Island of stability

Theory and origin

The possibility of an "island of stability" was first proposed by Glenn T. Seaborg in the late 1960s. The hypothesis is based upon the nuclear shell model, which implies that the atomic nucleus is built up in "shells" in a manner similar to the structure of the much larger electron shells in atoms. In both cases, shells are just groups of quantum energy levels that are relatively close to each other. Energy levels from quantum states in two different shells will be separated by a relatively large energy gap, so when the number of neutrons and protons completely fills the energy levels of a given shell in the nucleus, the binding energy per nucleon will reach a local maximum and thus that particular configuration will have a longer lifetime than nearby isotopes that do not possess filled shells.

Magic numbers

A filled shell would have "magic numbers" of neutrons and protons. This idea of a magic number derives from the counterpart of electron shells. The magic number for electron shells is 8. This completes the shell and makes it stable. Similarly, it is believed that there are complete shells in the nucleus that stabilize the nucleus. One possible magic number of neutrons for spherical nuclei is 184, and some possible matching proton numbers are 114, 120 and 126 – which would mean that the most stable spherical isotopes would be flerovium-298, unbinilium-304 and unbihexium-310. Of particular note is ²⁹⁸Fl, which would be "doubly magic" (both its proton number of 114 and neutron number of 184 are thought to be magic) and thus the most likely to have a very long half-life. (The next lighter doubly magic spherical nucleus is lead-208, the heaviest known stable nucleus and most stable heavy metal.)

Deformed nuclei

Studies from the early 1990s, and previous to that time, have shown that superheavy elements do not have perfectly spherical nuclei. A shell is considered stable when it is in a spherical form. A change in the shape of the nucleus changes the position of neutrons and protons in the shell, thus skewing the numbers. Recent research indicates that large nuclei are deformed, causing magic numbers to shift. A nucleus can have a magic number of neutrons or a magic number of protons. When the nucleus has magic numbers of both protons and neutrons, it can be said to be doubly magic. Hassium-270 is now believed to be a doubly magic deformed nucleus, with deformed magic numbers 108 and 162. It has a half-life of 3.6 seconds.

Experiments

Whilst elements with atomic numbers expected for the island of stability have been produced, the total nucleon count of these isotopes has been too low. These synthesised isotopes have contained too few neutrons to reach the supposed stable region. It is possible that these elements possess unusual chemical properties and, if they have isotopes with adequate lifespans, would be available for various practical applications (such as particle accelerator targets and as neutron sources as well). In particular, the very small critical masses of transplutonic elements (possibly as small as grams) implies that if stable elements could be found, they would enable small and compact nuclear bombs either directly or by serving as primaries to help ignite fission/fusion secondaries; this possibility motivated much of the early research in the 1960s and multiple nuclear tests by the United States (including Operation Plowshare) and the Soviet Union aimed at producing such elements.

Half-lives of the highest-numbered elements

All elements with an atomic number above 82 (lead) are unstable, and the "stability" (half-life of the longest-lived known isotope) of elements generally decreases with rising atomic numbers from the relatively stable uranium (92) upwards to the heaviest known element, oganesson (118). The longest-lived observed isotopes of each of the heaviest elements are shown in the following table.

(Note that for elements 108–118, the longest-lived known isotope is always the heaviest discovered thus far. This makes it seem likely that there are longer-lived undiscovered isotopes among the even heavier ones.)

For comparison, the shortest-lived element with atomic number below 100 is francium (element 87) with a half-life of 22 minutes.

The half-lives of nuclei in the island of stability itself are unknown since none of the isotopes that would be "on the island" have been observed. Many physicists think they are relatively short, on the order of minutes or days. Some theoretical calculations indicate that their half-lives may be long, on the order of 10⁹ years.

The alpha-decay half-lives of 1700 nuclei with 100 ≤ Z ≤ 130 have been calculated in a quantum tunneling model with both experimental and theoretical alpha-decay Q-values. The theoretical calculations are in good agreement with the available experimental data.

A possible stronger decay mode for the heaviest superheavies was shown to be cluster decay by Dorin N. Poenaru, R.A. Gherghescu, and Walter Greiner.

Islands of relative stability

232
Th (thorium), 235
U and 238
U (uranium) are the only naturally occurring isotopes beyond bismuth that are relatively stable over the current lifespan of the universe. Even bismuth was found to be slightly unstable in 2003, with an α-emission half-life of 7019190000000000000♠1.9×10¹⁹ years for 209
Bi. All elements beyond bismuth have relatively or very unstable isotopes: astatine, radon, and francium are extremely short-lived (and only have half-lives longer than isotopes of the heaviest elements found so far). Even thorium, with the largest known half-life in this region (7010140000000000000♠1.4×10¹⁰ years for 232
Th), is still about a billion times shorter than 209
Bi, so the main periodic table ends there.

By geographical analogy, bismuth is the shore edge of a continent. A continental shelf continues though, with shallows beginning at radium (see 'map' at right) that rapidly drop off again after californium. Significant islands appear at thorium and uranium, and with minor ones (i.e. neptunium, plutonium and curium) form an archipelago. All of this is surrounded by a "sea of instability". As can be seen from the table, there is a significantly large gap between the half-lives of the longest-lived actinide isotopes (the primordial ²³²Th, ²³⁸U, ²³⁵U, and ²⁴⁴Pu, and the long-lived ²³⁶U, ²⁴⁷Cm, and ²³⁷Np) and those of the others.

Current theoretical investigation indicates that in the region Z = 106–108 and N ≈ 160–164, a small ‘island/peninsula’ might be stable with respect to fission and beta decay, such superheavy nuclei undergoing only alpha decay. Also, 298
Fl is not the center of the magic island as predicted earlier. On the contrary, the nucleus with Z = 110, N = 183 (²⁹³Ds) appears to be near the center of a possible 'magic island' (Z = 104–116, N ≈ 176–186). In the N ≈ 162 region the beta-stable, fission survived 268
Sg is predicted to have alpha-decay half-life ~3.2 hours that is greater than that (~28 s) of the deformed doubly magic 270
Hs. The superheavy nucleus 268
Sg has not been produced in the laboratory as yet (2009). For superheavy nuclei with Z > 116 and N ≈ 184 the alpha-decay half-lives are predicted to be less than one second. The nuclei with Z = 120, 124, 126 and N = 184 (³⁰⁴Ubn, ³⁰⁸Ubq, and ³¹⁰Ubh) are predicted to form spherical doubly magic nuclei and be stable with respect to fission. Calculations in a quantum tunneling model show that such superheavy nuclei would undergo alpha decay within microseconds or less.

Synthesis problems

The manufacture of nuclei on the island of stability proves to be very difficult because the nuclei available as starting materials do not deliver the necessary sum of neutrons. For the synthesis of isotope 298 of flerovium, one could use an isotope of plutonium and one of calcium that together have a sum of at least 298 nucleons; for example, calcium-50 and plutonium-248. These and heavier isotopes are not available in measurable quantities, making production virtually impossible with current methods. The same problem exists for the other possible combinations of isotopes needed to generate elements on the island using target-projectile methods. It may be possible to generate the isotope 298 of flerovium, if the multi-nucleon transfer reactions would work in low-energy collisions of actinide nuclei. One of these reactions may be:

Hypothetical second island

At the 235th national meeting of the American Chemical Society in 2008, the idea of a second island of stability was presented by Yuri Oganessian. This new island would be centered on element 164 (unhexquadium), especially the isotope ⁴⁸²Uhq, with a stability similar to that of flerovium. It is thought that to be able to synthesize these elements, a new, stronger particle accelerator would be needed.