We report on the design and performance of a Mott polarimeter optimized for a nominal 5-MeV electron beam from the Continuous Electron Beam Accelerator Facility (CEBAF) injector. The rf time structure of this beam allows the use of time of flight in the scattered electron detection, making it possible to cleanly isolate those detected electrons that originate from the scattering foil, and resulting in measured scattering asymmetries which are exceptionally stable over a broad range of beam conditions, beam currents, and foil thicknesses. In two separate series of measurements from two different photocathode electron sources, we have measured the Mott scattering asymmetries produced by an approximately 86% transversely polarized electron beam incident on ten gold foils with nominal thicknesses between 50 and 1000 nm. The statistical uncertainty of the measured asymmetry from each foil is below 0.25%. Within this statistical precision, the measured asymmetry was unaffected by ±1-mm shifts in the beam position on the target foil, and by beam current changes and dead-time effects over a wide range of beam currents. The overall uncertainty of our beam polarization measurement, arising from the uncertainty in the value of the scattering asymmetry at zero foil thickness as determined from our fits to the measured asymmetries versus scattering foil thicknesses, the estimated systematic effects, and the (dominant) uncertainty from the calculation of the theoretical Sherman function, is 0.61%. A simulation of the polarimeter using geant4 has confirmed that double scattering in the target foil is the sole source of the dependence of the measured asymmetry on foil thickness, and gives a result for the asymmetry versus foil thickness in good agreement with both our measurements and a simple calculation. Future measurements at different beam energies and with target foils of different atomic numbers will seek to bound uncertainties from small effects such as radiative corrections to the calculation of the polarimeter analyzing power. A simultaneous high-precision measurement of the beam polarization with a different polarimeter, AESOP (Accurate Electron Spin Optical Polarimeter), under development at the University of Nebraska, clearly possible at the CEBAF accelerator, will allow a high-precision comparison of our measured asymmetries with theoretical calculations of the Mott analyzing power. Finally, the improved precision of the current Mott polarimeter along with similar improvements to other Jefferson Lab polarimeters warrants another precision comparison of all of these polarimeters when measuring a beam of the same polarization.

中文翻译：

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高精度5 MeV Mott旋光仪

我们报告了针对连续电子束加速器（CEBAF）注入器的标称5 MeV电子束优化的Mott旋光仪的设计和性能。该光束的rf时间结构允许在散射电子检测中使用飞行时间，从而可以干净地隔离那些源自散射箔的检测到的电子，并导致测量到的散射不对称性在很宽的范围内异常稳定光束条件，光束电流和箔片厚度。在来自两个不同的光电阴极电子源的两个单独的系列测量中，我们测量了入射在十个金箔上的约86％的横向极化电子束入射到标称厚度在50到1000 nm之间的Mott散射不对称性。每个箔片测得的不对称性的统计不确定度低于0.25％。在此统计精度范围内，所测量的不对称性不受目标箔片上光束位置±1 mm的偏移以及光束电流变化和宽范围光束电流上的停滞时间影响的影响。我们的光束偏振测量的整体不确定性，是由零箔厚度下的散射不对称值的不确定性所引起的，该不确定性是由我们对所测得的不对称性与散射箔片厚度的拟合度，估计的系统效应以及来自理论谢尔曼函数的计算结果为0.61％。偏振计的模拟使用 所测得的不对称性不受目标箔片上光束位置±1 mm的偏移以及光束电流变化和在较大范围内光束电流上的停滞时间影响的影响。我们的光束偏振测量的整体不确定性，是由零箔厚度下的散射不对称值的不确定性所引起的，该不确定性是由我们对所测得的不对称性与散射箔片厚度的拟合度，估计的系统效应以及来自理论谢尔曼函数的计算结果为0.61％。偏振计的模拟使用 所测得的不对称性不受靶箔上电子束位置±1 mm的偏移以及电子束电流变化和大范围电子束电流的死区时间影响。我们的光束偏振测量的总体不确定性，是由零箔厚度下的散射不对称值的不确定性所引起的，该不确定性是由我们对所测得的不对称性与散射箔片厚度的拟合度，估计的系统效果以及来自（理论谢尔曼函数的计算结果为0.61％。偏振计的模拟使用 由零箔厚度下的散射不对称值的不确定性（根据我们对测得的不对称性与散射箔片厚度的拟合确定），估计的系统效应以及理论谢尔曼函数的计算得出的（主要）不确定性引起0.61％。偏振计的模拟使用 由零箔厚度下的散射不对称值的不确定性（根据我们对测得的不对称性与散射箔片厚度的拟合确定），估计的系统效应以及理论谢尔曼函数的计算得出的（主要）不确定性引起0.61％。偏振计的模拟使用geant4已确认目标箔中的双重散射是所测得的不对称性与箔厚度的相关性的唯一来源，并给出了不对称性与箔厚度的关系，这与我们的测量结果和简单的计算结果非常吻合。未来在不同束能量和不同原子序数的目标箔下进行的测量将寻求限制不确定性，包括辐射校正等小影响到旋光仪分析能力的计算。内布拉斯加大学正在开发使用另一台旋光仪AESOP（精确电子自旋光学旋光仪）同时进行光束偏振的高精度测量，显然可以在CEBAF加速器上进行，可以将我们测得的不对称性与Mott分析能力的理论计算进行高精度比较。最后，当前Mott旋光仪的改进精度以及与其他Jefferson Lab旋光仪的类似改进，保证了在测量相同偏振光束时所有这些旋光仪的另一种精度比较。