Improved precision on the experimental E0 decay branching ratio of the Hoyle state
T. K. Eriksen, T. Kibédi, M. W. Reed, A. E. Stuchbery, K. J. Cook, A. Akber, B. Alshahrani, A. A. Avaa, K. Banerjee, A. C. Berriman, L. T. Bezzina, L. Bignell, … 展开作者
T. K. Eriksen, T. Kibédi, M. W. Reed, A. E. Stuchbery, K. J. Cook, A. Akber, B. Alshahrani, A. A. Avaa, K. Banerjee, A. C. Berriman, L. T. Bezzina, L. Bignell, J. Buete, I. P. Carter, B. J. Coombes, J. T. H. Dowie, M. Dasgupta, L. J. Evitts, A. B. Garnsworthy, M. S. M. Gerathy, T. J. Gray, D. J. Hinde, T. H. Hoang, S. S. Hota, E. Ideguchi, P. Jones, G. J. Lane, B. P. McCormick, A. J. Mitchell, N. Palalani, T. Palazzo, M. Ripper, E. C. Simpson, J. Smallcombe, B. M. A. Swinton-Bland, T. Tanaka, T. G. Tornyi, M. O. de Vries
备注：
Accepted for publication as a Regular Article in Phys. Rev. C on July 29 2020
恒星碳的合成完全通过$3α$过程发生，其中三个$α$粒子融合形成处于激发霍伊尔态的$^{12}$ C，然后电磁衰变到基态。霍伊尔态高于$α$阈值，恒星碳产生的速率取决于这种状态的辐射宽度。辐射宽度不能直接测量，必须通过结合三个单独测量的量来推导。其中一个量是霍伊尔态的$E0$衰变分支比，目前$10$辐射宽度的不确定性主要来自于这个比值的不确定性。$E0$的分支比是从$E0$和$E2$跃迁的一系列对转换测量中推导出来的，这些对转换测量分别减少了$^{12}$ C中的$0^+_2$ Hoyle态和$2^+_1$态。激发态由10.5 MeV束能量的$^{12}$ C $(p,p^\prime)$反应填充，并由澳大利亚国立大学的电子-正电子对光谱仪Super-e检测到。推导出的分支比需要了解两种状态的质子数，以及反应中$2^+_1$状态的排列。为此，还进行了质子散射和$γ$射线角分布实验.本文推导出了$Γ^{E0}_π/Γ=8.2(5)\times10^{-6}$的$E0$分支比，并在此基础上对文献值进行加权平均，给出了$Γ^{E0}_π/Γ=7.6(4)\times10^{-6}$的推荐值。$E0$分支比的新推荐值比以前采用的$Γ^{E0}_π/Γ=6.7(6)\times10^{-6}$值大约14%，而不确定性从9%降低到5%。
英文摘要：
Stellar carbon synthesis occurs exclusively via the $3α$ process, in which three $α$ particles fuse to form $^{12}$C in the excited Hoyle state, followed by electromagnetic decay to the ground state. The Hoyle state is above the $α$ threshold, and the rate of stellar carbon production depends on the radiative width of this state. The radiative width cannot be measured directly, and must instead be deduced by combining three separately measured quantities. One of these quantities is the $E0$ decay branching ratio of the Hoyle state, and the current $10$\% uncertainty on the radiative width stems mainly from the uncertainty on this ratio. The $E0$ branching ratio was deduced from a series of pair conversion measurements of the $E0$ and $E2$ transitions depopulating the $0^+_2$ Hoyle state and $2^+_1$ state in $^{12}$C, respectively. The excited states were populated by the $^{12}$C$(p,p^\prime)$ reaction at 10.5 MeV beam energy, and the pairs were detected with the electron-positron pair spectrometer, Super-e, at the Australian National University. The deduced branching ratio required knowledge of the proton population of the two states, as well as the alignment of the $2^+_1$ state in the reaction. For this purpose, proton scattering and $γ$-ray angular distribution experiments were also performed. An $E0$ branching ratio of $Γ^{E0}_π/Γ=8.2(5)\times10^{-6}$ was deduced in the current work, and an adopted value of $Γ^{E0}_π/Γ=7.6(4)\times10^{-6}$ is recommended based on a weighted average of previous literature values and the new result. The new recommended value for the $E0$ branching ratio is about 14% larger than the previous adopted value of $Γ^{E0}_π/Γ=6.7(6)\times10^{-6}$, while the uncertainty has been reduced from 9% to 5%.
