Nuclear Structure Features of Very Heavy and Superheavy Nuclei—Tracing Quantum Mechanics Towards the ‘Island of Stability’
Total Page：16
File Type：pdf, Size：1020Kb
Physica Scripta INVITED COMMENT Related content - Super-heavy nuclei Nuclear structure features of very heavy and Sigurd Hofmann superheavy nuclei—tracing quantum mechanics - New elements - approaching towards the ‘island of stability’ S Hofmann - Review of metastable states in heavy To cite this article: D Ackermann and Ch Theisen 2017 Phys. Scr. 92 083002 nuclei G D Dracoulis, P M Walker and F G Kondev View the article online for updates and enhancements. Recent citations - Production cross section and decay study of Es243 and Md249 R. Briselet et al - Colloquium : Superheavy elements: Oganesson and beyond S. A. Giuliani et al - Neutron stardust and the elements of Earth Brett F. Thornton and Shawn C. Burdette This content was downloaded from IP address 213.131.7.107 on 26/02/2019 at 12:10 | Royal Swedish Academy of Sciences Physica Scripta Phys. Scr. 92 (2017) 083002 (83pp) https://doi.org/10.1088/1402-4896/aa7921 Invited Comment Nuclear structure features of very heavy and superheavy nuclei—tracing quantum mechanics towards the ‘island of stability’ D Ackermann1,2 and Ch Theisen3 1 Grand Accélérateur National d’Ions Lourds—GANIL, CEA/DSM-CNRS/IN2P3, Bd. Becquerel, BP 55027, F-14076 Caen, France 2 GSI Helmholtzzentrum für Schwerionenforschung, Planckstr. 1, D-62491 Darmstadt, Germany 3 Irfu, CEA, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France E-mail: [email protected] Received 13 March 2017, revised 19 May 2017 Accepted for publication 13 June 2017 Published 18 July 2017 Abstract The quantum-mechanic nature of nuclear matter is at the origin of the vision of a region of enhanced stability at the upper right end of the chart of nuclei, the so-called ‘island of stability’. Since the 1960s in the early second half of the last century, various models predict closed shells for proton numbers 114–126 and neutron numbers such as 172 or 184. Being stabilized by quantum-mechanic effects only, those extremely heavy man-made nuclear species are an ideal laboratory to study the origin of the strong nuclear interaction which is the driving force for matter properties in many fields ranging from microscopic scales like hadronic systems to cosmic scales in stellar environments like neutron stars. Since the 1950s, experiments on the synthesis of new elements and isotopes have also revealed various exciting nuclear structure features. The contribution of Bohr, Mottelson and Rainwater with, in particular, the development of the unified model played an essential role in this context. Although not anticipated in the region of the heaviest nuclei, many phenomena were subsequently discovered like the interplay of collective features manifesting themselves e.g. in nuclear deformation, ranging from spherical to prolate and oblate shapes with the possible occurrence of triaxial symmetries, and single particle states and their excitation into quasiparticle configurations. The continuous development of modern experimental techniques employing advanced detection set-ups was essential to reveal these exciting nuclear structure aspects in the actinide and transactinide regions since the production cross-section becomes extremely small with increasing mass and charge. Further technological progress, in particular, high intensity stable ion beam accelerator facilities presently under construction, as well as potentially in the farther future radioactive neutron rich ion beams provide a high discovery potential for the basic understanding of nuclear matter. Keywords: nuclear structure, superheavy nuclei, decay spectroscopy, in-beam spectroscopy K-isomers, nuclear deformation (Some figures may appear in colour only in the online journal) 0031-8949/17/083002+83$33.001 © 2017 The Royal Swedish Academy of Sciences Printed in the UK Phys. Scr. 92 (2017) 083002 Invited Comment Contents 4.1. Discussion of the models 60 4.2. Exotic shapes and phenomena 61 1. Introduction 2 4.2.1. Oblate g.s. deformations 61 1.1. Bohr, Mottelson and Rainwater legacy 4.2.2. Octupole shapes 62 from the super-heavy nuclei (SHN) point 4.2.3. Gamma-vibrational states 62 of view 3 4.2.4. Superdeformed shapes 62 1.2. Theoretical background 5 5. Advances in experimental instrumentation 63 1.2.1. Limits of stability from the liquid 5.1. Future facilities and detection devices 63 drop model 5 5.1.1. Dubna: The SHE-factory 63 1.2.2. Nuclear structure shell effects 8 5.1.2. GANIL-SPIRAL2: S3 and 1.3. Brief review of experimental techniques 14 VAMOS-GFS 64 2. Decay spectroscopy after separation—DSAS 15 5.1.3. The cw-LINAC project for 2.1. Historic placement 15 GSI/FAIR 65 2.2. X-ray spectroscopy—Z identification for 5.1.4. AGFA at ANL 65 rutherfordium, dubnium and beyond 16 5.1.5. Gamma-ray tracking: AGATA 2.3. Experimental technique 17 and GRETA 66 2.3.1. Separation methods 18 5.2. Production and experimental techniques 67 2.3.2. Particle and photon detection 22 5.2.1. Isotope production 67 2.4. Accessible processes 25 5.2.2. Lifetimes, electromagnetic 2.5. Features to be investigated 28 moments, re-acceleration, direct reac- 2.5.1. Trends of single particle energies 28 tions, etc 68 2.5.2. K-isomers—a tool to scan the 6. Conclusion and open questions 69 region of deformed SHN 31 3. In beam spectroscopy 34 3.1. Global properties of rotational bands 35 1. Introduction 3.2. The pre-254No era 36 This review article will focus on the nuclear structure studies 3.2.1. Coulomb excitation 36 in the region of the heaviest nuclei on both the experimental 3.2.2. Transfer reactions 38 and the theoretical side. In the first section, we will introduce 3.2.3. Fusion-evaporation reactions 39 the basic theoretical and experimental aspects relevant for the heaviest nuclei. We will in particular highlight the contrib- 3.3. The 254No breakthrough 40 ution of Bohr, Mottelson and Rainwater to the progress in 3.4. Prompt spectroscopy techniques 41 theory. In the course of this review, reference to the literature 3.4.1. Gamma-ray spectroscopy 41 of these authors will be made whenever the concepts result from their work, thus emphasizing the profound legacy from 3.4.2. Conversion electron spectroscopy 43 these pioneers. Nuclear structure studies were historically first 3.5. Recent achievements 44 performed studying the decay of rare isotopes after their 3.5.1. Transfer reactions using light necessary extraction from the large background due to para- sitic reactions. This aspect of so-called decay spectroscopy beams 44 after separation (DSAS) will be discussed in section 2.Itis 3.5.2. Inelastic scattering and transfer only since the end of the last century that in-beam experi- reactions using Pb and Bi beams 44 ments could be performed for elements with atomic numbers greater than 100. These experiments revealing mainly the 3.5.3. Fusion evaporation reactions 45 collective structure will be developed in section 3. Theory 3.6. Deformation and deformed shell gaps 51 lessons and exotic phenomena will be discussed in section 4. 3.7. Evolution of moments of inertia, align- Section 5 will discuss recent and future advances in instru- mentation and production techniques as well as future facil- ment and pairing 54 ities. Open questions and perspectives will be debated in the 4. Theory lessons and exotic phenomena 60 concluding section. 2 Phys. Scr. 92 (2017) 083002 Invited Comment Figure 1. History of discovery and synthesis of heavy and SHEs. The subdivision in periods follows the spirit set by Peter Armbruster in [25]. 1.1. Bohr, Mottelson and Rainwater legacy from the super- decay from ‘thorium emanations’ [8]. In 1900, Villard observed heavy nuclei (SHN) point of view a new type of radiation emitted by radium [9], named γ-rays by Rutherford in 1909. In 1911, Bayer, Hahn and Meitner Since the early times of atomic and nuclear physics, heavy observed a fine structure of the decay of ‘radium B’ and ‘C’ elements have always been at the heart of some of the most 214 214 ( Pb and Bi) [10]. This turned out to correspond to the fundamental discoveries. Radioactivity, initially referred to as internal conversion process, as confirmed by Ellis in 1921, from ‘ ’ uranic rays , was discovered in 1896 by Becquerel, who ‘radium B’ studies [11, 12]. In the same year, Hahn discovered observed that uranium salts unexpectedly produced an image an isomer decay from ‘uranium X2’ to ‘uranium Z’ (214Pa [ ] on a photographic plate 1 . This was the beginning of a isomer decay) [13] and in 1929, Rosenblum discovered the α- fi scienti c adventure which should continue for more than a decay fine structure from ‘thorium C’ (212Bi) [14]. century, motivating generations of scientists to develop ever Later, in 1938, Hahn, Strassmann and Meitner discovered advancing technology and theoretical models, producing fission from the bombardment of 238U with neutrons [15, 16]. exciting science with the aim to reach the limits of nuclear A theory of fission was rapidly developed by Bohr and stability in terms of mass and atomic number. An overview Wheeler in a seminal paper [17], in which it was also shown over the episodes experienced so far in this continuing story is that the stability of the heaviest nuclei against fission is illustrated in figure 1. In 1898, Pierre and Marie Curie dis- governed by the value of Z 2 A, Z (A) being the number of covered the new elements Po and Ra in pitchblende ores protons (nucleons). Above the critical value ZA2 » 48, [2, 3]. In 1899, Rutherford isolated α and β radioactivities nuclei undergo spontaneous fission. As a consequence of this from uranium [4]. Alpha radiation was later identified as the critical value, nuclear matter cannot be extended limitlessly. emission of helium nuclei by Rutherford and Geiger [5], and In their original paper, Meitner and Frisch estimated this limit β radiation as electrons by Becquerel [6, 7].