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Island of stability

The island of stability is a term from nuclear physics that describes the possibility of elements with particularly stable "magic numbers" of protons and neutrons. This would allow certain isotopes of some transuranic elements to be far more stable than others; that is, decay much more slowly.

The idea of the island of stability was first proposed by Glenn T. Seaborg. The hypothesis is that the atomic nucleus is built up in "shells" in a manner similar to the 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 fill the energy levels of a given shell in the nucleus, the binding energy per nucleon will reach a local minimum and thus that particular configuration will have a longer lifetime than nearby isotopes that do not have filled shells.

A filled shell would have "magic numbers" of neutrons and protons. One possible magic number of neutrons is 184, and some possible matching proton numbers are 114, 120 and 126 — which would mean that the most stable possible isotopes would be ununquadium-298, unbinilium-304 and unbihexium-310. Of particular note is Ubh-310, which would be "doubly magic" (both its proton number of 126 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 nucleus is Lead-208, the heaviest stable nucleus and most stable heavy metal.) None of these transuranic isotopes has yet been produced, but isotopes of elements in the range between 110 through 114 are slower to decay than isotopes of nearby nuclei on the periodic table.

Half-lives of large isotopes

Fermium is the largest element that can be produced in a nuclear reactor. The stability (half-life of the longest-lived isotope) of elements generally decreases from element 101 to element 109 and then approaches an island of stability with longer-lived isotopes in the range of elements 111 and 114. The longest-lived observed isotopes are shown in the following table.

Isotopes of elements 100 through 118
Number	Name	Longest-lived isotope	Half-life of longest-lived isotope	Article
100	fermium	²⁵⁷Fm	101 days	Isotopes of fermium
101	mendelevium	²⁵⁸Md	52 days	Isotopes of mendelevium
102	nobelium	²⁵⁹No	58 minutes	Isotopes of nobelium
103	lawrencium	²⁶²Lr	3.6 hours	Isotopes of lawrencium
104	rutherfordium	²⁶⁷Rf	1.3 hours	Isotopes of rutherfordium
105	dubnium	²⁶⁸Db	29 hours	Isotopes of dubnium
106	seaborgium	²⁷¹Sg	1.9 minutes	Isotopes of seaborgium
107	bohrium	²⁷⁰Bh	61 seconds	Isotopes of bohrium
108	hassium	²⁷⁷Hs	16.5 minutes	Isotopes of hassium
109	meitnerium	²⁷⁸Mt	0.72 seconds	Isotopes of meitnerium
110	darmstadtium	²⁸¹Ds	11 seconds	Isotopes of darmstadtium
111	roentgenium	²⁸⁰Rg	3.6 seconds	Isotopes of roentgenium
112	ununbium	²⁸⁵Uub	29 seconds	Isotopes of ununbium
113	ununtrium	²⁸⁴Uut	0.49 seconds	Isotopes of ununtrium
114	ununquadium	²⁸⁹Uuq	2.6 seconds	Isotopes of ununquadium
115	ununpentium	²⁸⁸Uup	88 ms	Isotopes of ununpentium
116	ununhexium	²⁹³Uuh	61 ms	Isotopes of ununhexium
117	ununseptium	Yet unknown	N/A	Isotopes of ununseptium
118	ununoctium	²⁹⁴Uuo	0.89 ms	Isotopes of ununoctium

The half lives of elements in the island are uncertain. Many physicists think they are relatively short, on the order of minutes, hours, or perhaps days. However, some theoretical calculations indicate that their half lives may be long (some calculations put it on the order of 10⁹ years). It is possible that these elements could have unusual chemical properties, and, if long lived enough, various applications (such as targets in nuclear physics and neutron sources). However, the isotopes of several of these elements still have too few neutrons to be stable. The island of stability still hasn't been reached, since the island's "shores" have neutron richer nuclides than those produced.

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.

Island of relative stability

²³²Th (thorium), ²³⁵U and ²³⁸U (uranium) are the only naturally occurring isotopes beyond bismuth that are relatively stable over the current lifespan of the universe. Bismuth was found to be unstable in 2003, with an α-emission half-life of 1.9 × 10¹⁹ years for Bi-209. All other isotopes beyond bismuth are relatively or very unstable. So the main periodic table ends at bismuth, with an island at thorium and uranium. Between bismuth and thorium there is a sea trough of severe instability, which renders such elements as astatine, radon, and francium extremely short-lived relative to all but the heaviest elements found so far.

Current theoretical investigation indicates that in the region Z =106-108 and N~160-164 a small ‘island/peninsula’ might survive fission and beta-decay, and superheavy nuclei in this region might predominantly undergo alpha decay.. Also, ²⁹⁸114 is not the center of the magic island as predicted earlier. On the contrary, the nucleus with Z=110, N=183 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 ²⁶⁸106 is predicted to have alpha-decay half life ~3.2hrs that is greater than that (~28s) of the deformed doubly-magic ²⁷⁰108. The superheavy nuclei ²⁶⁸106 has not been produced in the laboratory as yet (2008). For superheavy nuclei with Z >116 and N ~184 the alpha-decay half-lives are predicted to be less than one second. The Z=120, 124, 126 with N=184 are predicted to form spherical doubly-magic nuclei and survive fission. Calculations in a quantum tunneling model show that such superheavy nuclei would undergo alpha-decay within microseconds or, less. .

Synthesis problems

Manufacturing nuclei in the island of stability may be very difficult, because the nuclei available would not deliver the necessary sum of neutrons. So for the synthesis of isotope 298 of element 114 by using plutonium and calcium, one would require an isotope of plutonium and one of calcium, which have together a sum of at least 298 nucleons (more is better, because at the nuclei reaction some neutrons are emitted). This would require for example in the case of synthesis of element 114 the usage of calcium-50 and plutonium-248. However these isotopes (and heavier calcium and plutonium isotopes) are not available in weighable quantities. This is also the case for other target/projectile-combinations.

However it may be possible to generate the isotope 298 of element 114, if nuclear transfer reactions would work. One of these reactions may be:

²⁰⁴Hg + ¹³⁶Xe → ²⁹⁸Uuq + ⁴⁰Ca + 2n