ATHENA experiment

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.

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.

An antihydrogen annihilation detector was situated coaxially with the mixing region, between the trap outer radius and the magnet bore.

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 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%.

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.