When an excited nucleus decays to a lower state, its energy may in turn excite an orbital electron in a process known as internal conversion. The energy transferred from the nuclear decay is much bigger than the typical binding energy of an electron and the electron can escape from the atom. Now an international team of researchers working at the GANIL accelerator in France have seen the first direct proof of what they call "bound internal conversion" (BIC).
The team fired tellurium ions into a thorium target. In the collisions the tellurium ions lose more electrons and their nuclei become excited. With up to 48 valence electrons missing, those that remain are bound so tightly to the nucleus that their binding energy starts to exceed the energy differences between nuclear excitation states. Then internal conversion of the nuclear decay energy to electron excitation doesn't kick the electron out of the ion - it is just excited to a higher energy state. The researchers saw characteristic X-rays emitted by the ions as the electronic vacancy was subsequently filled. Such photons could be due just to decays of excitations caused in the collisions. However, as the photons arrived after a time delay consistent with the nuclear state lifetime, the researchers believe that this is a definite signature of BIC.
Meanwhile, Japanese researchers have reported observations of the reverse effect - nuclear excitation by electron transition, the latest experiments using synchrotron radiation to excite gold atoms.
This coupling between electronic and nuclear states is suspected to account for the anomalous lifetimes of some nuclear excitations, and may have some bearing on the synthesis of elements inside stars, where highly-ionized atoms are likely to exist. AIP