Speaker
Toshio HYODO
(Slow positron applications at Slow Positron Facility of Institute of Materials Structure Science, KEK)
Description
Slow Positron Facility at KEK (High Energy Accelerator Research Organization) is a user dedicated facility with an energy-tunable (0.1 - 35 keV) slow positron beam created by a dedicated 50MeV linac [1]. It operates in a short pulse (width 1-12 ns, variable, \5\times 10 ^6\ e\^+\/s) and a long pulse (width 1.2 \mu s, \5\times 10 ^7\ e\^+\/s) modes of 50 Hz.
High energy positrons from pair creation are moderated by reemission after thermalization in W foils, which have negative positron work function of -3 eV. The positrons emitted at 3 eV are then electrostatically accelerated to a desired energy, which is variable from 50 eV to 35 keV depending on the requirement of each individual experiment. The accelerated beam is then magnetically transported.
A pulse-stretch section is installed in the middle of the beamline. It stretches the slow positron pulse of width 1.2 \mu s to a variable width from 10 \mu s up to 20 ms at beam energy of 5 keV. The stretched beam is used for the position-sensitive delay-line detector of the LEPD (low-energy positron diffraction) system. It will also be used for the future spectroscopy of the 511 keV annihilation \gamma rays and for the future construction of a short pulse producing system for the positron lifetime study.
Four beamline branches, SPF-A3, SPF-A4, SPF-B1 and SPF-B2, are available in the measurement area.
(1) A TRHEPD (total-reflection high-energy positron diffraction) station is connected to SPF-A3. TRHEPD is the positron counterpart of RHEED (reflection high-energy positron diffraction). The exceedingly high surface sensitivity of TRHEPD [2] determined the structure of Ge(001)-(4×2)-Pt nanowire after a decade from its discovery [3], rutile-TiO2(110)-(1×2) surface which had been under debate over the past 30 years [4], buckling and sheet-substrate distance of atomic sheet materials such as graphene [5], silicene [6], germanene [7], etc. synthesized on solid substrates.
(2) A LEPD station is recently constructed on SPF-A4.
(3) SPF-B1 is a general-purpose branch. A Ps– (positronium negative ion) station is currently connected. Ps– is a three-lepton (one positron and two electrons) bound system. Emission of Ps– from W surface is greatly enhanced by coating the surface with sub-monolayer of alkali metal. Laser photo-detachment of one of the electrons makes the ion neutral Ps [8]. By accelerating the ion before the photo-detachment energy-tunable Ps beam is created [9]. A spectroscopic study of the photo-detachment process verified a theoretically predicted shape resonance of 1Po symmetry near the Ps (n=2) formation threshold [10].
(4) Ps-TOF (positronium time-of-flight) station is connected to SPF-B2. It was found that alkali-metal-coating on W surface remarkably enhances Ps emission also [11]. The TOF spectroscopy of the energy distribution of the emitted Ps will give information the electronic states of the surface [12].
References
[1] T Hyodo, et al., J. Phys.: Conf. Ser. 262, 012026 (2011).
[2] Y. Fukaya, et al., Appl. Phys. Express 7, 056601 (2014).
[3] I. Mochizuki et al., Phys. Rev. B 85, 245438 (2012).
[4] I. Mochizuki et al., Phys. Chem. Chem. Phys. 18, 7085 (2016)
[5] Y. Fukaya, et al., Phys. Rev. B 88, 205413 (2013).
[6] Y. Fukaya, et al., Carbon 103, 1 (2016).
[7] Y. Fukaya, et al., 2D Materials, 3, 035019 (2016).
[8] K. Michishio, et al., Phys. Rev. Lett. 106 153401 (2011).
[9] K. Michishio, et al., Appl. Phys. Lett. 100 254102 (2012).
[10] K. Michishio, et al., Nature Communications 7, 11060 (2016).
[11] H. Terabe, et al., Surf. Sci., 641, 68 (2015).
[12] S. Iida, et al., J. Phys.: Condens. Matter 28, 475002 (2016).
Primary author
Toshio HYODO
(Slow positron applications at Slow Positron Facility of Institute of Materials Structure Science, KEK)
Co-authors
Dr
Izumi MOCHIZUKI
(High Energy Accelerator Research Organization (KEK))
Prof.
Nobukazu TOGE
(High Energy Accelerator Research Organization (KEK))
Prof.
Tetsuo SHIDARA
(High Energy Accelerator Research Organization (KEK))
