Nuclear Isomers: A Primer

Description

Nuclear isomers are the long-lived excited states of nuclei. Therefore, they constitute the meta-stable landscape of nuclei. The first isomer was probably identified as early as 1921. Since then, the number of isomers has been growing steadily picking up pace in recent times. Interest in nuclear isomers has grown in recent years for many reasons. The experimental capabilities to observe isomers have been expanding to cover a wider time scale. This has opened up new windows to observe and decipher the underlying nuclear structure and interactions. Further, the isomers are beginning to be seen as potential energy storage devices and nuclear clocks with a host of applications. Possible discovery of a gamma ray laser has also ignited many researches in this area. Isomers now cover the full nuclear landscape with structural peculiarities specific to each region of the nuclear chart. Exploring the nuclear isomers, therefore, provides a novel insight into the nuclear structure properties of that region. There could be many different reasons for the long lives of excited nuclear states, which lead to the classification of isomers. Isomers are broadly classified in to four classes: Spin isomers, shape isomers, fission isomers and K-isomers. Seniority isomers have also been identified which are often clubbed with the spin isomers. We discuss this classification and the underlying causes in detail. Many examples are considered to highlight the large variety of isomers. The range of half-lives covered by the isomers varies from billions of years to nano-seconds and even small. To understand this vast variation is a fascinating endeavor in itself. The angular momentum couplings, nuclear shapes, pairing etc. conspire together to give this vast range of half-lives. We go through these aspects in detail, highlighting the various selection rules at work. It is interesting that the nuclear shapes play an important role in many types of isomers. The spin isomers, which occur in spherical or, near-spherical nuclei, are generally confined to the magic numbers. Seniority isomers are largely found in semi-magic nuclei and should be explored in conjunction with the spin isomers. New developments in seniority and generalized seniority isomers are discussed in detail. As the nuclei deform; the nature of isomers changes. We take a close look into the decay properties of isomers in deformed nuclei, particularly the K isomers, the shape isomers and the fission isomers. While doing so, the theoretical and experimental developments of isomers are also addressed. A number of open questions are posed for possible new experiments and better understanding of the isomers. Foreword Preface Acknowledgements Contents 1 An Overview of Nuclear Isomers 1.1 Why a Primer on Nuclear Isomers 1.2 What are Nuclear Isomers? 1.3 Early History 1.4 Where are the Isomers Found? 1.5 Definition and Scope of Isomers 1.6 Half-Life of Isomers 1.7 Classification of Isomers and Hindrance Mechanisms 1.8 Systematic Features of Isomers 1.8.1 Half-Life Systematic 1.8.2 Spin Systematic 1.8.3 Multipolarity Systematic 1.8.4 Role of Pairing in Isomeric Energies References 2 Spin Isomers 2.1 Isomeric Transitions 2.2 Internal Conversion and Isomeric Half-Life 2.3 Islands of Spin Isomers Near Magic Numbers 2.3.1 g9/2 Spin Isomers 2.3.2 h11/2 Spin Isomers 2.3.3 i13/2 Spin Isomers 2.4 High-Spin Isomers Near the Proton Drip Line 2.5 Spin Isomers in 180mTa and the Only Natural Isomer 2.6 Spin Isomers in 208Pb 2.7 E5 Decaying Spin Isomers References 3 Seniority Isomers 3.1 Seniority and Seniority Isomers 3.1.1 Single-j Quasi-spin Scheme 3.1.2 Decay Properties 3.1.3 Magnetic Moments and g-Factors 3.2 Examples of Seniority Isomers and Their Moments 3.2.1 Seniority Isomers in N=50 Isotones 3.2.2 Seniority Mixing in 72,74Ni Isotopes 3.2.3 Seniority Isomers in 128Pd and 126Pd 3.2.4 Seniority Isomers in Pb Isotopes References 4 Generalized Seniority Isomers 4.1 Multi-j Quasi Spin Scheme 4.1.1 Decay Properties 4.1.2 Group Theoretical Understanding 4.1.3 Excitation Energies 4.2 Generalized Seniority in the Sn Isotopes 4.2.1 The 10+, 13-, and 15- Isomers 4.2.2 Comparison of Sn, Pb and N=82 Isomers 4.3 First 2+ and 3- States in Sn, Cd, Te Isotopes 4.3.1 Twin Asymmetric B(E2) Parabola in Sn Isotopes 4.3.2 Twin Asymmetric B(E2) Parabola in Cd and Te Isotopes 4.3.3 Inverted B(E3) Parabola in Sn Isotopes 4.3.4 Inverted B(E3) Parabola in Cd and Te Isotopes 4.4 Isomeric Moments 4.4.1 Quadrupole Moment of 11/2- States 4.4.2 Generalized Seniority Schmidt Model References 5 K-Isomers in Deformed Nuclei 5.1 The K-Quantum Number 5.2 Deformed Nilsson Model and High-K States 5.2.1 Quasi-particles 5.2.2 High-K MQP States and Isomers 5.3 Calculation of MQP States 5.3.1 Three-Quasiparticle States 5.3.2 MQP States 5.4 Some General Features of High-K States 5.4.1 K-Isomer in 250No 5.4.2 K-Mixing 5.4.3 K-Isomeric Rotational Band 5.5 Theoretical Treatments Used for K-Isomers References 6 Shape and Fission Isomers 6.1 Double-Hump Barrier and the Shell Corrections 6.2 Discovery of the Fission Isomers 6.3 Additional Features of Fission Isomers 6.4 The Low-Lying 0+ Shape Isomers 6.5 Half-Lives of the Shape Isomers 6.5.1 Projected Shell Model Calculations 6.5.2 Other Microscopic Calculations References 7 Unusual Isomers 7.1 Examples of Unusual Isomers 7.1.1 High Energy Isomers 7.1.2 Extremely Low Energy (ELE) Isomers 7.1.3 Very High-Spin Isomers 7.1.4 Very Long-Lived Isomers 7.1.5 Highest Multipolarity Isomers 7.1.6 100 % Proton Decaying Isomers 7.1.7 Highest Quasi-particle Isomer 7.2 ELE Isomers 7.3 A Specific Case of ELE Isomer in 229Th 7.4 β-Decaying Isomers References 8 Experimental Methods, Applications, Future Prospects 8.1 Experimental Methods and Applications 8.1.1 Gamma Ray Spectroscopy 8.1.2 Recoil/Fragment Mass Analyzers 8.1.3 Mass Measurement Techniques 8.1.4 Highly Charged Ion Storage Rings 8.1.5 Isomeric Targets and Isomeric Beams 8.1.6 Medical Applications of Isomers 8.1.7 Isomer Depletion by External Triggers 8.1.8 Moments and g-Factor Measurements 8.2 Future Applications of Isomers 8.2.1 Isomer Battery and Gamma Ray Laser 8.2.2 Nuclear Clock References Appendix Summary