Noble Gas Plasma Produced by Fission Fragments

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Abstract
The generation of a plasma by fission-fragment ionization of noble gases is reported. The experimental data consist of steady-state current-voltage (I-V) characteristics obtained with ceramic-metal ionization tubes which were operated in the high neutron flux region of a nuclear reactor. The observed dependence of I on V½ for applied potentials <100 V is shown to be consistent with the theory that the predominant charge loss in the plasma is volume recombination of diatomic ions and electrons. The ion generation rate is calculated from the energy loss rate of the fission fragments in the gas and I-V characteristics for the noble gases are computed. These characteristics, which do not contain any adjustable parameters, are in agreement with the experimental data within±10%. Data are presented for neon, argon, xenon, and neon-argon (Ar/Ne=10−3) for gas pressures in the range 30 to 400 Torr. Typically, for a gas pressure of 240 Torr and a neutron flux of 1013 cm−2·sec−1, the computed values for ion generation rate and electron density in the pure gases were ≈5×1016 cm−3·sec−1 and ≈3×1011 cm−3, respectively. For the neon-argon mixture, the electron density was estimated to be ≈7×1011 cm−3. This higher density arises not only because of additional production of Ar+ ions via neon metastables encountering argon neutrals, but also because of a reduction in the formation rate of Ar2+ ions (followed promptly by dissociative recombination). The point is that Ar++Ar+Ne→Ar2+ is unlikely compared with Ne++2Ne→Ne2+, for a small argon admixture.