Studies of electron density are reported for a quiescent neon-argon gas plasma generated by fission fragments in the core of a nuclear reactor. Reaction kinetic equations for the various ionic and excited species are solved self-consistently with electron-energy balance equations to yield values of electron density and temperature. It is shown that for containers of diffusion length of about 1.6 mm or greater, there exists a maximum value of electron density at a total gas pressure p≈90 Torr and [Ar]/[Ne]≈10−4; the dominant ion in this case is the atomic Ar+ ion. At low values of neutron flux (∼1010 cm−2 sec−1) and electron density (∼1010 cm−3), the electron temperature is computed to be at or near the gas temperature, but at high values of neutron flux (∼1013 cm−2 sec−1) and electron density (∼1012 cm−3), the electron temperature is higher than the gas temperature by an important amount (∼600°K). Inpile measurements of electron density using microwave techniques are in very good agreement with the theoretical predictions.

Determinations of primary α and secondary ω ionization coefficients in helium at pressures p0 near to atmospheric and low values of E/p0 (E is the electric field) show that α/p0 = f(E/p0) and ω/α = ϕ(E/p0) for 30 < p0d < 730 cm torr. The results obtained for ω/α and its dependence on E/p0 are shown to be adequately accounted for on the basis of the assumption that the predominant secondary ionization process in helium at high pressures is the destruction of metastable states in the gas with the consequent production of non-resonance photons which liberate secondary electrons at the cathode.

The note gives both theoretical cause and experimental verification for the manner in which electron densities ≈1012 cm−3 vary with total gas pressure and the percentage of trace gas in a Penning mixture ionized by fission fragments.

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.

An apparatus for the measurement of steady-state pre-breakdown ionization currents in helium at high pressures is described, and the sampling procedure necessary to obtain with it reproducible results discussed. The primary and secondary ionization coefficients determined from these measurements are given for 3 3 < E/p0 < 5.0 v cm-1 mmHg-1 and for 150 < p0 < 560 mmHg. The values of α are discussed in relation to previous theoretically computed values and the experimental values obtained at lower gas pressures but with similar values of E/p0. The values of the generalized secondary ionization coefficient are found to be relatively high (similar 0.1) and to increase as E/p0 decreases.

The secondary electron yield from fission fragments has been measured directly using an ultrahigh-vacuum diode containing uranium which was operated in a nuclear reactor. The current measured in this diode consisted of contributions from the positively charged fission fragments, secondary electron emission induced by fission fragments, and photoelectrons generated by gamma radiation. The secondary electron yield Δ was determined from the secondary electron current and the computed fission fragment emission rate. For a thick (>one range) uranium-nickel (5.7 wt. % nickel) alloy the value of Δ is 207±10.

IN recent years there has been considerable effort devoted to the study of the ionization processes which give rise to ionization growth in different gases. Of particular interest from the fundamental point of view is the growth of ionization in helium, because in this case quantum mechanical methods may be more easily applied than in the more complex case of diatomic gases. Consequently, direct comparison may be made between experimental data on ionization coefficients, for example, and the quantum mechanical computations. Despite its simple atomic structure, however, the existence of high-energy metastable states of the helium atom, as well as of the helium molecule, results in the possibility of highly complex ionization processes occurring in this gas1–3. Moreover, experimentally, helium is a difficult gas on which to make observations because it has the highest ionization potential (24.6 eV) of any known gas. Most common impurities have ionization potentials (<16 eV) appreciably lower than the ionization potential of helium, and are thus preferentially ionized by direct electron impact even when present in only small quantities. Furthermore, ionization of the impurity atoms and molecules can be produced by collisions of the second kind with helium atoms in the high-energy metastable states, and by high-energy photons produced in the gas by the transition of excited helium states (which all have energies > 20 eV) to the ground-state. The effects of impurities, even when present in quantities as low as 1 part in 106, become increasingly important as the parameter E/p (E, the electric field, p, the gas pressure) decreases; because, as E/p decreases, the mean electron energy decreases and the ratio of ionized impurity atoms to ionized helium atoms then increases. These considerable experimental difficulties probably explain why there are so far no published data on the measurement of ionization growth in helium at high gas pressures and low values of E/p.