I am currently calculating neutron yields from heavy ions impinging on an iron target. From observations the sum.lis file, I notice that the yield in the forward angle (0–15°) is lower than expected compared to the adjacent angular ranges (15–30°, 30–45°, etc.). This is unexpected, as one would typically anticipate a higher yield in the forward direction.
I would greatly appreciate your guidance on resolving this issue. In particular:
1.Could the lower-than-expected forward-angle yield is ok?
2.Is it possible that detector geometry or scoring definitions in the simulation are affecting the angular yield?
3.Could secondary scattering in the simulation reduce the forward yield relative to adjacent angles?
As you can see from your neutron fluence scoring by USRBIN, the forward direction is indeed favoured.
This is fully consistent with the USRBDX output, where you read the flux integrated over the solid angle, while what one expects to peak in the forward direction is the differential flux dN/dOmega. The integral is naturally affected by the fact that the solid angle in the forward direction is significantly smaller, as input in your USRBDX cards.
Moreover, USRBDX does not appear to be the right choice for your purposes, since the angle of the neutrons leaving the target through the lateral surface is measured with respect to the radial direction (normal to the surface) and not the beam direction. USRYIELD would allow you to get the neutron angular distribution dN/dOmega, as a function of the polar angle wrt the beam direction, with a single card instead of eight.
Thank you @Francesco Cerutti for the clarifications regarding the neutron fluence scoring. Your explanation about the difference between the integrated flux from USRBDX and the differential flux dN/dΩ, as well as the solid-angle considerations, is very helpful. I also appreciate the point about the angular reference frame and the suggestion to use USRYIELD instead of multiple USRBDX cards.
Could you please be able to explain how to update the *.inp accordingly, implementing the USRYIELD card for the neutron angular distribution with respect to the beam direction? It would help ensure that the scoring is set up correctly on my side.
Please look at the manual and take the polar angle in the lab frame in degrees as first quantity, with a suitable number of linear bins over the desired range. As second quantity (meant for filtering), you can just take the particle charge with unitary range from -0.5 to 0.5, so as not to discard any neutron and not to affect the normalization. Also, ask for plain double differential yield.