Polarimeter for High Energy Photons

自从托马斯·杰斐逊国家实验室(JLAB)的连续电子束加速器设施建成以来，对高能光子偏振的精确测量已经成为一个重要的课题。例如，JLAB的B厅的科学计划包括要求偏振测量精度至少为5%的基础实验，其中一些实验要求精度为1-2%。在计划中的D厅的物理计划也将在很大程度上依赖于线偏振光子，其偏振必须准确知道。尽管高能光偏振测量已经有40多年的历史，但它仍然是一个相对不发达的领域。对于高能光子，如JLAB产生的那些，所有已知的测量偏振的方法分析能力都很低，绝对精度低于批准和预期的物理计划所要求的。在这个项目中提出的旋光计在从几十MeV到几个GeV的能量范围内将具有无与伦比的实验不对称性，约为1.7。该旋光仪结构简单，结构紧凑，其成本将低于按任何现有方法制造的旋光仪。已经建造了两个原型并进行了测试。探测器和电子器件的性能达到了预期，测量分析能力也和预期的一样大。在Spring-8上使用这个偏振器，首次直接测量了能量在1.5到2.4 GeV范围内的线偏振光子产生的方位不对称对。然而，需要更多的改进才能生产出将在联合实验室永久使用的偏振器。该项目将承担这些改进工作，包括对生产理论和计算的额外工作，数据采集和分析软件，以及探测器硬件的开发。除了解决偏振测量方面的迫切需求，为JLAB的最终实验提供基础，该项目的开发将为NCCU带来新的设备，并增加学生参与研究的程度，促进来自弱势群体的本科生教育。

Since the completion of the Continuous Electron Beam Accelerator Facilityat Thomas Jefferson National Laboratory (JLAB), the precise measurement ofthe polarization of high energy photons has become an important topic.For example, the scientific program at JLAB's Hall B includes fundamentalexperiments that require polarization measurements to be accurate to atleast 5%, and some of them require an accuracy of 1-2%. The physicsprogram at the planned Hall D will also depend largely on linearlypolarized photons whose polarization must be accurately known. In spite of a history extending over 40 years, high energy photonpolarimetry is still a relatively undeveloped field. For high energyphotons, such as those produced at JLAB, all known methods for measuringpolarization have low analyzing power, and absolute accuracy below thatrequired for the approved and anticipated physics programs. Thepolarimeter proposed in this project will have an unsurpassed experimentalasymmetry of approximately 1.7 for an energy range from a few tens of MeVup to several GeV. The polarimeter is simple in construction, compact, andits costs will be lower than that for polarimeters built according to anyother available method. Two prototypes have already been built and tested. The performanceof the detectors and the electronics met expectations and the measuredanalyzing power was as large as predicted. The use of this polarimeter atSpring-8 enabled the first direct measurement of the azimuthal asymmetryin pair production by linear polarized photons in the energy range 1.5 to2.4 GeV. However, more improvements are needed to produce a polarimeterthat will be permanently in use at JLAB. This project will undertakethose improvements, including additional work on pair production theoryand calculations, data acquisition and analyzing software, and thedevelopment of detector hardware.In addition to solving urgent needs in polarimetry, providing the groundfor the ultimate experiments at JLAB, the development of this program willbring novel equipment to NCCU, and increase involvement of students inresearch, advancing the education of undergraduates from underrepresentedgroups.