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Nuclear astrophysics studied by neutrons

通过中子研究核天体物理学

基本信息

批准号：
08102004
负责人：
NAGAI Yasuki
金额：
$ 79.36万
依托单位：
Tokyo Institute of Technology
依托单位国家：
日本
项目类别：
Grant-in-Aid for Specially Promoted Research
财政年份：
1996
资助国家：
日本
起止时间：
1996 至 1998
项目状态：
已结题

项目摘要

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

近年来，人们致力于观测超金属贫乏恒星中丰富的重元素，以获得银河系早期历史中恒星核合成的信息。为了揭示银河系的历史，构建恒星演化和恒星核合成这些超金属缺陷恒星的实体模型至关重要。这些恒星的特点是具有很少的铁种子轻核，如C、O和Ne等。因此，如果这些核的中子俘获截面很大，这些核将充当中子毒物，并会大大降低重元素的产量。因此，有必要测量这些轻核在天体物理相关能量下的中子俘获(n，γ)截面，以构建上述模型。在本工作中，我们的目标是精确测量^<6,7>Li、^<13>C、^<16,18>O和^<20,22>Ne的(n,γ)截面，但尚未得到很好的研究。通过显着提高现有反康普顿NaI(Tl)谱仪的信噪比和γ射线探测效率，并新建气态氖样品液化器，测量成功。实际上，我们可以将信噪比提高 4 倍，将 γ 射线效率提高 2 倍。测量到的 ^7Li、^<13>C、^<16,18>O 和 ^<20,22>Ne 的横截面清楚地表明这些原子核在超金属贫乏恒星中充当中子毒物。关于原初 D 问题，我们测量了 D(n, γ) T 反应横截面。该反应非常重要，因为它是 D(n, γ)^3He 反应的镜像反应，它破坏了原始 D。我们的结果对于导出不受库仑效应影响的 D(p, γ)^3He 反应的 S 因子非常有用。至于 ^6Li(n, γ)^7Li 反应，我们的结果清楚地表明，尽管反应是通过 s 波中子俘获进行的，但激发函数并不遵循 l/v 定律预期的曲线。这些可能表明，尚未确定的连续态中的s_<1/2>粒子态是造成截面能量依赖性的原因。关于^7Li(n, γ)^8Li反应，我们首次使用反康普顿HpGe能谱仪测量了从中子俘获态到基态和第一激发态的γ射线分支比。结果给出了确定 ^7Be(p, γ)^8B 反应的 S_<17> 因子的重要信息。最后，我们还在 KENS 成功测量了绝对 keV 中子通量，结果表明散裂中子在核天体物理中的应用是有前景的。较少的