1. A method for analyzing spectral data produced by a detector that interrogates a substantially contained space for presence of one or more materials of interest, comprising: a. grouping materials expected to be in a detection path of the detector into one of a plurality of material groups based on similarities in cross sections of the materials; b. selecting data describing a representative energy versus cross section curve for each material group from cross sections of individual materials in the respective material group; c. computing interaction of each material of interest with one or more of the material groups using the representative energy versus cross section curve for the respective material groups to produce spectral data for the material of interest; d. generating a library of spectral data comprising the spectral data computed for each material of interest; and e. analyzing the spectral data produced by the detector against the library of spectral data to determine presence of a threat material in the space.