  Dual energy cargo inspection systems are sensitive to both the area density
and the atomic number of an imaged container due to the $Z$ dependence of
photon attenuation. The ability to identify cargo contents by their atomic
number enables improved detection capabilities of illicit materials. This work
introduces a novel method for atomic number reconstruction by minimizing the
chi-squared error between measured transparency values and a semiempirical
transparency model. This method is tested using two Geant4 Monte Carlo
simulated radiographic phantoms, demonstrating the ability to obtain accurate
material predictions on noisy input images, even in the presence of shielding.
Furthermore, we provide a simple procedure for porting this method to a
commercial system, requiring an approximate model of the scanner's beam spectra
and detector response, along with only three calibration measurements.
