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 Li2SO4is 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.
G. L. STUKENBROEKER, D. D. SMITH, G. K. WERNER, and J. R. MCNALLY,JR., "Spectro-Isotopic Assay of Lithium," J. Opt. Soc. Am. 42, 383-386 (1952)