The quest for stimulated energy release Nuclear isomers provide a form of energy storage . Whereas betadecaying radioactive nuclei can also store MeV energies (per nucleus) for long periods of time, nuclear isomers usually decay electromagnetically, by g-ray emission or internal conversion. This difference may offer improved opportunities for stimulating the decay of isomers . The interaction of visible or ultraviolet photons with nuclei has long been sought. In principle, low-energy (eV) photons should be able to initiate isomer decay, either by stimulated emission to a nearby lower-energy state, or by stimulated absorption to a nearby higher-energy state. In either case, if the stimulated transition is to a short-lived energy level, a cascade of g-rays could be released, with a total energy of several MeV. If this scheme were to be realized, then it would be possible to have a type of nuclear reservoir, where the energy could be released with a photon `switch' . It would not be necessary to wait for the intrinsic half-life to release the energy and, if the isomer were in a nucleus with a stable ground state, there would be no subsequent radioactive waste. Moreover, the potential development of a g-ray laser would be brought a significant step closer52. Such an idealized scenario has not yet been realized, and low-energy photons have yet to be proven to stimulate nuclear transitions; but this does not mean that there are no possibilities. In the search for stimulated transitions, the isomers studied to date have mostly been at low energies (that is, low on a nuclear energy scale). The 45-hour isomer in 229Th, at an excitation energy of 3.5 eV, has received special attention51,53,54 because the nuclear excitation is on a similar energy scale to atomic valence electrons. Although the stimulation of the 3.5-eV transition would release no additional energy, it is of fundamental interest for investigating photon±nucleus interactions . But the MeV isomers discussed here have the potential to release significant quantities of energy. A recent encouraging advance was the de-excitation55, using ,100-keV photons, of the 2.4-MeV, 31-year isomer in 178Hf , although this is still a long way from de-excitation with eV photons. At face value, the use of eV photons to stimulate nuclear transitions requires one or more states to lie within a few eV of the isomer. For low-lying isomers, this presents a general problem. A chance near-degeneracy is needed, such as that found exceptionally in 229Th. For highly excited isomers, however, with energies of several MeV, the isomers become embedded in a high statistical density of excited states. For example, at 5MeV in a nucleus such as 178Hf, there is on average about one state per eV, so the `chance' of a near-degeneracy becomes certain. Notwithstanding this certainty, the isomer would probably have very different spin quantum numbers from the background of statistical states (otherwise it would not be an isomer in the first place). To stimulate isomer de-excitation, additional processes may therefore be required; one such example is the assisted electronbridge mechanism51, in which excitation of atomic electrons can eliminate energy and spin mismatching between the isomeric state of a nucleus and the state to which it decays. There is, therefore, room for chance to play an important role in providing the right conditions for isomer formation , and for allowing transitions to be stimulated by low-energy photons . Many new isomers are predicted to exist in nuclei with about 180 nucleons. The insights already gained into the nucleus, together with the development of experimental facilities, suggest the possibility of wide technological benefits in the future.
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