Technology

The Passport Scanner is capable of non-intrusively detecting in two or three dimensions every element heavier than helium (Z>2) and it even distinguishes between different isotopes (nuclei with same number of protons but different number of neutrons) of a given element. It can do this through many inches of steel. How does it work?

A close analogy to Nuclear Resonance Fluorescence (NRF) is atomic resonance spectroscopy and imaging. For example, if a glass vial filled with an elemental gas or compound or a solid material is heated in a flame, a color characteristic of the material will be observed (e.g., the yellow glow of a sodium vapor lamp). If this light is passed through a prism (or diffraction grating), the observed light will be found in discrete wavelengths (colors) that completely characterize the element. These characteristic spectra from all elements have been measured and catalogued. Their applicability is universal. When looking at distant stars or galaxies, the observed spectral lines tell us what elements are in these distant bodies. An image of the elemental composition of a body (galaxy or sample under microscope) can be obtained by looking at the spectra from localized regions.

NRF is the nuclear analogue of atomic spectroscopy. Instead of exciting electrons and observing their emission spectra, NRF excites nuclei using a beam of high-energy photons (gamma rays) in the million electron volt range (MeV). Every atomic nucleus thus excited has an emission spectrum that uniquely identifies it. Nuclei of an element are determined by the number of protons (Z) in the nucleus (which is the same as the number of orbiting atomic electrons in a neutral atom). However, many elements have stable nuclei with different numbers of neutrons (neutrons have zero electric charge) representing different “isotopes” of the same element. The NRF states (“lines”) for isotopes of a given element are different and hence are distinguishable. This is an advantage over atomic spectroscopy since the atomic spectra are essentially unchanged for different isotopes. An important example of this is the two isotopes of uranium, 238U which cannot be made into a nuclear weapon and 235U which can. NRF imaging can tell the difference. The signature of any isotope is unique and unmistakable, enabling automated threat identification. The energy range of photons that excite NRF states is also the most penetrating. To our knowledge, these features all combine to make NRF the most powerful and useful tool for the non-intrusive inspection of cargo.

Overview of Passport Systems’ Nuclear Resonance Fluorescence (NRF) Technique

1	Photon beam is generated by an electron accelerator & target and collimated
2	Beam penetrates cargo, excites nuclear states and photons are emitted
3	Collimated detector captures emitted photons with information about the isotopic composition of the material in the intersected volume (voxel)
4	Unique signature is obtained for each isotope

A schematic of the NRF scanner is shown above.  An electron beam from a commercial electron accelerator, with energy from 3-8 MeV, impinges on a metal target producing high-energy photons. As this collimated beam of high-energy photons is scanned through the cargo, NRF states of all cargo isotopes in the beam path are simultaneously excited. The fluorescent photons from the de-excitation of these states are detected using a back-angle, segmented and collimated detector array, thereby yielding the 3-D distribution of all isotopes in the cargo. A high-resolution 2-D NRF absorption imager specifying the total amount of any isotope in the beam path and a conventional X-ray imager are also powerful components of the scanner (not shown).

Features of Passport’s Nuclear Resonance Fluorescence (NRF) Technology

• Each isotope has its own spectral signature – material composition – automated detection of threats

• Detects all elements with atomic number greater than Helium: High detection probability with low false positives

• Energy range suitable for penetrating dense cargo

• Real time identification with no dependency on image analysis