Science Sunday: Fact Or Fiction? Lead Can Be Turned Into Gold
Particle accelerators make possible the ancient alchemist’s dream—but at a steep cost. “It would cost more than one quadrillion dollars per ounce to produce gold by this experiment,» Seaborg told the Associated Press that year.
For hundreds of years alchemists toiled in their laboratories to produce a mythical substance known as the philosopher’s stone. The supposedly dense, waxy, red material was said to enable the process that has become synonymous with alchemy—chrysopoeia, the metamorphosis, or transmutation, of base metals such as lead into gold.
Alchemists have often been dismissed as pseudoscientific charlatans but in many ways they paved the way for modern chemistry and medicine. The alchemists of the 16th and 17th centuries developed new experimental techniques, medicines and other chemical concoctions, such as pigments. And many of them «were amazingly good experimentalists,” says Lawrence Principe, a chemist and science historian at Johns Hopkins University. “Any modern professor of chemistry today would be more than happy to hire some of these guys as lab techs.” The alchemists counted among their number Irish-born scientist Robert Boyle, credited as one of the founders of modern chemistry; pioneering Swiss-born physician Paracelsus; and English physicist Isaac Newton.
But despite the alchemists’ intellectual firepower and experimental acumen, the philosopher’s stone lay forever out of reach. The problem, Principe says, is that the alchemists did not yet know that lead and gold were different atomic elements—the periodic table was still hundreds of years away. Believing them to be hybrid compounds, and therefore amenable to chemical change in laboratory reactions, the alchemists pursued the dream of chrysopoeia to no avail.
With the dawn of the atomic age in the 20th century, however, the transmutation of elements finally became possible. Nowadays nuclear physicists routinely transform one element to another. In commercial nuclear reactors, uranium atoms break apart to yield smaller nuclei of elements such as xenon and strontium as well as heat that can be harnessed to generate electricity. In experimental fusion reactors heavy isotopes of hydrogen merge together to form helium. (An element is defined by the number of protons in its nucleus whereas an isotope of a given element is determined by the quantity of neutrons.)
But what of the fabled transmutation of lead to gold? It is indeed possible—all you need is a particle accelerator, a vast supply of energy and an extremely low expectation of how much gold you will end up with. More than 30 years ago nuclear scientists at the Lawrence Berkeley National Laboratory (LBNL) in California succeeded in producing very small amounts of gold from bismuth, a metallic element adjacent to lead on the periodic table. The same process would work for lead, but isolating the gold at the end of the reaction would prove much more difficult, says David J. Morrissey, now of Michigan State University, one of the scientists who conducted the research. “We could have used lead in the experiments, but we used bismuth because it has only one stable isotope,” Morrissey says. The element’s homogeneous nature means it is easier to separate gold from bismuth than it is to separate gold from lead, which has four stable isotopic identities.
Using the LBNL’s Bevalac particle accelerator, Morrissey and his colleagues boosted beams of carbon and neon nuclei nearly to light speed and then slammed them into foils of bismuth. When a high-speed nucleus in the beam collided with a bismuth atom, it sheared off part of the bismuth nucleus, leaving a slightly diminished atom behind. By sifting through the particulate wreckage, the team found a number of transmuted atoms in which four protons had been removed from a bismuth atom to produce gold. Along with the four protons, the collision-induced reactions had removed anywhere from six to 15 neutrons, producing a range of gold isotopes from gold 190 (79 protons and 111 neutrons) to gold 199 (79 protons, 120 neutrons), the researchers reported in the March 1981 issue of Physical Review C.
The amount of gold produced was so small that Morrissey and his colleagues had to identify it by measuring the radiation given off by unstable gold nuclei as they decayed over the course of a year. In addition to the several radioactive isotopes of gold, the particle collisions presumably produced some amount of the stable isotope gold 197—the stuff of wedding bands and gold bullion—but because it does not decay the researchers were unable to confirm its presence. “The stable isotope would have to be observed in a mass spectrometer,” Morrissey says, “but I think that the number of atoms was, and is still, below the level of detection by mass spec.”
Isolating the minute quantities of gold would be even more difficult using lead as a starting material, but smashing high-speed nuclei into a lead target would indeed complete the long-sought transmutation. Some of the collisions would be expected to remove three protons from lead, or one proton from mercury, to produce gold. “It is relatively straightforward to convert lead, bismuth or mercury into gold,” Morrissey says. “The problem is the rate of production is very, very small and the energy, money, etcetera expended will always far exceed the output of gold atoms.”
In 1980, when the bismuth-to-gold experiment was carried out, running particle beams through the Bevalac cost about $5,000 an hour, “and we probably used about a day of beam time,” recalls Oregon State University nuclear chemist Walter Loveland, one of the researchers on the project. Glenn Seaborg, who shared the 1951 Nobel Prize in Chemistry for his work with heavy elements and who died in 1999, was the senior author on the resulting study.
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