Construction of polarized slow-positron beams using a compact cyclotron

We have constructed a polarized positron beam using positrons provided from the β ⁺ decay of ²⁷ Si with the half-life of 4.1 s and the maximum β ⁺ energy of 3.85 MeV. This isotope is produced via the ²⁷ Al(p, n) ²⁷ Si reaction caused by proton irradiation using a compact proton cyclotron. The intensity of this beam is measured to be 5 × 10 ⁵ e ⁺ /s for protons with the energy of 18 MeV and the current of 30 μA. A pair of Monte Carlo simulation programs has been developed to calculate depolarization of positrons in our beam channel. One program simulates spin motion of the positron under electric and magnetic fields, while the other simulates depolarization due to multiple Coulomb scattering in matter. Using these simulators, we designed a special polarimeter by means of magnetic quenching of ortho-positronium and measured a quenched lifetime of ortho-positronium under the magnetic field of 0.4, 2.5 and 4.0 kG leading to the average polarization of 33.4 ± 5.3%, which is consistent with the estimation based on the Monte Carlo simulations.