Impact of pear-shaped fission fragments on mass-asymmetric fission in actinides
Impact of pear-shaped fission fragments on mass-asymmetric fission in actinides
复制标题
DOI：
10.1038/s41586-018-0780-0
复制
发表时间：
2018-04
期刊：
影响因子：
64.8
通讯作者：
G. Scamps;C. Simenel
中科院分区：
文献类型：
--
作者：
G. Scamps;C. Simenel
重（锕系）核的核裂变主要导致不对称的质量分裂。如果没有量子壳层效应，这些原子核将对称地裂变，因为量子壳层效应可以为它们的质量不对称形状提供额外的结合能。最强的壳层效应出现在球形核中，例如球形的“双魔”核（即它的原子数和中子数都是“魔”数）132 Sn，它包含50个质子和82个中子。然而，对裂变的系统研究表明，重裂变碎片的原子序数分布在Z = 52 ~ Z = 56之间，表明132 Sn中强壳效应不是影响锕系元素裂变的唯一因素。用自然界中观察到的不同Z值的裂变碎片来解释Z = 50时的强球壳效应是一个长期的难题。在这里，我们表明，最终的质量不对称的碎片也是由八极（梨形）变形，这已被实验证实最近在144 Ba（Z= 56），壳稳定八极变形的极少数核之一所提供的额外的稳定性。利用超流体裂变动力学的量子多体模型，我们发现，重裂变碎片主要产生52至56个质子，这是与大量的八极形变获得裂变的方式。这些有利于非对称裂变的八极形状是由Z = 52和Z = 56的变形壳引起的。相比之下，球形魔核对八极形变有很强的抵抗力，八极形变阻碍了它们作为裂变碎片的产生。这些发现可以解释在比铅轻的原子核中观察到的令人惊讶的不对称裂变。
Nuclear fission of heavy (actinide) nuclei results predominantly in asymmetric mass splits. Without quantum shell effects, which can give extra binding energy to their mass-asymmetric shapes, these nuclei would fission symmetrically. The strongest shell effects appear in spherical nuclei, such as the spherical ‘doubly magic’ (that is, both its atomic and neutron numbers are ‘magic’ numbers) nucleus132Sn, which contains 50 protons and 82 neutrons. However, a systematic study of fission has shown that heavy fission fragments have atomic numbers distributed aroundZ= 52 toZ= 56, indicating that the strong shell effects in132Sn are not the only factor affecting actinide fission. Reconciling the strong spherical shell effects atZ= 50 with the differentZvalues of fission fragments observed in nature has been a longstanding puzzle. Here we show that the final mass asymmetry of the fragments is also determined by the extra stability provided by octupole (pear-shaped) deformations, which have been recently confirmed experimentally around144Ba (Z= 56),, one of very few nuclei with shell-stabilized octupole deformation. Using a quantum many-body model of superfluid fission dynamics, we find that heavy fission fragments are produced predominantly with 52 to 56 protons, which is associated with substantial octupole deformation acquired on the way to fission. These octupole shapes, which favour asymmetric fission, are induced by deformed shells atZ= 52 andZ= 56. By contrast, spherical magic nuclei are very resistant to octupole deformation, which hinders their production as fission fragments. These findings may explain surprising observations of asymmetric fission in nuclei lighter than lead.