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 [Formula Presented] and [Formula Presented] 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 [Formula Presented]. The ratio of overall trapping times (mean lifetimes against annihilation), R=[Formula Presented]He)/[Formula Presented]He), has been determined to be 1.144 ± 0.009, which is in good agreement with a theoretical estimate yielding R=[[Formula Presented](p¯[Formula Presented]He)/ [Formula Presented](p¯[Formula Presented]He)[Formula Presented]=1.14, where [Formula Presented] denotes the reduced mass of the p¯[Formula Presented]<enspace>system. The presence of a short-lived component with a lifetime of 0.154±0.007 μs in the case of [Formula Presented] suggests that the p¯[Formula Presented]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 [Formula Presented] for [Formula Presented]. All the data can be fitted fairly well with simple three-level and four-level cascade models.