Effect of lattice compression on the Li7 recoil energy spectrum following electron capture decay of Be7

The significant increase in the ($L/K$) ratio of ${}^7\mathrm{Be}$ implanted in tantalum, compared to that in mercury telluride, has been quantitatively explained by using ab initio density functional calculations. The result highlights the effect of lattice compression on the ($L/K$) ratio of ${}^7\mathrm{Be}$. Predictions for the ($L/K$) ratio in several untested cases have also been provided. Density functional calculations were employed to understand the notable $\left(\approx 6\mathrm{eV}\right)$ downward shift observed in the $K$ capture to the nuclear ground state ($K$-GS) peak in the recoil energy spectrum of ${}^7\mathrm{Li}$ following the electron capture decay of ${}^7\mathrm{Be}$ implanted in a small tantalum lattice and the resulting reduced energy difference between the $K$-GS and $L$-GS peaks. No such anomaly was observed when ${}^7\mathrm{Be}$ was implanted in mercury telluride. The calculations predict that the chemical shifts of the ${}^7\mathrm{Li} 1s$ state for the implantation of ${}^7\mathrm{Li}$ in different media are $\approx \left(0-2\right)$ eV. We hypothesize that the observed large downward shift of the $K$-GS peak in the ${}^7\mathrm{Li}$ recoil spectrum might be attributed to the difference of Fermi energies of tantalum and mercury telluride.