Island of stability

Measured (boxed) and predicted (shaded) half-lives of isotopes, sorted by number of protons and neutrons. The expected location of the island of stability is circled.

In nuclear physics, the island of stability is the prediction that a set of heavy isotopes with a near magic number of protons and neutrons will temporarily reverse the trend of decreasing stability in elements heavier than Uranium. Although predictions of the exact location differ somewhat, Klaus Blaum expects the island of stability to occur in the region near the isotope ³⁰⁰Ubn. Estimates about the amount of stability on the island are usually around a half-life of minutes or days, with "some optimists" expecting half-lives of millions of years.

Although the theory has existed since the 1960s, the existence of such superheavy, relatively stable isotopes has not been demonstrated. Like the rest of the superheavy elements, the isotopes on the island of stability are practically never found in nature, and so must be created in an artificial nuclear reaction to be studied. However, scientists have not found a way to carry out such a reaction.

Theory and origin

One fact should be emphasized from the outset: while the various theoretical predictions about the superheavy nuclei differ as to the expected half-lives and regions of stability, all theoretical predictions are in agreement: superheavy nuclei can exist. Thus, the search for superheavy nuclei remains as a unique, rigorous test of the predictive power of modern theories of the structure of nuclei.

— Seaborg and Loveland, 1987.

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.

A filled shell would have "magic numbers" of neutrons and protons. 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 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 spherical nucleus is lead-208, the heaviest known stable nucleus and most stable heavy metal.)

Recent research indicates that large nuclei are deformed, causing magic numbers to shift. 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.

Isotopes have been produced with enough protons to plant them upon an island of stability but with too few neutrons to even place them upon the island's outer "shores". 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 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: 118. The longest-lived observed isotopes of each of the heaviest elements are shown in the following table.

Known isotopes of elements 83 through 118

Number	Name	Longest-lived isotope	Half-life	Article
83	Bismuth	209Bi	2 × 1019 years	Isotopes of bismuth
84	Polonium	209Po	130 years	Isotopes of polonium
85	Astatine	210At	8 hours	Isotopes of astatine
86	Radon	222Rn	3.824 days	Isotopes of radon
87	Francium	223Fr	22.0 min	Isotopes of francium
88	Radium	226Ra	1600 years	Isotopes of radium
89	Actinium	227Ac	21.77 years	Isotopes of actinium
90	Thorium	232Th	1.41 × 1010 years	Isotopes of thorium
91	Protactinium	231Pa	32800 years	Isotopes of protactinium
92	Uranium	238U	4.47 × 109 years	Isotopes of uranium
93	Neptunium	237Np	2.14 × 106 years	Isotopes of neptunium
94	Plutonium	244Pu	8.0 × 107 years	Isotopes of plutonium
95	Americium	243Am	7400 years	Isotopes of americium
96	Curium	247Cm	1.6 × 107 years	Isotopes of curium
97	Berkelium	247Bk	1000 years	Isotopes of berkelium
98	Californium	251Cf	900 years	Isotopes of californium
99	Einsteinium	252Es	470 days	Isotopes of einsteinium
100	Fermium	257Fm	100.5 days	Isotopes of fermium
101	Mendelevium	258Md	51.5 days	Isotopes of mendelevium
102	Nobelium	259No	58 minutes	Isotopes of nobelium
103	Lawrencium	266Lr	~11 hours	Isotopes of lawrencium
104	Rutherfordium	267Rf	~1.3 hours	Isotopes of rutherfordium
105	Dubnium	268Db	1.3 days	Isotopes of dubnium
106	Seaborgium	269Sg	~2.1 minutes	Isotopes of seaborgium
107	Bohrium	270Bh	61 seconds	Isotopes of bohrium
108	Hassium	277mHs	~12 minutes	Isotopes of hassium
109	Meitnerium	278Mt	7.6 seconds	Isotopes of meitnerium
110	Darmstadtium	281mDs	~3.7 minutes	Isotopes of darmstadtium
111	Roentgenium	281Rg	26 seconds	Isotopes of roentgenium
112	Copernicium	285mCn	~8.9 minutes	Isotopes of copernicium
113	Ununtrium	286Uut	19.6 seconds	Isotopes of ununtrium
114	Flerovium	289mFl	~1.1 minutes	Isotopes of flerovium
115	Ununpentium	289Uup	220 milliseconds	Isotopes of ununpentium
116	Livermorium	293Lv	61 milliseconds	Isotopes of livermorium
117	Ununseptium	294Uus	78 milliseconds	Isotopes of ununseptium
118	Ununoctium	294Uuo	890 microseconds	Isotopes of ununoctium

(Note that for elements 108–118, the longest-lived known isotope is always the heaviest or second-heaviest (111, 115) one discovered, making it likely that there are still longer-lived isotopes among the undiscovered 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.

Periodic table with elements colored according to the half-life of their most stable isotope.

  Stable elements.

  Radioactive elements with half-lives of over four million years.

  Half-lives between 800 and 34,000 years.

  Half-lives between 1 day and 103 years.

  Half-lives ranging between several minutes and 1 day.

  Half-lives less than several minutes.

Islands of relative stability

Region of relative stability: radium-226 to einsteinium-252

		88	89	90	91	92	93	94	95	96	97	98	99
154		Half-life Key   1   10  100    1k  10k 100k   1M  10M 100M   1G  10G (a)								250Cm		252Cf			154
153												251Cf	252Es		153
152										248Cm		250Cf			152
151										247Cm	248Bk	249Cf			151
150								244Pu		246Cm	247Bk				150
149										245Cm					149
148								242Pu	243Am	244Cm					148
147								241Pu	242m⁂	243Cm					147
146						238U		240Pu	241Am						146
145								239Pu							145
144						236U	237Np	238Pu							144
143						235U	236Np								143
142				232Th		234U	235Np	236Pu							142
141						233U									141
140		228Ra		230Th	231Pa	232U					Table Axes Neutrons (N) Protons (Z)				140
139				229Th											139
138		226Ra	227Ac	228Th											138
		88	89	90	91	92	93	94	95	96	97	98	99
Only nuclides with a half-life of at least one year are shown on this table.

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 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 (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. plutonium and curium) form an archipelago. All of this is surrounded by a "sea of instability".

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 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:

²⁴⁸
Cm
+ ²³⁸
U
→ ²⁹⁸
Fl
+ ¹⁸⁶
W
+ 2 ¹
₀n

Hypothetical second island

Recently, interest has been expressed over the possibility of a second island of stability. The idea of a second island of stability was presented by Yuri Oganessian at the 235th national meeting of the American Chemical Society. This new island would be centered around 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.

See also

• Island of inversion
• Table of nuclides

External links

• Six new isotopes of the superheavy elements discovered (Oct 26 2010, Physorg news. Inc chart of heavy nuclides)
• Exploring the island of superheavy elements (April 2010, re decay of 117; with chart)
• Hunting the biggest atoms in the universe (July 23, 2008; claimed finding natural atoms of 122 protons and 170 neutrons)
• The hunt for superheavy elements (April 7, 2008; prediction of seaborgium-290 half-life of 10⁸ years.)
• Here be stability (Nature Aug 2006 with JINR diagram of heavy nuclides and predicted IoS)
• Superheavy elements (Jul 2004 Yuri Oganessian of JINR )
• Uut and Uup Add Their Atomic Mass to Periodic Table (Feb 2004)
• Can superheavy elements (such as Z = 116 or 118) be formed in a supernova? Can we observe them? 2004 – "maybe"
• Second postcard from the island of stability (Oct 2001) nuclides with 116 protons and mass 292
• The synthesis of spherical superheavy nuclei in ⁴⁸Ca induced reactions Reports the 1999 synthesis of Z = 114, N + Z = 287.
• New elements discovered and the island of stability sighted (Aug 1999; includes report on article later retracted)
• First postcard from the island of nuclear stability (1999; first few Z = 114 atoms)
• NOVA: Island of Stability (2006. 13m TV segment, with transcript)
• New York Times editorial by Oliver Sacks regarding the Island of Stability theory (Feb 2004 re 113 and 115)
• Tendency equation and curve of stable nuclides
• Ununoctium Applications (diagram of IoS)