Isotope effects on delayed annihilation time spectra of antiprotonic helium atoms in a low-temperature gas

The delayed annihilation time spectra (DATS) of antiprotonic helium atoms have been studied in isotopically pure low-temperature $^{3}\mathrm{He}$ and $^{4}\mathrm{He}$ gas at various densities. The DATS taken at 5.8 K and 400 mbar are very similar in shape except for (i) a small difference in the time scale and (ii) the presence of a distinct fast decay component in the case of $^{3}\mathrm{He}$. The ratio of overall trapping times (mean lifetimes against annihilation), R=${\mathit{T}}_{\mathrm{trap}}$${(}^{4}$He)/${\mathit{T}}_{\mathrm{trap}}$${(}^{3}$He), has been determined to be 1.144 \ifmmode\pm\else\textpm\fi{} 0.009, which is in good agreement with a theoretical estimate yielding R=[${\mathit{M}}^{\mathrm{*}}$(p\ifmmode\bar\else\textasciimacron\fi{}${\mathrm{}}^{4}$He)/ ${\mathit{M}}^{\mathrm{*}}$(p\ifmmode\bar\else\textasciimacron\fi{}${\mathrm{}}^{3}$He)${]}^{2}$=1.14, where ${\mathit{M}}^{\mathrm{*}}$ denotes the reduced mass of the p\ifmmode\bar\else\textasciimacron\fi{}${\mathrm{He}}^{2+}$system. The presence of a short-lived component with a lifetime of 0.154\ifmmode\pm\else\textpm\fi{}0.007 \ensuremath{\mu}s in the case of $^{3}\mathrm{He}$ suggests that the p\ifmmode\bar\else\textasciimacron\fi{}${\mathrm{}}^{3}$${\mathrm{He}}^{+}$atom has a state of intermediate lifetime on the border between a metastable zone and an Auger-dominated short-lived zone. The fraction of antiprotons trapped in metastable states at 5.8 K and 400 mbar is lower by 22.2(4)% for $^{3}\mathrm{than}$ for $^{4}\mathrm{He}$. All the data can be fitted fairly well with simple three-level and four-level cascade models. \textcopyright{} 1996 The American Physical Society.