1. A method for analyzing spectral data produced by a detector that interrogates a substantially contained space for presence of one or more radioactive materials of interest, comprising: grouping materials that could be in a detection path of the detector into one of a plurality of material groups by evaluating energy versus photon cross section and/or energy versus neutron cross section of the materials and such that the plurality of material groups are representative of all materials that could be in the detection path of the detector, and wherein a cross section of a material is a likelihood of interaction between radiation and particles of the material; 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; at a computing device, computing interactions of each radioactive material of interest with the material groups using the representative energy versus cross section curve for each material group to produce spectral data for the radioactive material of interest; at a computing device, generating a library of spectral data comprising the spectral data computed for each radioactive material of interest; storing the library of spectral data in a data storage device; detecting radiation from the space with the detector and producing spectral data with the detector; and at a computing device, analyzing the spectral data produced by the detector against the library of spectral data to determine presence of a radioactive material of interest in the space.