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{{Short description|Collaboration at the Antiproton Decelerator}}
__FORCETOC__
The '''Antihydrogen Trap''' ('''ATRAP''') collaboration at the [[Antiproton Decelerator]] facility at [[CERN]], Geneva, was responsible for the [[AD-2 experiment]]. It was a continuation of the [[TRAP experiment|TRAP]] collaboration, which started taking data for the TRAP experiment (also known as the PS196 experiment) in 1985.<ref>{{Cite journal|last1=Gabrielse|first1=G.|last2=Fei|first2=X.|last3=Orozco|first3=L.|last4=Tjoelker|first4=R.|last5=Haas|first5=J.|last6=Kalinowsky|first6=H.|last7=Trainor|first7=T.|last8=Kells|first8=W.|date=September 1990|title=Thousandfold improvement in the measured antiproton mass|url=https://link.aps.org/doi/10.1103/PhysRevLett.65.1317|journal=Physical Review Letters|language=en|volume=65|issue=11|pages=1317–1320|doi=10.1103/PhysRevLett.65.1317|pmid=10042233|bibcode=1990PhRvL..65.1317G|issn=0031-9007}}</ref><ref>{{Cite web|title=Greybook: PS196|url=https://greybook.cern.ch/experiment/detail?id=PS196|url-status=live|access-date=2021-07-06|website=greybook.cern.ch|archive-url=https://web.archive.org/web/20210709184555/https://greybook.cern.ch/experiment/detail?id=PS196 |archive-date=2021-07-09 }}</ref> The TRAP experiment pioneered cold [[antiproton]]s, cold [[positron]]s, and first made the ingredients of cold [[antihydrogen]] to interact. Later ATRAP members pioneered accurate [[hydrogen]] [[spectroscopy]] and observed the first hot antihydrogen atoms.

==Experimental setup==
[[File:Atrap1.jpg|thumb|376x376px|ATRAP collaborators next to the apparatus that first trapped, cooled and stacked antiprotons at CERN's AD]]
ATRAP was a collaboration between physicists around the world with the goal of creating and experimenting with antihydrogen. ATRAP accumulates positrons emitted from a radioactive [[Sodium|<sup>22</sup>Na]] source.
There are two effective ways to slow down the fast positrons by inelastic processes. The ATRAP collaboration initially chose a different method to [[ATHENA experiment|ATHENA (AD-1)]].

=== Slowing down and trapping positron ===
The positrons which were emitted by the <sup>22</sup>Na were first slowed down with a 10 μm thick titanium foil and then passed through a 2 μm thick tungsten crystal.
Within the crystal there is a possibility that a positively charged positron and a negatively charged electron form a Rydberg [[positronium]] atom. In this process, the positrons lose much of their energy so that it is no longer necessary (as in ATHENA) to decelerate further with collisions in gas. When the loosely bound [[Rydberg atom|Rydberg]] positronium atom reaches the Penning trap at the end of the apparatus, it is [[Ionization|ionized]] and the positron is caught in the trap.
[[File:Atrap2.jpg|thumb|381x381px|A post-doctoral fellow on the ATRAP experiment, with the [[Penning trap]] apparatus for trapping antiprotons.]]
Since this method of positron accumulation was not particularly efficient, ATRAP switched to a Surko-type buffer gas accumulator as is now standard in experiments requiring large numbers of positrons.<ref>{{cite web|url=http://positrons.ucsd.edu/publications.php|title=UC San Diego: Department of Physics – Surko Plasma Research Group|access-date=21 December 2016}}</ref> This has led to the storage of the largest ever number of positrons in an [[Magnetic trap (atoms)|Ioffe trap]].<ref>{{cite journal |last1=Fitzakerley |first1=D. W. |display-authors=etal |collaboration=ATRAP Collaboration |year=2016 |title=Electron-cooled accumulation of {{val|4|e=9}} positrons for production and storage of antihydrogen atoms |journal=[[Journal of Physics B]] |volume=49 |issue=6 |article-number=064001|doi=10.1088/0953-4075/49/6/064001|bibcode=2016JPhB...49f4001F |s2cid=108283977 |url=http://ruj.uj.edu.pl/xmlui/handle/item/29326 }}</ref>

ATRAP has been terminated.<ref>{{Cite web |title=Experiments {{!}} CERN |url=https://home.cern/science/experiments |access-date=2024-09-11 |website=home.cern}}</ref> ATRAP had an Ioffe trap, which attempted to store the electrically neutral antihydrogen using a [[Quadrupole|magnetic quadrupole]] field. This was potentially possible because the magnetic moment of antihydrogen is non-zero. [[Spectroscopy|Laser spectroscopy]] was intended to be performed on antihydrogen stored in the Ioffe trap, however no publication was ever made.

==ATRAP collaboration==
The ATRAP collaboration comprised the following institutions:
{{columns-list|colwidth=30em|
*[[Harvard University]], US
*[[York University]], Canada
*[[University of Mainz]], Germany
*[[Forschungszentrum Jülich]], Germany
}}

== References ==
{{Reflist}}

== External links ==
* Record for [https://inspirehep.net/experiments/1110580 ATRAP] experiment on [[INSPIRE-HEP|INSPRE-HEP]]

[[Category:Particle experiments]]
[[Category:CERN experiments]]

ATRAP experiment - Revision history

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