  The smuggling of special nuclear materials (SNM) through international
borders could enable nuclear terrorism and constitutes a significant threat to
global security. This paper presents the experimental demonstration of a novel
radiographic technique for quantitatively reconstructing the density and type
of material present in commercial cargo containers, as a means of detecting
such threats. Unlike traditional techniques which use sources of bremsstrahlung
photons with a continuous distribution of energies, multiple monoenergetic
gamma radiography (MMGR) utilizes monoenergetic photons from nuclear reactions,
specifically the 4.4 and 15.1 MeV photons from the
$^{11}$B(d,n$\gamma$)$^{12}$C reaction. By exploiting the $Z$-dependence of the
photon interaction cross sections at these two specific energies it is possible
to simultaneously determine the areal density and the effective atomic number
as a function of location for a 2D projection of a scanned object. The
additional information gleaned from using and detecting photons of specific
energies for radiography substantially increases the resolving power between
different materials. This paper presents results from the imaging of mock cargo
materials ranging from $Z\approx5$--$92$, demonstrating accurate reconstruction
of the effective atomic number and areal density of the materials over the full
range. In particular, the system is capable of distinguishing pure materials
with $Z\gtrsim70$, such as lead and uranium --- a critical requirement of a
system designed to detect SNM. This methodology could be used to screen
commercial cargoes with high material specificity, to distinguish most benign
materials from SNM, such as uranium and plutonium.
