Isomer depletion as experimental evidence of nuclear excitation by electron capture

Experimental evidence is presented for isomer depletion through nuclear excitation by electron capture in molybdenum-93 nuclei. Exciting a nucleus of 93 Mo If an atomic nucleus absorbs an electron of the right energy, it can be excited to a higher-energy state. This process is called nuclear excitation by electron capture and, although it has been recognized theoretically, has not yet been observed. Chris Chiara and colleagues have now seen evidence of this effect in experiments on atoms of the isotope molybdenum-93. The effect could affect the survival of some isotopes inside stars, because the excitation can cause the states to be long-lived. The atomic nucleus and its electrons are often thought of as independent systems that are held together in the atom by their mutual attraction. Their interaction, however, leads to other important effects, such as providing an additional decay mode for excited nuclear states, whereby the nucleus releases energy by ejecting an atomic electron instead of by emitting a γ-ray. This ‘internal conversion’ has been known for about a hundred years and can be used to study nuclei and their interaction with their electrons 1 , 2 , 3 . In the inverse process—nuclear excitation by electron capture (NEEC)—a free electron is captured into an atomic vacancy and can excite the nucleus to a higher-energy state, provided that the kinetic energy of the free electron plus the magnitude of its binding energy once captured matches the nuclear energy difference between the two states. NEEC was predicted 4 in 1976 and has not hitherto been observed 5 , 6 . Here we report evidence of NEEC in molybdenum-93 and determine the probability and cross-section for the process in a beam-based experimental scenario. Our results provide a standard for the assessment of theoretical models relevant to NEEC, which predict cross-sections that span many orders of magnitude. The greatest practical effect of the NEEC process may be on the survival of nuclei in stellar environments 7 , in which it could excite isomers (that is, long-lived nuclear states) to shorter-lived states. Such excitations may reduce the abundance of the isotope after its production. This is an example of ‘isomer depletion’, which has been investigated previously through other reactions 8 , 9 , 10 , 11 , 12 , but is used here to obtain evidence for NEEC.