Precise measurement of stellar reaction rate of ^<12>C(α, γ)^<16>O with new experimental methods

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The ^<12>C(α, γ)^<16>O reaction plays crucial roles in stellar He-burning, because it governs ^<12>C/^<16>O ratio after core He-burning, abundances of intermediate-mass nuclei produced in He-burning, and the iron core mass just before the type-II supernova explosion. Therefore it is quite important to determine the reaction rate at the center-of-mass energy Ecm<1 MeV, which corresponds to the temperature of stellar interior.In order to determine the reaction rate accurately, we developed a new experimental method using a pulsed α beam and the prompt γ-ray detection technique.In 1999 we developed a measurement system, consisting of a high-sensitivity and low-background γ-ray spectrometer and a 2ns pulsed α beam. We carried out a test experiment to check the performance of the system, and found major background is caused by the fast neutrons, which are produced by the (α, n) reactions on boron isotopes contained in the carbon target.Therefore, in 2000, we made efforts to reduce the backgrounds due to those neutrons. We changed the material of the target chamber from stainless steel to aluminum, which has a small (n, γ) cross section. Here the λ rays from the (n, γ) reactions occurring near the target is quite dangerous, because the area around the target is directly viewed by the γ-ray detectors. With the above improvement, we could obtain the S/N ratios better than 18 and 360 with a natural carbon target and an enriched ^<12>C target, respectively. With the present result, it is expected that the ^<12>C(α, γ)^<16>O cross section can be measured accurately down to Ecm 〜900keV.In addition, we designed another new method, which measures the inverse reaction, i.e. the ^<16>O(γ, α)^<12>C reaction. In this method, we will use a laser-Compton backscattered γ-ray beam and a time projection chamber filled with CO_2 gas. We made an experimental plan, and proposed it to U.S.DOE.