Nuclear Resonance Fluorescence at MIT

The nucleus of every atomic species has uniquely characteristic low-lying energy states. These states are very narrow in energy and rarely more than a fraction of an electron volt wide. They can be excited by the absorption of photons of the correct energy. When the excited state decays the characteristic photons are radiated into all angular directions with respect to the incident beam. The angular distribution is a gently varying function of this angle. The radiated photons can be detected by high-resolution solid-state Ge detector-spectrometers and each nuclear species in the beam can be identified by the characteristic energy of the photons. Absorption of characteristic photon energies is also a possible method of material detection complimentary to the detection of radiated photons. The photon beam is ideally a beam of bremsstrahlung radiation, continuous in energy, and generated by bombarding a target with energetic electrons. Typical electron energies would be about 9 MeV, depending on the specific application. This method has the advantage that materials are transparent to the high-energy photons involved, and is thus able to investigate non-destructively containers of large extent filled with a large amount of diverse materials. Imaging can be accomplished by collimation of the detector views of the investigated sample and of the photon beam.

The technique can detect non-destructively the materials, their amount and their location in almost any container presently used in shipping on airplanes, trucks and on ocean-going vessels. This would encompass explosives of the chemical type and materials that would be used in manufacturing nuclear weapons as well. It can sense heavy metals as well as light elements and it can measure small samples as well as large ones because the gamma rays are very penetrating. The chemical compositions can be ascertained by material ratios. The technologies developed for neutrino physics experiments could be applied to cost-effective neutron detector systems. The applications are in (a) detection of smuggled neutron-emitting special nuclear materials (SNM), such as weapons grade plutonium and certain uranium compounds, (b) terrorist nuclear weapon threat detection, and (c) weapon accountability. Since the likely targets of interest would emit low fluxes of neutrons, and need to be detected at relatively long distances with short dwell times, any effective neutron detection system would in turn need to have a large surface area. The concept involves employing panel-like neutron detection modules to provide significantly improved solid-angle coverage at reduced cost as compared to competing technologies. This would allow the detection of neutron emitting materials or devices in venues such as train stations, airport concourses or taxiways, tunnels, border crossings, and exits to weapons or nuclear facilities.