Reaction Mechanism of ^4He Photodisintegration and Charge Symmetry

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The photonuclear reactions of ^4He in the energy region below the pion threshold are quite useful to investigate fundamental properties of few-nucleon systems and the nuclear charge symmetry. The present work was aimed to investigate the reaction mechanism of the photodisintegration of ^4He occurring in the energy region from 30MeV to 100MeV by measuring the reaction events precisely. For this purpose, we developed a new experimental method. In this method we used the tagged photon beam at Laboratory of Nuclear Science (LNS) of Tohoku University and a time projection chamber (TPC) filled with a gas mixture of He and CD_4. The photon beam impinged upon ^4He nuclei contained in TPC, and the charged particles were detected by TPC. So TPC served as an active target, and that provided extremely high detection efficiency of -100% and very large acceptance of almost 4π of the solid angle.Since TPC is a kind of drift chambers, an external trigger signal was needed to measure the tracks of the … More charged particles emitted from the reactions. But the LNS tagged photon beam is a continuous beam, it cannot be used as a source of the external trigger. So we installed a trigger counter (TR) made of plastic scintillators in TPC. TR could provide a prompt signal of the particles with the energies down to 1MeV, and the time resolution was 1.1ns(1σ). To study the performance of the measurement system, we made a test experiment at the LNS tagged photon facility We accumulated the raw data of about 400000events during the measuring time of 42300s. By requiring the coincidence between TPC and the photon tagging counter (TAG) in the offline analysis, the background events were efficiently reduced, and 20000events survived. Then we checked the tracks of the particles in the remaining event, and we could select the candidates of the photoreaction events of ^4He, D and ^<12>C, which were main compositions of the counter gas. The present result shows the good performance of our experimental method. More detailed analysis is now in progress. Less