Introduction
An isomer is an excited quantum-mechanical state of a nucleus, in which a combination of nuclear structure effects inhibits its decay and endows the isomeric state with a lifetime that is longer than expected (some examples are shown in Table 1). The strong force that binds a nucleus together and the exchange of the mediators of this force lead to immeasurably short lifetimes of the order of 10-24 s for most nuclear states. But known isomers in nuclei span the entire range of lifetimes from 1015 years for 180mTa (m = metastable) — longer than the accepted age of the universe — to an informal rule of thumb on the lower side of approximately 1 ns. This inhibition to decay leads to the storage of enormous amounts of energy in these states (104 or 105 times more than chemical energy release). The challenge and potential for scientific discovery today lie in the understanding of the formation of nuclear isomers (through a better comprehension of nuclear structure), the ability to excite and de-excite isomers at will for a broad range of applications from isomeric bombs to a clean source of energy, and the exploration of nuclei with isomeric states in nuclear astrophysics to determine how they affect the creation of the elements in the universe, and how they eventually contribute to the makeup of life in our cosmos.
Nuclear isomers continue to be a frontier area of nuclear physics research1, 2, 3. Recently, a workshop on nuclear isomers took place along with the 12th International Conference on Capture Gamma-Rays and Related Topics (CGS–12; 4–9 September 2005) at the University of Notre Dame. The lively workshop helped to frame the outstanding scientific questions on isomers, marking a new season in isomer research and providing even stronger impetus to the experimental nuclear physics efforts in studying the properties of exotic nuclei with radioactive-ion-beam facilities in the USA (National Superconducting Cyclotron Laboratory, Notre Dame, Oak Ridge National Laboratory, and the Rare Isotope Accelerator proposed by the Long Range Plan for Nuclear Physics), Canada (TRIUMF), Europe (Facility for Antiproton and Ion Research at GSI) and Asia (Japan Proton Accelerator Research Complex and China Institute of Atomic Energy).
Detailed nuclear structure studies are at the heart of understanding the formation of nuclear isomers with applications to many aspects of modern physics today. Nuclear isomers decay predominantly by electromagnetic processes (gamma decay or internal conversion). There are also well-known instances of the decay being initiated by the strong interaction (alpha-emission) or the weak interaction (beta-decay or electron capture). Decay by proton or neutron emission, or even by nuclear fission, is possible for some isomers. For example, the occurrence and decay of superheavy elements has been attributed entirely to quantum shell effects4. It seems that in superheavy elements there are long-lived isomers resulting from the special occupations of different neutron and proton shells. In a quantum system, the ground state is usually more stable than the excited states. In contrast, in superheavy nuclei, the isomeric states decrease the probability for both fission and alpha-decay, resulting in enhanced stability for these nuclei5.
But how do we excite nuclear isomers? Perhaps more importantly, how can we release on demand the energy stored in isomers? These are the questions central to experimental aspects of isomer studies in nuclei, and have implications for national security as well as technological applications. For example, 'decay on demand' for an isomer could provide the basis for a nuclear battery.
Attempts to trigger the energy release from isomers have been focused on the 31-year isomer in 178m2Hf. This isomer's conveniently long life and high excitation energy of 2.4 MeV make it particularly attractive for study. A large US effort involving three national laboratories and several universities set out to confirm results purported to demonstrate decay on demand, but were unable to confirm it, that is, the results reported to date do not seem to support the viability of the 31-year isomer as an economical energy source6. However, there has been renewed interest in both well-known as well as exotic schemes for isomer population and depopulation.
Triggering the 180mTa isomer by 2.8-MeV photons is well established7, 8. Currently, the search for other candidates continues with the development of different techniques to trigger the decay of isomers. One such technique is known as nuclear excitation by electronic transition (NEET), in which intense X-ray beams are used to create atomic or electronic vacancies in selected cases of matching atomic and nuclear energies. The probability for NEET obviously increases if the energy of the atomic transition is closely matched to the nuclear transition energy. There are also studies involving the excitation of isomers by electron capture as well as various other techniques exploring the mechanism or true potential for energy release.
Finally, isomers play a significant role in determining the abundances of the elements in the universe. In hot astrophysical environments, an isomeric state can communicate with its ground state through thermal excitations. This could alter significantly the elemental abundances produced in nucleosynthesis. The communication between the ground state of 26Al and the first excited isomeric state in this nucleus has the consequence that the astrophysical half-life for 26Al can be much shorter than the laboratory value9. We are just beginning to look at the impact that isomers have on various other nucleosynthesis processes such as the rapid proton capture process thought to take place on the accretion disks of binary neutron stars. The temperature and density conditions are such that rapid thermonuclear runaways are taking place. There are cases in which an isomer of sufficiently long lifetime (probably longer than microseconds) can change the paths of reactions taking place and lead to a different set of elemental abundances. This aspect of isomers is very much in its infancy10.
Esoteric nuclear structure studies are essential for understanding the formation and decay of isomers. The workshop made it clear that there is plenty of intellectual excitement about the possibility of triggering isomers with new techniques.
