Ups and Downs of Nuclear Isomers

Abstract

W hat could one do with a clean source of nuclear energy? That tantalizing question reflects a dream that some have hoped to realize by exploiting the energy-storage capabilities of nuclear isomers. But the hope hinges on facets of nuclear behavior that remain unknown despite decades of study. Electromagnetic transitions and b decay, the basic mechanisms that largely determine isomer halflives, are well understood. In many cases, however, it is still not possible to predict half-lives even to within an order of magnitude.Today's isomer research seeks a better understanding of the degrees of freedom that will reveal new aspects of nuclear structure and lead the way to new applications. In this article, we lay out the essential ingredients of nuclear isomerism and take a look at future possibilities. Metastable excited statesThe term "isomer" is borrowed from chemistry, where it refers to molecules that have different geometrical configurations of the same collection of atoms. Isomeric nuclei, as distinguished from isotopes, are different states of the same numbers of protons and neutrons. Whereas chemical isomers have energy states that are similar, sometimes identical, to each other, nuclear isomers always have different energies. Excitation energies can be as high as several MeV.An interesting example of a nuclear isomer is 99m Tc, an excited state of technetium-99. The "m" after the mass number denotes a metastable state-that is, a long-lived isomer. The half-life of 99m Tc is six hours and its excitation energy above the nuclear ground state of 99 Tc is 143 keV. By contrast, typical half-lives of excited nuclear states are about a picosecond. Isomers live at least a thousand times longer. The appellation is usually reserved for excited nuclear states that live longer than a nanosecond. The superscript "m" is even more restrictive; it's reserved for isomers with half-lives of more than a millisecond. If a nuclear species has more than one metastable isomer, an ordinal number after the "m" distinguishes between them in ascending order of excitation energy.Perhaps the most widely known nuclear isomer is the long-lived hafnium excitation 178m2 Hf. With a half-life of 31 years, it sits 2.4 MeV above the stable 178 Hf ground state. The exceptional combination of high excitation energy and conveniently long halflife has led to claims for practical applications that have lent the hafnium isomer unusual visibility (see PHYSICS TODAY, May 2004, page 21).Nature's sole example of an isomer long-lived enough to be called stable is 180m Ta. This tantalum isomer's half-life exceeds 10 15 years. One can only quote a lower lifetime limit, because the isomer's spontaneous decay has never been observed. But the isomer sits 77 keV above a ground state that is itself unstable, with a b-decay halflife of only eight hours.Isomers can lose their excess energy by the usual radioactive decay modes: a, b, or g. The favored mode in any particular case depends on the energies and quantum numbers of the states involved. Decay by neutron o...

Storage ring at HIE-ISOLDE

“…Moreover, both scenarios do have problems in describing the light p-nuclei with A < 100 correctly. Since the p-process in type II Supernovae is dominated by photo-disintegrations from heavy seeds, this model does not account for the relatively large abundances of 92,94 Mo and 96,98 Ru. An alternative origin of these nuclei could be imagined via (p,γ)-reactions.…”

“…The electromagnetically-induced release of isomer energy is a controversial topic [94] that could lead to novel energy-storage devices, but reliable measurements have been rare. This situation is set to change with the TSR at ISOLDE, where it will be possible to make detailed investigations of the de-excitation of a large range of isomers.…”

Study of the level structure of108Ag

“…This nucleus, 108 Ag, has a very long lived isomer with a half life T 1/2 = 438 years in the low-spin region. Isomers across the nuclear chart have always been a topic of immense interest and such long lived isomers are of particular relevance in exploiting their applications in the field of high energy density storage devices [4]. A spectroscopic measurement of states around the isomer is of vital importance due to its crucial role in estimating the depletion pathway of the isomer and the concerned rate.…”

Nuclear isomers: stepping stones to the unknown

“…Nuclear isomers continue to make key contributions to the development and understanding of nuclear structure physics [1,2]. Isomers are widely distributed in neutron number, proton number, excitation energy and angular momentum.…”