Speaker
Description
At the Slow Positron Facility (SPF) of the Institute of Materials Structure Science, KEK, high-intensity slow-positron beams are generated from accelerated electrons provided by a normal-conducting S-band linac (50 MeV, up to 900 W) and are supplied for the facility’s peer-reviewed user proposals [1]. In the long-pulse mode (pulse width 1.2 $\mu$s, repetition up to 50 Hz), a beam intensity of $1 \times 10^8$ slow-e$^+/$s is available and used with a transmission-type remoderator for surface structure analysis by positron diffraction experiments [2,3]. The short-pulse mode (pulse width 14 ns, repetition up to 50 Hz), with about one order of magnitude lower intensity, has been used for positronium (Ps) experiments [4-7].
Positron diffraction experiments at KEK-SPF have demonstrated the capability to precisely determine the atomic arrangements within the topmost 3–4 atomic layers of crystal surfaces. This region often exhibits relaxations or reconstructions that differ markedly from the bulk, and its detailed structure is difficult to determine by conventional methods. Positrons do not experience exchange interactions with electrons, leading to simple atomic scattering factors that allow accurate calculations. Moreover, owing to the positive charge of positrons, their diffraction is sensitive only to such very shallow layers.
At KEK-SPF, total-reflection high-energy positron diffraction (TRHEPD), the positron counterpart of reflection high-energy electron diffraction (RHEED) with grazing-incidence geometries, has produced numerous results over more than a decade [2]. More recently, a practical experimental station for low-energy positron diffraction (LEPD), the positron counterpart of low-energy electron diffraction (LEED) with normal incidence geometry, has been developed with a common sample-holder shared with the Photon Factory on the same campus of KEK Tsukuba [3]. It enables complementary measurements on identical samples with angle-resolved photoemission spectroscopy (ARPES), which probes surface electronic band structures.
In addition, the high-intensity slow-positron beams have supported fundamental physics studies on positronium. Following Ps time-of-flight (Ps-TOF) experiments [4,5] and investigations of Ps⁻ photodetachment and the generation of energy-tunable Ps beams [6], the first laser cooling of positronium down to 1K has been demonstrated [7].
In this presentation, we will describe the generation of high-intensity slow-positron beams, their transport to each experimental station, highlight primary user results in positron diffraction and positronium studies, and outline plans for further beam-intensity upgrades to meet future research needs.
References
[1] K. Wada et al., Eur. Phys. J. D 66, 37 (2012).
[2] Y. Fukaya et al., J. Phys. D: Appl. Phys. 52, 013002 (2019).
[3] K. Wada et al., e-J. Surf. Sci. Nanotechnol. 16 313 (2018); R. Ahmed et al., Nucl. Instrum. Meth. A, 1073, 170270 (2025).
[4] Y. Nagashima, Y. Morinaka, et al., Phys. Rev. B 58, 12676 (1998).
[5] A. Kawasuso, M. Maekawa, et al., J. Phys. B: At. Mol. Opt. Phys. 54, 205202 (2021).
[6] Y. Nagashima, Phys. Rep., 545, 95 (2014); K. Michishio, et al., Nat. Commun., 7, 11060 (2016).
[7] K. Shu, et al., Nature, 633, 793 (2024).
