Slow Positron Facility at IMSS Photon Factory
Toshio Hyodo and Ken Wada
Institute of Materials Structure Science
High Energy Accelerator Research Organization (KEK)
1-1 Oho, Tsukuba, Ibaraki, 305-0801, JapanThe Slow Positron Facility (SPF) at High Energy Accelerator Organization (KEK) is a part ofPhoton Factory (PF) of the Institute of Material Structure Science (IMSS). It is a user

dedicated facility with an energy tunable (0.1 - 35 keV) pulsed slow positron beam produced
by a dedicated 55MeV linac. The beam line branches have been used for the positronium
time-of-flight (Ps-TOF) measurements and the transmission positron microscope.
This year the slow positron generator (converter/moderator) system has been modified witha result of an order of magnitude increase of the slow positron intensity. This makes theaccumulation of the statistics of the data much easier. Also this year a 22Na-based slow
positron beam system has been transferred from RIKEN. This machine will be used for thecontinuous (DC) slow positron beam applications and for the orientation training of those
who are interested in beginning researches with a slow positron beam.
A remarkable scientific achievement this year is the success of the observation of the photodetachment of the positronium negative ion (Ps-) by Prof. Nagashima and co-workers. Nagashima

developed a method of creating a high fraction (1%) of Ps- by bombarding positrons on analkali metal coated W[1]. The Ps- is accelerated by an electric field and blue-shiftedannihilation γ-rays from the moving Ps- give a clear evidence for their existence. The photodetachment is performed by using the pulsed positron beam of this facility and an intense Qswitched ND:YAG laser beam synchronized to the positron bunch [2]. Observed reduction ofthe Doppler shifted Ps- signal indicates that the Ps- ions are converted into neutral Ps, ofwhich 3/4 is ortho-Ps which does not annihilate into 2γ [3]. During the forthcoming machinetimes it is planned to measure the cross section of the photo-detachment process and comparewith the theory.
A reflection high-energy positron diffraction (RHEPD [4,5]) measurement station has been
installed this year by Dr. Kawasuso and co-workers and ready to be used for themeasurements. High intensity of the positron beam of this facility is expected to improve thestatistics of their data drastically. Experiments planned include (i) observation of weak
RHEPD spots resulting from the formation of super-lattice structures on the topmost Si(111)7x7 surface, (ii) RHEPD determination of the atomic positions on low-dimensional surfaces,(iii) investigation the formation of the charge density waves on surface phase transitions, (iv)
observation of the spin splitting due to the surface Rashba effect, (v) observation of theRHEPD patterns from the reconstructed surface at various incident conditions, and (vi) direct
determination of the atomic positions on the topmost surface using the Patterson function.
[1]
[2]
[3]
[4]
[5]

Y. Nagashima, T. Hakodate, A. Miyamoto, et al., New J. Phys. 10 (2008) 123029.
T. Tachibana, K. Michishio, H. Terabe, et al., Nucl. Instr. and Meth. in Phys. Res. A 621 (2010)
670.
K. Michishio, T. Tachibana, H. Terabe, et al., in preparation.
A. Kawasuso and S. Okada, Phys. Rev. Lett. 81 (1998) 2695.
Y. Fukaya, A. Kawasuso and A. Ichimiya, Phys. Rev. B 79 (2009) 193310.