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
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.
References:
- Paul Preuss, "Six New Isotopes of the Superheavy Elements Discovered," Lawrence Berkeley Laboratory Press Release, October 26, 2010.
- P. A. Ellison, K. E. Gregorich, J. S. Berryman, D. L. Bleuel, R. M. Clark, I. Dragojevic, J. Dvorak, P. Fallon, C. Fineman-Sotomayor, J. M. Gates, O. R. Gothe, I. Y. Lee, W. D. Loveland, J. P. McLaughlin, S. Paschalis, M. Petri, J. Qian, L. Stavsetra, M. Wiedeking and H. Nitsche, "New Superheavy Element Isotopes: 242Pu(48Ca,5n)285114," Phys. Rev. Lett., vol. 105, no. 18 (October 29, 2010), document 182701 (4 pages).
- Paul Preuss, "Superheavy Element 114 Confirmed: A Stepping Stone to the Island of Stability," Lawrence Berkeley Laboratory Press Release, September 24, 2010.
- Katie Freeman, "New 'doubly magic' research reveals role of nuclear shell," Oak Ridge National Laboratory Press Release, June 1, 2010.
- "First superheavy element found in nature" (ArXiv Physics Blog, April 28, 2008).
- A. Marinov, I. Rodushkin, D. Kolb, A. Pape, Y. Kashiv, R. Brandt, R.V. Gentry, and H.W. Miller, "Evidence for a long-lived superheavy nucleus with atomic mass number A=292 and atomic number Z=~122 in natural Th," arXiv Preprint, April 24, 2008.
Permanent Link to this article
Linked Keywords: Particle accelerator; atom smasher; nuclei; nucleons; synthetic elements; half-life; Bohrium; isomer; Copernicium; Henry Moseley; Moseley's law; marginal utility; nuclear shell theory; island of stability; magic numbers; protons; neutrons; Periodic Table; ununquadium-298; unbinilium-304; unbihexium-310; Copernicium; atomic number; Ununquadium; Lawrence Berkeley National Laboratory; LBNL; Joint Institute for Nuclear Research; Dubna, Russia; Calcium-48; Plutonium-243; electronvolt; MeV; rotating anode X-ray tube; University of California, Berkeley; Lawrence Livermore National Laboratory; GSI Helmholtz Centre for Heavy Ion Research GmbH; Oregon State University; Institute for Energy Technology (Norway); Oak Ridge National Laboratory; ORNL; tin-132; Hebrew University of Jerusalem; mass spectrometer; thorium.