在$^{19}$ F中高于中子阈值的带电粒子分支比：在核心坍缩超新星中限制$^{15}$ N的产生
Charged-particle branching ratios above the neutron threshold in $^{19}$F: constraining $^{15}$N production in core-collapse supernovae
摘要：
在一些低密度石墨陨石颗粒中观察到的空间相关的$^{15}$ N和$^{18}$ O的过量，与核坍缩超新星期间氦燃烧壳中发生的核合成有关。已确定对$^{15}$ N和$^{18}$ O的最终丰度重要的两个反应是$^{18}$ F（$n,α$）$^{15}$ N和$^{18}$ F（$n,p$）$^{18}$ O。$^{18}$ F（$n,α$）$^{15}$ N和$^{18}$ F（$n,p$）$^{18}$ O反应的相对强度取决于相对的$α_0$和$p_0$从$^{19}$ F中高于中子阈值的状态衰变以及其它性质。带电粒子从这些高激发态衰变的实验数据缺乏或不一致。利用LiF靶和磁谱仪的质子非弹性散射进行了两个实验。第一个实验使用慕尼黑的高分辨率Q3 D摄谱仪来限制$^{19}$ F中能级的性质。为了测量$^{19}$ F中中子未结合能级的带电粒子衰变，进行了使用Orsay分极摄谱仪和硅探测器阵列的第二个实验。$^{19}$ F中的许多能级及其相应的带电粒子衰变已被确定。高于中子阈值的第一状态具有观察到的到$^{18}$ O基态的质子衰变分支，其位于中子阈值以上68 keV，而通常观察到从中子未结合能级的$α$粒子衰变比质子衰变强得多。观测到$^{19}$ F中的中子未结合能级主要通过$α$ -粒子发射衰变，支持了$^{18}$ F（$n,α$）$^{15}$ N在超新星氦燃烧壳层中产生$^{15}$ N的作用。需要改进的共振散射反应数据，以便能够准确地确定反应速率。
英文摘要：
Spatially-correlated overabundances of $^{15}$N and $^{18}$O observed in some low-density graphite meteoritic grains have been connected to nucleosynthesis taking place in the helium-burning shell during core-collapse supernovae. Two of the reactions which have been identified as important to the final abundances of $^{15}$N and $^{18}$O are $^{18}$F($n,α$)$^{15}$N and $^{18}$F($n,p$)$^{18}$O. The relative strengths of the $^{18}$F($n,α$)$^{15}$N and $^{18}$F($n,p$)$^{18}$O reactions depend on the relative $α_0$ and $p_0$ decays from states above the neutron threshold in $^{19}$F in addition to other properties. Experimental data on the charged-particle decays from these highly excited states are lacking or inconsistent. Two experiments were performed using proton inelastic scattering from LiF targets and magnetic spectrographs. The first experiment used the high-resolution Q3D spectrograph at Munich to constrain properties of levels in $^{19}$F. A second experiment using the Orsay Split-Pole spectrograph and an array of silicon detectors was performed in order to measure the charged-particle decays of neutron-unbound levels in $^{19}$F. A number of levels in $^{19}$F have been identified along with their corresponding charged-particle decays. The first state above the neutron threshold which has an observed proton-decay branch to the ground state of $^{18}$O lies 68 keV above the neutron threshold while the $α$-particle decays from the neutron-unbound levels are generally observed to be much stronger than the proton decays. Neutron-unbound levels in $^{19}$F are observed to decay predominantly by $α$-particle emission, supporting the role of $^{18}$F($n,α$)$^{15}$N in the production of $^{15}$N in the helium-burning shell of supernovae. Improved resonant-scattering reaction data are required in order to be able to determine the reaction rates accurately.
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