Speaker
Description
Attempts to utilize positronium (Ps)—a hydrogen-like bound state of an electron and a positron—as an energy-tunable beam have been ongoing since the 1980s. To achieve this, a technique was developed whereby a slow positron beam is injected into a dilute gas, where charge exchange generates a Ps beam [1]. Ps beams generated in this manner have been utilized for experiments involving specular reflection of Ps from LiF crystal surfaces [2] and for measuring scattering cross sections with gas molecules [3]. More recently, by interacting such generated Ps with circularly polarized microwaves, measurements of the spin polarization of the original slow positron beam have been performed [4].
We are conducting research to generate an energy-tunable Ps beam by producing positronium negative ions (Ps$^-$), in which a further electron is bound to Ps, accelerating them in an electric field, and then photodetaching the electron [5, 6]. This beam is utilized as an energy-tunable Ps beam. The Ps$^-$ ions are produced by irradiating a tungsten thin film, onto which Na has been deposited, with a slow positron beam. The Ps beam thus obtained can be generated in a clean environment, as it does not require charge exchange via gas, and enables to be generated in a higher energy range than when using gas. Furthermore, this beam possesses sufficient coherence to observe quantum interference.
We have used this beam to observe coherent resonant excitation of Ps [7] and measure Ps transmission through graphene [8]. Recently, through observing Ps diffraction through graphene, we have succeeded in observing quantum interference of Ps for the first time [9].
[1] B. L. Brown, Positron Annihilation (World Scientific, Singapore, 1985), p. 328.
[2] M. H. Weber et al., Phys. Rev. Lett. 61, 2542 (1988).
[3] S. Brawley et al., Science 330, 789 (2010).
[4] D. M. Newson and D. B. Cassidy, Phys. Rev. Lett. 133, 233004 (2024).
[5] Y. Nagashima, Phys. Rep. 545, 95 (2014).
[6] K. Michishio et al., Rev. Sci. Instrum. 90,023305 (2019).
[7] Y. Nagata et al., Phys. Rev. Lett. 124, 173202 (2020).
[8] R. Mikami et al., Eur. Phys. J. D 77, 205 (2023).
[9] Y. Nagata et al., submitted.
