Observation of uranium photo-fission by 16N decay gamma rays from water activated by D-T fusion neutrons

Radioactive 16N is generated in pure water (H2O) which has been exposed to D-T fusion neutrons with energies near 14 MeV. The principal neutron-activation reaction is 16O(n,p) 16N. Decay by β emission (100%) of the 7.13±0.02 s 16N activity is accompanied by production of energetic gamma rays (most of them with energies>6 MeV) that are associated with the prompt radiative de-excitation of various elevated energy levels of 16O populated through the process 16N(β-) 16O. The dominant gamma-ray emission branch energies (intensities) are 6.129 MeV (67.0±0.6%) and 7.115 MeV (4.9±0.4%). A particular given intensity refers to the percent of 16N decays that produce the indicated gamma ray. Both photons possess energies that are above the photo-fission threshold for the naturally occurring isotopes of uranium, namely, 234U, 235U, and 238U. This paper reports on an experiment that demonstrated 16N decay gamma-ray induced photo-fission in a commercial fission detector that contained 0.14 g of depleted uranium (predominantly 238U with a small trace of 235U). This detector was placed near a tubing system containing circulating water that had been activated by neutron bombardment as it passed near to the target of an intense D-T fusion neutron generator. The fission detector was situated in a shielded location remote from the source of primary D-T neutrons. Other possible mechanisms leading to the generation of the observed detector events were considered. Among these were natural background radiation (e.g., cosmic rays), delayed neutrons from the decay of 17N and 18N also produced in water by D-T neutrons, secondary neutrons from (γ,n) reactions in materials near the detector, and electronic noise or pulse pileup. All of these alternative possibilities were ultimately rejected as significant contributors to the experimental detector counts by a detailed examination of the pertinent nuclear data and by specific measurements carried out in the present study. Photo-fission of 238U remained as the predominant source of the recorded fission-detector events after elimination of these other possibilities as significant contributors of observed counts. The anticipated photo-fission yield was calculated based on consideration of the experimental setup, tabulated cross sections, and other pertinent nuclear data. This analysis yielded results which were found to be consistent with the present experimental observations to within the combined — rather large — uncertainties. This outcome further supports the conclusion that photo-fission events were indeed observed in this experiment. Some engineering implications of this investigation, and potential applications for the observed phenomenon in contemporary nuclear fusion technology, are also discussed.