How does a zinc-sulfide coated screen work, and how does it manage to detect high-frequency radiation, electrons, and helium? Could it be possible that a delta ray managed to somehow slip past detection?
For those wondering why zinc sulfide is important, I will note that "a zinc-sulphide screen in vacuum" is specifically called out in the original Geiger and Marsden papers on alpha particle scattering. It was already well accepted as the coating for the early cathode ray tubes, and zinc sulfide would become one of the main phosphors for CRTs for television. Phosphors remain very important in fluorescent and LED lighting to convert UV into visible light emission and create color balance.
If an incoming particle deposits energy into electronic states, populating higher energy levels, the relaxation back to the ground state may result in emission of a photon. This is also called scintillation when driven by ionizing radiation.
As @KyleKanos alludes to, the 'delta rays' as used in the early 20th century are actually electrons scattered by the primary alpha/beta/gammas, and may not have enough energy to make a nice bright scintillation that would be noticed by Geiger or Marsden as they sat in a dark room staring at the screen. These days of high efficiency CCDs, photomultipliers, channeltrons, etc. has made us experimenters soft...