June 30, 1998 OAK RIDGE, Tenn., June 25, 1998 -- Physicists from the Department of Energy's Oak Ridge Institute for Science and Education (ORISE) and Oak Ridge National Laboratory (ORNL) have identified the shortest-lived proton emitter known--the 33rd isotope of thulium (Tm), a bright, silvery rare-earth element that was discovered almost 120 years ago in erbium ore in Sweden. They also found that its nucleus is shaped like a soccer ball, not like a football, as previously thought.
The newly identified isotope, thulium-145, exists only a few millionths of a second in its normal "ground" state and then this unstable nucleus emits a proton. As a result, erbium-144, another rare-earth element, is formed. The measured half-life of 3.5 microseconds for thulium-145 makes it the shortest-lived of more than 20 known proton emitters.
"Because thulium-145 is so short lived and because of the emitted proton's kinetic energy that we measured, we determined that thulium-145 is shaped like a soccer ball, although source theories predict that this nucleus is shaped like a football," says Krzysztof Rykaczewski, an ORNL physicist and one of the discoverers of the new isotope.
Thulium-145 is very "proton-rich" for such a heavy nucleus--it has 69 protons and 76 neutrons. By contrast, natural thulium (thulium-169) has 69 protons and 100 neutrons. Thulium-169, when bombarded with neutrons in a nuclear reactor, is used as a source of X rays for medical purposes.
"When an unstable nucleus has more protons than usual for the number of neutrons present, the nucleus has less of a grip on its protons," says Ken Toth, an ORNL physicist and another discoverer of thulium-145. "Occasionally, in a really unstable nucleus, a proton will get around the other protons--confining repulsive forces and will escape the nucleus. We call this phenomenon 'tunneling through the Coulomb barrier.'"
Using the Holifield Radioactive Ion Beam Facility (HRIBF), the Recoil Mass Spectrometer (RMS), and detectors in ORNL's Physics Division, the researchers bombarded a molybdenum-92 target with a beam of stable nickel-58 ions to produce thulium-145, which lasts but a few fleeting moments. Because the experimental setup was tuned to observe very short-lived radioactivity, proton emission was detected.
"As soon as more intense radioactive ions beams are available at the new Holifield Radioactive Ion Beam Facility," says Toth, "it is likely that more proton emitters and other isotopes far from stability will be identified. Such discoveries using this newly operational, research tool will help DOE fulfill its mission of increasing scientific knowledge about the elements that make up the universe."
A report on the discovery was published in the March 19, 1998, issue of Physical Review C : Rapid Communications by Jon Batchelder, a postdoctoral scientist with ORISE; Krzysztof Rykaczewski and Ken Toth, both staff scientists in ORNL's Physics Division; Carrol Bingham, professor of physics at University of Tennessee at Knoxville and adjunct researcher at ORNL; Carl Gross of ORISE, who operates the RMS; and collaborators from other U.S. and United Kingdom laboratories.
ORISE was established by the U.S. Department of Energy to undertake national and international programs in education, training, health, and the environment. ORISE and its programs are operated by Oak Ridge Associated Universities (ORAU) through a management and operating contract with the Department of Energy (DOE). Established in 1946, ORAU is a consortium of 87 colleges and universities.
The research was supported partly by DOE's Office of Energy Research, Office of Basic Energy Sciences, Division of Nuclear Physics. About half the cost of the RMS, which was used for this research, was paid for by universities through ORAU.
ORNL, one of the Department of Energy's multiprogram national research and development facilities, is managed by Lockheed Martin Energy Research Corporation.
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