"flux" parameter of Source_flux. All the monitors simulate real
monitors/detectors, and compute neutron counts, not neutron fluxes.
Depending on your viewpoint, you can say that you are simulating a one
second experiment; then the monitors count neutrons per second.

What this means is that if you want the flux in n/cm**2, you need to
divide by the area of the monitor. Since the flux will probably depend
on the position, you really need to use a position-sensitive detector
and divide each bin by the pixel area. I suspect that the McStas flux
values given above are in fact the "raw" counts in monitor2.

When I divide by the monitor area, I get the following numbers:

                             Flux [neutrons/cm**2]
straight     3.43                1.71E7
focusing     2.87                1.79E7

These numbers match well with the Beamline results when compared relatively.

To get the units in neutrons/AA, it would be necessary to use an energy
sensitive detector and divide each bin by its wavelength range.

> calculated with McStas are and how they are calculated. Due to an erlier
> discussion about this matter on this newsgroup (11/23/98) I expected it
> to be n/cm**2/s/AA/ster.

Yes. That is the unit of the input parameter to the source. The unit for
the detector outputs is just counts (or counts/sec, if you like). I am
sorry about the confusion here.

> While the fluxes calculated by hand and calulated by Beamline fit well,
> from the numbers it seems as if McStas calculates fluxes with units
> n/cm**2/s!

As I said above, it is in fact just n/s, to get the 1/cm**2/AA you need
to use an energy sensitive detector and do the computation by hand.

I hope this clarifies matters? Otherwise, please ask again. Do you think
the way the monitors in McStas work should be changed to handle flux
like in Beamline, and if so, why?

 - Kristian.