中子诱发$^{140}$La复合态γ射线的角分布
Angular Distribution of $γ$-rays from Neutron-Induced Compound States of $^{140}$La
AI总结：
本文通过测量${}^{139}$La(n,$γ$)反应中γ射线角分布，定义并拟合了共振不对称度$A_{\mathrm{LH}}$的角依赖关系，在s波与p波干涉框架下解释了0.74 eV p波共振结果，并据此量化了p波共振附近时间反演不变性破坏的可能增强。
摘要：
利用锗探测器，研究了超热中子能区中，通过${}^{139}$La(n,$γ$)辐射俘获反应从中子诱发复合核态发射的各条γ射线角分布随入射中子能量的变化。定义不对称度$A_{\mathrm{LH}}$为$(N_{\mathrm L}-N_{\mathrm H})/(N_{\mathrm L}+N_{\mathrm H})$，其中$N_{\mathrm L}$和$N_{\mathrm H}$分别为一个中子共振低能区与高能区的积分，并发现$A_{\mathrm{LH}}$具有$(A\cosθ_γ+B)$的角依赖关系，其中$θ_γ$为γ射线发射角，在0.74 eV p波共振中$A= -0.3881\pm0.0236$，$B=-0.0747\pm0.0105$。在进入复合核态的入射道中s波与p波振幅干涉的框架内分析了这一角分布，并将其解释为与入射中子总角动量及弱矩阵元相联系的p波中子分宽度值，其背景是增强宇称破坏效应的机制。此外，我们利用该结果量化了在p波共振附近时间反演不变性破坏的可能增强程度。
英文摘要：
Angular distribution of individual $γ$-rays, emitted from a neutron-induced compound nuclear state via radiative capture reaction of ${}^{139}$La(n,$γ$) has been studied as a function of incident neutron energy in the epithermal region by using germanium detectors. An asymmetry $A_{\mathrm{LH}}$ was defined as $(N_{\mathrm L}-N_{\mathrm H})/(N_{\mathrm L}+N_{\mathrm H})$, where $N_{\mathrm L}$ and $N_{\mathrm H}$ are integrals of low and high energy region of a neutron resonance respectively, and we found that $A_{\mathrm{LH}}$ has the angular dependence of $(A\cosθ_γ+B)$, where $θ_γ$ is emitted angle of $γ$-rays, with $A= -0.3881\pm0.0236$ and $B=-0.0747\pm0.0105$ in 0.74 eV p-wave resonance. This angular distribution was analyzed within the framework of interference between s- and p-wave amplitudes in the entrance channel to the compound nuclear state, and it is interpreted as the value of the partial p-wave neutron width corresponding to the total angular momentum of the incident neutron combined with the weak matrix element, in the context of the mechanism of enhanced parity-violating effects. Additionally we used the result to quantify the possible enhancement of the breaking of the time-reversal invariance in the vicinity of the p-wave resonance.
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