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Experimental Nuclear Structure Studies in the Vicinityof the N = Z Nucleus 100Sn and in the ExtremelyNeutron Deficient 162Ta Nucleus
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Physics.ORCID iD: 0000-0001-6039-2067
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
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.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. Vol. 89, no 014301, viii, 75 p.0143011-0143019 p.
Series
TRITA-FYS, ISSN 0280-316X ; 14:02
Keyword [en]
Nuclear structure
National Category
Other Physics Topics
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-141421ISBN: 978-91-7595-007-5 (print)OAI: oai:DiVA.org:kth-141421DiVA: diva2:696728
Public defence
2014-02-28, FB52, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20140217

Available from: 2014-02-14 Created: 2014-02-14 Last updated: 2014-05-16Bibliographically 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, E-ISSN 1476-4687, 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.

Keyword
Shell-Model Description, Generalized Seniority, Rich Nuclei, Isospin, States
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-28905 (URN)10.1038/nature09644 (DOI)000285921600032 ()2-s2.0-78650974098 (Scopus ID)
Note
QC 20110501Available from: 2011-05-02 Created: 2011-01-24 Last updated: 2017-12-11Bibliographically 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)
Identifiers
urn:nbn:se:kth:diva-51357 (URN)10.1103/PhysRevC.84.064312 (DOI)000298130400001 ()2-s2.0-84855366031 (Scopus ID)
Funder
Swedish e‐Science Research Center
Note

QC 20120210

Available from: 2011-12-12 Created: 2011-12-12 Last updated: 2017-12-08Bibliographically approved
3. Character of particle-hole excitations in Ru-94 deduced from gamma-ray angular correlation and linear polarization measurements
Open this publication in new window or tab >>Character of particle-hole excitations in Ru-94 deduced from gamma-ray angular correlation and linear polarization measurements
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2014 (English)In: Physical Review C. Nuclear Physics, ISSN 0556-2813, E-ISSN 1089-490X, Vol. 89, no 1, 0143011-0143019 p.Article in journal (Refereed) Published
Abstract [en]

Linear polarization and angular correlations of γ-rays depopulating excited states in the neutron-deficient nucleus 9444Ru50 have been measured, enabling firm spin-parity assignments for several excited states in this nucleus. The deduced multipolarities of strong transitions in the yrast structure were found to be mostly of stretched M1, E1, and E2 types and, in most cases, in agreement with previous tentative assignments. The deduced multipolarity of the 1869 keV and the connecting 257 and 1641 keV transitions indicates that the state at 6358 keV excitation energy has spin parity 12−1 rather than 12+3 as proposed in previous works. The presence of a 12−1 state is interpreted within the framework of large-scale shell-model calculations as a pure proton-hole state dominated by the π(p−11/2⊗g−59/2) and π(p−13/2⊗g−59/2) configurations. A new positive-parity state is observed at 6103 keV and is tentatively assigned as 12+2. The 14−1 state proposed earlier is reassigned as 13−4 and is interpreted as being dominated by neutron particle-hole core excitations. The strengths of several E1 transitions have been measured and are found to provide a signature of core-excited configurations.

Keyword
Gamma-ray spectrosopy-polarization
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-141458 (URN)10.1103/PhysRevC.89.014301 (DOI)000332148300001 ()2-s2.0-84894470659 (Scopus ID)
Funder
Swedish Research Council, 621-2010-4723 621-2012-3805EU, FP7, Seventh Framework Programme, 262010
Note

QC 20140217. Author count: 35

Available from: 2014-02-17 Created: 2014-02-17 Last updated: 2017-12-06Bibliographically approved
4. Spectroscopy of the neutron-deficient N=50 nucleus Rh-95
Open this publication in new window or tab >>Spectroscopy of the neutron-deficient N=50 nucleus Rh-95
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2014 (English)In: Physical Review C. Nuclear Physics, ISSN 0556-2813, E-ISSN 1089-490X, Vol. 89, no 4, 044310- p.Article in journal (Refereed) Published
Abstract [en]

The neutron-deficient semimagic (neutron number N = 50) Rh-95 nucleus has been produced at high spins using the projectile-target system Ca-40 + Ni-58 at 125 MeV beam energy. The gamma-decays of levels populated by the 3p fusion evaporation reaction channel were studied using gamma-gamma coincidences, and 20 new gamma-ray transitions involving 15 new positive-and negative-parity states were observed. Spin and parity for many of the excited states were firmly deduced for the first time using the combined directional angular correlation and direction-polarization techniques. The observed structures are discussed within the framework of large-scale shell model calculations. E1 transition strengths were deduced and used together with the results of the shell model calculations to study the contribution of different particle-hole configurations, in particular for analyzing contributions from core-excited configurations.

Keyword
Level Structure, States
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-145265 (URN)10.1103/PhysRevC.89.044310 (DOI)000334300300002 ()2-s2.0-84899687197 (Scopus ID)
Funder
Swedish Research Council, 621-2010-4723 621-2012-3805EU, FP7, Seventh Framework Programme, 262010
Note

QC 20140516. Updated from manuscript to article in journal.

Available from: 2014-05-16 Created: 2014-05-15 Last updated: 2017-12-05Bibliographically approved
5. Lifetime measurement of the first excited 2(+) state in (108)Te
Open this publication in new window or tab >>Lifetime measurement of the first excited 2(+) state in (108)Te
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2011 (English)In: Physical Review C. Nuclear Physics, ISSN 0556-2813, E-ISSN 1089-490X, Vol. 84, no 4, 041306- p.Article in journal (Refereed) Published
Abstract [en]

The lifetime of the first excited 2(+) state in the neutron deficient nuclide (108)Te has been measured for the first time, using a combined recoil decay tagging and recoil distance Doppler shift technique. The deduced reduced transition probability is B(E2;0(g.s.)(+) -> 2(+)) = 0.39(-0.04)(+0.05)e(2)b(2). Compared to previous experimental data on neutron deficient tellurium isotopes, the new data point constitutes a large step (six neutrons) toward the N = 50 shell closure. In contrast to what has earlier been reported for the light tin isotopes, our result for tellurium does not show any enhanced transition probability with respect to the theoretical predictions and the tellurium systematics including the new data is successfully reproduced by state-of-the-art shell model calculations.

Keyword
TOTAL DATA READOUT, SPECTROMETERS, SPECTROSCOPY
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-52555 (URN)10.1103/PhysRevC.84.041306 (DOI)000296521000001 ()2-s2.0-82155162041 (Scopus ID)
Funder
Swedish Research Council, 623-2009-7340Swedish Research Council, 621-2010-3694Swedish Research Council, 2010-4723EU, FP7, Seventh Framework Programme, 262010
Note
QC 20111219Available from: 2011-12-19 Created: 2011-12-19 Last updated: 2017-12-08Bibliographically approved
6. Transition probabilities near Sn-100 and the stability of the N, Z=50 shell closure
Open this publication in new window or tab >>Transition probabilities near Sn-100 and the stability of the N, Z=50 shell closure
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2013 (English)In: Physical Review C. Nuclear Physics, ISSN 0556-2813, E-ISSN 1089-490X, Vol. 87, no 3, 031306- p.Article in journal (Refereed) Published
Abstract [en]

Recent B(E2; 0(g.s.)(+) -> 2(1)(+)) measurements in light tin isotopes have revealed surprisingly large values relative to standard shell model predictions, generating an unexpected asymmetry in the B(E2) values with respect to the neutron midshell. This effect has triggered various speculations as to its origin, such as a possible weakening of the N, Z = 50 shell closure. Here we present new shell model calculations to investigate the origin of the observed asymmetric character of the B(E2) values in the tin isotopes. By including the effects of the neutron g(9/2) orbital below the N = 50 shell gap it is shown that Pauli blocking effects may play an important role near the N = 50 shell closure. A new set of single-particle energies and monopole interactions, fitted to the experimental data in the region, together with the isospin-dependent effective charge suggested by Bohr and Mottelson is shown to reproduce the experimental transition rate values in the Sn isotopic chain.

Keyword
Generalized Seniority, Nuclei, Isotopes, Sn
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-121472 (URN)10.1103/PhysRevC.87.031306 (DOI)000316672700001 ()2-s2.0-84876008263 (Scopus ID)
Funder
Swedish Research Council, 621-2012-3805 621-2010-3694 621-2010-4723
Note

QC 20130502

Available from: 2013-05-02 Created: 2013-04-29 Last updated: 2017-12-06Bibliographically approved
7. gamma-ray linear polarization measurements and (g(9/2))(-3) neutron alignment in Ru-91
Open this publication in new window or tab >>gamma-ray linear polarization measurements and (g(9/2))(-3) neutron alignment in Ru-91
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2013 (English)In: Physical Review C. Nuclear Physics, ISSN 0556-2813, E-ISSN 1089-490X, Vol. 87, no 4, 044328- p.Article in journal (Refereed) Published
Abstract [en]

Linear polarization measurements have been performed for gamma rays in Ru-91 produced with the Ni-58(Ar-36,2p1n gamma)Ru-91 reaction at a beam energy of 111 MeV. The EXOGAM Ge clover array has been used to measure the gamma-gamma coincidences, gamma-ray linear polarization, and gamma-ray angular distributions. The polarization sensitivity of the EXOGAM clover detectors acting as Compton polarimeters has been determined in the energy range 0.3-1.3 MeV. Several transitions have been observed for the first time. Measurements of linear polarization and angular distribution have led to the firm assignments of spin differences and parity of high-spin states in Ru-91. More specifically, calculations using a semiempirical shell model were performed to understand the structures of the first and second (21/2(+)) and (17/2(+)) levels. The results are in good agreement with the experimental data, supporting the interpretation of the nonyrast (21/2(+)) and (17/2(+)) states in terms of the J(max) and J(max) - 2 members of the seniority-three nu(g(9/2))(-3) multiplet.

Keyword
High-Spin States, Compton Polarimeter, Level Structure, Beta-Decay, Spectroscopy, Detector, Nuclei, Spectrometer, Scattering, Moments
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-122512 (URN)10.1103/PhysRevC.87.044328 (DOI)000317866200004 ()2-s2.0-84877130355 (Scopus ID)
Funder
Swedish Research Council, 2007-4067 2008-5793
Note

QC 20130523

Available from: 2013-05-23 Created: 2013-05-23 Last updated: 2017-12-06Bibliographically approved

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