Uncovering Special Nuclear Materials by Low-energy Nuclear Reaction Imaging

Abstract

Weapons-grade uranium and plutonium could be used as nuclear explosives with extreme destructive potential. The problem of their detection, especially in standard cargo containers during transit, has been described as "searching for a needle in a haystack" because of the inherently low rate of spontaneous emission of characteristic penetrating radiation and the ease of its shielding. Currently, the only practical approach for uncovering well-shielded special nuclear materials is by use of active interrogation using an external radiation source. However, the similarity of these materials to shielding and the required radiation doses that may exceed regulatory limits prevent this method from being widely used in practice. We introduce a low-dose active detection technique, referred to as low-energy nuclear reaction imaging, which exploits the physics of interactions of multi-MeV monoenergetic photons and neutrons to simultaneously measure the material's areal density and effective atomic number, while confirming the presence of fissionable materials by observing the beta-delayed neutron emission. For the first time, we demonstrate identification and imaging of uranium with this novel technique using a simple yet robust source, setting the stage for its wide adoption in security applications.

Figure 1. Illustration of the imaging method using a low-energy nuclear reaction radiation source.

(a) Low-energy nuclear reaction imaging relies upon the source of monochromatic photons via a nuclear reaction between an ion accelerated to MeV-scale energy and a target. Gamma rays at discrete energies are produced from nuclear excited states of the product nucleus, with some reactions also producing neutrons. The collimated, penetrating radiation from the nuclear reaction source is used to perform transmission radiography of a shielded object, while neutron/gamma discriminating detectors detect the signature of nuclear fission. (b) Photon spectrum from the ¹¹B(d,nγ)¹²C reaction measured with a LaBr scintillation detector. The detector is capable of measurement of the 15.1 MeV peak despite of small crystal size. It is also able to resolve the full energy peaks (labeled as “f”) and single (“s”) and double (“d”) escape peaks. Also shown are “j” and “k” peaks corresponding to other nuclear transitions in the target. (c) Energy-dependent attenuation for several elements (4.438 MeV and 15.1 MeV gamma energies from the ¹¹B(d,nγ)¹²C reaction are shown as dashed lines).

Figure 2. Response of Cherenkov detectors to various materials of the same aerial density.

(a) Energy-dependent transmission measurement of several objects composed of different materials (a complete set of objects measured is provided in Supplementary Materials). The shaded regions of the spectrum are attributed to the Cherenkov detector response to 4.438 MeV and 15.1 MeV gamma rays produced in the ¹¹B(d,n)¹²C reaction. (b) The two characteristic energy regions in (a) are used to reconstruct the effective atomic number of eight test objects composed of different materials. The known photon interaction cross sections for 4.438 MeV and 15.1 MeV gamma rays are used for a comparison calculation and shown as red line. The reconstruction method for the measured Cherenkov radiation spectrum and the comparison transmission calculation are described in detail in the Supplementary Materials. Error bars calculated for the measured ratios are smaller than the plot markers (circles) and are therefore not shown.

Figure 3. Transmission imaging of a test object placed behind 14 inch-thick borated polyethylene.

(a) Photo of the uranium-containing object used for demonstration of transmission imaging. (b) Measured transmission integrated over the entire measured spectrum. (c) Effective atomic number, Z_eff, reconstructed from the measured spectrum (please see Supplementary Material for details of the reconstruction). (d) Schematic of the object. 1 and 2–uranium rods with aluminum cladding, lead, 3 – tungsten, 4 and 5–lead and aluminum plates, respectively. (e) Calculated theoretical transmission of the test object for the average photon energy. (f) Calculated effective atomic number, Z_eff, of the composite test object based on the known composition of the object. The indices on axes in (b,c,e and f) correspond to a step size of 25 mm in vertical direction, set by the detector pixel size (eight detectors were used in the array), and 3 mm in horizontal direction, set by the translation of the scanned object (total of 44 steps).

Figure 4. Observation of emission of beta delayed neutrons as a unique signature of material undergoing nuclear fission.

The interrogating beam is turned off at time = 0 s. (a) Temporal profile of delayed neutrons with a natural uranium target observed using a low-threshold composite fast neutron detector is in good agreement with a common parameterization into six delayed neutron groups (red line). (b) Temporal profile of delayed neutrons with a tungsten target using a low-threshold composite fast neutron detector shows no emission of delayed neutrons. More information on the neutron measurements is provided in Supplementary Materials.