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Fig. 1. Decay scheme of 180Ta
Click for larger image
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Nuclear reactions with high-spin excited states are among
the new topics of modern nuclear physics. Long-lived isomers
provide an opportunity for experimental study of structure peculiarities
in the level population of the reaction product and influence
of the initial quasiparticle configuration on the reaction cross-section
and angular distribution. These isomers usually have excitation
energies below 1 MeV and large angular momentum. But, attempts
to understand the long-lived highly excited isomers have been
hampered by the difficulty of producing this exotic form of
nuclear matter.
The predominant number of experiments with bremsstrahlung g
-rays and heavy ions have been carried out, as a rule, on stable
isotopes which have low ground state spins. Therefore, in photonuclear
reactions, for example, the states are excited with spin values
close to the ground state spin. The excitation mechanism of
these photo nuclear reactions is very well known for most isotopes
in the region of the Giant Dipole Resonance and these experimental
results are reflected in many reviews.
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However, one may essentially extend the possibilities of these
experiments if one uses a target of non-stable nuclei in isomeric
states. Nuclear experiments with high-spin isomers are important
to the investigation of nuclear structure effects. High-spin
targets may have significant absorption cross sections in comparison
with usual nuclei, due to the nuclear structure, possible alteration
of deformation and nuclear radius, and also the level density
of the compound nucleus. It is very important to establish the
existence of selective population levels in the residual nucleus
with structure similar to that of the initial nucleus. When
the initial nucleus has a high spin, one may expect preferable
populations of high spin levels in the residual nucleus. The
simplest method for obtaining the experimental information is
measuring the transition probability from the isomeric to the
ground state (the isomeric ratio).
At present, only one stable isotope (180mTa(Jp
=9-)) is available as a high-spin target and this
180mTa target has been used in a few interesting
experiments. The 180mTa being the only isomer abundant
in nature has a half-life of > 1015 years, abundance
of 1.2 10-4 in natural tantalum, spin and parity
9- and excitation energy of 73 keV. This isomeric
ratio at the de-excitation of 180Ta is practically
the same as that of the excitation of the isomers, with a small
difference between the ground and isomeric states (D
J=3-4). The large de-excitation cross-section of these
states in inelastic g -scattering
allows one to hope for successful measurements of a small number
of atoms (about 1015) in experiments with isomeric
targets. The interest in nuclear reactions with high-spin isomers
has been enhanced by the theoretical consideration of the K-mixing
in excited nuclei and by the possibility of an efficient pumping
process in the g -laser problem.
In the early 1990’s the idea of nuclear reactions with high-spin
isomeric targets 178m2Hf was conceived at the Joint
Institute for Nuclear Research, Russia. This four-quasiparticle
isomer (Jp =16+)
has an 2.45 MeV excitation energy and long enough half-life
(T1/2=31 y), suitable for its accumulation and use
as a target. Scientist did not doubt the physical productivity
of such studies, however enormous difficulties exist in the
methods of 178m2Hf production, purification, and
especially mass-separation of isomer from the ground state and
its target preparation.
This kind of experimental activity will start the new class
of photonuclear reactions on target nuclei with high angular
momentum and allow us studying fundamental questions in nuclear
physics. This proposal has not only pure nuclear significance
but is also related to the problem of gamma laser on nuclear
states and nuclear waste management.
Publication
A.P. Tonchev, Yu.P. Gangrsky, A.G. Belov. New Candidates for
High Spin Isomeric Targets. Nucl. Instr. and Methods in Phys.
Res. A, 422(1999) 532-535.
Yu.P. Gangrskii, V.E. Zhuchko, Novgorodov A.F., Tonchev
A.P. Isomers High-Enriched Targets for Nuclear Reactions. Radiochemistry,
40 (1998) 434-437. |
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