Development of the Gamma-ray detector for the measurement of helicity dependent photo-absorption cross-section for the nucleon at SPring-8.

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Measurements of the helicity dependent photo-absorption cross-section for the nucleon allow us to verify the GDH sum rule by accumulating the energy dependent data. The experiment to perform them is planned at Spring-8. The purpose of the current research is construction of the gamma-ray detector which is necessary in the experiment by performing Research and Development. The gamma-ray detector should have high efficiency even for the low energy gamma-rays (less than 100 MeV), small insensitive area and be capable of being operated under high magnetic field condition. In 2006, a series of basic test with cosmic rays has been carried out. In 2007, In 2007, construction of the prototype had been planned based on the results of the basic tests performed in the previous year. It turned out that it is necessary to have more ideas on the mechanical structure for achieving a small insensitive area. Basically, the large weight of the gamma-ray detector made of heavy metal such as lead, 1 ton for the current detector, gives problems. We corrected the design many times and evaluated the strength of the structure. And finally we successfully overcome the problems and determined the final design