  The smuggling of special nuclear materials (SNMs) across national borders is
becoming a serious threat to nuclear nonproliferation. This paper presents a
feasibility study on the rapid interrogation of concealed SNMs by combining
scattering and transmission nuclear resonance fluorescence (sNRF and tNRF)
spectroscopy. In sNRF spectroscopy, SNMs such as $^{235, 238}$U are excited by
a wide-band photon beam of appropriate energy and exhibit unique NRF
signatures. Monte Carlo simulations show that one-dimensional scans can realize
isotopic identification of concealed $^{235, 238}$U when the detector array
used for interrogation has sufficiently high energy resolution. The simulated
isotopic ratio $^{235}U/^{238}U$ is in good agreement with the theoretical
value when the SNMs are enclosed in relatively thin iron. This interrogation is
followed by tNRF spectroscopy using a narrow-band photon beam with the goal of
obtaining tomographic images of the concealed SNMs. The reconstructed image
clearly reveals the position of the isotope $^{235}$U inside an iron rod. It is
shown that the interrogation time of sNRF and tNRF spectroscopy is one order of
magnitude lower than that when only tNRF spectroscopy is used and results in a
missed-detection rate of 10$^{-3}$. The proposed method can also be applied for
isotopic imaging of other SNMs such as $^{239, 240}$Pu and $^{237}$Np.
