Gamma-Ray Resonant Absorption (GRA) is an automatic-decision radiographic screening technique that combines high radiation penetration with very good sensitivity and specificity to nitrogenous explosives. The method is particularly well-suited to inspection of large, massive objects (since the resonant γ-ray probe is at 9.17 MeV) such as aviation and marine containers, heavy vehicles and railroad cars. Two kinds of γ-ray detectors have been employed to date in GRA systems: 1) Resonant-response nitrogen-rich liquid scintillators and 2) BGO detectors. This paper analyses and compares
the response of these detector-types to the resonant radiation, in terms of single-pixel figures of merit. The latter are sensitive not only to detector response, but also to accelerator-beam quality, via the properties of the nuclear reaction that produces the resonant-γ-rays. Generally, resonant detectors give rise to much higher nitrogen-contrast sensitivity in the radiographic image than their non-resonant detector counterparts and furthermore, do not require proton beams of high energy-resolution. By comparison, the non-resonant detectors have higher γ-detection efficiency, but their contrast sensitivity is very sensitive to the quality of the accelerator beam. Implications of these detector/accelerator
characteristics for eventual GRA field systems are discussed.
A comparison is made between the techniques of Prompt Gamma Neutron Capture Analysis and Nuclear Resonance Absorption (NRA) of gamma-rays as applied to in-vivo determination of body elemental content in humans. Special emphasis is given to the quantification of body protein via the measurement of total body nitrogen. A proposed facility for in-vivo determination of nitrogen using the NRA method is described. The advantages and disadvantages of each method are listed and discussed.
The physical principles of the nuclear resonance absorption method and its application to explosives detection are described. In this method, the object to be tested is scanned by a beam of 9.17 MeV gamma rays, which undergoes resonant attenuation whenever the beam encounters regions of nitrogen concentration. This resonant component of attenuation is detected using an array of gamma ray detectors containing a nitrogen-rich medium. From the reconstructed spatial distribution of nitrogen density obtained in multiview scanning, the presence of an explosive can be determined.