I am trying to pick up secondary neutrons scattered on a water sphere with the help of the USRBDX card. At angles to the incident beam, I placed the cylinders as a detector, the surface of which is curved. Is it necessary to set the entire surface of the cylinder as the detector surface in the USRBDX card or only the surface that looks vertically in the direction
secondary neutrons ?
In this case, the detector will pick up secondary neutrons at a certain angle in relation to the incident beam, so there is no need to set the min and max value of the spatial angle or binning. Am I right?
Do I have to use a different Ebin for thermal and fast neutrons?
USRBDX scores neutrons which cross the boundary between two regions. Depending on how you build your geometry and how many regions you define, this region boundary can be limited to the circular face of the cylinder or coincide with the whole surface of the cylinder. Anyway, depending again on your geometry, neutrons may only cross the aforementioned circular face, rather than entering the cylinder elsewhere.
Said so, the input area is only a normalization factor, which can be safely left blank and just applied in the post-processing stage when plotting the results.
With a target sphere and an extended detector, the incoming neutron angle is not unique. If however the solid angle range corresponding to the cylindrical detector is small and you are not interested in discriminating the angle of the neutrons entering that detector, you can set loose limits (even 0 and 4pi) and asks for one single solid angle bin (with the appropriate solid angle normalization to be eventually applied in the post-processing stage).
While with the default group-wise treatment of <20 MeV neutrons their energy binning is pre-defined and cannot be changed, if selecting the point-wise treatment by means of the LOW-PWXS card, you can choose the (logarithmic) binning of your choice, which can suitably cover the whole energy range of neutrons.