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 ± 0.009, which is in good agreement with a theoretical estimate yielding R=[${\mathit{M}}^{\mathrm{*}}$(p¯${\mathrm{}}^4$He)/ ${\mathit{M}}^{\mathrm{*}}$(p¯${\mathrm{}}^3$He)${]}^2$=1.14, where ${\mathit{M}}^{\mathrm{*}}$ denotes the reduced mass of the p¯${\mathrm{He}}^{2+}$ system. The presence of a short-lived component with a lifetime of 0.154±0.007 μs in the case of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ suggests that the p¯${\mathrm{}}^3$ ${\mathrm{He}}^{+}$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. © 1996 The American Physical Society.