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Physical Review Letters

Spin-Orbit Quenching of Positronium during Atomic Collisions

Jim Mitroy, S. A. Novikov

Published May 8, 2003• 29 citations
Publisher

Abstract

When positrons are injected into a gas, 75% of the positronium (Ps) is likely to be formed as long-lived ortho-Ps. The main decay mechanisms for the ortho-Ps have been assumed to be natural decay of ortho-Ps and pickoff annihilation of the positron during Ps-atom collisions. A third possibility for annihilation is $\mathrm{o}\mathrm{r}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{\text{\ensuremath{-}}}\mathrm{P}\mathrm{s}\ensuremath{\rightarrow}\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{a}\mathrm{\text{\ensuremath{-}}}\mathrm{P}\mathrm{s}$ conversion due to the spin-orbit interaction between the atom and colliding Ps. This extra quenching mechanism may explain a number of phenomena observed in the annihilation spectrum of Kr and Xe, including the very small Ps fraction of 3% seen for Xe.

DOI
10.1103/physrevlett.90.183202

Related papers

Experimental Evidence for Spin-Orbit Interactions in Positronium-Xe Collisions

Haruo Saito, Toshio Hyodo

Spin-orbit interaction of positronium (Ps) with the surrounding atoms, predicted by Mitroy and Novikov [Phys. Rev. Lett. 90, 183202 (2003)], has been detected experimentally. We have found that the lifetime of the magnetically unperturbed ortho-Ps in Xe gas of 1 atm decreases significantly when a magnetic field of 1.0 T is applied. This decrease is attributed to the Ps spin conversion caused by spin-orbit interaction during Ps-Xe collision. The annihilation cross section of ortho-Ps due to this interaction has been determined to be 3 times as large as that expected by Mitroy and Novikov.

Physical Review Letters•Dec 19, 2006•30 citations

Conversion of ortho-Ps into para-Ps in collisions with atoms

Amanda Allen, G. F. Gribakin

The cross section for positronium spin-conversion by the spin-orbit interaction in collisions with atoms has been calculated. Mitroy and Novikov [Phys. Rev. Lett. 90, 183202 (2003)] predicted that this effect is responsible for the dearth of ortho-Ps in heavier noble gases, Kr and Xe. Our cross sections are broadly in agreement with their estimates and the measurements of Saito and Hyodo [Phys. Rev. Lett. 97, 253402 (2006)].

Journal of Physics Conference Series•Nov 1, 2009•0 citations

Study of the quenched lifetime of an interacting positronium gas

Omar Morandi, Paul-Antoine Hervieux, Giovanni Manfredi

By using a kinetic approach, we study the evolution of a gas composed of interacting ortho- and para-positronium atoms. We calculate the total lifetime of the gas and study the ortho-positronium quenching effect induced by the bi-atomic scattering mechanisms with spin exchange. We analyze a realistic situation where the positronium is formed by highly energetic positrons impinging on a solid surface. In the case of a spin-polarized source of positrons, the spinpolarization time of the final positronium gas is estimated.

Journal of Physics B Atomic Molecular and Optical Physics•Jul 28, 2014•6 citations

COOLING AND QUENCHING OF POSITRONIUM AND PROSPECT FOR A Ps BEC

Toshio Hyodo, Haruo Saito, Y. Nagashima +2 more

Realization of Bose-Einstein Condensate (BEC) of positronium (Ps), a purely leptonic particle-antiparticle bound system, is a challenging subject [1, 2]. Recently, Ivanov et al. [3] theoretically investigated the Ps-Ps scattering and showed theoretically that the scattering length for the o-Ps atoms is positive, indicating that a stable o-Ps BEC is actually possible. The condition for the BEC of the ideal Bose gas is λth > (2.612/n)1/3, where λth = h/ √ 2πmkT is the thermal de Broglie wavelength of the atom and n is the number density. This gives the critical temperature for the Ps BEC as Tc = 1.46×10−15n2/3, where Tc is in K and n is in m−3. This condition is much less severe than for the usual alkali atom BEC because λth is longer by five orders of magnitude. However, Ps has a finite lifetime: the lifetime of singlet state, para-positronium (p-Ps), is 125 ps and that of triplet state, ortho-positronium (o-Ps), is 142 ns. Thus a many-Ps system must be cooled within the lifetime of o-Ps. The short lifetime of Ps demands quick cooling and high density. The laser cooling which is employed for the accomplishment of alkali atom BEC is likely to be impractical because it takes longer time. It is also subject to relatively large recoil of the Ps atom when hit by a photon. Moreover, unless an effective method for trapping Ps in free space is developed, the system of Ps atoms diffuses and lowers its density. Platzmann and Mills [1] suggested to confine Ps in a small cavity of insulating material and let the Ps atoms give away their kinetic energy to the walls of the cavity. If one injects an intense polarized positron beam onto a material which has a micro cavity near surface, some of the positrons form Ps and are emitted into the cavity and stay there. This is expected because the work function of most of the material for Ps is negative, i.e., Ps atoms emitted into the cavity after thermalization in the material will never penetrate back into the material. They just hit the cavity walls and approach thermal equilibrium by exciting phonons. The probability of the annihilation of o-Ps with a surface electron is small if there are no paramagnetic centers on the cavity walls. It is necessary to use a polarized positron beam to make Ps. Otherwise all the o-Ps atoms are lost through mutual spin conversion into p-Ps [1, 4]. In this report we would like to discuss prospect of a Ps BEC with special attention to the following aspects [4]: (1) use of 18F for a source of an intense spin-polarized positron beams [5], (2) development of high efficiency Wmesh positron moderator, (3) emission of Ps from solid surface (4) the cooling of Ps in a cavity, and (5) the detection of a Ps BEC.

•Jan 1, 2002•0 citations

Pickoff and spin conversion of orthopositronium in oxygen

N. Shinohara, Naoki Suzuki, T. Chang +1 more

The quenching processes of the thermalized orthopositronium $(o\ensuremath{-}\mathrm{Ps})$ on an oxygen molecule have been studied by the positron annihilation age-momentum correlation technique. The Doppler-broadening spectrum of the 511-kev $\ensuremath{\gamma}$ rays from the $2\ensuremath{\gamma}$ annihilation of $o\ensuremath{-}\mathrm{Ps}$ in ${\mathrm{O}}_{2}$ has been measured as a function of the $o\ensuremath{-}\mathrm{Ps}$ age. The rate of the quenching, consisting of the pickoff and the spin conversion, is estimated from the positron lifetime spectrum. The ratio of the pickoff quenching rate to the spin-conversion rate is deduced from the Doppler broadening of the 511-kev $\ensuremath{\gamma}$ rays from the annihilation of the $o\ensuremath{-}\mathrm{Ps}.$ The pickoff parameter ${}^{1}{Z}_{\mathrm{eff}},$ the effective number of the electrons per molecule that contribute to the pickoff quenching, for ${\mathrm{O}}_{2}$ is determined to be $0.6\ifmmode\pm\else\textpm\fi{}0.4.$ The cross section for the elastic spin-conversion quenching is determined to be $(1.16\ifmmode\pm\else\textpm\fi{}0.01)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19} {\mathrm{cm}}^{2}.$

arXiv:physics/0011054•Physical Review A•Sep 10, 2001•33 citations

Determination and applications of enhancement factors for positron and ortho-positronium annihilations

Jim Mitroy

Electron-positron annihilation rates calculated directly from the electron and positron densities are known to underestimate the true annihilation rate. A correction factor, known as the enhancement factor, allows for the local increase of the electron density around the positron caused by the attractive electron-positron interaction. Enhancement factors are given for positrons annihilating with the $1s$ electron in H, ${\mathrm{He}}^{+}$, He, ${\mathrm{Li}}^{2+}$, and ${\mathrm{Li}}^{+}$. The enhancement factor for a free positron annihilating with ${\mathrm{He}}^{+}$ and He is found to be close to that of ortho-positronium (i.e., Ps in its triplet state) annihilating with these atoms. The enhancement factor for Ps-He scattering is used in conjunction with the known annihilation rate for pickoff annihilation to derive a scattering length of $1.47{a}_{0}$ for Ps-He scattering. Further, enhancement factors for ${e}^{+}$-Ne and ${e}^{+}$-Ar annihilation are used in conjunction with the pickoff annihilation rate to estimate scattering lengths of $1.46{a}_{0}$ for Ps-Ne scattering and $1.75{a}_{0}$ for Ps-Ar scattering.

Physical Review A•Dec 16, 2005•12 citations
When positrons are injected into a gas, 75% of the positronium (Ps) is likely to be formed as long-lived ortho-Ps. The main decay mechanisms for the ortho-Ps have been assumed to be natural decay of ortho-Ps and pickoff annihilation of the positron during Ps-atom collisions. A third possibility for annihilation is ortho\ensuremath-Ps\ensuremath→para\ensuremath-Ps\mathrm{o}\mathrm{r}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{\text{\ensuremath{-}}}\mathrm{P}\mathrm{s}\ensuremath{\rightarrow}\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{a}\mathrm{\text{\ensuremath{-}}}\mathrm{P}\mathrm{s}ortho\ensuremath-Ps\ensuremath→para\ensuremath-Ps conversion due to the spin-orbit interaction between the atom and colliding Ps. This extra quenching mechanism may explain a number of phenomena observed in the annihilation spectrum of Kr and Xe, including the very small Ps fraction of 3% seen for Xe.
Spin-orbit interaction of positronium (Ps) with the surrounding atoms, predicted by Mitroy and Novikov [Phys. Rev. Lett. 90, 183202 (2003)], has been detected experimentally. We have found that the lifetime of the magnetically unperturbed ortho-Ps in Xe gas of 1 atm decreases significantly when a magnetic field of 1.0 T is applied. This decrease is attributed to the Ps spin conversion caused by spin-orbit interaction during Ps-Xe collision. The annihilation cross section of ortho-Ps due to this interaction has been determined to be 3 times as large as that expected by Mitroy and Novikov.
By using a kinetic approach, we study the evolution of a gas composed of interacting ortho- and para-positronium atoms. We calculate the total lifetime of the gas and study the ortho-positronium quenching effect induced by the bi-atomic scattering mechanisms with spin exchange. We analyze a realistic situation where the positronium is formed by highly energetic positrons impinging on a solid surface. In the case of a spin-polarized source of positrons, the spinpolarization time of the final positronium gas is estimated.
Realization of Bose-Einstein Condensate (BEC) of positronium (Ps), a purely leptonic particle-antiparticle bound system, is a challenging subject [1, 2]. Recently, Ivanov et al. [3] theoretically investigated the Ps-Ps scattering and showed theoretically that the scattering length for the o-Ps atoms is positive, indicating that a stable o-Ps BEC is actually possible. The condition for the BEC of the ideal Bose gas is λth > (2.612/n)1/3, where λth = h/ √ 2πmkT is the thermal de Broglie wavelength of the atom and n is the number density. This gives the critical temperature for the Ps BEC as Tc = 1.46×10−15n2/3, where Tc is in K and n is in m−3. This condition is much less severe than for the usual alkali atom BEC because λth is longer by five orders of magnitude. However, Ps has a finite lifetime: the lifetime of singlet state, para-positronium (p-Ps), is 125 ps and that of triplet state, ortho-positronium (o-Ps), is 142 ns. Thus a many-Ps system must be cooled within the lifetime of o-Ps. The short lifetime of Ps demands quick cooling and high density. The laser cooling which is employed for the accomplishment of alkali atom BEC is likely to be impractical because it takes longer time. It is also subject to relatively large recoil of the Ps atom when hit by a photon. Moreover, unless an effective method for trapping Ps in free space is developed, the system of Ps atoms diffuses and lowers its density. Platzmann and Mills [1] suggested to confine Ps in a small cavity of insulating material and let the Ps atoms give away their kinetic energy to the walls of the cavity. If one injects an intense polarized positron beam onto a material which has a micro cavity near surface, some of the positrons form Ps and are emitted into the cavity and stay there. This is expected because the work function of most of the material for Ps is negative, i.e., Ps atoms emitted into the cavity after thermalization in the material will never penetrate back into the material. They just hit the cavity walls and approach thermal equilibrium by exciting phonons. The probability of the annihilation of o-Ps with a surface electron is small if there are no paramagnetic centers on the cavity walls. It is necessary to use a polarized positron beam to make Ps. Otherwise all the o-Ps atoms are lost through mutual spin conversion into p-Ps [1, 4]. In this report we would like to discuss prospect of a Ps BEC with special attention to the following aspects [4]: (1) use of 18F for a source of an intense spin-polarized positron beams [5], (2) development of high efficiency Wmesh positron moderator, (3) emission of Ps from solid surface (4) the cooling of Ps in a cavity, and (5) the detection of a Ps BEC.
The quenching processes of the thermalized orthopositronium $(o\ensuremath{-}\mathrm{Ps})$ on an oxygen molecule have been studied by the positron annihilation age-momentum correlation technique. The Doppler-broadening spectrum of the 511-kev $\ensuremath{\gamma}$ rays from the $2\ensuremath{\gamma}$ annihilation of $o\ensuremath{-}\mathrm{Ps}$ in O2{\mathrm{O}}_{2}O2​ has been measured as a function of the $o\ensuremath{-}\mathrm{Ps}$ age. The rate of the quenching, consisting of the pickoff and the spin conversion, is estimated from the positron lifetime spectrum. The ratio of the pickoff quenching rate to the spin-conversion rate is deduced from the Doppler broadening of the 511-kev $\ensuremath{\gamma}$ rays from the annihilation of the $o\ensuremath{-}\mathrm{Ps}.$ The pickoff parameter 1Zeff,{}^{1}{Z}_{\mathrm{eff}},1Zeff​, the effective number of the electrons per molecule that contribute to the pickoff quenching, for O2{\mathrm{O}}_{2}O2​ is determined to be $0.6\ifmmode\pm\else\textpm\fi{}0.4.$ The cross section for the elastic spin-conversion quenching is determined to be $(1.16\ifmmode\pm\else\textpm\fi{}0.01)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19} {\mathrm{cm}}^{2}.$
Electron-positron annihilation rates calculated directly from the electron and positron densities are known to underestimate the true annihilation rate. A correction factor, known as the enhancement factor, allows for the local increase of the electron density around the positron caused by the attractive electron-positron interaction. Enhancement factors are given for positrons annihilating with the 1s1s1s electron in H, He+{\mathrm{He}}^{+}He+, He, Li2+{\mathrm{Li}}^{2+}Li2+, and Li+{\mathrm{Li}}^{+}Li+. The enhancement factor for a free positron annihilating with He+{\mathrm{He}}^{+}He+ and He is found to be close to that of ortho-positronium (i.e., Ps in its triplet state) annihilating with these atoms. The enhancement factor for Ps-He scattering is used in conjunction with the known annihilation rate for pickoff annihilation to derive a scattering length of 1.47a01.47{a}_{0}1.47a0​ for Ps-He scattering. Further, enhancement factors for e+{e}^{+}e+-Ne and e+{e}^{+}e+-Ar annihilation are used in conjunction with the pickoff annihilation rate to estimate scattering lengths of 1.46a01.46{a}_{0}1.46a0​ for Ps-Ne scattering and 1.75a01.75{a}_{0}1.75a0​ for Ps-Ar scattering.