The poitron is the anti-particle of the electron. The Slow Positron Facility of the Photon Factory, equipped with a dedicated 55-MeV linac, provides a high intensity, pulsed slow positron beam. The beam is used for researches in solid state physics and atomic and molelcular physics.
The linac operates in a short-pulse mode and a long-pulse mode, at frequencies of up to 50 Hz. The intensity of the beam has been increased tenfold to 5 × 107 slow positrons/s in the long pulse mode after improving the converter/moderator assembly for producing slow positrons in 2010 . The pulse width in the short-pulse mode operation became adjustable from 1 to 10 ns in 2011, while that in the long-pulse mode remains at 1 μs. The facility is operated by a professor (Dr. T. Hyodo), an assistant professor (Dr. K. Wada), and a research associate (Dr. I. Mochizuki) who 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: 55 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. Form 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, which consists of two lattices of 25 micro m thick W foils. Part of the high energy positrons incident in the foil thermalize inside and then emitted from the surface. These are slow positrons.
- Features of the Slow Positron Beam
* High intensity: 5x107 slow e+ /s in long pulse mode and 5x106 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 for the development in the future.
* Pulses with width 1-10 ns: Convenient for positronium TOF, positronium lifetime measurements for porous materials, and synchronization with the irradiation with laser pulses.