| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
|
|
|
|
|
Counterpoints in Science Technetium
After vanishing from Earth more than four billion years ago, the element technetium now plays key roles in cosmology and medical imaging. Its amazing comeback is due to the work of Ernest O. Lawrence of the University of California, Berkeley, who in 1937 irradiated a piece of the element molybdenum in a cyclotron, a type of particle accelerator he had invented. Then he sent the radioactive metal to two Italian chemists, Carlo Perrier and Emilio Segré. They separated out a tiny amount of a substance never before seen. They called it technetium, from the Greek word meaning artificial. It was the first element to be re-created by humans. Scientists had been looking for this element since 1869, when Russian chemist Dmitri Mendeleev published his Periodic Table of the Elements. Modern versions of the Periodic Table hang in millions of classrooms throughout the world. The elements are lined up on the table so that those with similar chemical properties form columns. For instance, the column on the right side of the chart is made up of the inert gases-helium, neon, argon, krypton, xenon, and radon. Because of this periodic arrangement, Mendeleev expected that element 43 would resemble manganese, a grayish-white metal with a reddish tinge, which is directly above element 43 in the column. But the space for element 43 was blank. No such element was known. Intense searches during the next 50 years failed to turn up any of this mysterious stuff. Why did it take so much longer to find element 43 than the other 91 elements already well known to science? When Perrier and Segré finally isolated technetium in 1937, by chemically separating it from an irradiated mass of molybdenum, the puzzle of its elusiveness began to be understood. The material was radioactive, with a half-life of 2.6 million years. A half-life is the time it takes for 50 percent of a radioactive element to decay into its "daughter product," usually a different element. After about ten half-lives (in this case 26 million years), virtually nothing is left of the original material. Although 26 million years is a long time in human terms, it is a very short time in comparison with the 4.6 billion year age of the Earth. Any technetium that might have existed when the solar system and Earth were formed would long since have disappeared. The naturally occurring radioactive isotopes Potassium-40 and Uranium-238 have survived on Earth because they have half-lives greater than a billion years. Potassium-40 decays into Calcium-40 and Argon-40, while Uranium-238 decays by a complicated series of nuclear reactions into Lead-206. Atomic nuclei are made up of two different kinds of particles, protons and neutrons. The element number refers to the number of protons in its nucleus. For example, element number 1, hydrogen, has a nucleus consisting of a single proton. Element 2, helium, has two protons and two neutrons. The material found by Perrier and SegrŽ has 43 protons and 54 neutrons, and is designated Technetium-97. Other forms of technetium, called isotopes, have different numbers of neutrons, but all have 43 protons. Technetium is now known to have more than 20 isotopes. All are radioactive and unstable. Technetium-98 has the longest half-life, 4.2 million years, which is still too short to have survived to the present day: hence the gap in Mendeleev's Periodic Table. In the middle of the twentieth century, about the time technetium was being reincarnated on Earth, physicists and cosmologists were trying to explain the natural abundances of the different elements. Why, they wondered, does the observable universe seem to consist of about 80 percent hydrogen, 17 percent helium, and relatively tiny amounts of heavier elements such as iron, gold, and uranium? In techno-jargon, this is called "the nucleosynthesis problem." George Gamow and Fred Hoyle were two of the cosmologists competing to solve the nucleosynthesis problem in the 1940s. Gamow, a Russian, was a strong proponent of the idea that the vast universe had originated as a tiny point in which everything was squeezed together. As this point exploded outward, Gamow calculated it would have produced the same amounts of hydrogen and helium we actually observe. Hoyle scoffed at the notion of a catastrophically created universe, which he derisively called The Big Bang. Instead, Hoyle favored a "steady-state" universe that always looked the way it does now. He contended that all the elements heavier than helium had been cooked up in the furnaces of stars. Deep in the bellies of heavy stars the heat and gravitational pressure would smash together the nuclei of the light elements hydrogen and helium to create the heavier elements. It turned out that both of these protagonists were partly right. Gamow predicted the intense hot flash of the initial bang would cool off as the universe expanded and might still be present as cosmic background radiation that permeates space. This radiation was detected by Arno Penzias and Robert Wilson in 1965, giving a big boost to the Big Bang Theory. The Big Bang, however, does not explain how the heavy elements were created. We know that heavy elements like iron exist in many stars because the light from these stars contains the elements' characteristic wavelengths, which are as specific as fingerprints. But the spectral fingerprints of iron don't tell us whether the iron was made in those stars or whether it already existed when the stars first formed many billions of years ago. The riddle was solved in 1952, when Paul Merrill of the Mount Wilson Observatory identified the spectrum of technetium in certain red giant stars. Since all isotopes of technetium have short half-lives relative to the age of stars, its presence in red giants means it is constantly being created there. Hoyle was right about the stellar origin of the heavy elements, just as Gamow was right about The Big Bang origin of the light elements. If Merrill's observation was the smoking gun, an international collaboration earlier this year found the spent cartridges. Italian and American scientists examined meteorites containing stardust, tiny bits of giant stars that lived and died before the solar system formed. The scientists found that elemental abundances in stardust showed spectacular agreement with theoretical predictions-with one exception. There was too much Ruthenium-99, a steel-gray metal resembling platinum. The excess Ruthenium-99, however, could be accounted for if one assumes the original star contained a lot of Technetium-99, which decays with a half-life of 210,000 years into Ruthenium-99. In effect, the American and Italian investigators counted atoms that aren't there in stars that no longer exist. And technetium, like the dog that didn't bark in the Sherlock Holmes story, "The Adventure of Silver Blaze," again played a major role in solving a mystery. Our solar system was formed from the remains of an exploding supernova. Therefore, the Earth and its creatures are really stardust. The carbon that makes us organic, the calcium in our bones, the iron in our red blood cells, were all concocted in the guts of a gigantic stellar object and spewed out into space during its violent demise. Now that we can make technetium abundantly on Earth in nuclear reactors, does the element have any practical uses beside its role in cosmological understanding? Yes, it does. Technetium has at least two unique applications. In tiny amounts, it is the most powerful preventative of rust in steel known. When sprayed on carbon steel, a solution of five parts per million of pertechnetate, a salt of technetium will protect the steel from corrosion up to 250 ¼C. Why this coating works so well is not known. Unfortunately, since technetium is radioactive, this protection is limited to closed systems where people are not working. Technetium's second and most valuable application stems from the very fact that it is radioactive. Technetium made considerable impact on my own life and work starting in the 1960s. In medicine, we are always looking for better ways of taking pictures of internal organs like the heart, lungs, liver, and kidneys, and assessing their functions. Unlike better-known imaging procedures such as x-rays, CT scans and MRI scans, which focus on obtaining accurate anatomical pictures, nuclear studies emphasize organs' functions more than their appearance. Some radioactive tracers, for instance, are concentrated by the kidneys. After such a tracer is injected, computerized "gamma cameras" are used to track how fast the blood flows to each kidney, and the rates at which the tracer is excreted into the urine. This information is valuable in diagnosing kidney diseases and in guiding drug treatment after kidney transplants. Once, the main tracer used in nuclear medicine was Iodine-131, which has a half-life of eight days, delivers a fairly large dose of radiation to the patient, and does not produce very good pictures. By comparison, Technetium-99m (Tc-99m) is just about an ideal isotope for medical imaging. It is inexpensive and readily available, and has a half-life of only six hours. It delivers a much smaller radiation dose to patients than Iodine-131, and the resulting pictures are sharper and more detailed. Best of all, technetium can be "tagged" onto many different chemical compounds used in medical imaging. Tc-99m imaging agents can "light up" the thyroid, heart, lungs, bones, liver, spleen, kidneys, and various other organs, tell us how well they are performing their normal functions, and show disease processes. Today, probably 80 to 90 percent of all nuclear scans are done with this tracer. In a typical day as a nuclear medicine physician, I might see a patient with a lump on her thyroid gland. If the lump takes up Tc-99m in the same way as the rest of the gland, it's probably harmless. If it doesn't, a biopsy might be necessary to rule out cancer. Whole-body bone scans that look like miniature skeletons can show whether cancers of the breast or prostate have spread to the bones. Cancer shows up as black dots in the white bones. Men and women with suspected heart disease are scanned at rest and after treadmill exercise. If Tc-99m uptake in the heart is normal both at rest and during exercise, coronary disease is unlikely. If it is normal at rest but abnormal with exercise, the patient's coronary arteries have likely narrowed, and angioplasty or other surgery may be needed. Technetium, the first element to reappear after mankind rubbed the magic lamp of nuclear science, has earned its central position in the Periodic Table. The genie has rewarded us by revealing what goes on inside the stars and what goes on inside that peculiar end-product of nucleosynthesis, that animated hunk of stardust, the human body. Jerold Lowenstein is professor of medicine at the University of California, San Francisco. jlowen@itsa.ucsf.edu |
