30 research outputs found
Proton-Neutron Pairs in Heavy Deformed Nuclei
The microscopic justification of the emergence of SU(3) symmetry in heavy
nuclei remains an interesting problem. In the past, the pseudo-SU(3) approach
has been used, with considerable success. Recent results seem to suggest that
the key for understanding the emergence of SU(3) symmetry lies in the
properties of the proton-neutron interaction, namely in the formation of (S=1,
T=0) p-n pairs in heavy nuclei, especially when the numbers of valence protons
and valence neutrons are nearly equal. Although this idea has been around for
many years, since the introduction of the Federman-Pittel mechanism, it is only
recently that information about the p-n interaction could be obtained from
nuclear masses, which become available from modern facilities. Based on this
information, a new coupling scheme for heavy deformed nuclei has been suggested
and is under development.Comment: 12 pages, 3 table
Particle-hole symmetry breaking due to Pauli blocking
Particle-hole symmetry has been used on several occasions in nuclear
structure over the years. We prove that particle-hole symmetry is broken in
nuclear shells possessing the proxy-SU(3) symmetry. The breaking of the
symmetry is rooted in the Pauli principle and the short range nature of the
nucleon-nucleon interaction. The breaking of the symmetry explains the
dominance of prolate over oblate shapes in deformed nuclei and determines the
regions of prolate to oblate shape transitions in the nuclear chart.
Furthermore, it is related to the existence of specific regions of shape
coexistence across the nuclear chart, surrounded by regions in which shape
coexistence does not occur.Comment: 9 pages, 2 figures, to appear in HNPS: Advances in Nuclear Physics:
Proceedings of the 27th Annual Symposium of the Hellenic Nuclear Physics
Society (Athens, 2018), ed. T. Mertzimekis, G. Souliotis, and E. Styliari
Why do Nilsson quantum numbers remain good at moderate deformations?
The Nilsson model is a simple microscopic model which has been extensively
used over the years for the interpretation of a bulk of experimental results.
The single particle orbitals in this model are labeled by quantum numbers which
are good in the limit of large nuclear deformations. However, it is generally
admitted that these quantum numbers remain good even at moderate deformations.
We show that this fact is due to the existence of an underlying approximate
symmetry, called the proxy-SU(3) symmetry. The implications of proxy-SU(3) on
various aspects of nuclear structure will be discussed.Comment: 11 pages, 1 figure, to appear in Nuclear Theory '37, Proceedings of
the 37th International Workshop on Nuclear Theory (Rila 2018), ed. M.
Gaidarov and N. Minko
Nucleon numbers for nuclei with shape coexistence
We consider two competing sets of nuclear magic numbers, namely the harmonic
oscillator (HO) set (2, 8, 20, 40, 70, 112, 168, 240,...) and the set
corresponding to the proxy-SU(3) scheme, possessing shells 0-2, 2-4, 6-12,
14-26, 28-48, 50-80, 82-124, 126-182, 184-256... The two sets provide 0+ bands
with different deformation and band-head energies. We show that for proton
(neutron) numbers starting from the regions where the quadrupole-quadrupole
interaction, as derived by the HO, becomes weaker than the one obtained in the
proxy-SU(3) scheme, to the regions of HO shell closure, the shape coexistence
phenomenon may emerge. Our analysis suggests that the possibility for
appearance of shape coexistence has to be investigated in the following regions
of proton (neutron) numbers: 8, 18-20, 34-40, 60-70, 96-112, 146-168,
210-240,...Comment: 8 pages, 1 figure, to appear in HNPS: Advances in Nuclear Physics:
Proceedings of the 27th Annual Symposium of the Hellenic Nuclear Physics
Society (Athens, 2018), ed. T. Mertzimekis, G. Souliotis, and E. Styliari
Magic numbers for shape coexistence
The increasing deformation in atomic nuclei leads to the change of the
classical magic numbers (2,8,20,28,50,82..) which dictate the arrangement of
nucleons in complete shells. The magic numbers of the three-dimensional
harmonic oscillator (2,8,20,40,70...) emerge at deformations around
epsilon=0.6. At lower deformations the two sets of magic numbers antagonize,
leading to shape coexistence. A quantitative investigation is performed using
the usual Nilsson model wave functions and the recently introduced proxy-SU(3)
scheme.Comment: 9 pages, 4 figures, to appear in HNPS: Advances in Nuclear Physics:
Proceedings of the 27th Annual Symposium of the Hellenic Nuclear Physics
Society (Athens, 2018), ed. T. Mertzimekis, G. Souliotis, and E. Styliari
Proxy-SU(3) symmetry in heavy nuclei: Foundations
An approximate SU(3) symmetry appears in heavy deformed even-even nuclei, by
omitting the intruder Nilsson orbital of highest total angular momentum and
replacing the rest of the intruder orbitals by the orbitals which have escaped
to the next lower major shell. The approximation is based on the fact that
there is a one-to-one correspondence between the orbitals of the two sets,
based on pairs of orbitals having identical quantum numbers of orbital angular
momentum, spin, and total angular momentum. The accuracy of the approximation
is tested through calculations in the framework of the Nilsson model in the
asymptotic limit of large deformations, focusing attention on the changes in
selection rules and in avoided crossings caused by the opposite parity of the
proxies with respect to the substituted orbitals.Comment: 7 pages, 1 figure, 3 tables, Proceedings of the 4th Workshop of the
Hellenic Institute of Nuclear Physics on New Aspects and Perspectives in
Nuclear Physics (HINPw4),Ioannina, Greece, 5-6 May 2017, ed. A. Pako
A symmetry for heavy nuclei: Proxy-SU(3)
The SU(3) symmetry realized by J. P. Elliott in the sd nuclear shell is
destroyed in heavier shells by the strong spin-orbit interaction. However, the
SU(3) symmetry has been used for the description of heavy nuclei in terms of
bosons in the framework of the Interacting Boson Approximation, as well as in
terms of fermions using the pseudo-SU(3) approximation. We introduce a new
fermionic approximation, called the proxy-SU(3), and we comment on its
similarities and differences with the other approaches.Comment: 4 pages, 1 figure, Proceedings of the 4th Workshop of the Hellenic
Institute of Nuclear Physics on New Aspects and Perspectives in Nuclear
Physics (HINPw4),Ioannina, Greece, 5-6 May 2017, ed. A. Pako
Foundations of the proxy-SU(3) symmetry in heavy nuclei
We show that within the proxy-SU(3) scheme the wave functions of the normal
parity orbitals in a given nuclear shell are affected very little as a result
of the replacement of the abnormal parity orbitals by their 0[110] proxy-SU(3)
counterparts.Comment: 9 pages, 5 tables, to appear in the proceedings of the Workshop on
Shapes and Dynamics of Atomic Nuclei: Contemporary Aspects (SDANCA17, Sofia
2017), ed. N. Minko
Parameter-independent predictions for shape variables of heavy deformed nuclei in the proxy-SU(3) model
Using a new approximate analytic parameter-free proxy-SU(3) scheme, we make
predictions of shape observables for deformed nuclei, namely beta and gamma
deformation variables, and compare these with empirical data and with
predictions by relativistic and non-relativistic mean-field theories.Comment: 6 pages, 11 figures, Proceedings of the 4th Workshop of the Hellenic
Institute of Nuclear Physics on New Aspects and Perspectives in Nuclear
Physics (HINPw4),Ioannina, Greece, 5-6 May 2017, ed. A. Pako
Parameter free predictions within the proxy-SU(3) model
Using a new approximate analytic parameter-free proxy-SU(3) scheme, we make
predictions of shape observables for deformed nuclei, namely beta and gamma
deformation variables, and compare them with empirical data and with
predictions by relativistic and non-relativistic mean-field theories.
Furthermore, analytic expressions are derived for B(E2) ratios within the
proxy-SU(3) model, free of any free parameters, and/or scaling factors. The
predicted B(E2) ratios are in good agreement with the experimental data for
deformed rare earth nuclides.Comment: 12 pages, 5 figures, to appear in the proceedings of the Workshop on
Shapes and Dynamics of Atomic Nuclei: Contemporary Aspects (SDANCA17, Sofia
2017), ed. N. Minko