Discovery of a new super-heavy element sheds light on the basic organization of matter and strengthens the likelihood that still more massive elements may form an “island of stability”— a cluster of stable super-heavy elements that could form novel materials with exotic and as-yet-unimagined scientific and practical applications.
Vanderbilt physicist Joe Hamilton played a key role in the discovery of element 117, which has been created and identified by an international team of scientists from Vanderbilt, the Joint Institute for Nuclear Research (Dubna, Russia), Oak Ridge National Laboratory, Lawrence Livermore National Laboratory, and the Research Institute for Advanced Reactors (Dimitrovgrad, Russia). Vanderbilt Professor of Physics A.V. Ramayya also was a member of the discovery team.
Atomic nuclei consist of protons and neutrons. Elements are determined by the number of positively charged protons in their nuclei. Atoms with the same number of protons but different numbers of neutrons have the same chemical properties but weigh slightly differently and are called isotopes. The lightest natural element is hydrogen, with one proton. The heaviest natural element is uranium, with 92 protons.
Element 117 is the 26th new element that has been added to the periodic table since 1940. “These new elements provide important tests of nuclear theories,” says Hamilton. “The longer lifetimes of the new isotopes observed in our discovery of element 117 make it possible to study the chemistry of these super-heavy elements. These studies will test theoretical predictions that elements beyond 112 could have unexpected positions in the periodical table of elements.”
As scientists created heavier and heavier artificial elements, those elements became increasingly unstable until reaching a limit at element 113. Then the Dubna scientists developed their calcium-bombardment technique, and lifetimes began to climb. In recent years physicists at Dubna, led by Yuri Oganessian, have developed a method of making super-heavy elements by smashing calcium ions, with 20 protons, into various targets. In this way they had discovered elements 114, 115, 116 and 118. They were unable to create element 117, however, because the target required was itself a very exotic and expensive element, berkelium, with 97 protons.
“When I talked to the people at Oak Ridge National Laboratory who have the ability to create berkelium, I was told it would cost $3.5 million,” says Hamilton, the Landon C. Garland Distinguished Professor of Physics at Vanderbilt. “That was too much, so I kept looking.”
After several years of checking regularly with his Oak Ridge contacts, Hamilton finally saw the opportunity he was looking for when he heard they had agreed to make the element californium for a commercial project. He realized berkelium could be extracted at the same time, so he worked out a deal that would produce the target material.
The two-year experimental campaign began at the High Flux Isotope Reactor in Oak Ridge with a 250-day irradiation in the world’s most intense neutron flux that produced 22 milligrams of berkelium. This was followed by 90 days of processing at Oak Ridge to separate and purify the berkelium. The material was sent to Dimitrovgrad for target preparation and then transferred to Dubna, where it was placed in one of the world’s most powerful heavy ion accelerators. Six atoms of element 117 were produced after 150 days of bombardment. The data from these six events were analyzed at Dubna and Livermore, and the results were double-checked by the entire team.
A report of the discovery has been accepted for publication in the Physical Review Letters.