Tikalon Blog by Dev Gualtieri

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The Island of Stability

October 28, 2010

Atom smashing isn't just about splitting nuclei , it's also about building larger nuclei from smaller parts. Impacting the proper target with a selected nucleus will sometimes yield a nucleus with a larger number of nucleons that either of the originals. This is the way that synthetic elements , elements with too short of a lifetime to exist in nature, are made. The lifetimes of some of these are extremely short. For example, Bohrium (element 107) has an isomer , ^262mBh, with a half life of just 9.6 msec, and Copernicium (element 112) has an isotope, ²⁸²Cn, with only a 0.8 msec half life. Confirmation of such elements would not be possible without the pioneering work of the British physicist, Henry Moseley , in defining Moseley's law .

Henry Moseley

Henry Moseley

The creation of elements with such short lifetimes has always reminded me of the economic principle of marginal utility , since it seems that physicists are working harder and harder to create elements that last for shorter and shorter times. What motivates them is the idea that they might be able to create superheavy elements that are extremely stable, since nuclear shell theory predicts an island of stability centered around magic numbers of protons and neutrons . Lifetimes on this island may be minutes or days; or, these elements may be as stable as those that populate the lower regions of the Periodic Table . The roster of elements at the shoreline of this island includes ununquadium-298 (an isotope of element 114), unbinilium-304 (an isotope of element 120) and unbihexium-310 (an isotope of element 126). unbihexium-310 is believed to be especially stable, since it's "doubly magic." Its proton number of 126 and neutron number of 184 are both magic numbers.

There are some early indications that the island is near at hand. Copernicium (atomic number 112) is presently the highest-numbered named element, and the isotope ²⁸⁵Cn has a half-life of about thirty seconds. Ununquadium , the discovered element with atomic number 114, has an isotope, ²⁸⁹Uuq, with a half-life of about 2.6 seconds, and there's evidence of a ^289bUuq isomer with a 66 second half life.

Recently, physicists from Lawrence Berkeley National Laboratory (LBNL) observed six new superheavy isotopes in the disintegration of a new isotope of element 114.[1,2] It was an LBNL team that confirmed the existence of element 144 first reported by the Joint Institute for Nuclear Research (Dubna, Russia), so they had experience in creation of ununquadium (Uuq).[3] The Uuq is produced at LBNL by the impact of Calcium-48 ions into a Plutonium-243 target. Calcium was chosen as the impactor, since its nucleus is doubly-magic, having 20 protons and 28 neutrons. Being neutron-rich aids in its fusion with plutonium, which occurs at the relatively low center-of-target beam energy of 256 MeV . Since a lot of heat is generated by the impact of the calcium beam into the plutonium target, the plutonium was mounted on a rotating target to disperse heat, a technique used in rotating anode X-ray tubes .

The LBNL team observed the following decays

²⁸⁵Uuq → ²⁸¹Cn → ²⁷⁷Ds → ²⁷³Hs → ²⁶⁹Sg → ²⁶⁵Rf

Each step of the decay chain occurred via the emission of an alpha particle , as the decrement of four nucleons in each step indicates. The chain ended at rutherfordium , which split into two other nuclei by spontaneous fission . The copernicium isotope lived for less than 0.2 seconds, darmstadtium-277 lived just eight milliseconds. Hassium-273 remained for about a third of a second, seaborgium-269 had a rather long life of three minutes, five seconds. The rutherfordium-265 existed for two and a half minutes before its spontaneous fission. None of these elements are on the island of stability, but the experiment is important for another reason. Scientists need to develop their tools as well as perform experiments, and that's happening at LBNL. Research like this takes a lot of people, and there were 20 scientists on the team that performed this experiment, not only from LBNL, but from the University of California, Berkeley , Lawrence Livermore National Laboratory , the GSI Helmholtz Centre for Heavy Ion Research GmbH (Germany), Oregon State University , and the Institute for Energy Technology (Norway).

The goal, as predicted by theory, is ²⁹⁸Uuq, which would have 184 neutrons. This would be a doubly-magic nucleus that has full proton and neutron shells. Some models, however, predict a stable proton number of 120 or 126, which is quite a few elements removed from ununquadium (atomic number 114). Related research is underway at Oak Ridge National Laboratory (ORNL) to understand the nuclear shell model by experiments on the stability of tin-132 , which has a doubly-magic nucleus of 50 protons and 82 neutrons.[4]

As I discussed in a previous article (Superheavy Element Unbibium, April 29, 2008), a research team at the Hebrew University of Jerusalem caused a sensation a couple of years ago when they found evidence for a long-lived isotope of unbibium (Ubb, element 122).[5,6] They used a mass spectrometer to examine thorium . Unbibium would be chemically similar to thorium, and if it exists as a stable element, it would concentrate in the same ores as thorium. They found what appeared to be ²⁹²Ubb. Subsequent experiments with more sensitive equipment have not confirmed this result, but it's an interesting and less expensive technique for discovering the island elements.

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