Enhanced Electron-Capture Decay Rate of ${}^{\mathrm{7}}\mathrm{B}\mathrm{e}$ Encapsulated in ${\mathrm{C}}_{\mathrm{60}}$ Cages

The decay rate of ${}^{\mathrm{7}}\mathrm{B}\mathrm{e}$ electron capture was measured in ${\mathrm{C}}_{60}$ and Be metal with a reference method. The half-life of ${}^{\mathrm{7}}\mathrm{B}\mathrm{e}$ endohedral ${\mathrm{C}}_{\mathrm{60}}$ (${}^{\mathrm{7}}\mathrm{B}\mathrm{e}\mathrm{@}{\mathrm{C}}_{\mathrm{60}}$) and ${}^{\mathrm{7}}\mathrm{B}\mathrm{e}$ in Be metal (Be metal (${}^{\mathrm{7}}\mathrm{B}\mathrm{e}$)) is found to be $52.68\pm 0.05$ and $53.12\pm 0.05$ days, respectively. This amounts to a $0.83\%$ difference in electron-capture decay half-life between ${}^{\mathrm{7}}\mathrm{B}\mathrm{e}@{\mathrm{C}}_{60}$ and Be metal (${}^{\mathrm{7}}\mathrm{B}\mathrm{e}$). Our result is a reflection of the different electron wave functions for ${}^{\mathrm{7}}\mathrm{B}\mathrm{e}@{\mathrm{C}}_{60}$ inside ${\mathrm{C}}_{\mathrm{60}}$ compared to the situation when ${}^{\mathrm{7}}\mathrm{B}\mathrm{e}$ is in a Be metal.

Figure 1

HPLC elution curves of the soluble portion of the crude extracted in the proton irradiated sample of ${}^{\mathrm{7}}\mathrm{L}\mathrm{i}$ and ${\mathrm{C}}_{\mathrm{60}}$. The vertical axis shows the radioactivities of ${}^7\mathrm{B}\mathrm{e}$ in each fraction (solid circles) and an absorbance of UV chromatogram of ${\mathrm{C}}_{\mathrm{60}}$. The inserted figures are time-ordered snapshots of a simulation taken from Ref. 27, where ${}^{\mathrm{7}}\mathrm{B}\mathrm{e}$ hits the center of a six-membered ring of ${\mathrm{C}}_{\mathrm{60}}$ with 5 eV kinetic energy. One can see successful production of ${}^{\mathrm{7}}\mathrm{B}\mathrm{e}@{\mathrm{C}}_{60}$.

Figure 2

Typical $\gamma$-ray spectrum of ${}^{\mathrm{7}}\mathrm{B}\mathrm{e}$ in the ${}^{\mathrm{7}}\mathrm{B}\mathrm{e}@{\mathrm{C}}_{60}$ fraction.

Figure 3

Exponential decay curves of ${}^{\mathrm{7}}\mathrm{B}\mathrm{e}$ in samples of ${}^{\mathrm{7}}\mathrm{B}\mathrm{e}$ endohedral ${\mathrm{C}}_{\mathrm{60}}$ and Be metal (${}^{\mathrm{7}}\mathrm{B}\mathrm{e}$). Insets corresponding to the decay intervals of $0\sim 20$ days and $130\sim 160$ days are displayed with an expanded scale.