The poitron is the anti-particle of the electron. The Slow Positron Facility of the Photon Factory provides a high intensity, pulsed slow positron beam. The beam is used for researches in solid state physics and atomic and molelcular physics.
For positrons with their positive charge, it is straightforward to obtain information only for the atomic arrangement on the topmost surface. That is, positrons incident on a surface at a glancing angle below a critical angle are totally reflected at the first atomic layer and do not enter the crystal at all. By using this characteristic of positron beams, diffraction patterns reflecting the atomic positions at the topmost surface can be obtained by directing positrons with a given energy at a certain grazing angle to the crystal surface. The diffraction patterns can then be used to determine the atomic positions at the topmost surface. This forms the essence of the TRHEPD method.
A positron may be bound with an electron to form a positronium atom, a boud state like an hydrogen atom. It is also possible to form a positronium negative ion (Ps-) by binding one more electron. Ps- ions are emitted from a tungsten surface when it is bombarded with a slow positron beam. Its formation efficiency increases drastically when alkali metals such as Na and Cs is deposited on the surface. This station is for the formation of pulse Ps- by bombarding such a surface with a slow positron pulse and the applications. Recently an experiment of photodetachment of Ps- (dissociation into Ps and an electron by an irradiation of a laser beam) was successfully conducted.
Ps-TOF is a measurement of the energy of positronium emitted from a solid surface. The incident positrons hit the sample target and the ortho-Ps atoms emitted from the surface annihilate in flight. The annihilation gamma-rays are viewed by detector through lead slits and the time of the detection with reference to the positron injection time is recorded. The width and the frequency of the incident positron pulse are 1ns-10ns (variable) and 50 Hz, respectively.
Slow Positron Facility http://pfwww.kek.jp/slowpos/Engindex.html