1. A real-time method of identifying radioactive materials by their gamma-ray signatures comprising: providing at least one library of predetermined spectra for various nuclear material types; performing principal component analysis on the library to produce a corresponding transformed basis set representing the library and indexing each spectrum of the library in principal component space; projecting an unknown spectrum onto the principal component space; and determining the identity of the unknown spectrum by its proximity to clusters of the basis set in the principal component space.