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

[Kristian Nielsen]

> > 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?
> 

> It would be extremely useful to have monitors which calculate flux

> If you see the chance of writing a monitor, which computes flux
> densities automatically, it would be of really great help to us!

First we must decide on how "flux" is defined exactly.

You mentioned that in Beamline, the flux is given in the units of
n/cm**2/AA/s. Now, clearly this value will depend on the position in the
beam and the wavelength at which it is computed. So now it is I who is
confused about the units, which seems only fair after the confusion you
have been through :-)

I assume that Beamline gives you the average flux in the area under
consideration (eg. monitor/detector area or perhaps guide exit in
Beamline?). This makes sense and is easily handled by dividing neutron
counts with the monitor area.

But what about the wavelength dependence (as you yourself mention)? Can
you explain the precise meaning of the Beamline flux numbers you gave,

> Beamline:   Transmission (%)   Flux at Monitor2 (n/cm**2/AA/s)
> 
> straight     3.5                 1.75E+09
> focusing     2.9                 1.82E+09

in terms of wavelength dependence? More precisely, suppose I have a
neutron beam that is uniform on a 1 cm**2 area. The wavelength dependent
flux of the beam is given by a function f such that for a given
wavelength lambda in AA, the flux of the beam in n/cm**2/AA/s at that
lambda is f(lambda). Then what is the flux value corresponding to the
Beamline numbers?

I can see a couple of possibilities:

1. The average flux in a user supplied wavelength range, i.e.
     (integral(lmin,lmax) f(l) dl) / (lmax - lmin)

2. The maximum flux, i.e. the maximum of f (hm, that might be
   non-trivial to compute from monte carlo data)

and maybe others, but I would like to know what Beamline does.

Once we resolve this issue, I will be happy to provide you with a flux
monitor component for McStas.

 - Kristian.