The isotope spectrum shift in the 6707.8A resonance line of lithium has been used for the assay of the isotopes Li 6 and Li 7. A hollow cathode source loaded with 4 mg of Li2SO4 is excited at 35 ma using purified helium at about 2- to 3-mm pressure as support gas. Characteristics, such as tube geometry, pressure, sample life, flow rate of gas, current, and anode-cathode spacing have been studied to give reproducible line intensity ratios. Precision of the method based on a 95 percent confidence interval is about 6 percent of the Li 6 content or, by difference, about 0.5 percent of the Li 7 content in natural material.
Spectra are obtained with a 21-foot, 30,000 lines per inch grating spectrograph using first order for which the Li 6–Li 7 isotope shift of 0.159A is readily separated. A multiplier phototube (1P28) set on the 4603A lithium line is used in an integrating circuit as an exposure meter.
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