利用波荡器产生极化正电子
Undulator-Based Production of Polarized Positrons
摘要：
充分利用未来国际直线对撞机的物理潜力将需要使用极化电子束和正电子束。斯坦福直线加速器中心（SLAC）的E166实验演示了一种方案，其中电子束通过螺旋波荡器产生具有圆偏振的光子（其一次谐波谱扩展到7.9MeV），然后在薄靶中转换以产生纵向偏振的正电子和电子。实验是在46.6GeV的最终聚焦测试束（FFTB）中用一米长、400周期的脉冲螺旋波荡器进行的。在4.5 - 7.5MeV的五个正电子能量下进行了正电子极化的测量。此外，在6.7MeV的电子极化已被确定，并与铁磁流体操作的波荡器的效果也进行了研究。为了将测量结果与预期进行比较，使用升级版的Geant 4进行了详细的模拟，其中包括电子，正电子和光子与物质的主要偏振相关相互作用。测量结果与计算一致，对应于6 MeV附近的正电子的极化率为80%，7 MeV附近的电子的极化率为90%。
英文摘要：
Full exploitation of the physics potential of a future International Linear Collider will require the use of polarized electron and positron beams. Experiment E166 at the Stanford Linear Accelerator Center (SLAC) has demonstrated a scheme in which an electron beam passes through a helical undulator to generate photons (whose first-harmonic spectrum extended to 7.9MeV) with circular polarization, which are then converted in a thin target to generate longitudinally polarized positrons and electrons. The experiment was carried out with a one-meter-long, 400-period, pulsed helical undulator in the Final Focus Test Beam (FFTB) operated at 46.6GeV. Measurements of the positron polarization have been performed at five positron energies from 4.5 to 7.5MeV. In addition, the electron polarization has been determined at 6.7MeV, and the effect of operating the undulator with a ferrofluid was also investigated. To compare the measurements with expectations, detailed simulations were made with an upgraded version of Geant4 that includes the dominant polarization-dependent interactions of electrons, positrons, and photons with matter. The measurements agree with calculations, corresponding to 80% polarization for positrons near 6MeV and 90% for electrons near 7MeV.
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