Publications from GBAR Collaborators

The GBAR experiment is designed to measure the free fall acceleration of antihydrogen atoms almost at rest. We have developed a time-of-flight (TOF) detector for the measurement of free fall time and annihilation position of antihydrogen atoms as well as for the rejection of cosmic ray muons. A scintillator array as a part of the TOF detector was installed and tested at the GBAR experimental hall. The performance of the TOF detector studied by cosmic ray muons is reported in this paper.

B.H. Kim, J.J. Choi, J.W. Hwang, S.K. Kim, Y. Ko, A. Lee, B.C. Lee, H. Lee, J. Lee, K.H. Park, D. Won
Nuclear Inst. and Methods in Physics Research, A 973 (2020) 164162

We propose to use quantum interferences to improve the accuracy of the measurement of the free-fall acceleration g of antihydrogen in the gravitational behavior of antihydrogen at rest (GBAR) experiment. This method uses most antiatoms prepared in the experiment and it is simple in its principle, as interferences between gravitational quantum states are read out without transitions between them. We use a maximum likelihood method for estimating the value of g and assess the accuracy of this estimation by a Monte Carlo simulation. We find that the accuracy is improved by approximately three orders of magnitude with respect to the classical timing technique planned for the current design of the experiment.

P.-P. Crépin, C. Christen, R. Guérout, V. V. Nesvizhevsky, A.Yu. Voronin, and S. Reynaud
Phys. Rev. A 99, 042119 (2019)

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.

O. Morandi, P.-A. Hervieux, and G. Manfredi
J. Phys. B: At. Mol. Opt. Phys. 47 155202 (2014)

We simulate the thermalization process of a high energetic positronium gas trapped in a silica pore. The gas dynamics is reproduced by using a kinetic approach. Our approach includes the two-body scattering interaction and the exchange of energy between the atoms and the internal surface of the pore cavity. We discuss the formation of a quasi-equilibrium state induced by the fast internal thermalization of the gas and the evolution of the gas temperature. We estimate the time necessary to achieve the total thermalization of the gas. The reliability of our model is verified by comparing the numerical results with some experimental data.

O. Morandi, P.-A. Hervieux, and G. Manfredi
Eur. Phys. J. D 68, 84 (2014)

We investigate the possibility to produce a Bose-Einstein condensate made of positronium atoms in a porous silica material containing isolated nanometric cavities. The evolution equation of a weakly interacting positronium system is presented. The model includes the interactions among the atoms in the condensate, the surrounding gas of noncondensed atoms, and the pore surface. The final system is expressed by the Boltzmann evolution equation for noncondensed particles coupled with the Gross-Pitaevskii equation for the condensate. In particular, we focus on the estimation of the time necessary to form a condensate containing a macroscopic fraction of the positronium atoms initially injected in the material. The numerical simulations reveal that the condensation process is compatible with the lifetime of ortho-positronium.

O. Morandi, P.-A. Hervieux, and G. Manfredi, ,
Phys. Rev. A 89, 033609 (2014)

The evolution equation of a weakly interacting boson system is derived. The model includes the interaction between the atoms in the condensate and the surrounding gas of noncondensed particles. The Bogoliubov transformation is introduced in a full quantum context and the scattering kernel between dressed particles and the condensate phase is obtained. The final system is expressed by the Boltzmann evolution equation for noncondensed particles coupled to the Gross-Pitaevskii equation for the condensate. We consider an out-of-equilibrium situation that induces a fast production of condensed particles. We apply our model to study the condensation dynamics of positronium atoms by evaporation.

Omar Morandi, Paul-Antoine Hervieux and Giovanni Manfredi
Phys. Rev. A 88, 023618 (2013)

Non-neutral plasmas can be trapped for long times by means of combined electric and magnetic fields. Adiabatic cooling is achieved by slowly decreasing the trapping frequency and letting the plasma occupy a larger volume. We develop a fully kinetic time-dependent theory of adiabatic cooling for plasmas trapped in a one-dimensional well. This approach is further extended to three dimensions and applied to the cooling of antiproton plasmas, showing excellent agreement with recent experiments [Gabrielse et al., Phys. Rev. Lett. 106, 073002 (2011)].

G. Manfredi and P.-A. Hervieux
Phys. Rev. Lett. 109 (2012)

Electron–positron clusters are studied using a quantum hydrodynamic model that includes Coulomb and exchange interactions. A variational Lagrangian method is used to determine their stationary and dynamical properties. The cluster static features are validated against existing Hartree–Fock calculations. In the linear response regime, we investigate both dipole and monopole (breathing) modes. The dipole mode is reminiscent of the surface plasmon mode usually observed in metal clusters. The nonlinear regime is explored by means of numerical simulations. We show that, by exciting the cluster with a chirped laser pulse with slowly varying frequency (autoresonance), it is possible to efficiently separate the electron and positron populations on a timescale of a few tens of femtoseconds.

Giovanni Manfredi, Paul-Antoine Hervieux and Fernando Haas
New Journal of Physics 14 (2012) 075012

We report on muonium (Mu) emission into vacuum following μ⁺ implantation in mesoporous thin SiO₂ films. We obtain a yield of Mu into vacuum of (38±4) % at 250 K and (20±4) % at 100 K for 5 keV μ⁺ implantation energy. From the implantation energy dependence of the Mu vacuum yield we determine the Mu diffusion constants in these films: D^{250 K}_Mu = (1.6±0:1) × 10^-4 cm²/s and D^{100 K}_Mu = (4.2±0:5) × 10^-5 cm²/s. Describing the diffusion process as quantum mechanical tunnelling from pore to pore, we reproduce the measured temperature dependence ~T^3/2 of the diffusion constant. We extract a potential barrier of (0.3±0.1) eV which is consistent with our computed Mu work function in SiO₂ of [-0.3±-0.9] eV. The high Mu vacuum yield, even at low temperatures, represents an important step toward next generation Mu spectroscopy experiments.

A. Antognini et al.
Phys. Rev. Lett. 108 143401 (2012)

In this paper, we present measurements of the ortho-positronium (ortho-Ps) emission energy in vacuum from mesoporous films using the time-of-flight technique. We show evidence of quantum mechanical confinement in the mesopores that defines the minimal energy of the emitted Ps. Two samples with different effective pore sizes, measured with positron annihilation lifetime spectroscopy, are compared for the data collected in the temperature range 50–400 K. The sample with smaller pore size exhibits a higher minimal energy (73±5 meV), compared to the sample with bigger pores (48±5 meV), due to the stronger confinement. The dependence of the emission energy with the temperature of the target is modeled as ortho-Ps being confined in rectangular boxes in thermodynamic equilibrium with the sample. We also measured that the yield of positronium emitted in vacuum is not affected by the temperature of the target.

P. Crivelli, U. Gendotti, A. Rubbia, L. Liszkay, P. Pérez, and C. Corbel
Phys. Rev. A 81 (2010) 052703

We have measured the kinetic energy of positronium (Ps) atoms emitted into a vacuum from a porous silica film subsequent to positron bombardment, via the Doppler spread of the linewidth of the Ps 1³S-2³P transition. We find that the deeper in the target film that positrons are implanted the colder is the emitted Ps, an effect we attribute to cooling via collisions in the pores as the atoms diffuse back to the film surface. We observed a lower limit to the mean Ps kinetic energy associated with motion in the direction of the laser, Ex=42±3 meV, that is consistent with conversion of the confinement energy of Ps in the 2.7 nm diameter pores to kinetic energy in vacuum. An implication is that a porous sample would need to be composed of pores greater than around 10 nm in diameter in order to produce thermal Ps in vacuum with temperatures of less than 100 K. By performing Doppler spectroscopy on intense pulses of Ps we have experimentally demonstrated the production of many excited-state Ps atoms simultaneously, which could have numerous applications, including laser cooling and fundamental spectroscopic studies of Ps and the production of antihydrogen.

D. B. Cassidy, P. Crivelli, T. H. Hisakado, L. Liszkay, V. E. Meligne, P. Pérez, H. W. K. Tom, and A. P. Mills, Jr
Phys. Rev. A 81 (2010) 012715

We have installed in Saclay a demonstration setup for an intense positron source in November 2008. It is based on a compact 6 MeV electron linac to produce positrons via pair production on a tungsten target. A relatively high current of 0.15 mA compensates the low energy, which is below the neutron activation threshold. The expected production rate is 4 x 10¹¹ fast positrons per second. A set of coils is arranged to select the fast positrons from the diffracted electron beam in order to study the possibility to use a rare gas cryogenic moderator away from the main flux of particles. A first part of the commissioning of the linac has been performed. First attempts at measuring the fast positron flux are underway. This setup is part of a project to demonstrate the feasibility of an experiment to produce the H⁺ ion for a free fall measurement of neutral antihydrogen (H). Its small size and cost could be of interest for a university laboratory or industry for materials science applications.

P. Pérez et al.
Phys. Status Solidi C 6 (2009) 2462

The behavior of ortho-positronium (o-Ps) in mesoporous silica films implanted with low–energy positrons has been studied as a function of the film porous volume fraction. A lifetime spectrometer allowed determination of o-Ps annihilation decay both inside and outside of the film. A kinetic model is introduced that permits the determination of the yield and rate of escape of o-Ps into vacuum as well as the annihilation decay rate of the trapped o-Ps in the film. It is shown that these undergo a sudden change at a threshold porous volume fraction, above which the o-Ps escape rate to vacuum varies linearly with volume fraction.

L. Liszkay et al.
Appl. Phys. Lett. 95, 124103 (2009)

Positron annihilation gamma energy distribution, lifetime spectroscopy and time-of-flight method were used to study surfactant-templated mesoporous silica films deposited on glass. The lifetime depth profiling was correlated to Doppler broadening and 3γ annihilation fraction measurements to determine the annihilation characteristics inside the films. A set of consistent fingerprints for positronium annihilation, o-Ps reemission into vacuum, and pore size was directly determined. The lifetime measurements were performed in reflection mode with a specially designed lifetime spectrometer mounted on a slow positron beam system. The intensity of the 142 ns vacuum lifetime component was recorded as a function of the energy of the positron beam. In a film with high porosity a reemission efficiency of as high as 40 % was found at low positron energy. Positron lifetime in samples capped by a thin silica layer was used to determine the pore size. The energy of the reemitted o-Ps fraction was measured by a time-of-flight detector, mounted on the same s ystem, allowing determination of both o-Ps re-emission efficiency and energy in the same sample. We demonstrate the potential of the simultaneous use of different positron annihilation techniques in the study of thin porous films.

L. Liszkay et al.
Materials Science Forum 607, 30-33 (2009)

The reemission yield of ortho-positronium o-Ps into vacuum outside mesoporous silica films on glass is measured in reflection mode with a specially designed lifetime (LT) spectrometer. Values as high as 40 % are found. The intensity of the 142 ns vacuum LT is recorded as a function of reemission depth. The LT depth profiling is correlated to the 2 γ and 3 γ energy ones to determine the annihilation characteristics inside the films. Positron lifetime in capped films is used to determine the pore size. For the first time, a set of consistent fingerprints for positronium annihilation, o-Ps reemission into vacuum, and pore size, is directly determined in surfactant-templated mesoporous silica films.

L. Liszkay et al.
Appl. Phys. Lett. 92, 063114 (2008)