Herzberg R-D, Greenlees P T, Butler P A, Jones G D, Venhart M, Darby I G, Eeckhaudt S, Eskola K, Grahn T, Gray-Jones C, Hessberger F P, Jones P, Julin R, Juutinen S, Ketelhut S, Korten W, Leino M, Leppänen A-P, Moon S, Nyman M, Page R D, Pakarinen J, Pritchard A, Rahkila P, Sarén J, Scholey C, Steer A, Sun Y, Theisen Ch, Uusitalo J
Department of Physics, University of Liverpool, Liverpool L69 7ZE, UK.
Nature. 2006 Aug 24;442(7105):896-9. doi: 10.1038/nature05069.
A long-standing prediction of nuclear models is the emergence of a region of long-lived, or even stable, superheavy elements beyond the actinides. These nuclei owe their enhanced stability to closed shells in the structure of both protons and neutrons. However, theoretical approaches to date do not yield consistent predictions of the precise limits of the 'island of stability'; experimental studies are therefore crucial. The bulk of experimental effort so far has been focused on the direct creation of superheavy elements in heavy ion fusion reactions, leading to the production of elements up to proton number Z = 118 (refs 4, 5). Recently, it has become possible to make detailed spectroscopic studies of nuclei beyond fermium (Z = 100), with the aim of understanding the underlying single-particle structure of superheavy elements. Here we report such a study of the nobelium isotope 254No, with 102 protons and 152 neutrons--the heaviest nucleus studied in this manner to date. We find three excited structures, two of which are isomeric (metastable). One of these structures is firmly assigned to a two-proton excitation. These states are highly significant as their location is sensitive to single-particle levels above the gap in shell energies predicted at Z = 114, and thus provide a microscopic benchmark for nuclear models of the superheavy elements.
核模型的一个长期预测是，在锕系元素之后会出现一个长寿命甚至稳定的超重元素区域。这些原子核稳定性增强归因于质子和中子结构中的封闭壳层。然而，迄今为止的理论方法对于“稳定岛”的精确界限并未给出一致的预测；因此，实验研究至关重要。到目前为止，大部分实验工作都集中在通过重离子聚变反应直接产生超重元素，从而产生了质子数Z = 118以内的元素（参考文献4、5）。最近，对镄（Z = 100）之后的原子核进行详细光谱研究成为可能，目的是了解超重元素潜在的单粒子结构。在此，我们报告对锘同位素²⁵⁴No的此类研究，它有102个质子和152个中子，是迄今为止以这种方式研究的最重原子核。我们发现了三种激发结构，其中两种是同质异能的（亚稳态）。其中一种结构被明确归因为双质子激发。这些态非常重要，因为它们的位置对Z = 114时预测的壳层能量间隙之上的单粒子能级敏感，因此为超重元素的核模型提供了一个微观基准。
