Exciting times for heavy nuclei
Sep 1, 2000
A previously unknown nuclear phenomenon has been observed by an international collaboration of physicists. They found that the energy released from a nucleus relaxing to a lower energy state can be used to excite an orbiting electron into a higher energy state. Nuclear and atomic processes are not usually linked in this way, mainly because nuclear phenomena involve energies up to a million times greater than atomic effects. But Carreyre and co-workers have found an exception to the rule (2000 Phys. Rev. C 62 024311).
The researchers at the French GANIL accelerator studied heavy, highly charged ions, in which the attraction between the nucleus and the remaining electrons is extremely strong. As a result, the difference in energy between atomic states can actually be higher than different states in the nucleus. To observe the effect, the group fired high-energy tellurium-125 ions at a target to excite the tellurium nuclei and electrons. By analysing x-rays and gamma-rays emitted by the ions as they left the target, the team showed that as the nucleus returned to a lower energy state, a tightly bound inner electron was promoted to an outside orbit.
An important property of the new process is that it is strongly resonant: a small increase in the atomic excitation energy can produce a large variation in the half-life and rate of nuclear decay. Carreyre's team suspects that the transfer of energy between nucleus and surrounding electrons could account for apparent anomalies in the lifetimes of certain radioactive elements. Interestingly, the effect is exactly the reverse of a process recently observed, in which the nucleus is excited by a near-resonant electron transition (Kishimoto et al 2000 Phys. Rev. Lett. 85 1831).
The phenomenon may have important implications for astrophysics. The highly ionized gases, or plasmas, that exist inside stars contain nuclear species whose lifetimes may be affected by the process. If that is the case, this may change our understanding of how the elements are created inside stars.