库仑激发直接测量轻氡核的集体性
Collectivity in the light radon nuclei measured directly via Coulomb excitation
摘要：
背景资料：形状共存的重核构成了一个强大的挑战，国家的最先进的核模型，其中几个竞争的形状极小值被发现接近基态。研究这种现象的经典区域是在$Z=82$周围的区域和$N=104$的中子中壳层。目的：形状共存的证据已从$α$ -衰变测量，激光光谱和光束测量推断。虽然后者允许激发态和旋转能带结构的图案被映射出来，但对形状共存的详细理解只能来自电磁矩阵元素的测量。方法：在欧洲核子研究中心的REX-ISOLDE装置上，用低能库仑激发方法研究了$^{202}$ Rn和$^{204}$ Rn的二次放射性离子束。结果如下：对$^{202}$ Rn和$^{204}$ Rn提取了连接基态和第一激发态的电四极矩（$2^{+}_{1}$）矩阵元，对应于${B(E2;2^{+}_{1} \to 2^{+}_{1})=29^{+8}_{-8}}$ W.u.和$43^{+17}_{-12}$ W.u.，分别此外，在$^{202}$ Rn中确定了连接$2^{+}_{1}$状态与$4^{+}_{1}$和$2^{+}_{2}$状态的$E2$矩阵元素。在当前数据集中没有观察到激发态$0^{+}$，可能是由于在当前可用的束能量下的二阶过程的有限种群。结论：结果进行了讨论，在集体和变形的两个核的研究推断是弱的，如预期的低水平的能量计划。最先进的超越平均场模型的计算和过渡四极矩的大小也进行了比较，很好地再现。
英文摘要：
Background: Shape coexistence in heavy nuclei poses a strong challenge to state-of-the-art nuclear models, where several competing shape minima are found close to the ground state. A classic region for investigating this phenomenon is in the region around $Z=82$ and the neutron mid-shell at $N=104$. Purpose: Evidence for shape coexistence has been inferred from $α$-decay measurements, laser spectroscopy and in-beam measurements. While the latter allow the pattern of excited states and rotational band structures to be mapped out, a detailed understanding of shape coexistence can only come from measurements of electromagnetic matrix elements. Method: Secondary, radioactive ion beams of $^{202}$Rn and $^{204}$Rn were studied by means of low-energy Coulomb excitation at the REX-ISOLDE facility in CERN. Results: The electric-quadrupole ($E2$) matrix element connecting the ground state and first-excited $2^{+}_{1}$ state was extracted for both $^{202}$Rn and $^{204}$Rn, corresponding to ${B(E2;2^{+}_{1} \to 2^{+}_{1})=29^{+8}_{-8}}$ W.u. and $43^{+17}_{-12}$ W.u., respectively. Additionally, $E2$ matrix elements connecting the $2^{+}_{1}$ state with the $4^{+}_{1}$ and $2^{+}_{2}$ states were determined in $^{202}$Rn. No excited $0^{+}$ states were observed in the current data set, possibly due to a limited population of second-order processes at the currently-available beam energies. Conclusions: The results are discussed in terms of collectivity and the deformation of both nuclei studied is deduced to be weak, as expected from the low-lying level-energy schemes. Comparisons are also made to state-of-the-art beyond-mean-field model calculations and the magnitude of the transitional quadrupole moments are well reproduced.
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