Rydberg-positronium velocity and self-ionization studies in a 1T magnetic field and cryogenic environment

Abstract

We characterized the pulsed Rydberg-positronium production inside the Antimatter Experiment: Gravity, Interferometry, Spectroscopy (AEḡIS) apparatus in view of antihydrogen formation by means of a charge exchange reaction between cold antiprotons and slow Rydberg-positronium atoms. Velocity measurements on the positronium along two axes in a cryogenic environment (≈ 10 K) and in 1 T magnetic field were performed. The velocimetry was done by microchannel-plate (MCP) imaging of a photoionized positronium previously excited to the n = 3 state. One direction of velocity was measured via Doppler scan of this n = 3 line, another direction perpendicular to the former by delaying the exciting laser pulses in a time-of-flight measurement. Self-ionization in the magnetic field due to the motional Stark effect was also quantified by using the same MCP-imaging technique for Rydberg positronium with an effective principal quantum number n eff ranging between 14 and 22. We conclude with a discussion about the optimization of our experimental parameters for creating Rydberg positronium in preparation for an efficient pulsed production of antihydrogen.

Pulsed production of antihydrogen

“…The time of H production is defined by the laser firing time (known with few ns accuracy) and the transit time of Ps* toward the p cloud. The measured distribution of the velocity of Ps* in our apparatus 32 and the crosssection reported in refs. 26,27 imply that the corresponding distribution of the H formation time has a FWHM of ~80 ns, with an asymmetric tail due to the slowest Ps*, resulting in 90% of the H being produced within an ~250-ns-wide time window.…”

“…The on-axis MCP detector offered a very sensitive and high-resolution imaging diagnostics (with ~100 μm resolution 59 ) of the photodissociated e + , allowing the UV wavelength to be accurately set with the direct feedback of the excited Ps fraction alignment before H production trials. This detector also allowed a direct characterization 32 of the velocity distribution of Ps atoms emerging from the e + to Ps converter.…”

“…due to their motion in a magnetic field, which causes field ionization of sufficiently high n Ps Rydberg atoms 65,66 . n Ps was thus set to 17 during H production trials, a compromise between field-ionization losses (~30% of the total available atoms 32 ) and the n 4 Ps gain in the H formation cross-section 26 . Note that any field ionization of Ps*, at this addressed range around n Ps = 17, has a negligible impact on the H formation because it affects only those Ps* with strongly suppressed cross-section.…”

“…Given the typical time spread of the e + pulse hitting the target of <10 ns, and the Ps cooling time in the target of the order of ten ns, the major uncertainty on the time when H is formed is due to the spread in velocity, and thus in the time of flight, of the Ps* reaching the p cloud. With the Ps* velocity distribution measured in our apparatus 32 , this spread in time results in a formation time distribution with a FWHM of ~80 ns (see details in "Methods" section).…”

“…Each single Ps is given a velocity randomly extracted from the experimental distribution reported in ref. 32 and is propagated along a straight line with its standard lifetime in a vacuum (142 ns) unless it reaches some material surface.…”