Uniformly distributed and isotropic source

Dear FLUKA experts,
my goal is to have a source of monoenergetic photons which is uniformly distributed in a box-like volume and that emits isotropically.
I tried by using the isotropic option in the BEAM card and by setting the desired 2cmx5.08cmx5.08cm volume with the BEAMPOS CART-VOL type, as you can see in the figure:


However, I get this USRBIN photon scoring:

(2D projection over the y axis)

(1D projection over the z axis, the horizontal azis is the z axis)

I would expect a constant fluence between -2.54 and 2.54, right? Am I missing something?

Thank you in advance,

prova sorgente estesa.flair (8.8 KB)
prova sorgente estesa.inp (8.8 KB)

Dear Giacomo,

this is normal, since the photons started closer to the surfaces and corners, can leave the scoring region sooner on average, thus contributing less to the fluence.

You can verify the distribution of the actual starting position, by turning off the EMF transport, and scoring energy deposition instead of fluence. This will give an uniform distribution in the projections.


Dear David,
thank you once again for your quick answer.
I can confirm that what you are saying is true, here are the results:

2D energy USRBIN

1D energy USRBIN

Thank you,