Fraction of Stopped Antiprotons which Annihilate on Free Hydrogen in Propane

Antiprotons were stopped in a liquid-propane bubble chamber. In a sample of film containing 75 000 annihilations at rest, 21 examples of the reactions $\overline{p}p\to {\pi }^{+}{\pi }^{-}$ or $\overline{p}p\to K^{+}{K}^{-}$ were identified. This number, together with the previously measured branching ratios of these channels in liquid hydrogen, leads to a determination that (11±3)% of the antiprotons annihilate on free protons. The remaining 89% annihilate on bound nucleons in carbon. This fraction is markedly higher than the fraction of ${\pi }^{-}$ at rest which charge exchange on free protons in similar hydrocarbons. An explanation of the large difference is suggested.