Creation and Annihilation of Microclusters with an Impurity Core Ion in Low-Temperature Condensed Matter

引入超流氦的杂质离子形成带电的聚集体，称为雪球。它们被认为是通过电致伸缩而收紧的微团簇。在这个研究项目中，这种雪球是通过引入高能重离子碰撞产生的放射性离子而产生的。因此，短寿命离子，如^<12>B（半衰期20.4 ms）和^8Li（半衰期842 ms）被用作杂质核离子。利用从核心离子的原子核中探测β射线和α粒子的新方法，研究了雪球的产生和湮灭过程。实验研究沿着两条线进行：1)确定了成核概率；形成雪球的数量与撞击超流氦的离子数量之比，是通过观察来自^<12>B的衰变射线的数量来测量的。2)杂质核离子中保留的核极化是寻找雪球中直接邻近杂质离子的氦原子相的一个很好的探针。我们的结果反映了雪球的结构很可能是固体实体，原子结构是高度对称的，正如人们通常认为的那样，二十面体取向可能是核心离子周围原子构型的候选之一。实验研究在RCNP，大阪大学进行，部分在RIKEN使用回旋加速器系统进行。

Impurity ions introduced in superfluid helium form charged aggregations called snowballs. They are considered microclusters tightened through electrostriction.In this research project, such snowballs are created through introduction of radioactive ions produced in high-energy heavy-ion collisions. Thus short-lived ions, such as ^<12>B (halflife 20,4 ms) and ^8Li (halflife 842 ms) are used as impurity core ions. The processes of creation and annihilation of snowballs are studied employing novel methods of detecting beta rays and alpha particles from the nuclei of the core ions.The experimental studies were proceeded along two lines, 1) nucleation probability has been determined ; the ratio of number of snowballs formed to number of impinging ions into superfluid helium, has been measured by observing the numbers of decay beta rays from ^<12>B.2) The preservation of nuclear polarization in impurity core ions is a good probe to seek into the phase of helium atoms as directly neighboring the impurity ions in snowballs. Our result reflects that the structure of a snowball is most probably of solid entity and the atomic structure is highly symmetric, as often presumed, the icosahedral orientation may be one of the candidates for the atomic configuration around the core ion.The experimental studies are carried out at RCNP,Osaka University and partly at RIKEN using the cyclotron systems.