Physics:ATHENA experiment

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Short description: Antimatter research project

ATHENA, also known as the AD-1 experiment, was an antimatter research project at the Antiproton Decelerator at CERN, Geneva. In August 2002, it was the first experiment to produce 50,000 low-energy antihydrogen atoms, as reported in Nature.[1][2] In 2005, ATHENA was disbanded and many of the former members of the research team worked on the subsequent ALPHA experiment and AEgIS experiment.

Experimental setup

An actual matter-antimatter annihilation due to an atom of antihydrogen in the ATHENA experiment. The antiproton produces four charged pions (yellow) whose positions are given by silicon microstrips (pink) before depositing energy in CsI crystals (yellow cubes). The positron also annihilates to produce back-to-back gamma rays (red).

The ATHENA apparatus comprised four main subsystems: the antiproton catching trap, the positron accumulator, the antiproton/positron mixing trap, and the antihydrogen annihilation detector. All traps in the experiment were variations of the Penning trap, which uses an axial magnetic field to transversely confine the charged particles, and a series of hollow cylindrical electrodes to trap them axially. The catching and mixing traps were adjacent to each other, and coaxial with a 3 T magnetic field from a superconducting solenoid.[3][4]

The positron accumulator had its own magnetic system, also a solenoid, with a field strength of 0.14 Tesla. A separate cryogenic heat exchanger in the bore of the superconducting magnet cooled the catching and mixing traps to about 15 K. The ATHENA apparatus featured an open, modular design that allowed experimental flexibility, particularly in introducing large numbers of positrons into the apparatus.[5][6]

Catching trap

The catching trap slowed, trapped, cooled, and accumulated antiprotons. To cool antiprotons, the catching trap was first loaded with 3×108 electrons, which cooled by synchrotron radiation in the 3 Tesla magnetic field. Typically, the AD delivered 2×107 antiprotons having kinetic energy 5.3 MeV and a pulse duration of 200 ns to the experiment at 100 s intervals. The antiprotons were slowed in a thin foil and trapped using a pulsed electric field. The antiprotons lost energy and equilibrated with the cold electrons by Coulomb interaction. The electrons were ejected before mixing the antiprotons with positrons. Each AD shot resulted in about 3×103 cold antiprotons for interaction experiments.[7]

Positron accumulator

The positron accumulator slowed, trapped and accumulated positrons emitted from a radioactive source (1.4×109 Bq 22Na). Accumulation for 300 s yields 1.5×108 positrons, 50% of which were transferred to the mixing trap, where they cooled by synchrotron radiation.[8]

Mixing trap

The mixing trap had the axial potential configuration of a nested Penning trap, which permitted two plasmas of opposite charge to come into contact. In ATHENA, the spheroidal positron cloud could be characterized by exciting and detecting axial plasma oscillations. Typical conditions were: 7×107 stored positrons, a radius of 2 – 2.5 mm, a length of 32 mm, and a maximum density of 2.5×108 cm−3. An antihydrogen annihilation detector was situated coaxially with the mixing region, between the trap outer radius and the magnet bore.

Antihydrogen annihilation detector

The detector was designed to provide unambiguous evidence for antihydrogen production by detecting the temporally and spatially coincident annihilations of the antiproton and positron when a neutral antihydrogen atom escaped the electromagnetic trap and struck the trap electrodes. An antiproton typically annihilates into a few charged or neutral pions. The charged pions were detected by two layers of double-sided, position sensitive, silicon microstrips. The path of a charged particle passing through both layers could be reconstructed, and two or more intersecting tracks allowed determination of the position, or vertex, of the antiproton annihilation. The uncertainty in vertex determination was approximately 4 mm and is dominated by the unmeasured curvature of the charged pions' trajectories in the magnetic field. The temporal coincidence window was approximately 5 microseconds. The solid angle coverage of the interaction region was about 80% of 4π.[9]

A positron annihilating with an electron yields two or three photons. The positron detector, comprising 16 rows each containing 12 scintillating, pure cesium-iodide-crystals, was designed to detect the two-photon events, consisting of two 511 keV photons which are always emitted back-to-back. The energy resolution of the detector was 18% full width half maximum at 511 keV, and the photo-peak detection efficiency for single photons was about 20%. The maximum readout rate of the whole detector was about 40 Hz. Ancillary detectors included large scintillator paddles external to the magnet, and a thin, position sensitive, silicon diode through which the incident antiproton beam passed before entering the catching trap.

To produce antihydrogen atoms, a positron well in the mixing region was filled with about 7×107 positrons and allowed to cool to the ambient temperature (15 kelvin). The nested trap was then formed around the positron well. Next, approximately 104 antiprotons were launched into the mixing region by pulsing the trap from one potential configuration to another. The mixing time is 190 s, after which all particles were dumped and the process repeated. Events triggering the imaging silicon detector (three sides hit in the outer layer) initiated readout of both the silicon and the CsI modules.

Using this method, ATHENA could produce – for the first time – several thousands of cold antihydrogen atoms in 2002.[10]

ATHENA collaboration

Members of the ATHENA collaboration gathered to celebrate the successful production of thousands of antihydrogen atoms, on 20th September 2002

The ATHENA collaboration comprised the following institutions:[11]


References

  1. ↑ "Thousands of cold anti-atoms produced at CERN" (Press release). CERN. 18 September 2002.
  2. ↑ Amoretti, M. (2002). "Production and detection of cold antihydrogen atoms". Nature 419 (6906): 456–459. doi:10.1038/nature01096. PMID 12368849. Bibcode: 2002Natur.419..456A. https://cds.cern.ch/record/581488. 
  3. ↑ Amsler, C.; Bonomi, G.; Fontana, A.; Kellerbauer, A.; Lagomarsino, V.; Rizzini, E. Lodi; Rotondi, A.; Testera, G. et al. (2014-08-10). "The ATHENA experiment for the study of antihydrogen" (in en). International Journal of Modern Physics A 29 (20): 1430035. doi:10.1142/S0217751X1430035X. ISSN 0217-751X. Bibcode: 2014IJMPA..2930035A. https://www.worldscientific.com/doi/abs/10.1142/S0217751X1430035X. 
  4. ↑ "Ask The Experts". Scientific American 293 (3): 124. September 2005. doi:10.1038/scientificamerican0905-124. ISSN 0036-8733. Bibcode: 2005SciAm.293c.124.. http://dx.doi.org/10.1038/scientificamerican0905-124. 
  5. ↑ Fujiwara, M. C.; Amoretti, M.; Amsler, C.; Bonomi, G.; Bouchta, A.; Bowe, P. D.; Canali, C.; Carraro, C. et al. (2008-07-30). "Temporally Controlled Modulation of Antihydrogen Production and the Temperature Scaling of Antiproton-Positron Recombination" (in en). Physical Review Letters 101 (5): 053401. doi:10.1103/PhysRevLett.101.053401. ISSN 0031-9007. PMID 18764390. Bibcode: 2008PhRvL.101e3401F. https://link.aps.org/doi/10.1103/PhysRevLett.101.053401. 
  6. ↑ Berg, M.; Haack, M.; Körs, B. (2004-06-01). "Brane/flux interactions in orientifolds". Fortschritte der Physik 52 (67): 583–589. doi:10.1002/prop.200310148. ISSN 0015-8208. Bibcode: 2004ForPh..52..583B. http://dx.doi.org/10.1002/prop.200310148. 
  7. ↑ Cesar, C. L. (2005). "Cold Antihydrogen at ATHENA: Experimental Observation and Beyond" (in en). AIP Conference Proceedings (Rio de Janeiro (Brazil): AIP) 770: 33–40. doi:10.1063/1.1928839. Bibcode: 2005AIPC..770...33C. http://aip.scitation.org/doi/abs/10.1063/1.1928839. 
  8. ↑ Funakoshi, R.; Amoretti, M.; Bonomi, G.; Bowe, P. D.; Canali, C.; Carraro, C.; Cesar, C. L.; Charlton, M. et al. (2007-07-19). "Positron plasma control techniques for the production of cold antihydrogen" (in en). Physical Review A 76 (1): 012713. doi:10.1103/PhysRevA.76.012713. ISSN 1050-2947. Bibcode: 2007PhRvA..76a2713F. https://link.aps.org/doi/10.1103/PhysRevA.76.012713. 
  9. ↑ Amoretti, M.; Amsler, C.; Bonomi, G.; Bouchta, A.; Bowe, P.D.; Carraro, C.; Cesar, C.L.; Charlton, M. et al. (February 2004). "Production and detection of cold antihydrogen atoms" (in en). Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 518 (1–2): 244–248. doi:10.1016/j.nima.2003.10.072. Bibcode: 2004NIMPA.518..244A. https://linkinghub.elsevier.com/retrieve/pii/S0168900203028067. 
  10. ↑ Amoretti, M. (February 2004). "The ATHENA antihydrogen apparatus". Nuclear Instruments and Methods in Physics Research Section A 518 (3): 679–711. doi:10.1016/j.nima.2003.09.052. Bibcode: 2004NIMPA.518..679A. 
  11. ↑ "The ATHENA Collaboration". CERN. http://athena-positrons.web.cern.ch/ATHENA-positrons/wwwathena/collaboration.html. 

External links



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