Radioactive Decay Speedup At T=5 K: Electron-Capture Decay Rate Of Be-7 Encapsulated In C-60

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