Exotic Nuclear Excitations: The Transverse Wobbling Mode in 135 PR

ISBN-13: 9783319532394 / Angielski / Twarda / 2017 / 112 str.

James Till Matta

Opis

While chirality has been observed in numerous nuclei from many regions of the nuclear chart, wobbling, prior to this work, had only been observed at high spins in super deformed bands in five nuclei confined to the Z~70, N~90 region. Additionally, this dissertation establishes a new interpretation for the wobbling phenomenon.

Spis treści

CHAPTER 1: INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1 Rotation of Deformed Nuclei . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 Axial Deformation . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.2 Triaxiality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1.2.1 Chirality . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1.2.2 Wobbling . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1.2.3 Triaxiality in the A 130 Region . . . . . . . . . . . 4

1.2 Previous Cases of Wobbling . . . . . . . . . . . . . . . . . . . . . . . 5

1.3 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

CHAPTER 2: NUCLEAR MODELS FOR HIGH SPIN PHENOMENA . . . . 10

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2 The Shell Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3 The Deformed Shell Model . . . . . . . . . . . . . . . . . . . . . . . . 15

2.3.1 Parameterization of Deformation . . . . . . . . . . . . . . . . 15

2.3.2 The Nilsson Model . . . . . . . . . . . . . . . . . . . . . . . . 17

2.4 Collective Rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.4.1 Rigid Triaxial Rotor Model (TRM) . . . . . . . . . . . . . . . 20

2.4.2 Quasiparticle + Triaxial Rotor (QTR) . . . . . . . . . . . . . 25

2.4.3 Pairing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.4.4 Tilted Axis Cranking . . . . . . . . . . . . . . . . . . . . . . . 29

2.4.4.1 Rotation About A Principal Axis . . . . . . . . . . . 33

2.4.4.2 Rotation About A Tilted Axis . . . . . . . . . . . . . 35

2.5 The Wobbling Mode in Nuclei . . . . . . . . . . . . . . . . . . . . . . 36

2.5.1 Models of the Wobbling Mode . . . . . . . . . . . . . . . . . . 36

2.5.1.1 Simple Wobbling . . . . . . . . . . . . . . . . . . . . 36

2.5.1.2 Transverse Wobbling . . . . . . . . . . . . . . . . . . 39

2.5.1.3 Longitudinal Wobbling . . . . . . . . . . . . . . . . . 42

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2.5.2 Signatures of Wobbling . . . . . . . . . . . . . . . . . . . . . . 43

CHAPTER 3: EXPERIMENTAL METHODS . . . . . . . . . . . . . . . . . . 44

3.1 Heavy-ion Fusion-evaporation Reaction . . . . . . . . . . . . . . . . . 44

3.1.1 Creation and Decay of the Compound Nucleus . . . . . . . . . 44

3.1.2 Choice of Beam and Target . . . . . . . . . . . . . . . . . . . 48

3.2 Gamma-ray Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . 49

3.2.1 Gamma-ray Interaction with Matter . . . . . . . . . . . . . . 49

3.2.2 High Purity Germanium (HPGe) Detectors . . . . . . . . . . . 53

3.2.3 Escape Suppression with BGO Detectors . . . . . . . . . . . . 58

3.2.4 The Gammasphere Detector Array . . . . . . . . . . . . . . . 60

3.2.5 Indian National Gamma Array (INGA) . . . . . . . . . . . . . 62

3.3 Experimental Details . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

3.3.1 ATLAS/Gammasphere . . . . . . . . . . . . . . . . . . . . . . 65

3.3.2 TIFR - BARC Pelletron LINAC / INGA . . . . . . . . . . . . 66

3.4 Data Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

3.4.1 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

3.4.2 Level Scheme Scheme Determination . . . . . . . . . . . . . . 73

3.4.3 Background Subtraction . . . . . . . . . . . . . . . . . . . . . 75

3.4.3.1 Symmetric Gates . . . . . . . . . . . . . . . . . . . . 75

3.4.3.2 Asymmetric Gates . . . . . . . . . . . . . . . . . . . 76

3.5 Directional Correlation of Gamma-rays from Oriented Nuclei (DCO) . 80

3.5.1 Angular Distributions . . . . . . . . . . . . . . . . . . . . . . 84

3.5.2 DCO Ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

3.5.3 Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

CHAPTER 4: TRANSVERSE WOBBLING IN 135Pr . . . . . . . . . . . . . . 90

4.1 Level Scheme of 135Pr . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

4.1.1 Angular Distributions and DCO-like Ratios . . . . . . . . . . 98

4.1.2 Polarizations . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

4.2 Description by Theory . . . . . . . . . . . . . . . . . . . . . . . . . . 103

4.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

CHAPTER 5: SUMMARY AND OUTLOOK . . . . . . . . . . . . . . . . . . 111

APPENDIX A: GAMMASPHERE RING AND DETECTOR INFORMATION 113

APPENDIX B: INGA RING AND DETECTOR INFORMATION . . . . . . 117

APPENDIX C: NEGATIVE PARITY LEVEL AND TRANSITION INFORMATION

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . .

 

O autorze

Dr James Matta was awarded the PhD degree by the University of Notre Dame in 2015, and is now with Oak Ridge National Laboratory.

Blurb

The work presented in this thesis established the existence of wobbling at low spin and low deformation in the Z~60, N~76 nuclear region.  This opens the region to further searches for wobbling and shows that wobbling is not confined to a particular quasiparticle orbital, spin or deformation. 

While deformed nuclei usually have axial shape, triaxial shapes have been predicted at low to moderate spins in certain regions of the nuclear chart (e.g. Z~60, N~76 and Z~46, N~66).  Observation of one of the fingerprints of triaxiality, chirality and wobbling, guarantees that the nucleus is axially asymmetric.  While chirality has been observed in numerous nuclei from many regions of the nuclear chart, wobbling, prior to this work, had only been observed at high spins in super deformed bands in five nuclei confined to the Z~70, N~90 region.

Additionally, this dissertation establishes a new interpretation for the wobbling phenomenon.  It shows for the first time that the nucleon aligns to the short axis, which explains the decrease in wobbling energies with angular momentum seen on this and all previous wobbling nuclei while still explaining the observed B(E2_out)B(E2_in) ratios.  This is a new phenomenon, which is in contrast to the increase of the wobbling energies predicted by Bohr and Mottelson.