Define a neutron source

Dear expert,
I want to define a neutron source.A cylindrical surface, the inner surface radiates neutrons isotropically to the inner space,please see the below picture.
But it seems wrong from the second picture,could you help me?

mixscint.flair (4.3 KB)

Dear Xiong,

it is not possible to limit the isotropic source distribution only to the direction towards the inside of the surface by using only the standard FLUKA cards. You need to implement the required direction sampling using a source routine.

See the following example:
mixscint.flair (4.4 KB)
source_newgen.f (19.2 KB)


Dear@horvathd ,
Thank you for your answer,I will try it later.