PORTLAND, Ore. -- You might have thought that scientists had already cataloged the 118 natural elements--from hydrogen to silicon to uranium--and their various isotopes. Unfortunately, only the first eight elements--hydrogen to oxygen--have had all their isotopes recorded. Recently, Michigan State University's National Superconducting Cyclotron Laboratory (NSCL) began smashing atoms in hopes of cataloging the isotopes for the other 110 elements on the periodic table. So far, they've found three never-before-observed isotopes of silicon, aluminum and magnesium.
"We are doing classical atom smashing--a beam of particles accelerated to half the speed of light is broken up against a target while a tandem mass spectrometer separates out all the different kinds of particles and identifies them," said Dave Morrissey, a professor at Michigan State University (Lansing). "Our new isotopes only last a fraction of a second before undergoing radioactive decay, but by confirming their existence, we are helping other scientists design new materials and understand astronomical events, like supernovas."
The three new isotopes discovered at the NSCL are silicon-44, aluminum-42 and magnesium-40. Silicon found in nature has only three stable isotopes: silicon-28 (92 percent in nature) silicon-29 (four percent) and silicon-30 (four percent). Magnesium also has three stable isotopes: magnesium-24 (79 percent), magnesium-25 (10 percent) and magnesium-26 (11 percent). Aluminum has only single stable isotope--aluminum-27--but exists in radioactive isotopes ranging from aluminum-21 up to aluminum-43, the latter observed in a single glimpse for the first time at NSCL. By discovering all the isotopes for an element, design engineers can more accurately tailor future materials to an application's needs.
"How heavy you can make an isotope is a fundamental limit that needs to be charted out," said Morrissey. "Just as the total number of elements is limited to about 118, each chemical element has a heaviest-possible isotope. So, in a way, we are trying to find out what the fundamental limits of nature are."
Pioneering atom smashers
Elements are defined by how many protons they have, which range from one to 118. But the number of neutrons that can accompany those protons is variable, and, thus, the number of isotopes of each element is variable. Theorists have attempted to write formulae to deduce which isotopes are possible, but without much success, leaving it to pioneering atom smashers to fill-in the blanks with empirical evidence.
The isotopes were made by starting with a beam of the heaviest stable isotope of calcium-48 accelerated to half the speed of light, then smashed against a heavy tungsten target, breaking it apart into hundreds of different elements, most of which are unstable. These unstable atoms offer brief glimpses into nature's catalog of all possible isotopes, enabling the scientists to filter out those rare ones that help construct their catalog of isotopes.
During the recent round of isotope discoveries, the Michigan State team jury-rigged their cyclotron to twice filter the atom fragments it creates. The success of this technique in glimpsing aluminum-43 for the first time has prompted the scientists to propose making the two-stage filtration system a permanent part of the cyclotron accelerator.
The heaviest isotope for each element is called its "drip line," because there will be examples of isotopes at all levels between the lowest and highest isotope. The numbers given after an element--for instance, carbon-12 or carbon-13--indicate the number of protons and neutrons in the atom. Carbon, for instance, has 15 isotopes, from carbon-8 to its highest value, or drip line, of carbon-22. Only two of carbon's 15 isotopes are stable--carbon-12 and carbon-13. The rest are radioactive and decay by beta emission. About 98 percent of naturally occurring carbon is carbon-12, but about two percent is the heavier carbon-13.
For silicon, aluminum and magnesium, the NSCL team reports raising their drip lines--to silicon-44, aluminum-42 and magnesium-40. The team also observed aluminum-43, but only in a single experiment and is therefore seeking the verification of repeatability. Converting a one pound block of normal aluminum-27--with 13 protons and 14 neutrons--into aluminum-43--with 13 protons and 30 neutrons--would cause it to weigh almost 1.6 pounds.