Electron-capture decay rate of ⁷Be encapsulated in a C₇₀ fullerene cage

Abstract

The decay rate of ⁷Be electron capture in C₇₀ and Be metal was measured employing a reference method. The half-life (T_{1/2}) of ⁷Be endohedral C₇₀ (⁷Be@C₇₀) was found to be T_{1/2} = 52.49±0.04 d at room temperature (T = 293 K) and T_{1/2} = 52.42±0.04 d at liquid helium temperature (T = 5 K). Furthermore, the T_{1/2} of ⁷Be in Be metal was T_{1/2} = 53.25±0.04 d at room temperature (T = 293 K) and T_{1/2} = 53.39±0.03 d at liquid helium temperature (T = 5 K). These values for ⁷Be@C₇₀ at T = 5 K are approximately 1.6%(1.8%) smaller than those for ⁷Be in Be metal at T = 293 K (T = 5 K), indicating the difference in the electron wave functions for ⁷Be inside C₇₀ and ⁷Be in C60 and Be metal. The average charge transfer from the L(2s) electrons of the ⁷Be atom influences such variations in the decay constant (λ = ln2/T_{1/2}) in the environment. The experimental and theoretical investigations revealed that the change in the EC-decay rate of ⁷Be could be largely related to the potential configurations and the environment inside C60 and/or C₇₀ cages. The motion of ⁷Be inside cages was found to be restricted according to temperature.