Positronium Doppler velocimetry

Positronium (Ps) is the bound state of an electron and a positron. Being a two-particle purely leptonic system, Ps is a system with a huge potential in different areas, including the precise determination of the... [ view full abstract ]

Positronium (Ps) is the bound state of an electron and a positron. Being a two-particle purely leptonic system, Ps is a system with a huge potential in different areas, including the precise determination of the fine-structure constant, the study of the coupling of neutral antimatter in the earth’s gravitational field or photon entanglement in the gamma ray spectral range through annihilation. In AEgIS (Antihydrogen Experiment: gravity, interferometry, spectroscopy), Ps is used to produce antihydrogen through charge exchange, i.e., a Ps atom is used to transfer a positron to an antiproton. In the ground state, para-Ps has a lifetime of 125 ps, which makes it difficult to study or manipulate or even transfer from the production area to the antiproton trap. In order to extend the lifetime of Ps, we use two-photon excitation to prepare Ps Rydberg atoms. This also has the advantage of increasing the cross section of the charge exchange reaction.

We report on the characterization of a positronium source using Doppler velocimetry. Nanosecond bunches of positrons are implanted in a nanochannel target, converting the positron into positronium in a reflection geometry. A UV laser pulse is used to excite ground state positronium to the n = 3 level, which is further ionized with a strong mid-infrared laser field. The photopositrons thus emitted are trapped in the magnetic field lines of a 1-tesla magnet and imaged on an MCP, phosphor screen and CCD device. This technique allows spatially resolving the transverse and longitudinal velocity of the positronium cloud in a highly efficient way. While scanning the UV wavelength, different portions of the positronium cloud are ionized according to their Doppler shift in the transverse direction. Scanning the IR wavelength also affects the measurement, as approaching the threshold Rydberg states of the coldest fraction start being stable in the 1 T magnetic field, contrary to hotter fractions that auto-ionize due to the motional Stark effect. We measure temperatures ranging from hundreds of K to 1000 K.