Showing papers on "Positronium published in 2018"

Experimental progress in positronium laser physics

Abstract: The field of experimental positronium physics has advanced significantly in the last few decades, with new areas of research driven by the development of techniques for trapping and manipulating positrons using Surko-type buffer gas traps. Large numbers of positrons (typically ≥106) accumulated in such a device may be ejected all at once, so as to generate an intense pulse. Standard bunching techniques can produce pulses with ns (mm) temporal (spatial) beam profiles. These pulses can be converted into a dilute Ps gas in vacuum with densities on the order of 107 cm−3 which can be probed by standard ns pulsed laser systems. This allows for the efficient production of excited Ps states, including long-lived Rydberg states, which in turn facilitates numerous experimental programs, such as precision optical and microwave spectroscopy of Ps, the application of Stark deceleration methods to guide, decelerate and focus Rydberg Ps beams, and studies of the interactions of such beams with other atomic and molecular species. These methods are also applicable to antihydrogen production and spectroscopic studies of energy levels and resonances in positronium ions and molecules. A summary of recent progress in this area will be given, with the objective of providing an overview of the field as it currently exists, and a brief discussion of some future directions.

Feasibility studies of the polarization of photons beyond the optical wavelength regime with the J-PET detector

Abstract: J-PET is a detector optimized for registration of photons from the electron–positron annihilation via plastic scintillators where photons interact predominantly via Compton scattering. Registration of both primary and scattered photons enables to determinate the linear polarization of the primary photon on the event by event basis with a certain probability. Here we present quantitative results on the feasibility of such polarization measurements of photons from the decay of positronium with the J-PET and explore the physical limitations for the resolution of the polarization determination of 511 keV photons via Compton scattering. For scattering angles of about 82 $$^\circ $$ (where the best contrast for polarization measurement is theoretically predicted) we find that the single event resolution for the determination of the polarization is about 40 $$^\circ $$ (predominantly due to properties of the Compton effect). However, for samples larger than ten thousand events the J-PET is capable of determining relative average polarization of these photons with the precision of about few degrees. The obtained results open new perspectives for studies of various physics phenomena such as quantum entanglement and tests of discrete symmetries in decays of positronium and extend the energy range of polarization measurements by five orders of magnitude beyond the optical wavelength regime.

Feasibility study of the positronium imaging with the J-PET tomograph.

Abstract: A detection system of the conventional PET tomograph is set-up to record data from e+ e- annihilation into two photons with energy of 511 keV, and it gives information on the density distribution of a radiopharmaceutical in the body of the object. In this paper we explore the possibility of performing the three gamma photons imaging based on ortho-positronium annihilation, as well as the possibility of positronium mean lifetime imaging with the J-PET tomograph constructed from plastic scintillators. For this purposes simulations of the ortho-positronium formation and its annihilation into three photons were performed taking into account distributions of photons' momenta as predicted by the theory of quantum electrodynamics and the response of the J-PET tomograph. In order to test the proposed ortho-positronium lifetime image reconstruction method, we concentrate on the decay of the ortho-positronium into three photons and applications of radiopharmaceuticals labeled with isotopes emitting a prompt gamma quantum. The proposed method of imaging is based on the determination of hit-times and hit-positions of registered photons which enables the reconstruction of the time and position of the annihilation point as well as the lifetime of the ortho-positronium on an event-by-event basis. We have simulated the production of the positronium in a cylindrical phantom composed of a set of different materials in which the ortho-positronium lifetime varied from 2.0 ns to 3.0 ns, as expected for ortho-positronium created in the human body. The presented reconstruction method for total-body J-PET like detector allows to achieve a mean lifetime resolution of about 40 ps. Recent Positron Annihilation Lifetime Spectroscopy measurements of cancerous and healthy uterine tissues show that this sensitivity may allow to study the morphological changes in cell structures.

Frequency Tunable sub-THz Gyrotron for Direct Measurements of Positronium Hyperfine Structure

Abstract: The feasibility of a high-power sub-THz gyrotron with smooth wideband frequency tuning suitable for direct measurement of the positronium hyperfine structure is demonstrated numerically using both averaged equations and PIC-code simulations. Analytical estimates show that the frequency-tunable powerful sub-THz radiation can be generated through the excitation of high-order axial modes in a gyrotron with short cavity driven by an electron beam with high current. Simulations show that an output power of 0.5–1 kW can be obtained at a frequency of about 0.2 THz within a 10-GHz band which are the parameters needed for testing of quantum electrodynamics predictions through the spectroscopy of positronium.

Producing long-lived 2 3S positronium via 3 3P laser excitation in magnetic and electric fields

Abstract: Producing positronium (Ps) in the metastable 23S state is of interest for various applications in fundamental physics. We report here on an experiment in which Ps atoms are produced in this long-lived state by spontaneous radiative decay of Ps excited to the 33P level manifold. The Ps cloud excitation is obtained with a UV laser pulse in an experimental vacuum chamber in presence of guiding magnetic field of 25mT and an average electric field of 300Vcm−1. The evidence of the 23S state production is obtained to the 3.6σ level of statistical significance using a novel analysis technique of the single-shot positronium annihilation lifetime spectra. The dynamic of the Ps population on the involved levels has been studied with a rate equation model.

Calculations of positronium-atom scattering using a spherical cavity

Abstract: Positronium (Ps) scattering by noble-gas atoms (He, Ne, Ar, Kr, and Xe) is studied in the frozen-target approximation and with inclusion of the van der Waals interaction. Single-particle electron and positron states in the field of the target atom are calculated, with the system enclosed by a hard spherical wall. The two-particle Ps wave function is expanded in these states, and the Hamiltonian matrix is diagonalized, giving the Ps energy levels in the cavity. Scattering phase shifts, scattering lengths, and cross sections are extracted from these energies and compared with existing calculations and experimental data. Analysis of the effect of the van der Waals interaction shows that it cannot explain the recent experimental data of Brawley et al. for Ar and Xe [Phys. Rev. Lett. 115, 223201 (2015)].

Spin polarization of graphene and h -BN on Co(0001) and Ni(111) observed by spin-polarized surface positronium spectroscopy

Abstract: In spin-polarized surface positronium annihilation measurements, the spin polarizations of graphene and $h$-BN on Co(0001) were higher than those on Ni(111), while no significant differences were seen between graphene and $h$-BN on the same metal. The obtained spin polarizations agreed with those expected from first-principles calculations considering the positron wave function and the electron density of states from the first surface layer to the vacuum region. The higher spin polarizations of graphene and $h$-BN on Co(0001) as compared to Ni(111) simply reflect the spin polarizations of these metals. The comparable spin polarizations of graphene and $h$-BN on the same metal are attributed to the creation of similar electronic states due to the strong influence of the metals: the Dirac cone of graphene and the band gap of $h$-BN disappear as a consequence of $d\ensuremath{-}\ensuremath{\pi}$ hybridization.

Commissioning of the J-PET detector in view of the positron annihilation lifetime spectroscopy

Abstract: The Jagiellonian Positron Emission Tomograph (J-PET) is the first PET device built from plastic scintillators. It is a multi-purpose detector designed for medical imaging and for studies of properties of positronium atoms in porous matter and in living organisms. In this article we report on the commissioning of the J-PET detector in view of studies of positronium decays. We present results of analysis of the positron lifetime measured in the porous polymer. The obtained results prove that J-PET is capable of performing simultaneous imaging of the density distribution of annihilation points as well as positron annihilation lifetime spectroscopy.

Feasibility studies of the polarization of photons beyond the optical wavelength regime with the J-PET detector

Abstract: J-PET is a detector optimized for registration of photons from the electron-positron annihilation via plastic scintillators where photons interact predominantly via Compton scattering. Registration of both primary and scattered photons enables to determinate the linear polarization of the primary photon on the event by event basis with a certain probability. Here we present quantitative results on the feasibility of such polarization measurements of photons from the decay of positronium with the J-PET and explore the physical limitations for the resolution of the polarization determination of 511 keV photons via Compton scattering. For scattering angles of about 82 deg (where the best contrast for polarization measurement is theoretically predicted) we find that the single event resolution for the determination of the polarization is about 40 deg (predominantly due to properties of the Compton effect). However, for samples larger than ten thousand events the J-PET is capable of determining relative average polarization of these photons with the precision of about few degrees. The obtained results open new perspectives for studies of various physics phenomena such as quantum entanglement and tests of discrete symmetries in decays of positronium and extend the energy range of polarization measurements by five orders of magnitude beyond the optical wavelength regime.

Positronium collisions with rare-gas atoms: Free-electron gas plus orthogonalizing pseudopotential model

Abstract: Positronium collisions with rare-gas atoms are treated using the free-electron-gas approximation for exchange and correlation potential. The results confirm the absence of the Ramsauer-Townsend minimum in elastic scattering cross sections, but show lower cross sections in the lower-energy region when compared to previous pseudopotential calculations. This is explained by a more attractive ab initio correlation potential as compared to the previously used empirical potential. The results in the thermal-energy region agree very well with most swarm measurements for all rare-gas atoms. At higher energies, the results are compared with beam experiments and agreement for heavier rare-gas atoms Ar, Kr, and Xe is found to be very good. For He and Ne, some discrepancies with beam measurements are observed. This is explained by a poorer performance of the free-electron-gas potentials, based on the statistical Thomas-Fermi model, for systems with fewer electrons.

The positronium hyperfine structure: Progress towards a direct measurement of the 23S1 → 21S0 transition in vacuum

Abstract: We present the current status for the direct measurement of the positronium hyperfine structure using the 2S1 → 2S0 transition. This experiment, currently being commissioned at the slow positron beam facility at ETH Zurich, will be the first measurement of this transition and the first positronium hyperfine splitting experiment conducted in vacuum altogether. This experiment will be free of systematic effects found in earlier experiments, namely the inhomogeneity in static magnetic fields and the extrapolation from dense gases to vacuum. The achievable precision is expected to be on the order of 10 ppm while the systematic uncertainty is estimated to be within a few ppm. This would allow to check recent bound state QED calculations and a 3σ discrepancy with earlier experiments.

Positron scattering from pyridine.

Abstract: We present a range of cross section measurements for the low-energy scattering of positrons from pyridine, for incident positron energies of less than 20 eV, as well as the independent atom model with the screening corrected additivity rule including interference effects calculation, of positron scattering from pyridine, with dipole rotational excitations accounted for using the Born approximation. Comparisons are made between the experimental measurements and theoretical calculations. For the positronium formation cross section, we also compare with results from a recent empirical model. In general, quite good agreement is seen between the calculations and measurements although some discrepancies remain which may require further investigation. It is hoped that the present study will stimulate development of ab initio level theoretical methods to be applied to this important scattering system.

Laser excitation of positronium produced in various materials at a range of temperatures

Abstract: Methods and techniques in the production, manipulation, and accumulation of positrons in a `Surko-type' buffer-gas trap are presented The pulsed output of the trap is optimised for laser excitation of positronium (Ps) The techniques of positron time compression and single-shot positron annihilation lifetime spectroscopy (SSPALS) are discussed in detail and used throughout the study A small array or Ps converter materials have been characterised at various temperatures Ps cooling as a function of positron implantation is presented and discussed Direct laser irradiation of the samples, and the effect on the Ps yield has also been studied In some cases, particularly at cryogenic temperatures, laser induced paramagnetic centres are formed and this has a detrimental effect on the Ps yield as they cause triplet Ps to convert to the shorter-lived singlet state Whereas in single crystal semiconductors, the laser has an enhancing effect due to an exciton-like positron-electron surface state Many of the Ps converter materials studied here are formed from porous structures The long term confinement of Ps within isolated cavities has also been observed, where a lifetime measurement technique using an excitation laser is presented General methods of Ps-laser spectroscopy are also presented and the applications of which are discussed including laser-enhanced time-of-flight spectroscopy (LEPTOF), and the production, and electrostatic guiding of highly excited Rydberg Ps Using these techniques a crossed beam scattering experiment involving Rydberg Ps, electrons, and Argon ions was carried out This work presents important considerations for experiments to create a laser-cooled Bose-Einstein condensate (BEC) of Ps in an engineered porous material at cryogenic temperatures

State-selective electric-field ionization of Rydberg positronium

Abstract: We report experiments in which positronium (Ps) atoms, optically excited to Rydberg states with principal quantum numbers $n$ in the range 18--25, were selectively ionized by both static and pulsed electric fields. The experiments were modeled using Monte Carlo simulations that include tunnel ionization rates calculated for hydrogen and scaled by the Ps reduced mass. Our measurements exhibit a small disagreement with the calculated tunnel ionization rates. Despite this we show that the electric fields in which different Ps states are ionized are sufficiently separated to allow selective field-ionization methods to be used in typical experimental conditions.

Effect of positron-alkali metal atom interactions in the diffuse interstellar medium

Abstract: In the Milky Way galaxy, positrons, which are responsible for the diffuse 511 keV gamma ray emission observed by space-based gamma ray observatories, are thought to annihilate predominantly through charge exchange interactions with neutral hydrogen. These charge exchange interactions can only take place if positrons have energies greater than 6.8 eV, the minimum energy required to liberate the electron bound to the hydrogen atom and then form positronium, a short-lived bound state composed of a positron-electron pair. Here we demonstrate the importance of positron interactions with neutral alkali metals in the warm interstellar medium (ISM). Positrons may undergo charge exchange with these atoms at any energy. In particular, we show that including positron interactions with sodium at solar abundance in the warm ISM can significantly reduce the annihilation timescale of positrons with energies below 6.8 eV by at least an order of magnitude. We show that including these interactions in our understanding of positron annihilation in the Milky Way rules out the idea that the number of positrons in the Galactic ISM could be maintained in steady state by injection events occurring at a typical periodicity $g\mathrm{Myr}$.

Towards total-body modular PET for positronium and quantum entanglement imaging

Abstract: The purpose of the reported research is (i) the elaboration of the new imaging method based on the in-vivo measurement of properties of positronium produced inside patient during positron emission tomography, and determination of correlations between properties of positronium inside the cancer tissues and histopathological characteristics of cancers, as well as (ii) exploration of possibilities of the determination of the linear polarization of annihilation photons and development of novel prognostic indicators for cancer diagnostics based on the quantum information from (multipartite) entanglement of photons originating from the positronium decay.During PET diagnosis positronium may be trapped inside free volumes between and within molecules of the examined patient. Currently, in the PET technique, the phenomenon of positronium production is neither recorded nor used for imaging. Yet in about 40% cases, the electron-positron annihilation proceeds in the tissue via creation of positronium. The properties of positronium (such as e.g. mean lifetime or ratio of decay rates into two and three photons) depend on the size of the free volumes between atoms and there are indications that they are correlated with the stage of the development of metabolic disorders of the human tissues. Therefore, an image of properties of positronium formed inside the human body may deliver new information complementary to SUV index and useful for the diagnosis.Moreover, recent theoretical studies have proven that the entanglement in the three-photon state from the decay of ortho-positronium survives surprisingly also for mixed scenarios expected in human tissues. Hence, detecting entanglement of photons originating from positronium may enable the extraction of quantum properties of the surrounding tissue environment.We discuss (i) results of the feasibility studies of the positronium mean-lifetime image reconstruction with the total-body PET scanner from plastic scintillators, as well as (ii) results of pilot studies of the mean lifetime of positronium in the healthy and tumorous tissues operated from the patients. Performed experiments show that properties of positronium atoms in uterine tissues operated from human patients reveals meaningful differences between healthy and tumorous tissues. We also discuss results of the feasibility studies of the polarization of annihilation photons with the J-PET tomograph in which annihilation photons interact predominantly via Compton scattering. Registration of both primary and scattered photons enables to determinate the linear polarization of the primary photon on the event by event basis and hence enables to witness the entanglement of annihilation photons in polarization based on Mutually Unbiased Bases. The performed simulations indicate that in the future with the total-body PET and improved time resolution it shall be feasible to reconstruct images of positronium properties in-vivo during the routine PET diagnosis.

Excitation of positronium: from the ground state to Rydberg levels

Abstract: Following bombardment of a mesoporous silica sample by positrons ejected from a two-stage buffer gas trap, ortho-positronium (o-Ps) was emitted into vacuum with an efficiency of around 27-28%. This ensemble, with a density close to 10 m−3, was then irradiated by multiple solid state-based laser beams up to 50 ns after positron implantation. Tunable over a wide range (230-2200 nm), the lasers have enabled excitation of the o-Ps Lyman-α transition, followed by subsequent excitation and, if desired, ionisation to vacuum. Excitation from the 2P state to intermediate states in the principal quantum number, nPs, range 3 ≤ nPs ≤ 18 has been achieved with efficiencies nPs ex & 80%, whilst the excitation efficiency of ground state o-Ps to the 2P state of ex ∼ 13% is currently limited by a mismatch between the Doppler broadening of the 1S-2P transition and the 225 GHz laser bandwidth at 243 nm. † Current address: IRFU, CEA, University Paris-Saclay F-91191 Gif-sur-Yvette Cedex, France Page 1 of 13 AUTHOR SUBMITTED MANUSCRIPT JPHYSB-104049.R1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 A cc ep te d M a us cr ip t Excitation of positronium: from the ground state to Rydberg levels 2

Commissioning of the J-PET detector in view of the positron annihilation lifetime spectroscopy

Abstract: The Jagiellonian Positron Emission Tomograph (J-PET) is the first PET device built from plastic scintillators. It is a multi-purpose detector designed for medical imaging and for studies of properties of positronium atoms in porous matter and in living organisms. In this article we report on the commissioning of the J-PET detector in view of studies of positronium decays. We present results of analysis of the positron lifetime measured in the porous polymer. The obtained results prove that J-PET is capable of performing simultaneous imaging of the density distribution of annihilation points as well as positron annihilation lifetime spectroscopy.

Erratum: Search for CP Violation in Positronium Decay [Phys. Rev. Lett. 104, 083401 (2010)].

Abstract: This corrects the article DOI: 10.1103/PhysRevLett.104.083401.

Feasibility Study of the Time Reversal Symmetry Tests in Decay of Metastable Positronium Atoms with the J-PET Detector

Abstract: This article reports on the feasibility of testing of the symmetry under reversal in time in a purely leptonic system constituted by positronium atoms using the J-PET detector. The present state of symmetry tests is discussed with an emphasis on the scarcely explored sector of leptonic systems. Two possible strategies of searching for manifestations of violation in nonvanishing angular correlations of final state observables in the decay of metastable triplet states of positronium available with J-PET are proposed and discussed. Results of a pilot measurement with J-PET and assessment of its performance in reconstruction of three-photon decays are shown along with an analysis of its impact on the sensitivity of the detector for the determination of - violation sensitive observables.

Antihydrogen formation in low-energy antiproton collisions with excited-state positronium atoms

Abstract: The convergent close-coupling method is used to obtain cross sections for antihydrogen formation in low-energy antiproton collisions with positronium (Ps) atoms in specified initial excited states with principal quantum numbers ni ≤ 5. The threshold behaviour as a function of the Ps kinetic energy, E, is consistent with the 1/E law expected from threshold theory for all initial states. We find that the increase in the cross sections is muted above ni = 3 and that here their scaling is roughly consistent with ${n_{i}^{2}}$ , rather than the classically expected increase as ${n_{i}^{4}}$ .

Study of elastic and inelastic cross sections by positron impact on inert gases

Abstract: In this article, a modified computational method recently introduced is used for the calculation of total, positronium (Ps) formation and ionization cross sections including direct and total ionization cross sections for positron scattering from noble gases. The incident positron is assumed to have energies over a wide range from ~5 eV to 5 keV. The positron–atom interaction potential is developed under an optical potential framework and the computations of cross sections for each process are performed by introducing appropriate absorption thresholds. The calculated results obtained by employing this modified approach are found to be in reasonably good agreement with most of the existing data.

A study on the performance of an electrostatic focusing mirror for Rydberg positronium

Abstract: Recently, we demonstrated an electrostatic mirror that focuses a beam of Rydberg positronium atoms over a 6 m path to a 32 ± 1 mm FWHM diameter spot on a position sensitive detector. The mirror is comprised of 360 wires arranged in the shape of a nearly-cylindrical revolved truncated ellipse ~96 mm in radius, with potentials of equal and opposite magnitude applied to alternating wires to create a short-ranged electric field that decreases in magnitude exponentially with e-folding length = 0.53 mm. Here, we explore in detail the observed resolution and discuss the factors contributing to its broadening from the ideal point focus of a perfect embodiment of the mirror. Improvements to the design are considered, with the aim to achieve a mirror with a resolution of <0.5 mm, which is necessary for a proposed measurement of the gravitational deflection of positronium.

Exchange and correlation in positronium-molecule scattering

Abstract: Exchange and correlations play a particularly important role in positronium (Ps) collisions with atoms and molecules, since the static potential for Ps interaction with a neutral system is zero. Theoretical description of both effects is a very challenging task. In the present work we use the free-electron-gas model to describe exchange and correlations in Ps collisions with molecules similar to the approach widely used in the theory of electron-molecule collisions. The results for exchange and correlation energies are presented as functions of the Fermi momentum of the electron gas and the Ps incident energy. Using the Thomas-Fermi model, these functions can be converted into exchange and correlation potentials for Ps interaction with molecules as functions of the distance between the projectile and the target.

Two-center convergent close-coupling approach to positron–helium-ion collisions

Abstract: We generalize the two-center convergent close-coupling approach of Kadyrov and Bray [Phys. Rev. A 66, 012710 (2002)] for positron scattering on neutral targets to charged targets, and apply it to positron scattering on the singly charged helium ion. Where possible internal consistency checks are used to validate the two-center method, which required a significant redevelopment of the positronium-formation matrix elements, by comparison with a single-center approach. Only the two-center approach explicitly yields positronium formation, and so it also provides a mechanism to calculate positronium scattering on $\ensuremath{\alpha}$ particles. As yet there are no experiments for the calculated processes, but there are some previous theoretical calculations, with which comparison is mixed.

Photodetachment of the Positronium Negative Ion with Excitation in the Positronium Atom

Abstract: Lyman-α radiation ( 2 P → 1 S ) has been seen from astrophysical sources and the sun. The line shape of this transition has been measured recently in Ps atoms both inside and outside a porous silica target. In the photodetachment of Ps−, the residual Ps atom can be left in the 2P state instead of the 1S state giving rise to positronium Lyman radiation at 2432 A0. Photodetachment cross sections of Ps− have been calculated when the Ps atom is left in nP states, n being 2, 3, 4, 5, 6 and 7, using the asymptotic form of the bound-state wave function and a plane wave for the final state wave function, following the approach of Ohmura and Ohmura [Phys. Rev. 1960, 118, 154] in the photodetachment of H−.

Structure of LiPs ground and excited states

Abstract: The lithium atom in its ground state can bind positronium (Ps) forming LiPs, an electronically stable system. In this study we use the fixed node diffusion Monte Carlo method to perform a detailed investigation of the internal structure of LiPs, establishing to what extent it could be described by smaller interacting subsystems. To study the internal structure of positronic systems we propose a way to analyze the particle distribution functions: We first order the particle-nucleus distances, from the closest to the farthest. We then bin the ordered distances obtaining, for LiPs, five distribution functions that we call sorted distribution functions. We used them to show that Ps is a quite well-defined entity inside LiPs: The positron is forming positronium not only when it is far away from the nucleus, but also when it is in the same region of space occupied by the $2s$ electrons. Hence, it is not correct to describe LiPs as positronium ``orbiting'' around a lithium atom, as sometimes has been done, since the positron penetrates the electronic distribution and can be found close to the nucleus.