Study of a Polarized Positron Source for Linear Colliders

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Studies were done at the Accelerator Test Facility (ATF) in KEK.As the first step, we performed the experiment to generate positrons via pair-creation from Compton scattered gamma-rays. In the experiment, we aimed to confirm that number of positron generated by our apparatus agreed to the design value. The 1.28 GeV electron beam extracted from the ATF collided with the laser beam of 2nd harmonic of the Nd : YAG laser. The energy of a laser photon is 2.33 eV.This collision generated gamma-rays which maximum energy was 56 MeV.The gamma-rays were injected to the 1-mm-thick tungsten target to generate positrons. The number of generated positrons was about 250 at just downstream of the target. This value agreed the design value within the expected error. The results of the experiment was published in "Nuclear Instruments and Methods in Physics Research". We did not measure the polarization of the positrons in this experiment.From the experience of the first step, we recognized that the existence of the background particles from the ATF accelerator was the serious obstruction against the polarization measurement. Therefore, as the second step, we performed following three developments.1. Development of the detector which has no sensitivity to low energy particles (most of background particles have low energy), but has high sensitivity to positrons generated by Compton scattered gamma-rays.2. Development of the detailed simulation code for polarization measurement.3. Construction of shields around the beam pipe to reduce the background particles.Through R/Ds of above three subjects we did accumulate large amount of data and experiences which are necessary to achieve the Polarization measurement. We believe those are of great use when we will perform further investigation to realize the polarize the polarized positron source.