The reason that the BC-270 is relatively gamma immune is due to its construction. The zinc sulfide scintillating layer is thin, and the rest of the device is made of more or less inert material of low atomic number. Gammas will tend to pass right through the plastic and the scintillating screen without causing a lot of activity. Recoil protons generated by fast neutron scattering have a much shorter range and end up exciting the phosphor screen. The Hornyak button works in much the same way, except that the ZnS particles are dispersed throughout the plastic matrix rather than concentrated in a screen. This causes some loss of photons due to diffusion.
Created on Tuesday, January 16, 2001 3:34 PM EDT by Richard L. Hester