absolute flux and M2

[Kristian Nielsen]

> Hello Kristian,

> now that I'm using the E_monitor I'm a little bit confused about the
> output. The E_monitor's output consist of 4 numbers in the file. They
> should be Energy, number of neutrons, sum of the weights and sum of
> the squared weights (E, N, I, M2). I'm not sure how to read these
> values when I use Source_flux. Then M2 is much higher than I.
> E.g. source_flat:
> 
> 80.992 425 0.00177358 2.13035e-08
> 
> same file, but source_flux (flux=3.63E+12):
> 
> 80.992 384 127625 1.14823e+08

This looks fine, but I think I understand your confusion. A common
mistake (at least I made it many times while working on McStas) is to
think of the weights as probabilities, ie. numbers smaller than
one. Weights are NOT probabilities and they may well be greater than
one. For example, for the Source_flux example above, N=384 and I=127625,
so the average neutron weight is a bit more than 300.

In fact, we can see that between the two sets of numbers, I increses by
a factor of about 10**8, while M2 increses with a factor of about
10**16 = (10**8)**2. This fits well since M2 is the sum of the squares
of the weights (I = sum(p_i) and M2 = sum(p_i**2)). You still get a good
estimate of the statistical error from the square root of M2.

Does that clarify matters for you?

> The values for I can be transformed using flux and solid
> angle (2.387E-4). How about M2?

The square root of M2 can be transformed the same way as I, I should think.

> What are the units of I and M2 for any monitor when I use source_flux?
> I assume it is n/cm**2/s/AA? 

It is n/cm**2/time/st/AA, where the 'st' means steradian (units of solid
angle), and 'time' is the same unit as the one supplied for the 'flux'
parameter of the component (McStas does not really have any comcept of
the duration of an experiment).

When you say 'transform using solid angle', I am not quite sure what you
mean; the Source_flux component itself makes a correction for the solid
angle in which neutrons are emitted ... ?

> According to the manual all monitors should give N, I and M2 in the
> output. But using PSD_monitor or Divergence_monitor I have only one
> number in the file? The E_monitor is the only one which produces all 3
> numbers?

Yes, the manual should probably be clearer about this. The
two-dimensional detectors do not output N and M2, since that would mess
up the output (a M x N x 3 array is not easily representable in plain
ASCII format). The output file format should improve vastly once we
switch to NeXus. For now, the two-dimensional detectors store the N and
M2 arrays as output parameters, so you can insert code in the FINALLY
section of the instrument definitions to write them out to separate
files. Let me know if you need more assistance on this.

> Here two smaller remarks for any future revision of the manual:
> 4.5.6. not dE but nchan hast to be given
> 4.1.2 wrong formula for I in last sentence

Ok, thanks.

 - Kristian.