Led by Dr. Kosuke Morita at the RIKEN Nishina Center for Accelerator-based Science, a group of scientists specializing in the superheavy elements have established the clearest evidence yet for the synthesis of the a new element with the temporary name of ununtrium (element 113). Claims of discovering a new element in the 21st century are usually the result of lengthy experiments involving new detection methods and element 113, which was first reported in 2003, has been particularly elusive.
Ununtrium is part of a group called superheavy or transuranic elements (those with atomic numbers greater than 92 – the atomic number of uranium). To date, all the transuranic elements having 93-117 protons have been discovered and confirmed (although 115 and 117 are still not officially recognized), save for element 113. Until now.
Transuranics, are difficult to produce and difficult to detect and verify. While the lighter transuranics can be produced in nuclear reactors through neutron absorption (curium with 100 protons was discovered in fallout from the first thermonuclear weapon test), heavier elements can only be made in particle accelerators when a rapidly moving ion strikes a target atom, and the two nuclei fuse together. For example, the nuclear fusion of californium (98 protons) and carbon (six protons) creates rutherfordium (104 protons).
When a new element is discovered, modern practice is that the IUPAC (International Union of Pure and Applied Chemistry) vets all such claims, and usually requires confirmation of a group's work before announcing the existence of a new element. With the number of accelerator facilities capable of reproducing transuranic element synthesis decreasing, IUPAC has decided not to insist on independent confirmation, although they still prefer such.
Element 113 was first reported in 2003 as a decay product of element 115, in 2004 the synthesis of ununtrium was claimed by Dr. Morita's group at RIKEN, resulting from the fusion of zinc ions (30 protons) with bismuth atoms (83 protons), but these early observations did not provide enough evidence to claim that atoms of element 113 had been produced.
There isn't time to do anything with a new transuranic element except to watch it decay. This typically gives two pieces of information – the decay times in milliseconds (ms) and the energy of any radiated particle. In most transuranic elements the decay chain is quite long. Element A decays into B, B into C, and so forth until at last a stable nucleus is formed (often of lead). But what if you don't know what A is? Then B has a particular relationship to A, depending on the type of decay.
For example, alpha decay is the emission of an alpha particle containing 2 protons and two neutrons, so if A undergoes alpha decay, B has an atomic number 2 smaller and a mass number (total number of nucleons in the nucleus) 4 smaller than A. At this point watching A decay into B tells you that something was made that undergoes alpha decay, but you really still don't know much more about A, unless there is something characteristic about B – usually how B decays (or C or D...). Once you recognize a known isotope with some degree of certainty, you can work back through the decay chain to discover the identies of the earlier isotopes.
An example of a decay chain with an anchor is the decay of americium-241, the isotope used in smoke detectors:
- 95 Am 241 produces 93 Np 237 through alpha decay
- 93 Np 237 produces 91 Pr 233 through alpha decay
- 91 Pr 233 produces 92 U 233 through beta decay (emission of an electron)
Still with us? OK, now assume that you don't know about americium, neptunium, or protactinium, but you are familiar with uranium isotopes. You start with candidate isotope A. You observe two alpha decays followed by a beta decay, giving you isotope D. Chemical analysis shows that isotope D can be chemically separated along with uranium, so D is a uranium isotope. When you observe the separated D decaying by emission of alpha particles with an energy of 4.909 MeV, you realize that D must be uranium-233 – the only uranium isotope that decays in that manner.
Now you go back up the decay chain. The U-233 came from the beta decay of C. Beta decay is the sign of a neutron changing into a proton, so C was an isotope with 91 protons and 142 neutrons. C came from the alpha decay of B. Adding the two protons and two neutrons tells us B was an isotope with 93 protons and 146 neutrons. B in turn came from the alpha decay of A, so A must have been an isotope with 95 protons and 146 neutrons, for a mass number of 241. In this process we have "discovered" protactinium, neptunium, and americium.
Returning now to Dr. Morita's 2004 claim of creating element 113 at RIKEN. His group bombarded a target of bismuth atoms (having 83 protons and 126 neutrons) with a beam of zinc ions (having 30 protons and 40 neutrons). The beam energy was 349 MeV (roughly 5 MeV per nucleon) and the total number of zinc ions striking the target was 1.7 x 10^20, or about 200 milligrams. They found one solid candidate for for an atom of element 113. They observed three successive alpha decays, having energies and delay times of 11.68 MeV and 0.344 milliseconds (ms), 11.15 MeV and 9.26 ms, and 10.03 MeV and 7.16 ms respectively.
If we try to identify the candidate isotope by working back up the decay chain, we have to ask if the 10.03 MeV decay with a decay time of 7.16 ms is sufficiently unique that an identification can be claimed. Three successive alpha decays from the desired 113-165 isotope would give us an isotope of element 107 having 159 neutrons. This would be bohrium-266, which would have resulted from an alpha decay of meitnerium-270. Unfortunately, there is no independent measurement of the decay properties of Mt-270, so this decay chain has no anchor, and cannot form the basis for claiming a discovery.
In 2005, four examples of a four alpha decay chain was detected by Morita's group at Riken, presumably leading to dubnium-262 with 105 protons and 157 neutrons. This last product was observed to decay by spontaneous fission, and dubnium-262 does decay by spontaneous fission, but also by alpha decay and by electron capture. Among the four events only the spontaneous fission decay was observed, which, as spontaneous fission is so common in transuranic elements, is insufficient information to identify dubnium-262. Hence the identification of element 113 was insufficiently justified by the data. The decay chain still lacked an anchor.
In Morita's recent set of experiments, the anchor has been found. Using the same isotopes and energies as before, a candidate was observed to undergo a series of six consecutive alpha decays, as shown in the figure above. The energy and half-life for the alpha decay of dubnium-262 are 8.45 MeV and 40 s, while the dubnium-262 candidate had an energy of 8.63 MeV and 126 s. The energy and half-life for the alpha decay of lawrencium-258 are 8.60 MeV and 4.2 s, while the lawrencium-258 candidate had an energy of 8.66 MeV and 3.78 s. Both of these data points are consistent with the known decays. As a result, they are both anchors for the decay chain, meaning that the discovery of ununtrium-278 has been confirmed.
Now Dr. Morita will await an official IUPAC announcement that element 113 has indeed been found. This will probably take a couple of years, during which time he can contemplate what name to choose for the first transuranic element discovered in Asia.
"For over 9 years, we have been searching for data conclusively identifying element 113, and now that at last we have it, it feels like a great weight has been lifted from our shoulders," Morita said. "I would like to thank all the researchers and staff involved in this momentous result, who persevered with the belief that one day, 113 would be ours. For our next challenge, we look to the uncharted territory of element 119 and beyond."
The following video presents more of the story of confirming element 113.
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