In a triumph marked by years of unremitting experiments, scientists at the Riken national research institute have synthesized new atomic element 113, making Japan the first Asian country to have the honor of naming a new element.
Solid evidence obtained from the experiments, which lasted for nine years, was the decisive factor allowing the Riken group to win the rights at the end of last year over a rival team of U.S. and Russian researchers.
The Japanese researchers, led by Kosuke Morita, who serves both as a group director at Riken and as a professor of nuclear physics at Kyushu University, attempted to synthesize element 113 through the method of bombarding the nucleus of bismuth (atomic number 83) with an accelerated nucleus of zinc (atomic number 30) to trigger nuclear fusion.
The team faced two major challenges: to determine optimal bombardment conditions and to successfully detect the product element.
Speeds of bombardment that were too high ended up destroying the atomic nucleus, whereas speeds that were too low fell short of causing reactions. The scientists relied on experiments to synthesize known elements and other means to adjust the speed until they discovered that 10 percent of the speed of light was the optimal choice.
Successful bombardment does not necessarily produce element 113. The bombardment was attempted hundreds of quintillions of times (where a quintillion is a million times a trillion), but all that produced only three atoms over the course of nine years.
Those atoms were captured by the gas-filled recoil ion separator (GARIS), a detector developed by Morita, which uses electromagnets to sort particles that fly about during the experiments. Its structure was designed to efficiently detect the target element.
An atom of element 113, once synthesized, decays with a discharge of alpha rays after an average time of 0.002 second. It continues to decay consecutively, turning each time into a different element. Success in tracking down that process to elements with well-known decay characteristics earned Riken the naming rights.
“(Riken’s) victory was brought about by unambiguous demonstration of the connections to known atomic nuclei,” said Toshimitsu Yamazaki, a professor emeritus of nuclear physics with the University of Tokyo, who sits on a working party of the International Union of Pure and Applied Chemistry, which is tasked with assigning the naming rights. “The Japanese researchers produced something reliable, be it in small numbers.”
The rival team of U.S. and Russian scientists took the approach of accumulating a large number of cases and using circumstantial evidence to offer proof. They bombarded heavier elements, such as americium (atomic number 95), with calcium (atomic number 20) to synthesize elements 115 and 117 in the first place, and tried to find element 113 during the processes of decay.
The U.S. and Russian researchers used reactions that are called “hot nuclear fusion,” which can produce element atoms in large numbers but are prone to cause nuclear fission along the way. That approach has the drawback of added difficulty in tracking down connections to known elements than in Riken’s approach, which uses “cold nuclear fusion” reactions.
But elements 115, 117 and 118, whose naming rights were assigned to the U.S.-Russian team on the same occasion, were officially recognized by way of the first approach. Riken’s method is inefficient in synthesizing heavier elements, whereby it loses its competitive edge.
Riken would be hard-pressed to discover element 119 and subsequent elements under its current approach.
So far, 118 have been named. The larger the atomic number, the heavier and the more prone the atomic elements are to breaking up.
Neptunium (atomic number 93) and subsequent elements are seldom found in nature and have therefore been synthesized artificially. How far the table of elements continues is a longstanding subject in chemistry and physics.
Discovering unknown elements with long lives represents another scientific challenge for the future. Theory predicts there will be elements with lives ranging between one day and several years, unlike element 113, which breaks up in a tiny fraction of a second. The prediction, known under the name of the “island of stability,” is believed to present one of the largest conundrums of nuclear physics.
“We will think about working efficiently, with an eye toward international cooperation,” Morita said.