This station is for the analysis of the atomic configuration of a crystal surface by using the total-reflection high-energy positron diffraction (TRHEPD). In this method, a positron beam is incident on a sample surface with a small glancing angle. When the angle is smaller than a certain critical angle, the beam is totally reflected, making it possible to determine the surface structure accurately. The experiment started with a beam with a collimator in 2011. A brightness enhancement unit using a W transmission-type positron remoderator was installed in 2012.
This station is for low-energy positron diffraction (LEPD) measurements. LEPD is a positron version of low-energy electron diffraction (LEED). In this method, a positron beam is incident perpendicularly on a sample surface. The perpendicular incidence has an advantage that it is applicable a crystal surface with poor smoothness or much smaller sample. In addition, since the positron is repelled by the positive charge of the nucleus and does not interact with the inner shell electrons, the angular dependence of the atomic scattering factor is as simple as that of X-rays, enabling high-precision atomic arrangement analysis. Brightness enhancement using a Ni transmission-typs positron remoderator is applied.
The station is for a purpose that authorized users use by connecting their own measurement chambers with equipment necessary for their experiments. Experiments on laser cooling of positronium (Ps) have been conducted since FY2017. Ps is a hydrogen atom-like bound state in which a positron and an electrons are bound. It is regarded as a hydrogen isotope whose mass is 1/1000 of that of hydrogen. In this experiment, a large amount of Ps atoms are created in the voids in porous materials. Until 2016, Ps- (positronium negative ion)-related experiments were conducted. Ps- is one in which another electron is added to Ps. A positron version of H- (hydride ion). This experiment was conducted by a research group that discovered that Ps- was efficiently reemitted from alkali-metal-deposited W surface when slow positrons were incident on it.
This station is for the 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 γ-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.