Gammasphere 上裂变丰中子核核结构若干前沿领域的新进展(Ⅰ: 第一和第二部分)(英文)
New Insights into Frontier of Nuclear Structure of Neutronrich Nuclei by Means of Prompt Fission γ Ray Measurements at Gammasphere(Ⅰ: Section 1 & Section 2)
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摘要: 使用Gammasphere多探测器系统对252Cf裂变源瞬发γ射线进行γ-γ-γ和γ-γ(θ)符合测量， 裂变丰中子原子核核结构若干前沿领域的深入研究获得了新的进展。高达5.7×1011以上的三重和更高重符合事件的数据统计， 以及更少压缩的三维数据为宽广未知丰中子核区的寻找和研究提供了有利的条件。在具有重要物理意义的若干丰中子核区首次建立， 或显著扩展了一批包括转晕态和转晕附近能态的高自旋能级纲图。
在偶-偶丰中子核110, 112Ru和108Mo中鉴别出了手征对称破缺结构。丰中子 110, 112Ru附近核的三轴形变基态具有最低的能量， 在它们之中已确认了接近最大值的三轴形变。在这些Ru和Mo同位素中观察到的手征双线能带展示出手征破缺的一切特征， 特别是其理想的能量简并， 表明它们在迄今已报道的手征破缺结构中， 具有最好的手征特性。研究了手征结构从具有γ软度的108Ru到具有大三轴形变的110, 112Ru的过渡。斜轴推转（TAC）和随机相近似（RPA）理论计算成功地拟合了在这些偶偶丰中子核中观察到的手征双线能带的特性， 并指定其为软手征振动态。在这些偶偶核中观察到的手征破缺不可能归纳为奇奇核中那样的简化的几何图像。前者来自闭壳外所有中子的相互作用。
对双幻核132Sn附近N=83同中素链的系统研究为这个极富吸引力的核区提供了大量新的谱学信息。N=83 同中素135Te(Z=52), 136I(Z=53), 137Xe(Z=54),138Cs(Z=55) 和139Ba(Z=56)的最新能级信息， 特别是首次建立的138Cs高自旋能级纲图和壳模型理论计算表明，Z=50质子闭壳外少数g7/2价质子激发同N=82中子闭壳外之唯一f7/2价中子的耦合对该核区能级结构具有关键作用。观察到了132Sn和208Pb附近核区谱学信息的相似性和相对应的三粒子和五粒子态。在135Te中观察到了磁转动， 这是在双幻核132Sn附近观察到的首例磁转动。New insights have been gained into the frontiers of nuclear structure of neutronrich nuclei by means of γ-γ-γ and γ-γ(θ) coincidences of prompt γ rays emitted in the spontaneous fission of 252Cf at Gammasphere. Over 5.7×1011 tripleand higher fold coincidence events and the lesscompressed cube data provide excellent conditions for searches and studies over a wide unknown range with more neutron excess. Highspin yrast and near yrast level schemes of neutronrich nuclei in regions of physics interest have been identified for the first time, or extensively extended and expanded compared to previous preliminary measurements ......Abstract: New insights have been gained into the frontiers of nuclear structure of neutronrich nuclei by means of γ-γ-γ and γ-γ(θ) coincidences of prompt γ rays emitted in the spontaneous fission of 252Cf at Gammasphere. Over 5.7×1011 tripleand higher fold coincidence events and the lesscompressed cube data provide excellent conditions for searches and studies over a wide unknown range with more neutron excess. Highspin yrast and near yrast level schemes of neutronrich nuclei in regions of physics interest have been identified for the first time, or extensively extended and expanded compared to previous preliminary measurements.
Chiral symmetry breaking was recently identified in eveneven neutronrich110, 112Ru and 108Mo isotopes. The former have the largest lowering of ground state energy when axial symmetry is broken, and near maximum triaxiality was deduced in the isotopes. By exhibiting all the fingerprints for chiral doubling, especially the best energy degeneracy, the doublet bands observed in these Ru and Mo isotopes are the best examples of chiral properties reported in this region. The evolution of chirality from γsoft 108Ru to triaxial 110, 112Ru is proposed. Tilted axis cranking (TAC) calculations extended by random phase approximation (RPA) calculations can explain the features of the doublet bands in terms of a soft chiral vibration for these eveneven nuclei. The chirality in these eveneven nuclei cannot be reduced to the simple geometrical picture as in oddodd nuclei. Instead, in these eveneven nuclei the tendency to chirality comes about from the interplay of all the neutrons in the open shell.
Systematic studies of the N=83 isotonic chain in the vicinity of the doublymagic 132Sn have yielded a wealth of spectroscopic information in this attractive region. The new data of N=83 isotones 135Te (Z=52), 136I (Z=53), 137Xe (Z=54), 138Cs (Z=55) and 139Ba (Z=56), especially the observation of the longsought level scheme of 138Cs, and shell model calculations, indicate the key role played by the coupling of the excitations of the few g7/2 valence protons outside the Z=50 major shell closure and the f7/2 valence neutrons outside the N=82 major shell closure. Resemblance of spectroscopy and counterparts were observed between the 132Sn region and 208Pb region. Tilted rotation(magnetic rotation) was observed in 135Te, which is the first observation of magne tic rotation in the vicinity of 132Sn.
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