This post concerns having a way to communicate a nuclear explosion in your area via HAM or whatever other commo device you might have available.
The purpose of the mask is primarily to filter the air you breathe. Inhalation of radioactive particulates will kill you from the inside out.
Gamma Radiation is the big killer in a nuclear fallout context. Unhindered by a barrier, Gama particles have a range of a 1/2 mile.