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Idaho Accelerator Home Nuclear Reactions on High-Spin Isomeric Targets
 

Fig. 1. Decay scheme of 180Ta
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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.



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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