A high-quality and energy-tunable positronium beam system employing a trap-based positron beam

We constructed a new apparatus, built upon a trap-based slow positron beam, for the production of a collimated, energy-tunable positronium beam under ultra-high vacuum conditions employing the photodetachment of positronium negative ions. A slow positron generator consisting of a ²²Na radioisotope (20 mCi) combined with a buffer-gas positron trap is employed to generate high-quality, nano-second positron bursts with a repetition rate of 1 Hz-1 kHz. The positron bursts are focused onto an efficient positron-to-positronium negative ion converter, a Na-coated W thin film in a transmission geometry, using a magnetic lens system. The ions emitted from the opposite surface of the film are electrostatically accelerated to a given energy and photodetached by a pulsed infrared laser to form a mono-energetic positronium beam with kinetic energies of 0.2 keV-3.3 keV. The achieved detection rate of Ps atoms is 23 cps at the energy of 3.3 keV with a signal-to-background ratio as high as 300. The energy spread of the beam was evaluated by comparing the result of the time-of-flight measurements and particle-tracking simulations. With the use of a collimator of 1 mm diameter, a coherent beam with an angular divergence of less than 0.3° is obtained. The obtained Ps beam, having a much higher quality than those reported hitherto, will open up a new field of experimental investigations, such as Ps interacting with a variety of materials and fundamental studies on Ps spectroscopy.