1. A real-time method of identifying radioactive materials by their gamma-ray signatures on a computerized gamma-ray signature identification system, comprising: receiving an unknown gamma-ray spectrum as input into the computerized gamma-ray signature identification system; providing in the computerized gamma-ray signature identification system at least two libraries of predetermined spectra for various nuclear material types with each library having spectral entries which share a common pre-defined parameter that is different from and independent of other libraries; in a computer processor of the computerized gamma-ray signature identification system, performing principal component analysis on each of said at least two libraries to produce corresponding transformed basis sets representing the libraries and indexing each spectrum of the libraries in principal component space; in the computer processor, projecting an unknown spectrum onto the respective principal component space of said at least two libraries; and in the computer processor, determining a first identity of the unknown spectrum based on the unknown spectrum's proximity to clusters of a basis set in the principal component space of a first one of said at least two libraries; in the computer processor, determining a second identity of the unknown spectrum based on the unknown spectrum's proximity to clusters of a basis set in the principal component space of a second one of said at least two libraries; and presenting the determined identity of the unknown spectrum to a user as the first identity based on the assumption that the unknown spectrum meets the criteria of a first pre-defined parameter and as the second identity based on the assumption that the unknown spectrum meets the criteria of a second pre-defined parameter.