Polarimeter for High Energy Photons

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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.