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Positronium

Positronium formation in mesoporous materials for Antihydrogen production
Antihydrogen production by charge exchange reaction between positronium (Ps) atoms and antiprotons is the main process considered for antihydrogen production in AEgIS.
Below will be discussed the formation of Ps in mesoporous materials.


Positronium
Ps is a system consisting of an electron and a positron bound together into an "hydrogenoid atom". Ground statePs is formed for 75% as ortho-Ps (spin 1) and for 25% as para-Ps (spin 0). Para-Ps annihilates in two gamma rays with 511 keV each and ortho-Ps in vacuum is required to annihilate into three gamma rays at least, with a maximum energy of 511 keV each and with a total energy of 1022 keV (total rest mass). We are interested only in Ps emitted by the converter as ortho-Ps with a characteristic lifetime in vacuum of 142 ns, since the lifetime of para-Ps is too short (125 ps) to allow the necessary laser excitation before annihilation. The ortho-Ps lifetime can be shortened by collisional pick-off annihilation and ortho-para conversion (both processes lead to annihilation in two gamma rays) in ranges typically from a fraction of nanosecond to tens of nanoseconds.

Positronium in porous materials


Schematic views of systems formed by nanometric solids interspersed with empty regions: a) a random structure in a disordered SiO2 skeleton (e.g. silica Aerogel or compressed powder); b) ordered Si nano-channels with SiO2 at the surface.

A strategy to obtain Ps in vacuum is using mesoporous materials with pores open to the surface. Porous materials are necessary not only to form a high yield of Ps atoms but also to cool Ps through collisions with the inner walls of the pores. Ps atoms are emitted from the pore walls with high kinetic energy (1-3 eV). The collisions between Ps and the internal surface of the pores involve weak coupling to phonons or other surface modes. Low Ps kinetic energies are desirable for AEgIS. The velocity distribution of the Ps atoms coming out of the target should be the order of 104 m/s to allow Ps laser excitation to a Rydberg state (Ps*) and for efficient Hformation, which requires that the relative velocity of antiprotons and Ps* must be not higher than the classical orbital velocity of the positron in the Rydberg Ps atom. Efficient formation of cooled Ps atoms is a requisite for the production of antihydrogen. The thermalization of Ps atoms is possible by means of thousands of collisions with the walls of the pores. A careful choice of the materials used to convert bare positrons in Ps atoms is required to provide the appropriate morphology for efficient cooling. Not only is the morphology of the pores at nanometric scale an important aspect, but also the chemical composition of the pore walls. The figure shows schematic views of two nanometric porous solids, which represent two alternative strategies for AEgIS: samples with wide and interconnected random pores (panel a) and ordered structures with SiO2 at the surface of the pores (panel b).


Aerogel

Aerogel is a manufactured material with the lowest bulk density of any known porous solid. It is derived from a gel in which the liquid component of the gel has been replaced with a gas. The result is an extremely low-density solid, with a notable effectiveness as a thermal insulator.
Aerogels are produced by extracting the liquid component of a gel through supercritical drying. This allows the liquid to be slowly drawn off without causing the solid matrix in the gel to collapse from capillary action, as would happen with conventional evaporation.

The research works about Ps production in Aerogel are in collaboration with the Jet Propulsion Laboratory (NASA). 
(see some characteristics)

AEgIS collaboration (Antimatter Experiment: Gravity, Interferometry, Spectroscopy - CERN)

  1. G. Consolati, R. Ferragut, A. Galarneau, F. Di Renzo, and F. Quasso. Mesoporous materials for antihydrogen productionChemical Society Reviews (I.F.: 28.76042, 3821 (2013) doi: 10.1039/c2cs35454c pdf
  2. D. Krasnicky, S. Aghion (AEgIS col.): AEgIS experiment commissioning at CERN, AIP Conf. Proc. 1521, 144 (2013) doi: http://10.1063/1.4796070pdf
  3. M. Doser et al. (AEgIS col.): Exploring the WEP with a pulsed cold beam of antihydrogenClass. Quantum Grav. 29   184009 (2012) doi:10.1088/0264-9381/29/18/184009pdf
  4. R. Ferragut et al. (AEgIS col.): Antihydrogen physics: gravitation and spectrometry in AEgIS. Canadian J. Physics 89, 17 (2011) doi:10.1139/P10-099pdf
  5. F. Moia, R. Ferragut, A. Dupasquier, M.G. Giammarchi, G.Q. Ding, Thermal production of positronium in porous alumina, Eur. Phys. J. D 66, 124 (2012). doi: 10.1140/epjd/e2012-20637-0pdf
  6. A. Kellerbauer et al. (AEgIS col.): The AEGIS experiment at CERN: Measuring the free fall of antihydrogenHyperfine Interact. 209, 43 (2012) doi:10.1007/s10751-012-0583-xpdf
  7. R. Ferragut, A. Dupasquier, A. Calloni, G. Consolati, F. Quasso, M.P. Petkov, S.M. Jones, A Galarneau and F Di Renzo, Homogeneous porous silica for positronium production in AEgIS, J. Phys.: Conf. Ser. 262, 012020 (2011) doi:10.1088/1742-6596/262/1/012020pdf
  8. R. Ferragut, A. Calloni, A. Dupasquier, G. Consolati, F. Quasso, M.G. Giammarchi, D. Trezzi, W. Egger, L. Ravelli, M.P. Petkov, S.M. Jones, B. Wang, O.M. Yaghi, B. Jasinska, N. Chiodini and A. Paleari:Positronium Formation in Porous Materials for Antihydrogen ProductionJ. Phys.: Conf. Ser. 225, 012007 (2010) doi:10.1088/1742-6596/225/1/012007pdf
  9. M. Doser et al. (AEgIS col.): Measuring the fall of antihydrogen: the AEgIS experiment at CERN. Physics Procedia 17 (2011) 49 doi:10.1016/j.phpro.2011.06.016pdf
  10. M. G. Giammarchi et al. (AEgIS col.): AEGIS at CERN: Measuring Antihydrogen Fall in Proceedings of the Fifth Meeting on CPT and Lorentz Symmetry, ed. V. Alan Kostelecký (World Scientific Publishing. Singapore, 2011). pdf
  11. C. Canali et al. (AEgIS col.): The Aegis Experiment (Antimatter Experiment: Gravity, Interferometry, Spectroscopy), in Astroparticle, Particle and Space Physics, Detectors and Medical Physics Applications.Vol. 5. ed. C. Leroy, P. G. Rancoita, M. Barone, A. Gaddi, L. Price and R. Ruchti. (World Scientific Publishing. Singapore, 2010) pp 185-189 doi:10.1142/9789814307529_0031pdf
  12. D. Fabris et al. (AEgIS col.): The AEGIS detection system for gravity measurements, Nuclear Physics A834, 751c (2010) doi:10.1016/j.nuclphysa.2010.01.136pdf
  13. G. Bonomi et al. (AEgIS col.): Measuring the antihydrogen fallHyperfine Interact. 193, 297 (2009). doi:10.1007/s10751-009-0015-8pdf
  14. M. G. Giammarchi et al. (AEgIS col.): Efficient Rydberg positronium laser excitation for antihydrogen production in a magnetic fieldHyperfine Interact. 193, 321 (2009) doi:10.1007/s10751-009-0018-5pdf
  15. G. Testera et al. (AEgIS col.): Formation of a cold antihydrogenbeam in AEGIS for gravity measurements,AIP Conf. Proc. 1037, 5 (2008) doi:10.1063/1.2977857pdf

   

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