1. A method of determining and displaying three dimensional images of presences of at least one specified nuclear isotope in at least a portion of a target of interest, comprising: a) illuminating at least a part of the target with an incident photon beam within a predetermined energy range, wherein the incident photon beam comprises photons traveling in at least one first predetermined direction which impinge upon the target at least one first predetermined surface location, and wherein the predetermined energy range includes energies at which nuclear resonant fluorescence scattering can take place in each of the at least one specified nuclear isotopes and energies at which it cannot; b) detecting in a transmission detector array a plurality of photons from said incident photon beam, wherein the transmission detector array is disposed such that at least a portion of said incident photon beam traversing the target is incident on the array, and wherein the transmission detector array determines an energy of at least some photons incident on it with accuracy to permit determination of whether the said photon energy corresponds to an energy at which nuclear resonant fluorescence scattering can take place from one of the at least one specified nuclear isotope; c) repeating steps a) and b) for a predetermined selection of additional directions and surface locations; d) based upon numbers of photons detected in the transmission detector array, with energies at which nuclear resonant fluorescence scattering can take place from each of the at least one specified nuclear isotope, determining through use of tomographic computation methods the three dimensional image of the presence of each of the at least one specified nuclear isotope in at least the portion of the target of interest; and e) displaying the three dimensional images on an output device to a user.