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Evidence for a spin-aligned neutron-proton paired phase from the level structure of 92Pd
KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnfysik.ORCID-id: 0000-0003-1771-2656
KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnfysik.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnfysik.ORCID-id: 0000-0003-1996-0805
KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnfysik.
Vise andre og tillknytning
2011 (engelsk)Inngår i: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 469, nr 7328, s. 68-71Artikkel i tidsskrift (Fagfellevurdert) 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.

sted, utgiver, år, opplag, sider
2011. Vol. 469, nr 7328, s. 68-71
Emneord [en]
Shell-Model Description, Generalized Seniority, Rich Nuclei, Isospin, States
HSV kategori
Identifikatorer
URN: urn:nbn:se:kth:diva-28905DOI: 10.1038/nature09644ISI: 000285921600032Scopus ID: 2-s2.0-78650974098OAI: oai:DiVA.org:kth-28905DiVA, id: diva2:414099
Merknad
QC 20110501Tilgjengelig fra: 2011-05-02 Laget: 2011-01-24 Sist oppdatert: 2017-12-11bibliografisk kontrollert
Inngår i avhandling
1. In-Beam Spectroscopy of the Neutron Deficient Nuclei 92Pd and 162Ta
Åpne denne publikasjonen i ny fane eller vindu >>In-Beam Spectroscopy of the Neutron Deficient Nuclei 92Pd and 162Ta
2011 (engelsk)Licentiatavhandling, med artikler (Annet vitenskapelig)
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.

 

sted, utgiver, år, opplag, sider
Stockholm: KTH Royal Institute of Technology, 2011. s. viii, 48
Serie
Trita-FYS, ISSN 0280-316X ; 2011:61
HSV kategori
Identifikatorer
urn:nbn:se:kth:diva-50501 (URN)978-91-7501-210-0 (ISBN)
Presentation
2011-12-20, FD5, AlbaNova, Roslagstullsbacken 21, Stockholm, 10:30 (engelsk)
Opponent
Veileder
Merknad
QC 20111212Tilgjengelig fra: 2011-12-12 Laget: 2011-12-06 Sist oppdatert: 2011-12-12bibliografisk kontrollert
2. Experimental Nuclear Structure Studies in the Vicinityof the N = Z Nucleus 100Sn and in the ExtremelyNeutron Deficient 162Ta Nucleus
Åpne denne publikasjonen i ny fane eller vindu >>Experimental Nuclear Structure Studies in the Vicinityof the N = Z Nucleus 100Sn and in the ExtremelyNeutron Deficient 162Ta Nucleus
2014 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

This work covers spectroscopic studies of nuclei from different regions of the Segré chart whose properties illustrate the delicate balance between the forces in the atomic nucleus. Studies of nuclei far from stability offer new insights into the complex 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. The bound 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 have prevented a firm interpretation of their structure even for the lowest 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; the EXOGAM-Neutron Wall-DIAMANT setup, and a prolonged experimental running period. 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, an unexpected effect of enhanced np correlations for N = Z nuclei in the immediate vicinity of the doubly magic nucleus 100Sn.

Excited states in 94Ru and 95Rh nuclei close to the double magic shell  Z = N = 50 have been studied in order to untangle the ambiguity of the spin and the parity of the lowest-lying states. The observed yrast structures are compared to results of large-scale shell model (LSSM) calculations and the strengths of hindered E1 transitions are used as a sensitive test of the LSSM parameters. The effect of single-particle-hole excitations is discussed in terms of the strength of hindered E1 transitions.

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.

sted, utgiver, år, opplag, sider
Stockholm: KTH Royal Institute of Technology, 2014. s. viii, 75
Serie
TRITA-FYS, ISSN 0280-316X ; 14:02
Emneord
Nuclear structure
HSV kategori
Forskningsprogram
Fysik
Identifikatorer
urn:nbn:se:kth:diva-141421 (URN)978-91-7595-007-5 (ISBN)
Disputas
2014-02-28, FB52, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (engelsk)
Opponent
Veileder
Merknad

QC 20140217

Tilgjengelig fra: 2014-02-14 Laget: 2014-02-14 Sist oppdatert: 2014-05-16bibliografisk kontrollert

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