Neutron-based inspection techniques
The non-intrusive inspection of concealed threats (primarily explosives) using neutron- and photon-based techniques started seriously over three decades ago following the string of aviation bombings in the 70s and 80s that culminated with the Air India and Pan Am 101 bombings in 1985 and 1988, respectively. The aim of such inspection is to look for explosives, narcotics, and nuclear material in objects as small as a briefcase, or as large as a truck or a marine shipping container. The size of detectable threats, the impact of the amount and type of benign material surrounding it, the measurement time, and the concept of operation vary for different cases. The basis, however, is always the same: search for the elemental signatures and deduce from them the presence or absence of a threat. The chemical compositions of threat materials are sufficiently different from benign materials. Hence, it is possible to distinguish between the two by determining the concentration of some or most elements in a defined ‘inspection zone.’
Neutron- and photon-based techniques detect the presence of threats through a variety of nuclear reactions. These can be categorized by the type of probing source particle, by its energy and time profile, and by the type of signature particles and their energy. Neutron-in, gamma-ray-out techniques, typically generated by (n,γ) or (n,n'γ) reactions, employ gamma rays produced through neutron interactions in the inspected objects. On the other hand, neutron-in, neutron-out methods, or (n,n), employ neutron broad resonance elastic scattering and (possibly) neutron backscattering while photon-in, photon-out techniques, or (γ,γ), use photon resonance scattering. Finally, the neutron-in or photon-in, fission-signatures-out (neutrons and/or gamma-rays) approach is used for the detection of fissile materials.
Over the years, the progress made in nuclear-inspection techniques was reflected in the diversity of neutron sources used. At first, californium-252 (Cf-252) neutron sources were employed. Then, commercial low-voltage electronic neutron generators (ENG) based on the (d,D) or (d,T) reactions (d being deuteron, D deuterium, and T tritium) were used with increasing reliability. Systems with more intense sources (using higher energy deuteron accelerators with variable time profile of neutron pulses, including nanosecond pulsing) extended the capabilities of material detection by a quantum leap, though with certain increase in cost and complexity. Recently, commercial electron linear accelerators (linac), used as intense x-ray sources for container and truck radiography inspection, have been modified to also serve as intense pulsed neutron sources by converting part of the x-rays into neutrons.
Besides the increased selection and applications of neutron sources, a considerable array of large and efficient gamma-ray detectors are being employed to increase the quality and quantity of elemental signatures (more full-energy peak for the high-energy elemental gamma rays and higher count-rates, respectively). Examples include the 6in-diameter × 6in-long, or even larger, sodium iodide (NaI) and other detectors such as cesium iodide (CsI), bismuth germanate (BGO), emerging lanthanum(III) bromide (LaBr3), and so forth. The multiple detectors are placed around the inspected container to assure that the explosive can be detected regardless of their location.
Moreover, methods to handle multiple detectors, high-rate digitization, and corrections for spectral distortion due to high count rates, have been successfully developed affording shorter measurement time and higher sensitivity. Other recent advancements include faster nuclear electronics and more rapid digital-data processing. Nuclear electronics today can process data at rates up to 1GHz. Such frequencies allow for signature measurement in higher-count-rate environments present when high-flux radiation sources are used for inspection.
On the downside, neutron sources cause temporary deleterious effects to gamma-ray detectors. Ways to cope, minimize, and correct for these harmful effects are being developed in order to further extend the capabilities of neutron-based inspection.
Another significant advancement made in recent years is the full use of the energy and time correlation afforded by the pulsed nature of ENG. Reactions occurring during the fast neutron pulse—mostly inelastic scattering when a 14MeV (d,T) source is employed—are significantly different from the thermal-neutron-capture reactions prevailing immediately after the pulse. These are themselves distinct from the charged-particle-activation reactions prevailing at a later time, several milliseconds after the pulse. New electronics have been capable of tagging events with both time and energy data at very high rates to achieve this and extract all the available signatures.
We have successfully used this technique to generate gamma-ray spectra that are dominated by different nuclear reactions, thus providing more information about the object being inspected. The ability to use a full two-parameter (time and energy) analysis of the gamma-ray spectra resulting from the injection of a fast neutron pulse into an inspected cargo, offers important improvements to the system performance. This assures that the highest signal at the lowest background is achieved. Collecting spectra following the complete decay of the thermal neutrons allows the detection of any neutron-induced activation. An example is that of oxygen through neutron-in, proton-out reactions of neutrons with energies above 10MeV. The activation spectrum on its own provides an alternative signature for oxygen. It also enables the subtraction of this and other activations as backgrounds from the capture (n,γ) spectrum. Figure 1 shows the spectra measured over three time intervals for a cargo of copy paper, which also contained an ammonium nitrate sample (to simulate explosives).
Other recent advances and continued development include using neutron or nuclear techniques to detect nuclear materials. We have successfully measured prompt and delayed signatures from fission using the differential die-away analysis technique employing an efficient fast-neutron detector created from moderated He-3 neutron detectors. In addition, we have shown that threshold activation detectors (such as fluorinated liquid scintillators) have success in detecting prompt fission neutrons after activating the detector material and measuring the resulting beta-decay within it. Similarly to a gamma-ray detector, these systems can detect delayed gamma rays from fission. More information on these topics can be found in recent publications.1,2 The fundamentals and applications of active detection of nuclear materials are described in detail in a 1981 text book.3
In summary, recent progress in neutron- and photon-based inspection techniques significantly improved the detection of various threats from explosives to nuclear. Innovations in this field are currently being implemented in existing commercial and new prototype inspection systems. These areas represent the focus of our future work.
Tsahi Gozani is chief scientist and CEO/president emeritus of Rapiscan Advanced Nucleonics Division and Ancore corporation. He has 45 years of experience in nuclear-based inspection technologies with more than 250 papers and books published. He also testified before the US government on this matter.
