ISOTOPE SHIFT BETWEEN THE GROUND LEVEL OF NE-20 AND NE-22

From a comparison of self-absorption of resonance radiation in pure 20Ne gas and in a fifty-fifty mixture of 20Ne and 22Ne gas, an upper limit is found for the isotope shift of a number of neon resonance lines. It appears that these shifts are considerably smaller than the full widths at half maximum of the single Doppler-broadened absorption lines at room temperature. An upper limit for the difference in energy between the ground level of 20Ne and that of 22Ne could be obtained using published values for excited-state level shifts. This upper limit (5 GHz) indicates that the difference in normal mass shift and the difference in specific mass shift of the 20Ne and 22Ne ground levels are of different sign and equal magnitude.