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In-Beam Spectroscopy of the Neutron Deficient Nuclei 92Pd and 162Ta
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Physics.
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Studies of nuclei far from stability offers new insights into the complete nucleon many-body problem. In nuclei with equal neutron and proton numbers (N=Z), the unique nature of the atomic nucleus as an object composed of two distinct types of fermions can be expressed as enhanced correlations arising between neutrons and protons occupying orbitals with the same quantum numbers. Such correlations have since several decades been predicted to favour a new type of nuclear superfluidity; isoscalar neutron-proton pairing, in addition to normal isovector pairing which dominates the structure of most known nuclei. Despite many experimental efforts these predictions have not been confirmed. The N=Z nuclei with mass number A>90 can only be produced in the laboratory at very low cross sections. The related problems of identifying and distinguishing such reaction products and their associated gamma rays from the vast array of N>Z nuclei that are present in much greater numbers have prevented observation of their low-lying excited states until recently. In the present work the experimental difficulties of observation of excited states in the N=Z=46 nucleus 92Pd have been overcome through the use of a highly efficient, state-of-the-art detector system and a prolonged experimental running period. The lowest excited states in 92Pd was empirically observed via detection of gamma rays emitted in the fusion-evaporation reaction together with detection of charged particles and neutrons in the ancillary detector system. The level spacings in the ground state band of 92Pd give the first experimental evidence for a new spin-aligned neutron-proton (np) paired phase. These findings reconcile with nuclear shell model calculations which predicts an unexpected effect of enhanced np correlations for N=Z nuclei in the immediate vicinity of the doubly magic nucleus 100Sn. Excited states of the odd-odd nucleus 162Ta have been observed using the JUROGAM/RITU experimental set-up. This nucleus is located in a transitional region in the nuclide chart which is between near-spherical nuclei and well-deformed nuclei, offering the possibility to study the emergence of collective phenomena and nuclear deformation (in particular the degree of triaxiality). The results, which are interpreted in the framework of the cranked shell model with total Routhian surface calculations suggest an almost axially symmetric nuclear shape. The energy staggering between the signature partners of the yrast rotational bands has been deduced for eight odd-odd isotopes in the neighborhood of 162Ta nucleus and the special observed feature of signature inversion for these nuclei is discussed.


Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , viii, 48 p.
Trita-FYS, ISSN 0280-316X ; 2011:61
National Category
Other Physics Topics
URN: urn:nbn:se:kth:diva-50501ISBN: 978-91-7501-210-0OAI: diva2:462182
2011-12-20, FD5, AlbaNova, Roslagstullsbacken 21, Stockholm, 10:30 (English)
QC 20111212Available from: 2011-12-12 Created: 2011-12-06 Last updated: 2011-12-12Bibliographically approved
List of papers
1. Evidence for a spin-aligned neutron-proton paired phase from the level structure of 92Pd
Open this publication in new window or tab >>Evidence for a spin-aligned neutron-proton paired phase from the level structure of 92Pd
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2011 (English)In: Nature, ISSN 0028-0836, Vol. 469, no 7328, 68-71 p.Article in journal (Refereed) Published
Abstract [en]

Shell structure and magic numbers in atomic nuclei were generally explained by pioneering work(1) that introduced a strong spin-orbit interaction to the nuclear shell model potential. However, knowledge of nuclear forces and the mechanisms governing the structure of nuclei, in particular far from stability, is still incomplete. In nuclei with equal neutron and proton numbers (N = Z), enhanced correlations arise between neutrons and protons (two distinct types of fermions) that occupy orbitals with the same quantum numbers. Such correlations have been predicted to favour an unusual type of nuclear superfluidity, termed isoscalar neutron-proton pairing(2-6), in addition to normal isovector pairing. Despite many experimental efforts, these predictions have not been confirmed. Here we report the experimental observation of excited states in the N = Z = 46 nucleus Pd-92. Gamma rays emitted following the Ni-58(Ar-36,2n)Pd-92 fusion-evaporation reaction were identified using a combination of state-of-the-art high-resolution c-ray, charged-particle and neutron detector systems. Our results reveal evidence for a spin-aligned, isoscalar neutron-proton coupling scheme, different from the previous prediction(2-6). We suggest that this coupling scheme replaces normal superfluidity (characterized by seniority coupling(7,8)) in the ground and low-lying excited states of the heaviest N = Z nuclei. Such strong, isoscalar neutron-proton correlations would have a considerable impact on the nuclear level structure and possibly influence the dynamics of rapid proton capture in stellar nucleosynthesis.

Shell-Model Description, Generalized Seniority, Rich Nuclei, Isospin, States
National Category
Physical Sciences
urn:nbn:se:kth:diva-28905 (URN)10.1038/nature09644 (DOI)000285921600032 ()2-s2.0-78650974098 (ScopusID)
QC 20110501Available from: 2011-05-02 Created: 2011-01-24 Last updated: 2014-02-17Bibliographically approved
2. High-spin study of 162Ta
Open this publication in new window or tab >>High-spin study of 162Ta
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2011 (English)In: Physical Review C. Nuclear Physics, ISSN 0556-2813, E-ISSN 1089-490X, Vol. 84, no 6, 064312- p.Article in journal (Refereed) Published
Abstract [en]

Excited states in the odd-odd neutron deficient nucleus (162)Ta (Z = 73, N = 89) have been studied for the first time. The gamma spectroscopy analysis using gamma - gamma - gamma coincidences revealed a strongly coupled rotational structure that was established up to large angular momentum states. The rotational band was assigned to the configuration pi h(11/2)[514]9/2 circle times nu i(13/2)[660]1/2 based on its rotational and electromagnetic properties. The data are interpreted within the framework of total Routhian surface calculations, which suggests an axially symmetric shape with a gamma-softminimum at beta(2) approximate to 0.16 and gamma approximate to 6 degrees. The crossing of the signature partners observed in heavier (N >= 91) odd-odd nuclides in this mass region is found to be absent at N = 89. This might be correlated with a change in S-band structure above the paired band crossing at these neutron numbers.

National Category
Subatomic Physics
Research subject
SRA - E-Science (SeRC)
urn:nbn:se:kth:diva-51357 (URN)10.1103/PhysRevC.84.064312 (DOI)000298130400001 ()2-s2.0-84855366031 (ScopusID)
Swedish e‐Science Research Center

QC 20120210

Available from: 2011-12-12 Created: 2011-12-12 Last updated: 2015-04-29Bibliographically approved

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