^3He and ^2H measurements and what they tell us about cosmic ray propagation in the galaxy

We have compared new observations of the ^3He/^4He and ^2H/^4He ratios is cosmic rays made by the I-Max experiment with calculations of the interstellar production of secondary ^3He and ^2H. Within an uncertainty of +/- 10% these measurements imply an interstellar path length that is the same as that required to produce the observed cosmic ray B/C ratio. Thus cosmic ray He nuclei must have generally the same propagation history as heavier nuclei such as C, O and Fe. More subtle effects related to the actual path length distribution of the different primary species, which could provide information on the source distribution and how the diffusion varies with distance above the galactic plane, for example, will require that the cross sections into ^3He and ^2H - both in interstellar space and in the atmosphere be known to within a few percent.