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利用激光谱测量62–80Zn核素超精细结构的实验是在欧洲核子中心(CERN)的ISOLDE在线同位素分离装置实验终端COLLAPS上完成的。不稳定的Zn同位素是通过1.4 GeV的质子束轰击厚
$ \rm UC_x $ 靶反应产生,再利用激光离子源[25]选择性电离到Zn的离子态(Zn Ⅱ)后进行后续质量分离。为了压制荧光测量方法的本底[22],在将Zn 离子束传输到实验终端前,先经过射频四极冷却聚束器[26]冷却并积累200 ms,并提供宽度为5 μs的Zn离子脉冲。脉冲离子束在激光谱实验终端经过电荷交换室(内部充有高密的钠蒸汽)被中性化,使得Zn原子最外层电子布居在$4s4p\;^3P_2$ 亚稳态。连续钛宝石激光器提供的基频光(利用高精度波长计锁频在961.450 8 nm)经倍频系统后产生480.725 4 nm 波长的光束。倍频后的激光波长匹配Zn原子的$4s4p\;^3P_2 \to 4s5s\;^3S_1$ 跃迁频率。当激光束与中性化后的Zn脉冲原子束在激光谱装置共线传输时,Zn原子核外的电子被共振激发到$4s5s\;^3S_1$ 激发态。激发态电子退激产生荧光被由四套光电倍增管探测器组成的荧光探测系统收集。在具体的实验中,激光频率固定不变,通过调节Zn束流的能量并利用多普勒效应来改变原子吸收的激光频率,从而达到电子能态超精细能级共振的目的。实验中通过电子学和数据获取系统得到收集的荧光强度和束流能量(原子吸收激光频率)的函数谱图,即超精细结构谱图 [27]。后续的数据分析过程中,通过最小$ \chi ^2 $ 法[28]来系统地拟合62–80Zn同位素的超精细结构谱,即可精确提取超精细结构常数和同位素移位等原子物理参量。这些参量与原子核的性质直接相关,由此可以模型无关地提取62–80Zn同位素的自旋、磁矩、电四极矩及均方根电荷半径参量(表1)。Isotope $I^{\pi}$ $ \mu_{\rm exp}/\mu_{\rm N}$ $ Q_{s,\rm exp}/\rm b$ $\delta \langle r^{2}\rangle^{68,A}$/fm2 Isotope $I^{\pi}$ $ \mu_{\rm exp}/\mu_{\rm N}$ $ Q_{s,\rm exp}/\rm b$ $\delta \langle r^{2}\rangle^{68,A}$/fm2 62Zn 0+ – – $-0.493$(3)[52] 73Zn $1/2^{-}$ +0.5585(5) – 0.318(3)[37] 63Zn $3/2^{-}$ $-0.282(1)$ +0.20(2) $-0.389(9)$[43] 73mZn $5/2^{+}$ $-0.8527(14)$ +0.43(4) 0.322(6)[37] 64Zn $0^{+}$ – – $-0.279(4)$[34] 74Zn $0^{+}$ – – 0.375(4)[44] 65Zn $5/2^{-}$ +0.7695(16) –0.024(15) $-0.257(7)$[25] 75Zn $7/2^{+}$ $-0.7887(9)$ +0.16(2) 0.349(3)[51] 66Zn $0^{+}$ – – $-0.121$(4)[16] 75mZn $1/2^{-}$ +0.5580(9) – 0.373(6)[51] 67Zn $5/2^{-}$ +0.875 479(9) +0.122(10) $-0.089(6)$[8] 76Zn $0^{+}$ – – 0.421(4)[57] 68Zn $0^{+}$ – – 0 77Zn $7/2^{+}$ –0.9074(1) +0.48(4) 0.440(5)[64] 69Zn $1/2^{-}$ +0.557(2) – 0.026(6)[9] 77mZn $1/2^{-}$ +0.562(2) – 0.455(11)[64] 69mZn $9/2^{+}$ –1.1613(7) –0.39(3) 0.073(3)[8] 78Zn $0^{+}$ – – 0.474(3)[70] 70Zn $0^{+}$ – – 0.142(3)[15] 79Zn $9/2^{+}$ $-1.1866$(10) +0.40(4) 0.461(3)[77] 71Zn $1/2^{-}$ +0.551(1) – 0.227(7)[23] 79mZn $1/2^{+}$ $-1.018$(1) – 0.639(8)[75] 71 mZn $9/2^{+}$ –1.049(1) –0.26(3) 0.191(3)[23] 80Zn $0^{+}$ – – 0.465(4)[84] 72Zn $0^{+}$ – – 0.292(3)[30]
Exotic Nuclear Structure of Neutron-rich Zn Isotopes
doi: 10.11804/NuclPhysRev.37.2019CNPC34
- Received Date: 2019-12-30
- Rev Recd Date: 2020-05-28
- Available Online: 2020-09-30
- Publish Date: 2020-09-20
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Key words:
- laser spectroscopy /
- nuclear spin /
- nuclear moment /
- charge radius /
- shell evolution /
- shape coexistence /
- deformation
Abstract: Nuclear structure of unstable nuclei, in particularly the nuclei near the magic number, has been one of the hot topics of nuclear physics study. Near the neutron magic number N=40, 50, rich nuclear structure phonomania appeared in the nickel region, in particularly for the neutron-rich isotopes, have stimulated intensive investigation from both theoretical and experimental aspects. In order to gain a better understanding of the nuclear structure in the nickel region, we choose to study the properties of neutron-rich Zn(Z=30) isotopes. In this paper, after a simple introduction of the laser spectroscopy experiment of Zn isotopes at CERN-ISOLDE, we reviewed the nuclear spins, magnetic moment, electric quadrupole moment and root mean square charge radius of the ground and long-lived isomeric states of 62–80Zn isotopes. Based on these properties, together with shell-model calculation from different interactions, we discussed systematically the nuclear structure phenomena, such as the shell structure evolution, magicity, deformation and shape coexistence, and the cross-shell excitation of correlated nucleons. At the end, on the basis of the current experimental data and nuclear structure information, as well as the theoretical prediction of energy level evolution of N=51 isotones in nickel region, we propose to measure the basic properties of 81,82Zn nuclei at the collinear resonance ionization spectroscopy setup at ISOLDE-CERN.
Citation: | Shujing WANG, Shiwei BAI, Xiaofei YANG. Exotic Nuclear Structure of Neutron-rich Zn Isotopes[J]. Nuclear Physics Review, 2020, 37(3): 291-300. doi: 10.11804/NuclPhysRev.37.2019CNPC34 |