Island of superheavy elements

Z= 114 was mentioned as an alternative, with reference to unpublished calculations by H. Meldner, who presented his results [4] later at the Why and How. .. symposium 1966 [5], the seminal event for superheavy element research. Simultaneously, A. Sobiczewski et al. [6] also derived, that 114 should be the next magic proton number. Other groups using different theoretical approaches soon agreed. A fantastic perspective was, thus, opened an island of superheavy elements located not too far from the then heaviest known element, 103, and, hence, perhaps within reach. 

Fig. 3. Allegorical view of heavy-ion accelerator projects launched in the early seventieths for a journey to the island of superheavy elements. The flags indicate characteristic projectile beams offered by the facilities, see text. Cartoon provided by G.N. Flerov [13].

The expected island of superheavy elements near Z = 114 and A = 184 has now been reached with respect to the atomic number Z, whereas there is still a gap in the neutron number. This gap can only be surmounted by fusion of neutron-rich nuclei. 

FIGURE 8.1 The periodic system of elements as conceived by the Frankfurt school in the late sixties. The islands of superheavy elements (Z = 114, N = 184,196 and Z = 164, N = 318) are shown as dark hatched areas. 

With the addition of microscopic effects to the nuclear droplet, which add stabilization (Strutinsky 1967), the question arose as to whether superheavy nuclei can exist only due to shell stabilization beyond the macroscopic limit estimated by Meitner and Frisch. In a first semi-empirical approach the possible evidence for an island of superheavy nuclei was predicted for 126 protons and 184 neutrons (Myers and Swiatecki 1966). A series of more basic calculations followed to confirm the hypothesis of shell stabilization playing a predominant role in the existence of superheavy elements (Mosel and Greiner 1969 Nilssen et al. 1969 Fizet and Nix 1972). Most of these calculations predicted Z= 114 and N= 184 as the center of the island of superheavy nuclei located in the sea of macroscopic instability, well separated from the transuranium elements. The situation remained practically unaltered until 1984. With the discovery of element 108 (Miinzenberg et al. 1984b) it became clear that the island of superheavy elements is connected to the transuranium elements by a bridge of shell-stabilized nuclei. 

Myers and Swiatecki (1966) and Meldner (1967) predicted that an Island of Superheavy Elements well beyond uranium might exist around elements with atomic numbers 114 or 126. This raised the possibility that very long-lived SHEs might still exist on earth after having been formed during the last nucleosynthesis in the solar system some 4.5 billion years ago. Later theoretical studies based on new theories of nuclear structure (Strutinsky 1966 Nilsson et al. 

Investigation of physical and chemical properties of recently synthesized, relatively long-living isotopes of superheavy elements (SHEs) with nuclear charges Z=105 to 116 [1, 2, 3, 4] and their compounds is of fundamental importance. Their measured lifetimes may reach several hours and the nuclei near the top of the island of stability are predicted to exist for many years. The experimental study of the SHE properties is very difficult be-... 

A brief and good account of the manufacture of superheavy elements, and the search for the island of stability. Is given by R. Stone, Science, 278 (1997), 571, and Science, 283 (1999), 474. The topic is discussed in more detail in G. T. Seaborg and W. D. Loveland, The search for new elements . In N. Hall (ed.). The New Chemistry (Cambridge Cambridge University Press,... 

Oganessian, Yuri Ts., Vladimir K. Utyonkov, and Kenton J. Moody. Voyage to Superheavy Island. Scientific American 282 (January 2000) 63-67. This is a highly accessible discussion of the synthesis of superheavy elements using both cold fusion and hot fusion techniques. 

The nuclear models that resulted in the prediction of an island of superheavy nuclei have evolved in response to experimental measurements of the decay properties of the heaviest elements. While the prediction of a spherical magic N = 184 is robust and persists across the models [8], the shell closure associated with Z — 114 is weaker, and different models place it at higher atomic numbers, from Z = 120 to 126 [60-69] or even higher [70] (see Nuclear Structure of Superheavy Elements ). Interpretation of the decay properties of the heaviest elements may support this [71, 72], but the most part decay and reaction data do not conclusively establish the location of the closed proton shell. Because of this, the domain of the superheavy elements can be considered to start at approximately Z = 106 (seaborgium), the point at which the liquid-drop fission barrier has vanished [9]. For our purposes, the transactinide elements (Z > 103) will be considered to be superheavy (see Nuclear Structure of Superheavy Elements ). 

Since the radioactive half-lives of the known transuranium elements and their resistance to spontaneous fission decrease with increase in atomic number, the outlook for the synthesis of further elements might appear increasingly bleak. However, theoretical calculations of nuclear stabilities, based on the concept of closed nucleon shells (p. 13) suggest the existence of an island of stability around Z= 114 and N= 184. Attention has therefore been directed towards the synthesis of element 114 (a congenor of Pb in Group 14 and adjacent superheavy elements, by bombardment of heavy nuclides with a wide range of heavy ions, but so far without success. 

Superheavy Elements Island of Stability. LBNL Image Library. http //www.imglib.Ibl.gov. ImgLib/COLLECTlONS/ BERKELEY-LAB/SEABORG-ARCHIVE (accessed October 23, 2005). 

One isotope of element 114, with 184 neutrons, is predicted to be another doubly magic nucleus, and is therefore expected to sit right in the middle of an island of stability in the space of superheavy nuclei (Fig. 13). Nuclear scientists suspect that it may have a half-life of as much as several years. Element 114 has thus become a kind of Holy Grail for element-makers. If it turns out to be stable, this would show that these researchers are not necessarily doomed to search for increasingly fleeting glimpses of ever heavier and less stable new elements. There might be undiscovered elements out there that you can (in principle, at least) hold in your hand. 

We now know these predictions were wrong, in part. While we believe there are a group of superheavy nuclei whose half-lives are relatively long compared to lower Z elements, we do not believe they form an island of stability. Rather, we picture them as a continuation of the peninsula of known nuclei (Fig. 15.1 lb). We also believe that their half-lives are short compared to geologic time scales. Therefore, they do not exist in nature. The most stable of the superheavy nuclei, those with Z = 112, N 184, are predicted to decay by a-particle emission with half-lives of 20 days. 

The experimental work of the last two decades has shown that cross sections for the synthesis of the heaviest elements decrease almost continuously. However, recent data on the synthesis of element 114 and 116 in Dubna using hot fusion seem to break this trend when the region of spherical superheavy elements is reached. Therefore a confirmation is urgently needed that the region of spherical SHEs has finally been reached and that the exploration of the island has started and can be performed even on a relatively high cross section level. 

The Flerov Laboratory of Nuclear Reactions (FLNR) in Dubna, Russia, has recently announced the observation of relatively long-lived isotopes of elements 108, 110, 112, 114, and 116 [63-66] confirming the over 30 years old theoretical prediction of an island of stability of spherical superheavy elements. Due to the half-lives of the observed isotopes in the range of seconds to minutes, chemical investigations of these heaviest elements in the Periodic Table appear now to be feasible. The chemistry of these elements should be extremely interesting due to the predicted dramatic influence of relativistic effects [67], In addition, the chemical identification of the newly discovered superheavy elements is highly desirable as the observed decay chains [63-66] cannot be linked to known nuclides which has been heavily criticized [68,69],... 

The term "superheavy elements" was first coined for elements on a remote "island of stability" around atomic number 114 (Chapter 8). At that time this island of stability was believed to be surrounded by a "sea of instability". By now, as shown in Chapter 1, this sea has drained off and sandbanks and rocky footpaths, paved with cobblestones of shell-stabilized deformed nuclei, are connecting the region of shell-stabilized spherical nuclei around element 114 to our known world. 

The islands of relative stability are shown in Fig. 14.9. The stability gap around mass number 4 = 216 is evident from this figure nuclides with half-lives > 1 s do not exist for = 216. The search for superheavy elements concentrates on the islands around Z = 108 and Z= 114. At neutron numbers A = 162 the nuclei should exhibit a high degree of deformation, whereas spheric nuclei are expected for N = 184. 

These first results were very promising and stimulated a very extensive but up to now unsuccessful search for superheavy elements in nature. A most comprehensive review of this subject was given by G. Herrmann (32). But, besides spontaneous fission, a nucleus can decay by other decay modes like a decay, decay, or electron capture. The most comprehensive study of half-lives in the first superheavy island was performed by Fiset and Nix 33). Figure 4 is taken from their work. 

Chemists will synthesize millions of new compounds tailored for a wide spectrum of practical uses. Nuclear chemists will be involved in the synthesis of additional chemical elements, hopefully in the region of the superheavy elements predicted to exist in the island of stability. ... 

Most superheavy elements exist for only a tiny fraction of a second. Thirty seconds is a very long life span for a superheavy element. This long life span of element 114 points to what scientists have long suspected that an island of stability would be found beginning with element 114. Based on how long element 114 lasted, their predictions may have been correct. However, scientists still must try to confirm that element 114 was in fact created. The results of a single experiment are never considered valid unless the experiments are repeated and produce the same results. 

The scientific interest in the elements since the middle of the twentieth century has been closely linked to the development of physics and the demands of the Cold War arms race. As chemists and physicists looked into the interior of the atom, they learned more about the structure of matter, and that knowledge, in turn, allowed a degree of control over the creation of matter. Nuclear reactors can now produce both useful and deadly materials even as they put a new source of power in to the hands of people around the world. The atom smashers, as the cyclotrons and accelerators were sometimes called, made new elements possible, and scientists continues to explore the elements, looking for the islands of stability among the superheavy elements. 

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