Setting the probe sphere radius positive in TGEO (word: _PROBE_SPHERE_RAD) e.g.:-
$geom_probe_sphere_rad 900.creates an imaginary spherical surface. Tracks passing through this surface generate null verteces i.e. ones with the same region on both sides. This feature is implimented in the GEO routine GE_NEXT, and consequently the sphere can probe every region. This includes regions internal to the PMT, although in this case, Cerenkov photon will not see this surface, as they are transported by code that uses an optimised geometry that bypasses the GEO interface. This feature was introduced to help study the flux of background gammas from the rock, although it may also prove useful in any particle transport study at points in the detector away from physical boundaries.