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Measurements are reported of uniform-field breakdown voltages and of effective primary and secondary ionization coefficients in oxygen in the range 0.825*1018<or=N<or=9.90*1018 cm-3, 1.1*10-15<or=E/N<or=2.1*10-15 V cm-2, where N is the gas number density and E is the electric field strength. Oscillographic observations show that, over the range covered, the secondary electrons are produced only by photoelectric processes. Also reported are theoretical values of the secondary ionization coefficient, calculated using a one-dimensional model. The results show that for N=0.825*1018 cm-3 the secondary process can be described in terms of a single radiation component with an absorption coefficient of 38 cm-1 (at N=25.1*1018 cm-3). The secondary process appears to be mainly cathode photoemission but photo-ionization of the gas also plays a significant part. For N=3.30*1018 cm-3, on the other hand, the calculated and measured secondary coefficients differ considerably and additional radiation components must be assumed to be active.

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Drift velocities were measured for ions of both polarities in unpurified cylinder SF6, at pressures ranging from 50 to 700 Torr. The ions were produced in electron avalanches occurring in the high-field region close to a protrusion on one electrode of a uniform-field discharge gap, and the drift velocities were measured in the uniform-field region, where the electric field strength was below the minimum value required for avalanche growth. Negative ions were studied over the E/p range from 33 to 100 V cm-1 Torr-1: their behaviour suggests a possible transition in the nature of the ion in the range 64-82 V cm-1 Torr-1, but this is not entirely clear. Positive ions were studied over the E/p range from 40 to 120 V cm-1 Torr-1 and their drift velocity was found to vary linearly over this range. Empirical equations are given for the measured values. It appears that these may safely be extrapolated to higher pressures provided that the compressibility of the gas is taken into account. The presence of variable amounts of impurities did not affect the results significantly.