Since the completion of the Continuous Electron Beam Accelerator Facility at Thomas Jefferson National Laboratory (JLAB), the precise measurement of the polarization of high energy photons has become an important topic. For example, the scientific program at JLAB's Hall B includes fundamental experiments that require polarization measurements to be accurate to at least 5%, and some of them require an accuracy of 1-2%. The physics program at the planned Hall D will also depend largely on linearly polarized photons whose polarization must be accurately known. In spite of a history extending over 40 years, high energy photon polarimetry is still a relatively undeveloped field. For high energy photons, such as those produced at JLAB, all known methods for measuring polarization have low analyzing power, and absolute accuracy below that required for the approved and anticipated physics programs. The polarimeter proposed in this project will have an unsurpassed experimental asymmetry of approximately 1.7 for an energy range from a few tens of MeV up to several GeV. The polarimeter is simple in construction, compact, and its costs will be lower than that for polarimeters built according to any other available method. Two prototypes have already been built and tested. The performance of the detectors and the electronics met expectations and the measured analyzing power was as large as predicted. The use of this polarimeter at Spring-8 enabled the first direct measurement of the azimuthal asymmetry in pair production by linear polarized photons in the energy range 1.5 to 2.4 GeV. However, more improvements are needed to produce a polarimeter that will be permanently in use at JLAB. This project will undertake those improvements, including additional work on pair production theory and calculations, data acquisition and analyzing software, and the development of detector hardware. In addition to solving urgent needs in polarimetry, providing the ground for the ultimate experiments at JLAB, the development of this program will bring novel equipment to NCCU, and increase involvement of students in research, advancing the education of undergraduates from underrepresented groups.
