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Physical Review A

Analysis of the lifetimes and fractions of antiprotons trapped in metastable states of antiprotonic helium

Isao Shimamura, Mineo Kimura

Published Dec 1, 1994• 2 citations
Publisher

Abstract

A recent paper by T. Yamazaki and K. Ohtsuki [Phys. Rev. A 45, 7782 (1992)] discusses properties of antiprotonic helium (${\mathrm{He}}^{2+}$p\ifmmode\bar\else\textasciimacron\fi{}${\mathit{e}}^{\mathrm{\ensuremath{-}}}$) formed in collisions of antiprotons (p\ifmmode\bar\else\textasciimacron\fi{}) with helium. Here we point out problems with approximations and assumptions used in deriving the results of that paper.

DOI
10.1103/physreva.50.5346

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Atomic approaches in metastable antiprotonic helium atoms. Reply to Analysis of the lifetimes and fractions of antiprotons trapped in metastable antiprotonic helium states by I. Shimamhura and M. Kimura

Toshimitsu Yamazaki, Kazumasa Ohtsuki

•Aug 1, 1994•0 citations

Reply to ‘‘Analysis of the lifetimes and fractions of antiprotons trapped in metastable states of antiprotonic helium’’

Toshimitsu Yamazaki, Kazumasa Ohtsuki

Shimamura and Kimura have submitted the preceding Comment [Phys. Rev. A 50, 5346 (1994)], (to be referred to as SK), which criticizes our recent paper [T. Yamazaki and K. Ohtsuki, Phys. Rev. A 45, 7782 (1992)] (to be referred to as YO). In this Reply the authors clarify the purpose of YO and complement its essential points, thus showing that the criticisms of SK are inappropriate.

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Physical Review Letters•Feb 14, 2005•45 citations
A recent paper by T. Yamazaki and K. Ohtsuki [Phys. Rev. A 45, 7782 (1992)] discusses properties of antiprotonic helium (He2+{\mathrm{He}}^{2+}He2+p\ifmmode\bar\else\textasciimacron\fi{}e\ensuremath−{\mathit{e}}^{\mathrm{\ensuremath{-}}}e\ensuremath−) formed in collisions of antiprotons (p\ifmmode\bar\else\textasciimacron\fi{}) with helium. Here we point out problems with approximations and assumptions used in deriving the results of that paper.
Calculations of low-energy collisions of antiprotons p‾\overline{p}p​ with helium atoms in the ground (11S)(1\phantom{\rule{4pt}{0ex}}^{1}\mathit{S})(11S) and metastable (23S(2\phantom{\rule{4pt}{0ex}}^{3}\mathit{S}(23S and 21S)2\phantom{\rule{4pt}{0ex}}^{1}\mathit{S})21S) states are carried out by using a semiclassical method and by further assuming that one of the two electrons is passive. Antiproton capture to form antiprotonic helium $(\ensuremath{\rightarrow}\overline{p}{\mathrm{He}}^{+}+e)$ and breakup ionization $(\ensuremath{\rightarrow}\overline{p}+{\mathrm{He}}^{+}+e)$ are investigated. The energy dependence of the capture and ionization cross sections and the distributions of the product states are presented and compared with previous theoretical calculations. For the metastable helium targets, most of the p‾He+\overline{p}{\mathrm{He}}^{+}p​He+ atoms produced in the capture have long lifetimes against subsequent Auger decays and can be observed experimentally. For the ground-state targets, however, the formation of such long-lived states is rather a rare event at low collision energies.
The time evolution of the state populations in the $v\ensuremath{\equiv}n\ensuremath{-}\ensuremath{\ell}\ensuremath{-}1=2$ and 3 metastable cascades of antiprotonic helium (p‾4He+{\overline{p}}^{4}{\mathrm{He}}^{+}p​4He+) atoms were studied using laser spectroscopy. The antiprotonic states $(n,\ensuremath{\ell})=(39,35)$, (40,36), and (41,37) in the v=3v=3v=3 cascade were estimated to initially contain, respectively, $(0.28\ifmmode\pm\else\textpm\fi{}0.04)%$, $(0.06\ifmmode\pm\else\textpm\fi{}0.02)%$, and $(0.02\ifmmode\pm\else\textpm\fi{}0.01)%$ of the antiprotons stopped in the helium target, while the corresponding values for states (37,34)(37,34)(37,34), (38,35)(38,35)(38,35), and (39,36)(39,36)(39,36) in the v=2v=2v=2 cascade were $(0.21\ifmmode\pm\else\textpm\fi{}0.04)%$, $(0.49\ifmmode\pm\else\textpm\fi{}0.07)%$, and $(0.19\ifmmode\pm\else\textpm\fi{}0.07)%$. The shortening of the state lifetimes due to collisions between p‾He+\overline{p}{\mathrm{He}}^{+}p​He+ and helium atoms was studied. As the atomic density of the target was increased from $\ensuremath{\rho}=2\ifmmode\times\else\texttimes\fi{}{10}^{20}$ to $2\ifmmode\times\else\texttimes\fi{}{10}^{21}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, the lifetime of state (37,34)(37,34)(37,34) decreased by an order of magnitude, whereas the higher states (38,35)(38,35)(38,35) and (39,35)(39,35)(39,35) were relatively unaffected. Between densities $\ensuremath{\rho}=6\ifmmode\times\else\texttimes\fi{}{10}^{20}$ and $2\ifmmode\times\else\texttimes\fi{}{10}^{22}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, a short-lived component with a time constant $\ensuremath{\tau}\ensuremath{\sim}0.2\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ appeared at early times in the delayed annihilation time spectrum of p‾He+\overline{p}{\mathrm{He}}^{+}p​He+, while the total fraction of long-lived antiprotons decreased from 3.0 to 2.52.5%2.5. These effects were caused by the lifetime-shortening of specific p‾He+\overline{p}{\mathrm{He}}^{+}p​He+ states such as (37,34)(37,34)(37,34).
A semiclassical, many-body atomic model incorporating a momentum-dependent Heisenberg core to stabilize atomic electrons is used to study antiproton capture on helium. Details of the antiproton collisions leading to eventual capture are presented, including the energy and angular-momentum states of incident antiprotons which result in capture via single- or double-electron ionization, i.e., into He2+{\mathrm{He}}^{2+}He2+p\ifmmode\bar\else\textasciimacron\fi{} or He+{\mathrm{He}}^{+}He+p\ifmmode\bar\else\textasciimacron\fi{}, and the distribution of energy and angular-momentum states following the Auger cascade. These final states are discussed in light of recently reported anomalously long-lived antiproton states observed in liquid He [Iwasaki et al., Phys. Rev. Lett. 67, 1246 (1991)].
Cold, two-body antiprotonic helium ions p‾ 4He2+\overline{p}\text{ }^{4}\mathrm{H}\mathrm{e}^{2+}p​ 4He2+ and p‾ 3He2+\overline{p}\text{ }^{3}\mathrm{H}\mathrm{e}^{2+}p​ 3He2+ with 100-ns-scale lifetimes, occupying circular states with the quantum numbers ni=28−−32{n}_{i}=28--32ni​=28−−32 and ${\ensuremath{\ell}}_{i}={n}_{i}\ensuremath{-}1$ have been observed. They were produced by cooling three-body antiprotonic helium atoms in an ultra-low-density helium target at temperature $T\ensuremath{\sim}10\text{ }\text{ }\mathrm{K}$ by atomic collisions, and then removing their electrons by inducing a laser transition to an autoionizing state. The lifetimes of p‾ 3He2+\overline{p}\text{ }^{3}\mathrm{H}\mathrm{e}^{2+}p​ 3He2+ against annihilation induced by collisions were shorter than those of p‾ 4He2+\overline{p}\text{ }^{4}\mathrm{H}\mathrm{e}^{2+}p​ 4He2+, and decreased for larger-ni{n}_{i}ni​ states.