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A Beachhead on the Island of Stability Yuri Ts A beachhead on the island of stability Yuri Ts. Oganessian, and Krzysztof P. Rykaczewski Citation: Physics Today 68, 8, 32 (2015); doi: 10.1063/PT.3.2880 View online: https://doi.org/10.1063/PT.3.2880 View Table of Contents: https://physicstoday.scitation.org/toc/pto/68/8 Published by the American Institute of Physics ARTICLES YOU MAY BE INTERESTED IN What every physicist should know about string theory Physics Today 68, 38 (2015); https://doi.org/10.1063/PT.3.2980 The formation of the heaviest elements Physics Today 71, 30 (2018); https://doi.org/10.1063/PT.3.3815 The secret of the Soviet hydrogen bomb Physics Today 70, 40 (2017); https://doi.org/10.1063/PT.3.3524 The laws of life Physics Today 70, 42 (2017); https://doi.org/10.1063/PT.3.3493 Hidden worlds of fundamental particles Physics Today 70, 46 (2017); https://doi.org/10.1063/PT.3.3594 The image of scientists in The Big Bang Theory Physics Today 70, 40 (2017); https://doi.org/10.1063/PT.3.3427 A beachhead on the island of stability Yuri Ts. Oganessian and Krzysztof P. Rykaczewski Recent experiments to synthesize the heaviest elements have dramatically expanded the periodic table and the Segrè chart of nuclides. emember learning the periodic table of el- Despite much recent progress, scientists are ements in high school? Our chemistry still learning how to consistently describe the struc- teachers explained that the chemical prop- ture and properties of atoms and nuclei. Experimen- R erties of elements come from the electronic tal studies of the heaviest elements and nuclei con- shell structure of atoms. Our physics tinue to yield new data that challenge and inform teachers enriched that picture of the atomic world theoretical models of atomic and nuclear structures. by introducing us to isotopes and the Segrè chart of The overarching questions are far from trivial. nuclides, which arranges them by proton number Z Where is the end of the periodic table? What is the and neutron number N. heaviest nucleus? How do properties evolve for ex- Nuclear binding energies and nuclear shell treme numbers of protons, neutrons, and electrons? structures provide organizing principles for under- How do relativistic electrons in outer atomic orbits standing the properties of nuclei and the directions influence the size and chemical behavior of the heav- of radioactive decay. Protons and neutrons occupy a iest atoms? Do superheavy elements (SHEs), say range of nuclear states with unevenly spread ener- those beyond Z = 100, follow the well-established gies. Groups of those states with relatively close en- group structure of the periodic table, or do they ex- ergies form so-called shells separated by large energy hibit unexpected chemical and nuclear properties? gaps, analogous to electron shells in atomic physics. In the 1960s, developments by Vilen Strutinsky1 At certain numbers of protons and neutrons, and others in the understanding of the nuclear struc- called magic numbers, nuclear shell closures occur, ture of lighter nuclei led to predictions by Adam and consequently, a large amount of energy is Sobiczewski and his collaborators on new nuclear needed to excite a nucleon to the distant higher shell closures.2 At about the same time, two groups— level. Closed nuclear shells enhance nuclear stabil- William Myers and Władysław Świątecki, and ity. (See the article by David Dean, PHYSICS TODAY, Victor Viola Jr and Glenn Seaborg—independently November 2007, page 48.) predicted the existence of heavy nuclei that would occupy a so-called island of stability on the Segrè Yuri Ts. Oganessian is the scientific leader of the Flerov chart.3 Since that time, the concept of an island of Laboratory of Nuclear Reactions at the Joint Institute for stability has dominated the physics of superheavy Nuclear Research in Dubna, Russia. Krzysztof P. Rykaczewski nuclei. is a senior researcher at Oak Ridge National Laboratory in Figure 1 shows the grand nuclear landscape as Oak Ridge, Tennessee. it is understood today. A handful of magic numbers 32 August 2015 Physics Today www.physicstoday.org Figure 1. The grand nuclear landscape. Nuclei that have been experimentally identified are inside the yellow region, whereas nuclei only predicted to exist are roughly indicated by the green area. Black squares mark stable isotopes. Magic proton and neutron numbers, at which nuclei have enhanced stability, are indicated by red lines. The star labeled SHE indicates the region of superheavy elements. (Courtesy of Witold Nazarewicz). 2015, page 40), researchers have artificially created 26 new elements and hundreds of isotopes, all by using nuclear reactions to modify the properties of WINSLOW HOMER existing nuclei. Attempts to change the nuclear properties of materials are not new. Historically, the have been well established: 2, 8, 20, 28, 50, and 126 hope of creating precious metals like gold out of (neutrons only). Modern theoretical approaches, more common ones like tin or lead drove such al- supported by new experimental data, also point to chemical ambitions. the existence of long-lived nuclei at and around the The dreams of medieval alchemists have nearly magic neutron number N = 184. Presently, 177 is the come true in the modern era of accelerators and nu- largest experimentally observed neutron number4,5— clear reactions.7 For example, one can fuse two 294 293 74 in isotopes 117 and livermorium-293 (116 Lv). metallic atoms, a germanium-74 ( 32Ge) projectile 124 74 Although there is no consensus on the next with a tin-124 ( 50Sn) target. When the 32Ge has a magic atomic number Z, theoretical predictions kinetic energy of 300 MeV, which corresponds to 198 * place it between 114 and 126. In fact, in the vicinity about 9% of the speed of light, a 82Pb nucleus is of N = 184 and Z = 114 to 126, with very high neu- created in the nuclear fusion. 198 * tron and proton numbers and large densities of That initial 82Pb nucleus has a mass number neutron and proton states, the stability of nuclei equal to the sum of the Ge projectile’s and the Sn tar- might be enhanced without sizeable energy gaps.6 get’s mass numbers. The asterisk indicates that the Current experimental results are consistent nucleus—which is created “hot” with an excitation with the existence of an extended island of super- energy of about 50 MeV, corresponding to a temper- heavy nuclei that are more resistant to radioactive ature of more than 1010 K—is a compound nucleus, decay and have much longer half-lives than some- one that is not fully bound. It promptly evaporates what lighter isotopes, as shown in figure 2. Indeed, several neutrons to cool down, and different Pb iso- some models predict half-lives up to a million years topes, called fusion-evaporation residues, are cre- for new superheavy nuclei, and some even calculate ated in the process. 198 * them at around Earth’s age. In the 82Pb example, the evaporation of four 194 Extensive chemistry studies have searched for neutrons to produce 82Pb accounts for about 60% traces of SHEs in geothermal waters, rare ores, and of the evaporation residues. Within a few tens of 194 cosmic rays. However, to date all known elements minutes, beta decay transforms the 82Pb into thal- 194 beyond uranium (Z = 92) have been manmade first, lium-194 ( 81Tl). Then a second beta decay turns 194 194 in laboratories using large-scale devices like nuclear 81Tl into mercury-194 ( 80Hg), a nucleus with a reactors and particle accelerators or in the violent 520-year half-life. The nuclear alchemist seeking Au conditions of nuclear explosions. has to wait quite some time for the next beta decay 194 194 into 79Au. Unfortunately, 79Au is an unstable iso- Fusion evaporation tope, with a half-life of only 38 hours. 194 Since the discovery of nuclear fission in 1938 (see the The beta decay of 79Au does create stable and 194 article by Michael Pearson, PHYSICS TODAY, June even more precious platinum-194 ( 78Pt), but at www.physicstoday.org August 2015 Physics Today 33 Island of stability 104 cross section for each nuclear reaction channel, 1 h drops exponentially with increasing projectile 103 atomic number. N = 162111 N = 184 107 The last and most challenging cold-fusion ex- 2 10 209 70 FE (s) 113 1 min periment, which used a Bi target and Zn projec- N = 152 83 30 101 tiles, ran for nine years, from 2003 to 2012, at the 109 115 RIKEN research institute in Japan, with nearly 600 100 1 s days of beam on target. That tour-de-force effort re- Y HALF-LI sulted in the observation8 of three decay chains of −1 117 10 the isotope 278113. The deduced cross section for the production of 278113 via cold-fusion reaction is ex- 10−2 tremely small, about 20 fb (1 fb = 10−39 cm2), which is 10−3 1 ms less than 1/1012 of the total reaction cross section. ALPHA-DECA In the late 1990s, researchers at the Joint Insti- 10−4 tute for Nuclear Research (JINR) in Russia success- fully applied a new method to create the heaviest el- 10−5 ements and nuclei to date.5 Instead of stable target 150 155 160 165 170 175 180 185 208 209 NEUTRON NUMBER N nuclei, like 82Pb and 83Bi, radioactive actinide ma- 238 249 terials from 92U to californium-249 ( 98Cf) were Figure 2. The dependence of alpha decay half-lives on neutron number. used as targets and irradiated with calcium-48 48 Solid and open symbols correspond to nuclei with odd and even numbers (20Ca) ions, which have both magic proton (Z = 20) of neutrons, respectively.
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