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Prompt Proton Decay of a
Well-Deformed Band in 58Cu
Atomic nuclei can assume a variety of shapes that range from spherical to deformed football-shaped, pancake-shaped, or sometimes pear-shaped. Spherical shapes are found at or near the shell closures, i.e. in nuclei having a "magic" number of protons or neutrons, while deformation sets in as one approaches the middle of the shells. Since the nucleus 58Cu has only one more proton and neutron than its doubly-magic neighbor 56Ni, it is an excellent example of a spherical nucleus.
The spherical states in 58Cu have their angular momenta generated by aligning the spins of the individual nucleons (j1, j2, etc.) along a common axis, as schematically shown in the accompanying figure (dark red). These states lie in the lowest or first minimum in the potential energy surface. However, well deformed states in 58Cu also exist, albeit at higher energies and spins, in the so-called second minimum of the energy potential. One such band, corresponding to collective rotation of the nucleus as a whole, was recently identified in 58Cu (depicted in dark blue in the figure). The very fast gamma rays that connect the adjacent, regularly spaced members of this collective sequence, or band, are shown in purple in the gamma-energy (Eg) spectrum in the bottom panel.
A unique and surprising feature of this band is its decay mode. Commonly nuclei that are trapped in the second minimum eventually make a transition to the first minimum by emitting gamma rays, although they may, in principle, also decay by emission of beta rays or even by fission when the nucleus is very heavy. However, in addition to the common mode of gamma decay, 58Cu was observed to decay to its neighboring nucleus 57Ni by emitting a proton. These two distinct decay modes may be inferred from the spectra (opposite). The 4171 keV gamma ray (pink) that links the deformed and spherical states in 58Cu and the subsequent gamma rays emitted from the spherical states (red) are clearly present (lower panel). But, surprisingly, gamma rays originating from 57Ni (green) are also observed in this spectrum. Their intensity indicates that some members of the collective band decay by emission of protons. This was confirmed experimentally by detection of a 2.4 MeV proton line (the blue peak in the upper right panel) that is emitted from the lowest member of the collective band in 58Cu. (The upper panel in this figure displays the broad proton energy (Ep) spectrum that is commonly observed in these reactions.) The final state in 57Ni that is fed by this proton decay may have some special properties since the decay does not populate other states (i.e. those within the yellow area) that could have also been fed.
This unprecedented decay mode represents a very interesting quantum mechanical process -- the emission of such a proton is forbidden in classical mechanics. Theoretical studies of this process will help elucidate how the proton "tunnels" through a potential energy barrier in a deformed nucleus.