Moderation and Cascade Times of Negative Pions and Negative Kaons in Liquid Helium

We have measured separately the mean moderation times $T_{\pi }^{}{}_{}{}^{\mathrm{He}}\left(v_A\right)$ and $T_K^{}{}_{}{}^{\mathrm{He}}\left(v_A\right)$, spent by ${\pi }^{-}$ and $K^{-}$, respectively, in atomic orbits before nuclear capture in the Northwestern helium bubble chamber. It was necessary to resolve the velocities of ${\pi }^{-}$ (or $K^{-}$)down to $v\approx 0.02c$. The resolution was obtained in the pion case by measuring the ${\mu }^{-}$ range from ${\pi }^{-}-{\mu }^{-}-e^{-}$ decays and in the kaon case by the kinematic analysis of the $\tau$-decay mode. We obtained the following results: $T_{\pi }^{}{}_{}{}^{\mathrm{He}}\mathrm{}(0.02c)=(3.19\mathrm{}\pm 0.23)\times {10}^{-10\mathrm{}}sec,$ $T_K^{}{}_{}{}^{\mathrm{He}}\mathrm{}(0.02c)=(2.4\mathrm{}\pm 0.4)\times {10}^{-10\mathrm{}}\sec .\mathrm{}$