1. A method of analyzing material in a plurality of voxels of a target, the method comprising: (a) illuminating the said plurality of voxels of the target with a photon beam, such that the said beam enters and exits a first voxel selected from said plurality of voxels, and upon exiting said first voxel enters and exits remaining voxels selected from said plurality of voxels in a predetermined sequence; (b) measuring with at least one photon detector at least one energy spectrum of photons scattered from each of the said selected voxels; (c) determining in a processor, using the said at least one measured energy spectrum, an average atomic number in each of the said selected voxels; (d) determining a flux of photons incident on the first selected voxel; (e) for the first selected voxel, using the determined photon flux, the determined average atomic number, the measured at least one energy spectrum, and predetermined values of a scattering kernel, estimating in the processor an average mass in the said first selected voxel; (f) for each of said selected remaining voxels, in the predetermined sequence in which the beam transits them, (i) determining in the processor a flux of photons incident on the selected remaining voxel, based upon the photon flux incident on a prior voxel in the predetermined sequence, the determined average atomic number in the prior voxel, the estimated average mass in the prior voxel, and predetermined values of a scattering kernel; and (ii) estimating in the processor an average mass in the said selected remaining voxel, using the incident flux into the selected remaining voxel, the average atomic number for the selected remaining voxel, the at least one energy spectrum for the said selected remaining voxel, and predetermined values of a scattering kernel; (g) estimating in the processor a flux of photons exiting from a final selected voxel, based upon the photon flux incident on the said voxel, the determined average atomic number in the said voxel, the estimated average mass in the said voxel, and predetermined values of a scattering kernel; (h) measuring in a detector a measured exit flux exiting the target; (i) computing in the processor a difference between the estimated exit flux and the measured exit flux; (j) computing in the processor a corrected estimated average mass in each selected voxel based upon the computed difference between the estimated exit flux and the measured exit flux; and (k) based upon the corrected estimated average mass computed, taking an action chosen from the group consisting of: scanning at least one of the plurality of voxels at a higher resolution; performing a NRF scan of at least one of the plurality of voxels; scanning at least one of the plurality of voxels with another scanning method; displaying in an output device an image of at least a part of the target to an operator; and notifying the operator by means of an output device of a potential threat.