Nuclear isomers or
metastable nuclear states have been identified in many nuclei throughout the periodic
table1,2. O. Hahn, co-discoverer of nuclear fission phenomenon, is credited
with the discovery of nuclear isomerism way back in 1921 and its interpretation in terms
of inhibited gamma decay was provided by Weizsacker in 1936. Even though isomers occur in
different types3 – shape isomers (decay inhibited due to shape
mismatch), spin isomers (spin mismatch) and K-isomers (change in spin orientation with
respect to axis of symmetry), the discussion here will be limited to spin isomers. In the
early days of nuclear physics research, the study of isomers generated a lot of interest,
because of the rather peculiar situation of having two nuclei with same Z and A values but
different spin values. The nuclear isomers have half-lives at least 100 times more than
typical gamma-decay times (~ 10–14 s) and their occurrence is
widespread in odd-A nucleus with odd-N (neutron) number
just below the magic numbers, N = 50,82,126
(ref. 2). Formation of isomers requires availability of widely different j (angular momentum) orbits for the nucleons
(protons and neutrons) at nearby energies in the shell model description of nuclei. This
situation is prevalent in the medium–heavy nuclei mentioned above. Some examples of
isomers may be given: 178Hf nucleus has an isomer, 178mHf, located
at Ex = 2.4 MeV with
Jp = 16+ and
half-life of 31 years. 180Ta has an isomer, 180mTa, with Ex = 0.075 MeV, Jp = 9– and
half-life value of 1015 years and it ranks amongst the largest half-lives
known. The isomers decay predominantly by electromagnetic processes (gamma decay or
internal conversion). There are also instances where it has been observed that the decay
is initiated by strong interaction (a or proton emission) or by weak
interaction (b-decay of electron capture).
Nuclear isomers are
usually produced through fusion reactions. When a swift moving heavy ion fuses with a
moderately heavy target nucleus, a compound nucleus is formed at high excitation energies
and with large angular momenta. The hot compound nucleus deexcites by emitting particles
and gamma rays. In this process, the decaying nucleus might populate an isomeric state and
its subsequent decay gets inhibited due to large spin change involved between the isomeric
state and the lower energy state(s). Recently, partial fusion or break-up fusion reactions
involving weakly bound projectiles (like 6,7Li, 9Be) have been found
to be not only favourable but perhaps the only way to populate some of the spin isomers3.
Scattering reactions utilizing the strong Coulombic interaction between two heavy nuclei
have also been employed to produce these metastable states.
Recently, there has
been renewed interest in the study of isomers, with the prospects of realizing enormous
amount of stored energy in nuclei through the accelerated decay of isomer. If one has to
manipulate the isomer to deexcite on a faster time scale, then it is essential to have a
better understanding of nuclear structure of not only the isomer but also the neighbouring
excited states. In principle, low energy (eV–keV) photons should be able to initiate
an accelerated decay of isomer either by stimulated emission to a nearby lower energy
state or absorption to a nearby higher energy state. In essence, the meta-stable state can
be made to shed its stored energy on a faster timescale, resulting in considerable energy
gain. From level density considerations, availability of nearby excited states having
favourable spin values through which isomer could decay, is expected to be more probable
in the case of isomers occurring at high excitation energies. Very recently encouraging
results have been reported in several efforts made to initiate a faster decay of isomer.
Collins et al.4 have observed
accelerated decay of 2.4 MeV, 31-year-old isomer in Hf (178mHf) induced by
~ 40 keV photons from a dental machine. They have reported increased decay rate
of the isomer by 4 ± 2%. According to Collins et al., this amounts to an energy gain of 60! The 229mTh
isomer at Ex = 3.5 ± 1.0 eV with a half-life of
45 h has received special attention because the excitation energy involved here is
similar to the values one encounters in atomic excitations. Shaw et al.5 have critically examined the UV
spectrum from 233U and found a transition at 391 nm. According to the authors,
this line could be a good candidate to be identified with the 3.5 eV isomer decay
from 229mTh initiated by light. These studies are of basic interest from the
point of view of understanding light (laser)–nucleus interaction. A complete
understanding of this will lead to greater prospects of realizing the stored energy of
isomer in a cheap way by use of low energy light sources. A beginning has been made in
comprehending the ‘isomer laboratory’ and the ongoing efforts to tap the
reservoir of stored energy through the isomer route hold promise for future.
1. de Shalit, A. and
Feshbach, H., Theoretical Nuclear Physics, John
Wiley and Sons, New York, 1990, vol. 1; Segre, E., Nuclei
and Particles, W. A. Benjamin, Inc., New York, 1964.
2. Goldhaber, M. and Hill,
R. D., Rev. Mod. Phys., 1952, 24, 179.
3. Walker, P. and
Dracoulis, G., Nature, 1999, 35, 399; Dracoulis, G., et al., J.
Phys. G. Nucl. Particle Phys., 1997, 23, 1191; Walker, P., Phys. World, May 1999, p. 19.
4. Collins, C. B. et al., Phys.
Rev. Lett., 1999, 82, 695.
5. Shaw, R. W., et al., Phys.
Rev. Lett., 1999, 82, 1109.
S. Kailas is in the Nuclear Physics
Division, Bhabha Atomic Research Centre, Mumbai 400 085, India.