This station is for structural analysis of crystal surfaces and low-dimensional materials formed on the surfaces by total-reflection high-energy positron diffraction (TRHEPD, Trept). TRHEPD is the positronic version of reflection high-energy electron siffraction (RHEED). Since the iner electrostatic potential of every material is positive, positrons incident at a glancing angle (the angle between the incident beam and the surface) smaller than a certain critical angle will be totally reflected, giving positional information only for the atoms on the top surface. If the positron beam is incident at a glancing angle greater than the critical angle, it will enter the material, but it will be refracted in the direction toward the surface. As a result, information on the location of atoms just below the surface can also be obtained with high accuracy. A high intensity positron beam with a small diameter and high collimation required In the TRHEPD experiment is obtained by the brightness-enhancement by using a transmission-type remoderator. This station is also equipped with a RHEED system. The RHEED patterns of the samples prepared and processed in situ are observed before or after the TRHEPD measurement to check the surface condition. (The symmetry of the diffraction pattern tells the symmetry of the atomic arrangement, but the intensity of the diffraction spots needs to be analyzed to determine the atomic coordinates.)
This station is for low-energy positron diffraction (LEPD) measurements. LEPD is the positron counterpart 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 to a crystal surface with poorer smoothness or a much smaller sample than TRHEPD is. 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. This enables highly accurate atomic arrangement analysis by LLEPD. Brightness of the beam is enhanced by using a Ni transmission-type positron re-moderator.
This is a station where users can connect the measurement chambers with the measurement devices for their own experiments. Experiments on laser cooling of positronium (Ps) have been conducted since FY2017. Until FY2016, experiments related to Ps- (positronium negative ion, the positron counterpart of H- (hydride)) were conducted. The experiments were conducted by a group of researchers who discovered that injection of a slow positron beam onto an alkali-metal coated tungsten surface results in efficient emission of Ps-.
This is a station for a positronium (Ps) time-of-flight (Ps-TOF) experiment. In a Ps-TOF experiment the γ-rays from the self-annihilation of the Ps emitted from the sample surface on irradiation with a pulsed slow-positron beam is detected at a certain distance from the surface. Only those γ rays that enter the slit placed perpendicularly to the traveling direction of the Ps are detected. From the difference between the time of detection and the time when the positrons are injected, the speed of the Ps and therefore its emission energy can be determined. This provides information on the electronic states of the surface through the mechanism of Ps production.
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