OK, I thought it was some 3d graphics presentation you were looking for.
So the mask blocks the rays that have been emitted but have not passed through the subject.
If the subject can move, then you will need some way to adapt the mask model parametrically, with respect to the orientation of the cone.
After giving it some thought, I believe Inventor can create an ellipse constrained to 3 points, which would allow you to create the projection of the cone with just a small number of construction lines.
Perhaps the mask should be close to the isotope container. It would require much less lead. Or would that cause significant diffraction?
STF