The precise measurement of electron capture decay rate of ^7Be encapsulated in C_<60>

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The electron capture (EC) decay rate of ^7Be in C_<60> at the temperature of liquid helium (T=5K) was measured and compared with the rate in Be metal at T=293K. We found that the half-life of ^7Be in endohedral C_<60>(^7Be@C_<60>) at a temperature close to T=5K is 52.47±0.04 days, a value that is 0.34% faster than that at T=293K. In this environment, the half-life of ^7Be is nearly 1.5% faster than that inside Be metal at room temperature (T=293K). We then interpreted our observations in terms of calculations of the electron density at the ^7Be nucleus position inside the C_<60>; further, we estimate theoretically the temperature dependence (at T=0K and 293K) of the electron density at the Be nucleus position in the stable center inside C_<60>. The theoretical estimates were almost in agreement with the experimental observations.