Nuclear astrophysics studied by neutrons

点击翻译按钮获取中文摘要

Recently a lot of efforts are being made to observe an abundance of heavy elements in ultra metal deficient stars to obtain information of stellar nucleosynthesis in the early history of Galaxy. In order to reveal the history of Galaxy it is crucial to construct solid models of stellar evolution and stellar nucleosynthesis these ultra metal deficient stars. These stars are characterized to have very few iron seed light nuclei such as C, O and Ne etc. Hence, these nuclei would act as a neutron poison and would strongly reduce the production yields of heavy elements, if the neutron capture cross section of the nuclei would be large. Therefore, it is necessary to measure the neutron capture (n, γ) cross section of these light nuclei at astrophysically relevant energy to construct the models mentioned above.In the present work we aimed at accurately measuring the (n, γ) cross section of ^<6,7>Li, ^<13>C,^<16,18>O and ^<20,22>Ne, which have not been well studied. The measurement has been ca … More rried out successfully by significantly improving the signal to noise ration and the γ・ray detection efficiency of the existing anti-Compton NaI(Tl) spectrometer, and by newly constructing the liquefier for gaseous Ne sample. Actually we could improve the signal to noise ration by a factor 4 and the γ-ray efficiency by a factor 2. The measured cross sections of ^7Li,^<13>C, ^<16,18>O and ^<20,22>Ne clearly indicate that these nuclei act as a neutron poison in ultra metal deficient stars.Concerning the primordial D problem we measured the D(n, γ) T reaction cross section. The reaction is quite important, since it is a mirror reaction of the D(n, γ)^3He reaction, which destroys the primordial D. Our result would be quite useful to derive the S-factor of the D(p, γ)^3He reaction free from a Coulomb effect. As for the ^6Li(n, γ)^7Li reaction, our result clearly indicates that the excitation function does not follow the curve expected from a l/v law, although the reaction proceeds via an s-wave neutron capture. These could suggest that the s_<1/2> particle state in the cotinuum state, which has not yet been identified, is responsible for the energy dependence of the cross section. Concerning the ^7Li(n, γ)^8Li reaction we have for the first time measured the γ-ray branching ratio from the neutron capturing state to the ground and first excited states using an anti Compton HpGe spectrometer. The result gives a vital information to determine to S_<17> factor for the ^7Be(p, γ)^8B reaction. Finally we have also measured successfully the absolute keV neutron flux at KENS and the result indicates that the use of spallation neutrons for nuclear astrophysics is promising. Less