Radiography studies with gamma rays produced by 14-MeV fusion neutrons

摘要

Oxygen contained in pure water has been activated via the (sup 16)O(n, p)(sup 16)N reaction using 14-MeV neutrons produced at a neutron generator with the (sup 3)H(d,n)(sup 4)He source. Photons of 6.129 and 7.115 MeV, generated by the decay of 7.13-second (sup 16)N, were then used to demonstrate the feasibility of employing highly penetrating, nearly monoenergetic gamma rays for radiography studies of thick, dense objects composed of elements with medium to relatively high atomic numbers. A simple radiography apparatus was constructed by circulating water continuously between a position near the target of the neutron generator and a remote location where photon transmission measurements were conducted. A sodium iodide scintillator was employed to detect the photons. Pulses equivalent to photon energies smaller than 2.506 MeV (corresponding to the cascade sum of 1.333- and 1.173-MeV gamma rays from the decay of 5.271-year (sup 60)Co) were rejected by the electronics settings in order to reduce background and improve the signal-to-noise (S/N) ratio. Respectable S/N ratios on the order of 20-to-1 were achieved with this setup. Most of the background (N) could be attributed to ambient environmental radiation and cosmic-ray interactions with the lead shielding and detector. Four representative objects were examined by photon radiography in this study. This demonstrated how such interesting features as hidden holes and discontinuities in atomic number could be easily identified from observed variations in the intensity of transmitted photons. Some advantages of this technique are described, and potential applications are suggested for a future scenario where fusion reactors are used to generate electric power and very intense sources of high-energy photons from (sup 16)N decay are continuously available as a byproduct of the reactor cooling process.