Density Functional Study on Compounds to Accelerate the Electron Capture Decay of 7Be

In radionuclide compounds undergoing electron capture (EC) decay, the electron density at the nucleus (ρ(0)) and half-life of the nucleus are inversely proportional. Thus, the decay can be accelerated by changing the chemical or physical conditions. A previous study reported a 1.1–1.5% reduction in the half-life of 7Be encapsulated in C60 compared with 7Be metal. However, 7Be was inserted into the fullerene using the rebound energy of the nuclear reaction, which may not be a practical method. This paper elucidates the mechanism of ρ(0) change in various Be compounds from density functional calculations and attempts to propose better systems that show faster EC decay (larger ρ(0)) and/or that are easier to generate than Be in C60. In typical Be compounds, ρ(0) decreases because Be donates electrons to other atoms through chemical bonds and, thus, is not effective. Among the various Be-encapsulated fullerenes (C20–C180), the largest increase in ρ(0) was obtained for C50 fullerene, but the magnitude was almost similar to that of C60. As new systems, we propose Be-encapsulated rare gas solids, which would be generated only by applying high pressure. An increase in ρ(0) from Be metal in the range 2–10%, which depends on the lattice constant, is obtained.在发生电子俘获（electron capture, EC）衰变的放射性核素化合物中，原子核处的电子密度（ρ(0)）与原子核半衰期呈反比关系。因此，可通过改变化学或物理条件加速该衰变过程。既往研究显示，与金属态7Be相比，封装于C60中的7Be半衰期缩短了1.1%~1.5%。但此前的研究借助核反应反冲能量将7Be嵌入富勒烯，该方法实用性不足。本文通过密度泛函计算阐明了各类铍（Be）化合物中ρ(0)变化的机制，并尝试提出更优体系：这类体系要么可实现更快的EC衰变（即具备更高的ρ(0)），要么比C60封装的7Be更易制备。在典型铍化合物中，由于铍通过化学键向其他原子转移电子，其ρ(0)会降低，因此这类体系效果不佳。在各类封装铍的富勒烯（C20~C180）中，C50富勒烯体系的ρ(0)增幅最大，但增幅幅度与C60体系几乎持平。作为新型体系，本文提出了封装铍的稀有气体固体，此类体系仅可通过施加高压制备。结果表明，相较于金属态铍，该体系的ρ(0)可提升2%~10%，提升幅度随晶格常数变化而改变。