Atomic physics

Atomic physics

The research in the Physics of Elementary Particles has been the central topic of the Collaboration PS201, OBELIX. In order to use hydrogen targets with densities till one thousandth of the usual one, where the annihilations happen at rest in hydrogen mainly in P wave, it was necessary to study the energy loss of antiprotons in this gas from the beginning kinetic energy of some MeVs to the capture at some eVs, i.e. about 6 orders of magnitude. The Group of Brescia has introduced therefore a new methodology for the appraisal of stopping power (the energy loss for braking), using the space-time reconstruction of the annihilations of antiprotons with the resolution of the OBELIX apparatus.
This work has allowed to obtain the only existing measures for the two simples structures: the hydrogen molecule (H2) and the simple helium atom of helium (He) in the whole range of energy. The most important results have been the matter of two papers on Physical Review Letter in 1995 and in 2002, while the whole treatment is collected in about ten articles, whose we report the conclusive ones.

  • A. Bianconi et al., Antiproton slowing down, capture, and decay in low-pressure helium gas Phys. Rev. A 70, 032501 (2004)
  • E. Lodi Rizzini et al., Antiproton spopping power in He in the energy range 1-900 KeV and the Barkas effectPhys. Lett. B 599, 190 (2004)
  • E. Lodi Rizziniet al. Barkas effect for antiproton stopping in H2 Phys. Rev. Lett. 89, 18 (2002)

In the picture the stopping power of the antiproton in hydrogen is drawn with continuous line, while that one of the proton is marked with dotted line. stopping power

In this field the Group of Brescia has also supplied important and sometimes unique informations on the length of the “cascade” which brings the antiproton from the quantic numbers (n,l) of capture followed by the annihilation on the nucleus, proton (p) or the alpha particle (α), respectivly in the case of hydrogen or helium.
Also this sector has been illustrated with about ten articles on the most important reviews. These activities of Atomic Phisycs have been at the base of the choice to activate the research for the production of antiatoms of hydrogen H.
The relative international collaboration was established in 1995 and was called ATHENA, labelled with the acronym AD1 by the CERN. The data taking took place in 2002, 2003 and 2004.
The clear indication of antihydrogen production  appeared in August 2002 and was communicated to the world scientific community on September 18th, when an article (sent by the collaboration on August 28th) appeared on the scientific journal Nature.
In the following articles published in Physical Review Letters and in other leading international journals are being published the results of the analysis aimed at characterizing the antihydrogen atom (pe+) formation process starting from the superposition of positron (e+) and antiproton (p) clouds in a suitable trap.
distribuzione dell'angolo sotteso da due elementi del rivelatore di raggi gamma colpiti contemporaneamente al rivelatore di particelle cariche  In the following picture, we reproduce the characteristic signature of the antihydrogen atom production (Nature, september 2002).
You can observe the peculiar peak of the antiproton annihilation events, coming from the negative nuclei of the antihydrogen atoms, in temporal coincidence with the relative positron annihilation happening on an electron of the matter costituting the trap wall.
In this trap antiprotons and positrons are superimposed in order to make them bind and form antihydrogen.
The positron-electron (e+e-) annihilation feature is the opening angle between the two 511 keV photons of 180 degrees, i.e. its cosine equals -1.

  • L. V. Jørgensen et al., New Source of Dense, Cryogenic Positron Plasmas Phys. Rev. Lett. 95, 025002 (2005)
  • N. Madsen et al., Spatial Distribution of Cold Antihydrogen Formation Phys. Rev. Lett. 94, 033403 (2005)
  • M. C. Fujiwara et al., Three-Dimensional Annihilation Imaging of Trapped Antiprotons Phys. Rev. Lett. 92, 065005 (2004)
  • M. Amoretti et al., Antihydrogen production temperature dependence Phys. Lett. B583, 59-67 (2004)
  • M. Amoretti et al., High rate production of antihydrogen Phys. Lett. B578, 23-32 (2004)
  • M. Amoretti et al., The ATHENA antihydrogen apparatus Nucl. Inst. Meth. Phys. Res. A 518, 679-711 (2004)
  • M. Amoretti et al., Positron Plasma Diagnostic and Temperature Control for Antihydrogen Production Phys. Rev. Lett. 91, 055001 (2003)
  • M. Amoretti et al., Complete Nondestructive Diagnostic of Nonneutral Plasmas Based on the Detection of Electrostatic Modes Phys. Plasma 10, 3056 (2003)
  • M. Amoretti et al., Production and detection of cold antihydrogen atoms Nature 419, 456 (2002)

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