Dear @thanapong,
Neutron coordinates around the target cladding is exactly what I would expect. In fact, from your previous questions, I thought that this was what you wanted. Of course, if you score the position of every neutron at the moment it leaves the target cladding, its coordinates will be at the surface of the target cladding.
I just took a quick look to the paper you mention. From section 2.1, and in particular from the sentence
“By specifying some logical definitions in the program (in this case, specifying the particles that cross from the target area to outside of the target), the variable information provided in FLUSCW is returned”
I think (but I may be perfectly wrong) that they are doing the same thing as you do now: score the particles as they cross from the target area (which in your case I assume to be target + cladding) to outside of the target (your VOID), in order to later on use this information as an input to continue the simulation using a different software.
In your input file there are only the target and its cladding, surrounded by vacuum. In this case, once the neutrons leave the target cladding and enter the VOID, they will just travel to infinite without interacting (being eventually discarded once they enter the blackhole surrounding the fluka geometry). Therefore, unless I am missing something, there is no point in scoring the neutron properties somewhere else in the VOID, specially if the target is in reality surrounded by something else and you will use the results as the input source for a second simulation. Said that, you can always create another region around, fill it up with vacuum, and score the neutrons at its boundary.
If what you want to get from FLUKA are the position, direction, energy, etc. of every neutron leaving your target cladding and entering your VOID region, you are doing it correctly as far as I can tell. If you want a different thing please elaborate it being as specific as possible so I can try to help you.
Regarding your mgdraw.f routine: probably you are aware, but if not please note that you are writing ETRACK, which is the total energy of the particle (see comments inside /include/trackr.inc). Before, in your fluscw.f, you were writing PLA, which is the particle laboratory momentum (GeV/c) or kinetic energy (GeV) (see comments inside the routine).
Hope it helps,
Francisco