Photomultiplier tubes can detect some of these photons, but of course most of the photons shoot off in directions away from the photomultiplier and are never detected.

  Not quite true.  Scintillators are usually surrounded by a reflective surface to
correct this problem.  For example, see the structure of any of the Harshaw
hermetically sealed NaI (Tl) scintillation crystals.
  Regards, Alan