E1/E2 Ratio of ^<12>C(α, γ)^<16>O Reaction in Energy Regin of Astrophysical Interest

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Although the rate of the α-capture reaction, ^<12>C+α→^<16>O+γ, is of crucial importance in the scenario of evolution of main sequence stars, the experimental determination has long been hampered by great technical difficulties arising from extremely low cross section. The aim of the present investigation was to facilitate the rate measurement by detecting ^<16>O ions from the inverse kinematics reaction ^<12>C(^4He, ^<16>O) γ and to determine the E1/E2-capture ratio from the γ-ray angular distribution extracted from the analysis of the ^<16>O energy spectrum. In order to overcome the experimental difficulty, many technical developments were carried out both in the tandem accelerator and in the measuring system.1.Developments of the tandem acceleratorTo cope with very small cross section, the accelerator developments were carried out to make high-current beams with good quality available. A new acceleration mode of strong focusing principle was developed and a large aperture gas charge … More -stripper was installed. These altogether achieved 1000 times increase in the low energy ^<12>C beam intensity and 2-5 times improvement in the beam emittance. Beam pulsing was also made available by a newly developed buncher-chopper system.2.Development of the measuring systemIn order to separate ^<16>O ions from the high flux of beam particles, a new recoil separator was constructed and established as an excellent tool for the measurement by the detailed ion-optical investigation. A newly developed ^4He gas target was also installed. The method of measuring γ-ray angular distribution from the particle energy spectrum was examined using the ^<16>O (^<13>C, ^<12>C[2+])^<17>O and ^<16>O (^<ll>B, ^<12>C[2+] )^<15>N reactions and was found to work quite effectively in attaining the aim of the measurement.3.Test measurement and experimentThe test measurements were carried out and it was observed that three kinds of backgrounds were present in the particle spectrum. The origins were identified and two of them were successfully eliminated. To remove the last one, the beam line is being remodeled. The measurement has been started at a high energy of E_<cm>=2.4MeV, where the yield is expected relatively high. The energy of the measurement will be gradually reduced along with the repetition of test and improvement as above. Less