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Collective Excitations in Transitional Nuclei Studied by Means of gamma-ray Spectroscopy and Lifetime Measurements
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Physics.ORCID iD: 0000-0003-2843-0582
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Despite that it is more than 100 years since the atomic nucleus was first dis- covered by Ernest Rutherford and coworkers, many of its features still elude our understanding. The fact that the fundamental interactions between the nuclear constituents; nucleons, and ultimately quarks, are not yet known in detail, and the complexity of the nuclear many-body system compound the great challenges facing theoretical interpretations of experimental data. It is therefore important to focus on distinct phenomena where experimental mea- surements can be compared with theoretical predictions, providing stringent tests of theory. One such area is the nuclear phenomenology of collective excitations related to rotations and vibrations of the nucleus as a whole, and how such modes of excitation may develop from the interactions between a few nucleons occupying single-particle orbits outside closed shells.

This thesis is devoted to experimental studies of excited states in the 99Tc, 162W, and 166Re nuclei. These nuclei lie in “transitional” regions of the Segrè chart, where collective excitation mechanisms start becoming important when adding valence nucleons outside closed neutron and proton shells. Such nuclei are important for testing state-of-the-art theoretical models. The excited states of the nuclei studied in the present work were populated using heavy-ion fusion-evaporation reactions. Prior to the present work, high-spin data were still lacking in 99Tc since in the previous works [1–5] excited states were only populated up to around 3 MeV. Two collective bands have been extended to intermediate angular momentum states in the present work. The experimental results were compared with the systematics of other technetium isotopes, evaluating electromagnetic characteristics based on semiclassical calculations and the particle-rotor model. The 162W and 166Re nuclei are situated in the very neutron-deficient 160-170 mass region, requiring special techniques for identifying their excited-state structures. The level scheme of 162W was first reported by Dracoulis et al. in 1993 [6], where the first excited 2+ state was assigned to be 450 keV above the ground state. However, the lack of selectivity made the γ-ray identification for higher energy levels of 162W uncertain. In this work, the highly selective recoil-decay tagging technique was used to uniquely identify γ-ray transitions from excited states in 162W and to construct its level scheme. In addition, the experimental setup enabled a precise determination of the half-life of the α-decaying ground state of 162W. The α-formation probability for 162W was extracted from the measurement of the half-life and systematical comparisons with the neighboring nuclei were performed. Two rotational-like bands were identified in 166Re for the first time and the lifetimes of the lowest three excited states in band (1) were measured using the recoil distance Doppler shift method. The results were compared with theoretical calculations based on a semiclassical approach, the particle-rotor model, and the tilted axis cranking model in a relativistic mean field approach. The microscopic mechanisms (configuration and alignment, etc) of the rotational bands were interpreted under the framework of total Routhian surface predictions and cranked shell model calculations.

 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , xiv, 52 p.
Series
TRITA-FYS, ISSN 0280-316X ; 2015:31
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-170923ISBN: 978-91-7595-622-0 OAI: oai:DiVA.org:kth-170923DiVA: diva2:841156
Public defence
2015-08-25, Sal FB54, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:10 (English)
Opponent
Supervisors
Note

QC 20150710

Available from: 2015-07-10 Created: 2015-07-10 Last updated: 2017-02-23Bibliographically approved
List of papers
1. Collective band structures in the Tc-99 nucleus
Open this publication in new window or tab >>Collective band structures in the Tc-99 nucleus
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2015 (English)In: Physical Review C. Nuclear Physics, ISSN 0556-2813, E-ISSN 1089-490X, Vol. 91, no 5, 054314Article in journal (Refereed) Published
Abstract [en]

Excited states in Tc-99 with energies up to 6 MeV have been populated using the Zr-96(Li-7, 4n)Tc-99 reaction with a laboratory beam energy of 35 MeV. Coincident gamma rays from excited nuclei produced in the reactions were detected using an array of coaxial, planar, and clover-type high-purity germanium detectors. A total of 60 new gamma-ray transitions and 21 new levels are identified and placed into a new level scheme. Two collective bands assigned to be built on the pi g(9/2)[422]5/2(+) and pi p(1/2)[301]1/2(-) Nilsson configurations have been extended with spins up to 35/2 and 33/2 h, respectively. Backbending and signature inversion have been observed in the yrast band. The large signature splitting of the positive-parity band in Tc-99 may be caused by a triaxial deformation, which agrees well with the electromagnetic properties, theoretical calculations based on total Routhian surface, and triaxial particle-rotor model calculations.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-168692 (URN)10.1103/PhysRevC.91.054314 (DOI)000354558000001 ()2-s2.0-84929359307 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20150608

Available from: 2015-06-08 Created: 2015-06-08 Last updated: 2017-12-04Bibliographically approved
2. First identification of rotational band structures in Re-166(75)91
Open this publication in new window or tab >>First identification of rotational band structures in Re-166(75)91
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2015 (English)In: Physical Review C. Nuclear Physics, ISSN 0556-2813, E-ISSN 1089-490X, Vol. 92, no 1, 014310Article in journal (Refereed) Published
Abstract [en]

Excited states in the odd-odd, highly neutron-deficient nucleus Re-166 have been investigated via the Mo-92(Kr-78, 3p1n)Re-166 reaction. Prompt gamma rays were detected by the JUROGAM II. gamma-ray spectrometer, and the recoiling fusion-evaporation products were separated by the recoil ion transport unit (RITU) gas-filled recoil separator and implanted into the Gamma Recoil Electron Alpha Tagging spectrometer located at the RITU focal plane. The tagging and coincidence techniques were applied to identify the gamma-ray transitions in Re-166, revealing two collective, strongly coupled rotational structures, for the first time. The more strongly populated band structure is assigned to the pi h(11/2)[514]9/2(-) circle times vi(13/2)[660]1/2(+) Nilsson configuration, while the weaker structure is assigned to be built on a two-quasiparticle state of mixed pi h(11/2)[514]9/2(-) circle times v[h(9/2)f(7/2)]3/2(-) character. The configuration assignments are based on the electromagnetic characteristics and rotational properties, in comparison with predictions from total Routhian surface and particle-rotor model calculations.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-171888 (URN)10.1103/PhysRevC.92.014310 (DOI)000358021600002 ()
Funder
Swedish Research Council, 621-2010-3694
Available from: 2015-08-14 Created: 2015-08-10 Last updated: 2017-12-04Bibliographically approved
3. Recoil-decay tagging spectroscopy of 16274W88
Open this publication in new window or tab >>Recoil-decay tagging spectroscopy of 16274W88
(English)In: Physical Review C. Nuclear Physics, ISSN 0556-2813, E-ISSN 1089-490XArticle in journal (Refereed) Accepted
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-170928 (URN)
Note

QP 201507

Available from: 2015-07-10 Created: 2015-07-10 Last updated: 2017-12-04Bibliographically approved
4. Lifetime measurements in166Re: collective vs magnetic rotation
Open this publication in new window or tab >>Lifetime measurements in166Re: collective vs magnetic rotation
(English)Manuscript (preprint) (Other academic)
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-170929 (URN)
Note

QS 2015

Available from: 2015-07-10 Created: 2015-07-10 Last updated: 2015-07-10Bibliographically approved

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