A low upper-limit on the lithium isotope ratio in HD140283

We have obtained a high-S/N (900-1100), high-resolving-power (R = 95 000) spectrum of the metal-poor subgiant HD 140283 in an effort to measure its Li-6/Li-7 isotope ratio. From a 1-D atmospheric analysis, we find a value consistent with zero, Li-6/Li-7 = 0.001, with an upper limit of Li-6/Li-7 < 0.018. This measurement supersedes an earlier detection (0.040 +/- 0.015(1 sigma)) by one of the authors. HD 140283 provides no support for the suggestion that Population II stars may preserve their Li-6 on the portion of the subgiant branch where Li-7 is preserved. However, this star does not defeat the suggestion either; being at the cool end of the subgiant branch of the Spite plateau, it may be sufficiently cool that Li-6 depletion has already set in, or the star may be sufficiently metal poor that little Galactic production of Li-6 had occurred. Continued investigation of other subgiants is necessary to test the idea. We also consider the implications of the HD 140283 upper limit in conjunction with other measurements for models of Li-6 production by cosmic rays from supernovae and structure formation shocks.