Nuclear physicists in Japan have shown that the electron-capture decay rate of radioactive beryllium-7 can be increased by almost 1% by placing it inside a carbon-60 "cage". This is the largest change in the decay rate of an element ever observed. Although the resulting reduction in the radioactive half-life is unlikely to help with the problem of storing nuclear waste, the cages could be used as tracers in medical radiotherapy (T Ohtsuki et al. 2004 Phys. Rev. Lett. 93 112501).
Beryllium-7 can decay into lithium-7 through a process in which the beryllium nucleus captures one of its own electrons. This electron is then absorbed into the nucleus, where it combines with a proton to form a neutron. Increasing the density of electrons surrounding the nucleus can increase electron-capture decay rates because more electrons are likely to be captured and absorbed. The electron density can be increased by external factors, such as chemical environment or pressure.
Tsutomu Ohtsuki and co-workers at Tohoku University and Yokohama National University began by inserting beryllium-7 atoms into carbon-60 cages using a nuclear recoil implantation technique. Next, they measured the decay rate of the encapsulated beryllium — denoted 7Be@C60 — with a gamma-ray detector and found that its half-life was 52.68 days. This was 0.83% shorter than the half-life they measured for pure beryllium (53.12 days). The half-life of a radioisotope is defined as the time it takes for half of the original amount of material to decay.
The team says that the faster decay rate can be explained by the fact that the dense cloud of electrons in the carbon-60 cage increases the electron density at the nucleus. Moreover, the particular orbits of these electrons further increase the electron density at the nucleus.
Increasing the decay rate by just 0.83% will have little effect on radioisotopes with half-lives of thousands or millions of years. However, Ohtsuki and co-workers say that their results will help identify the sort of environments that increase the decay rate. These could include the high-pressure conditions found inside neutron stars.
