The ATHENA experiment at the CERN antiproton decelerator
produced for the first time cold antihydrogen atoms in large
quantities in a nested penning trap by overlapping antiproton
and positron clouds.
Once formed, antihydrogen is electrically neutral. Not confined
anymore, it annihilates on the walls of the trap.
The antihydrogen detector measured the annihilation vertex of the
pions and detected the two 511 keV from electron-positron annihilation.
The angle between the two photons (180 degrees) was measured
by using the annihilation vertex.
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Sketch of the ATHENA antihydrogen detector (left).
Photograph with one row of CsI crystals prior to installation (right)
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Electronic reconstruction of an antihydrogen annihilation on the trap wall.
The yellow lines show the tracks from three charged pions, the red lines the
directions of the two 511 keV annihilation photons which are emitted
in opposite directions.
Further reading:
M. Amoretti et al. (ATHENA Collaboration), Nature 419 (2002) 456
C. Amsler, G. Bonomi, A. Fontana, A. Kellerbauer, V. Lagomarsino, E. Lodi Rizzini,
A. Rotondi, G. Testera, L. Venturelli, N. Zurlo
International Journal of Modern Physics A29 (2014) 1430 pdf