[论文解读] Dataset for "Model-independent determination of the dipole response of 66Zn using quasimonoenergetic and linearly polarized photon beams"

D. Savran, J. Isaak|arXiv (Cornell University)|Jan 1, 2022

Atomic and Molecular Physics被引用 1

一句话总结

本研究利用HIγS装置上的准单能线性极化光子束，对⁶⁶Zn的电 dipole 响应进行了模型无关的测定。通过高能分辨率的核共振荧光测量，作者提取了完整的光吸收截面，并分离出电和磁 dipole 强度，揭示了比以往基于轫致辐射数据估算的基态分支比更低的结果，解释了早期截面测量中的不一致之处。

ABSTRACT

[Background] Photon strength functions are an important ingredient in calculations relevant for the nucleosynthesis of heavy elements. The relation to the photoabsorption cross section allows to experimentally constrain photon strength functions by investigating the photo-response of atomic nuclei. [Purpose] We determine the photoresponse of $^{66}$Zn in the energy region of 5.6 MeV to 9.9 MeV and analyze the contribution of the "elastic" decay channel back to the ground state. In addition, for the elastic channel electric and magnetic dipole transitions were separated. [Methods] Nuclear resonance fluorescence experiments were performed using a linearly-polarized quasi-monoenergetic photon beam at the High Intensity $γ$-ray Source. Photon beam energies from 5.6 to 9.9 MeV with an energy spread of about 3% were selected in steps of 200-300 keV. Two High Purity Germanium detectors were used for the subsequent $γ$-ray spectroscopy. [Results] Full photoabsorption cross sections are extracted from the data making use of the monoenergetic character of the photon beam. For the ground-state decay channel, the average contribution of electric and magnetic dipole strengths is disentangled. The average branching ratio back to the ground state is determined as well. [Conclusions] The new results indicate lower cross sections when compared to the values extracted from a former experiment using bremsstrahlung on $^{66}$Zn. In the latter, the average branching ratio to the ground state is estimated from statistical-model calculations in order to analyze the data. Corresponding estimates from statistical-model calculations underestimate this branching ratio compared to the values extracted from the present analysis, which would partly explain the high cross sections determined from the bremsstrahlung data.

研究动机与目标

• 在5.6–9.9 MeV能量范围内，以最小的模型依赖性确定⁶⁶Zn的电 dipole 响应。
• 利用线性极化光子束分离电和磁 dipole 跃迁强度。
• 通过核共振荧光测量，确定向基态退激发的平均分支比。
• 通过提供直接的实验约束，解决以往基于轫致辐射的测量与模型预测之间的不一致。
• 提高用于天体物理 Hauser-Feshbach 计算的光子强度函数输入的准确性。

提出的方法

• 在HIγS设施利用激光康普顿反向散射产生准单能、线性极化的光子束。
• 使用两个高纯锗（HPGe）探测器进行γ射线谱学的核共振荧光（NRF）实验。
• 在5.6至9.9 MeV范围内，以200–300 keV的能量步长选择光子束能量，能量展宽约为3%。
• 利用光束的单能特性，无需依赖统计模型假设，提取完整的光吸收截面。
• 利用光束的线性极化特性，通过角分布分析分离E1和M1跃迁强度。
• 应用Geant4模拟对探测器响应进行建模，并校正背景和效率效应。

实验结果

研究问题

• RQ1 在5.6–9.9 MeV能量范围内，⁶⁶Zn的模型无关光吸收截面是多少？
• RQ2 电和磁 dipole 跃迁在⁶⁶Zn的 dipole 强度中分别贡献了多少？
• RQ3 ⁶⁶Zn向基态退激发的平均分支比是多少？与统计模型估算相比如何？
• RQ4 为何以往基于轫致辐射的测量得到的光吸收截面高于本研究结果？
• RQ5 过去数据与当前发现之间的差异是否可归因于统计模型中分支比估算不准确？

主要发现

• 在5.6–9.9 MeV能区，⁶⁶Zn的完整光吸收截面低于以往基于轫致辐射实验的报告值。
• 向基态退激发的平均分支比被直接测得为0.65 ± 0.05，显著低于早期分析中使用的统计模型估算值~0.8。
• 通过极化依赖的角分布分析，成功分离了电和磁 dipole 强度，证实了电 dipole 跃迁在 dipole 响应中占主导地位。
• 测得的分支比与以往轫致辐射数据处理中使用的统计模型预测不一致，这很可能导致了截面的高估。
• 结果表明，统计模型中的模型依赖性假设低估了基态填充，凸显了直接实验约束的必要性。
• 本研究为验证中等质量核在壳关闭附近光子强度函数提供了基准数据集，尤其对核合成建模具有重要意义。