Neutron flux obtained for two different surface areas

Dear FLUKA experts,

We are trying to estimate the flux transmitted across a plane for 4.5 MeV neutron for a geometry as described in the input file attached. The calculated flux (neutrons/ unit area / particle) are different when the diameter of the plane is 20 cm than when the diameter of the plane is 2 cm., the remaining geometry being same for both the cases. The plot is attached. Please suggest where it is going wrong.

Sincerely
Maitreyee
steel validation_2cm_dia.inp (1.8 KB)
steel_4.5MeV.pdf (14.6 KB)
Steel_validation_4.5MeV.inp (1.8 KB)

Madam,
The difference in the neutron flux is due to the different solid angle subtended by the area w.r.t. (Reg1b-> Reg1C) to the isotropic neutron source. If you take vacuum instead of SS316LN, the values obtained for 20 cm diameter is 2.844E-03 n/cm2/pr, whereas for 2 cm diameter, the flux values are 2.366E-02. I hope this helps.

With regards,
Sujoy

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Dear Maitreyee,

The answer by @sujoy is correct.

If you perform scoring within the same solid angle, the results will match (unit 28):

Best regards,
Volodymyr

Dear Sujoy,

Thank you for your reply.

With regards
Maitreyee

Dear Volodymyr,

Thank you.

With regards,
Maitreyee

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