[Gate-users] Hollow sphere (shell) in water phantom: is it empty or filled with water?

[Bryan McIntosh]

In my experience with GATE, a daughter volume with any space inside it
is filled with the parent material, such as when you make a detector
volume that's the daughter of the world.  For a world made of air,
spaces between detector modules would be filled with air, as an
example.  So, if that behaviour holds true, then making a gold shell
inside a water phantom would lead to a gold shell containing water.
If you want the shell to contain air or vacuum, then you can put a
daughter sphere of vacuum/air inside the shell; that should displace
the water.

When confining a source, the source must overlap the confining region.
 The source can be physically larger than the confined area (but not
too much, otherwise GATE gets cranky).  If you want to confine the
source to be inside your gold shell, I recommend confining the source
to the air space inside the shell rather than the shell itself.  If
you were to confine the source activity to the shell, it would emit
from the volume between the rmin and rmax values you defined instead
of inside the volume you enclosed with the shell.

If you need additional clarification, let me know!

-Bryan