As shown in the figure, it is a spherical shell model. Assuming that the center of the spherical shell is an isotropic neutron source, if I want to get the neutron flux at various thickness positions in the shield, how should I set the USRBIN card?
It is not possible in FLUKA to declare a spherical binning using the built-in USRBIN options. Cylindrical binning is possible, however, see the screenshot attached for an example. With a cylindrical binning you could take a thin slice of bins close to z=0 and then do a radial projection to obtain the approximate fluence as function of the radius.
If you don’t find this option satisfying for your purposes, you could use a user-defined binning by writing your own musrbr.f, lusrbl.f and fusrbv.f routines and selecting type=“special” in flair. Depending on your implementation, those routines could return the radial, phi and theta bins. See sections 13.2.9 and 7.20.USRBIN of the FLUKA manual for details.
For this spherical shell model, can I create a cube inspection area in Cartesian coordinate system, so that the spherical shell is inside the inspection area? Finally, the calculation results of spherical shell model are obtained.
With FLUKA, you can create detector for fluence in Cartesian coordinates using the USRBIN card. Create the detector centered on the sphere with half-widths corresponding to the sphere radius. See the example below.
Then you can consider transforming the Cartesian bin contents into a spherical equivalent, like I think you suggest. FLUKA doesn’t offer a tool for such a conversion, you would need to write your own script to analyse the ASCII detector output.
If you are just interest in the spectra of particles at different radii, you might also consider to divide you spherical region in several concentric spherical regions and add USRBDX estimators between each contiguous shell.