Hello @thanapong,
I will need you to further clarify what you want to obtain to try to provide an optimal solution.
In your previous question you attached an image with a fraction of a paper saying “ …The information on spallation neutrons passing through the target surface into the core is recorded … ” and you mentioned that you were following that paper. If the neutron positions are recorded when passing through the surface, they will be at the surface by definition, so you cannot get them all over your volume.
Neutrons will be moving through your geometry, and you need to choose when to obtain their properties (e.g. position), since they will obviously be in constant change.
Maybe what you want to do is to score their properties at the moment when the neutrons are created, this way you will obtain them distributed in the volume where they are created.
A completely different thing is what is shown in Fig. 2 and 3. of the paper you mention in this post. As stated in the paper they are fluxes shown as function of neutron energy and solid angle. They are most probably obtained using a simple USRBDX scoring. Below a presentation that may help you to understand how to use this built-in scoring:
I would appreciate if you can comment further on what do you want to obtain, but focussing on the physical meaning rather than if it should look as a volume or a surface.
Kind regards,
Francisco