The Hamamatsu photomultipliers can be modelled as a combination of a sphere,
radius approximately 
cm, centred at the origin, combined with a torus of
circular cross-section. For simplicity, the structure is assumed to be
symmetric with respect to a reflection in the z = 5.09 cm plane with
exceptions due to the base and the lower end of the dynode stack.
The exception to the symmetry is the upper portion of the
dynode stack, which is modelled as a cylinder. It
should be noted that the spheres completely enclose the tori, which
implies that the most efficient way of searching for an intersection
is to check the sphere first. If it cannot be hit, then neither can
the torus. The inner surface of the glass envelope is assumed to be a
constant 0.3 cm from the outer surface. The photocathode is assumed to
be situated on the inner surface 0.5 cm above the equator. Below the
equator, the inner surface is assumed to be coated with aluminium.
However, there is an overlap region (which extends approximately 2.5
cm below the equator), where the photocathode is laid down on the
aluminium.