The positron is the anti-particle of the electron. The Slow Positron Facility of the Institute of Materials Structure Science, equipped with a dedicated 50-MeV electron linac, provides a high intensity, pulsed slow positron beam. The beam is used for researches in solid state physics and atomic and molecular physics.
The electron linac operates in a short-pulse mode and a long-pulse mode, at frequencies up to 50 Hz. The present intensity of the beam is 1 × 108 slow positrons/s in the long pulse mode after renewing the converter/moderator assembly for producing slow positrons in 2020. The pulse width in in the long-pulse mode remains at 1.2 µs, while that in the short-pulse mode is adjustable from 1 to 10 ns. The staff operate the facility to accommodate the research projects of the users from other universities and institutes and conduct their own research as well.
- Method for producing positrons
The electrons accelerate by a dedicated linac are injected into a Ta converter (4 mm thick).
* Linac acceleration energy: 50 MeV, Power 600 W
* Pulse operation: Frequency 50 Hz, Short pulse (1-10 ns variable), Long pulse (1.2 µs）
Bremsstrahlung (high energy X-ray) is produced by the deflection of the electron in the electric field of a Ta nucleus. The X-ray photon may interact with another Ta nucleus and a positron electron pair is created.
- Method to slow-down the positrons
The high energy of the positrons produced by the pair creation process can be moderated by using W thin films whose work function of the positron is negative. Some amount of energy is required to take an electron out of a metal since the work function for the electron is positive. In contrast, the work function for the positron is positive or negative, depending on the species of metals. From the surface of the metals whose positron work function is negative, such as W, Ni, Cu, positrons returned to the surface after thermalization are spontaneously emitted.
A tungsten (W) moderator assembly consists of two lattices of 25 µm thick W foils. Part of the high energy positrons incident in the foil thermalize inside and then emitted from the surface as "slow positrons".
- Features of the Slow Positron Beam
* High intensity: 1x108 slow e+ /s in long pulse mode and 1x107 slow e+ /s in short pulse mode
* Transported with variable energy up to 35 keV: highly versatile and UHV compatible because the beam line, experiment chamber and detectors may be grounded. This is unique for high intensity positronium beam facility in the world.
* Standardized beam line branching unit: effective up to 35 keV positron beam: Highly flexible for the development in the future.
* Pulses with width 1-10 ns: Convenient for positronium TOF and synchronization with the irradiation with laser pulses.