Island of stability

In nuclear physics, the island of stability is the prediction that a set of superheavy nuclides with magic numbers of protons and neutrons will temporarily reverse the trend of decreasing stability in elements heavier than uranium. Various predictions have been made regarding the exact location of the island of stability, though it is generally thought to center near copernicium and flerovium isotopes (such as 291Cn, 293Cn, and 298Fl) approaching the predicted closed shell at neutron number N = 184. It is thought that the closed shell will confer additional stability towards fission, while also leading to longer half-lives towards alpha decay. While these effects are expected to be greatest near atomic number Z = 114 and N = 184, the region of increased stability is expected to encompass several neighboring elements, and there may also be additional islands of stability around heavier nuclei that are doubly magic (having magic numbers of both protons and neutrons). Estimates of the stability of the elements on the island are usually around a half-life of minutes or days; however, some estimates predict half-lives of millions of years. Although the nuclear shell model predicting magic numbers has existed since the 1940s, the existence of long-lived superheavy nuclides has not been definitively demonstrated. Like the rest of the superheavy elements, the nuclides on the island of stability have never been found in nature; thus, they must be created artificially in a nuclear reaction to be studied. Scientists have not found a way to carry out such a reaction; it is likely that new types of reactions will be needed to populate nuclei near the center of the island. Nevertheless, the successful synthesis of superheavy elements up to oganesson in recent years demonstrates a slight stabilizing effect around elements 110–114 that may continue in unknown isotopes, supporting the existence of the island of stability. The composition of an atomic nucleus is determined by the number of protons Z and the number of neutrons N, which sum to mass number A. The atomic number Z determines the position of an element in the periodic table, but the more than 3000 nuclides are commonly represented in a chart with Z and N for its axes and the half-life for radioactive decay indicated for each unstable nuclide (see figure). 252 nuclides are thought to be stable (having never been observed to decay), and these follow a general trend in which the number of neutrons rises more rapidly than the number of protons. The last element in the periodic table that has a stable isotope is lead (Z = 82), with stability generally decreasing in heavier elements. The half-lives of nuclei also decrease when there is a lopsided neutron-proton ratio, such that the resulting nuclei have too few or too many neutrons to be stable. The stability of a nucleus is determined by its binding energy, with higher binding energy conferring greater stability. The binding energy per nucleon increases with atomic number to a broad plateau around A = 60, then declines. If a nucleus can be split into two parts that have a lower total energy (a consequence of the mass defect resulting from greater binding energy), it is unstable. The nucleus can hold together for a finite time because there is a potential barrier opposing the split, but this barrier can be crossed by quantum tunnelling. The lower the barrier and the masses of the constituents, the greater the probability per unit time of a split. Protons in a nucleus are bound together by the strong force, which counterbalances the Coulomb repulsion between positively charged protons. In heavier nuclei, larger numbers of neutrons are needed to reduce repulsion and confer additional stability. Even so, as physicists started to synthesize elements that are not found in nature, they found the stability decreased as the nuclei became heavier. Thus, they speculated that the periodic table might come to an end. The discoverers of plutonium (element 94) considered naming it 'ultimium', thinking it was the last. Following the discoveries of heavier elements, of which some decayed in microseconds, it then seemed that instability with respect to spontaneous fission would limit the existence of heavier elements. In 1939, an upper limit was estimated around element 104, and later, it seemed that element 108 might be the limit. The possible existence of superheavy elements with atomic numbers well beyond that of uranium had been suggested as early as 1919, when German physicist Richard Swinne proposed that superheavy elements around Z = 108 were a source of radiation in cosmic rays. Although he did not make any definitive observations, he hypothesized in 1931 that transuranium elements around Z = 100 or Z = 108 may be relatively long-lived and possibly exist in nature. In 1955, John Archibald Wheeler also proposed the existence of these elements; he is credited with the first usage of the term 'superheavy element' in a 1958 paper published alongside Frederick Werner. However, this idea did not attract wide interest until a decade later, after improvements in the nuclear shell model. In this model, the atomic nucleus is built up in 'shells', analogous to electron shells in atoms. Independently of each other, neutrons and protons have energy levels that are normally close together, but after a given shell is filled, it takes substantially more energy to start filling the next. Thus, the binding energy per nucleon reaches a local maximum and nuclei with filled shells are more stable than those without. This theory of a nuclear shell model originates in the 1930s, but it was not until 1949 that Maria Goeppert Mayer and Johannes Hans Daniel Jensen et al. independently devised the correct formulation. The numbers of nucleons for which shells are filled are called magic numbers. Magic numbers of 2, 8, 20, 28, 50, 82 and 126 have been observed for neutrons, and the next number is predicted to be 184. Protons share the first six of these magic numbers, and 126 has been predicted since the 1940s. Nuclides with a magic number of each are referred to as 'doubly magic' and are more stable than nearby nuclides as a result of greater binding energies. In the late 1960s, more sophisticated shell models by William Myers and Władysław Świątecki, and by Heiner Meldner, taking into account Coulomb repulsion, changed the prediction for the next proton magic number from 126 to 114. Some Russian physicists argued for the existence of the doubly magic nuclide 298Fl (Z = 114, N = 184), rather than 310Ubh (Z = 126, N = 184) which was predicted to be doubly magic as early as 1957. Subsequently, estimates of the proton magic number have ranged from 114 to 126, and there is still no consensus. Myers and Świątecki appear to have coined the term 'island of stability', and Glenn Seaborg, later a discoverer of many of the superheavy elements, quickly adopted the term and promoted it. Myers and Świątecki also proposed that some superheavy nuclei would be longer-lived as a consequence of a higher fission barrier. Further improvements in the nuclear shell model by Vilen Strutinsky led to the emergence of the macroscopic-microscopic method which takes into consideration both smooth trends characteristic of the liquid drop model and local fluctuations such as shell effects. This approach enabled Sven Nilsson et al., as well as other groups, to make the first detailed calculations of the stability of nuclei within the island.