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 ³He and ⁴He 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 ³He. The ratio of overall trapping times (mean lifetimes against annihilation), R=Tₜᵣₐₚ{(}⁴$He)/${T}ₜᵣₐₚ(³He), has been determined to be 1.144 ±{} 0.009, which is in good agreement with a theoretical estimate yielding R=[M*(p{}⁴He)/ M*(p{}³He)]²=1.14, where M* denotes the reduced mass of the p{}He²⁺system. The presence of a short-lived component with a lifetime of 0.154±{}0.007 {μ}s in the case of ³He suggests that the p{}³{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 ³than for ⁴He. All the data can be fitted fairly well with simple three-level and four-level cascade models. {} 1996 The American Physical Society.