Chasing Atoms

Thanks to Sam Hurst. The chemical composition of matter is now an open book. He can detect impurities as small as a single atom.

By Hans Christian Von Baeyer|Wednesday, July 01, 1992
RELATED TAGS: SUBATOMIC PARTICLES

The town of Oak Ridge, Tennessee, was built by the atom. Two generations ago the site was an isolated forest in a valley between the Great Smoky Mountains and the Cumberland range. Then, in 1942, it was selected as the headquarters for the Manhattan Project, the American effort to build an atomic bomb. A security fence was erected and a boomtown of 75,000 workers, at three secret fissionable materials plants, mushroomed behind it. When the first atom bomb exploded in the desert of New Mexico in the summer of 1945, the town's purpose was achieved.

Soon after the war the fence came down, and the shopping centers, housing developments, fast-food restaurants, and four-lane highways of contemporary America began to move in. Today the town bears little resemblance to its wartime appearance, but its economy is still tied to the atom. The bomb factory continued production throughout the cold war, and its research division, the Oak Ridge National Laboratory, became a world leader in the development of peaceful uses of atomic energy. The words nuclear and radiation inspire no terror in the people of Oak Ridge because most of them work in jobs involving scientific, environmental, and medical applications of nuclear technology. Oak Ridge is the site of an atomic energy museum and a nuclear-research consortium that comprises 62 American universities. For physicists the name Oak Ridge represents a welcome counterpoint to the horrors evoked by the words Hiroshima and Chernobyl.

I went to Oak Ridge to meet Sam Hurst, a physicist-entrepreneur who has taken atomic tools out of the exotic surroundings of academic research laboratories and into the doctor's office and the factory. He is the inventor of a reliable method of counting atoms that has applications in the most diverse fields of science and technology. Besides learning about that technique, however, I wanted to experience an atmosphere in which atoms were ordinary, everyday objects, like cups and saucers and grains of sand. In Oak Ridge atoms seemed more manifest than elsewhere in the world; in Ernest Rutherford's words, you could almost see the jolly little beggars.

Hurst met me at Atom Sciences Inc., the business he set up in order to commercialize his ideas. It is located in the Ridgeway shopping center, past a supermarket and next to a contractor's office and a store enigmatically called Family Tailor & Gift Shop. Hurst, a native of Pineville, Kentucky, is short and trim. In a commemorative photo of a visit by Jimmy Carter, the two men resemble each other in stature, but Hurst's mien is more serious than the president's; with reading glasses, his somber look gives him a somewhat owlish appearance. His sentences are delivered with a mountain twang, and they come slowly--each one chosen with deliberation for accuracy and succinctness, and interspersed with thoughtful pauses. He comes across as a beguiling combination of internationally renowned scientist, homespun philosopher, and country boy made good.

Hurst's career unfolded in four different arenas of scientific research: the government laboratory, the academy, the private sector, and the think tank. After developing the universal atom counting method at Oak Ridge National Laboratory, and cofounding Atom Sciences, he joined the faculty of the University of Tennessee in nearby Knoxville, where he helped create a research institute devoted to the extension and worldwide promulgation of atom counting.

Hurst patiently told me the story of atom counting as he showed me around the little empire he built. As we wandered through a warren of small rooms, each one dedicated to a separate operation, in which half a dozen young technicians sat behind computer consoles or fiddled with vacuum pumps and dye lasers, Hurst had a joking greeting or technical remark for most of them. In every available corner there were samples of materials that had been sent here for analysis--a canister of air extracted from bubbles in a glacier in Alberta, a bottle of water from Tunisia, a box of semiconductor chips from the Soviet Union. The place was an international smorgasbord of atoms.

Like many inventions, the universal method of counting atoms was born of necessity. In 1970, as leader of a group at the Oak Ridge National Laboratory charged with investigating certain nuclear reactions, Sam Hurst encountered a problem. He believed that an anomaly in his data resulted from the presence of impurities in his materials, but the levels of contamination were so small as to be undetectable by any known method. Chemists routinely deal with quantities in ppm--parts per million--but in view of the vast number of atoms in a drop of water, a ppm is not really a small amount. Hurst believed that the impurities he was dealing with were well below the ppm level and decided to figure out how to detect them.

He knew that modern electronics can reliably count small numbers of electrons, down to a single particle. So, he reasoned, if one electron is stripped from the outer shell of each atom, and then the electrons are counted, the number of atoms will be known. Furthermore, lasers can be used as versatile and efficient guns for knocking electrons out of atoms; only the method is not in the least bit selective, but counts all types of atoms indiscriminately. A sufficiently powerful laser will remove electrons from every atom it illuminates, which, of course, is counterproductive; the point is to detect only atoms of a specific kind in order to measure minute impurities.

Then Hurst had an inspiration. Suppose, he thought, the laser is adjusted so that it doesnít knock the electron all the way out of the atom, but only a little more than halfway up the energy staircase. Since the sizes of the energy steps are different for each atom, and modern lasers are finely tunable in energy (or, equivalently, in frequency, wavelength, and color), the excitation of selected atoms can be achieved with exquisite discrimination. A laser adjusted to raise an electron in, say, an aluminum atom will have no effect at all on a nearby atom of oxygen.

The last step then follows easily. The laser delivers such enormous numbers of photons with each burst that after all the atoms of a certain type have been excited, there will still be plenty of photons left over. Every atom that has had its energy increased will then absorb a second photon of the same kind, which will knock the electron the rest of the way up the energy staircase and over the top, and thus completely out of the atom, where it is finally counted.

The crux of the method lies in the first step, which represents a resonance between the laser light and a specific type of atom. Tuning a laser to excite only selected atoms is very much like tuning a radio by establishing a resonance between the receiver and the stationís emitter. Just as a good radio can discriminate between a multitude of adjacent stations, a good laser can select among a multitude of different atoms. When Hurst and his team realized, in 1975, that with some variations the method could be adapted to count concentrations down to single atoms of any element, they applied for a patent that was granted one year later. Additional patents on various refinements followed. By 1975 Hurst and his colleagues had succeeded in detecting a single cesium atom in a background of 1019, or 10 billion billion, argon atoms. Today Hurst and Atom Sciences are the reigning experts in the counting business.

The success of the new invention in finding a few atoms among trillions of others began to attract attention, and Hurst's method was soon adopted by laboratories throughout the world to tackle a great variety of practical problems. One satisfied customer is the computer industry. As integrated circuits on computer chips become ever smaller, the likelihood that minute faults in the material will spoil their delicate electrical performance becomes increasingly serious. Some modern electronic components are so small that a few foreign atoms can cause them to malfunction. This problem, which Hurst calls single atom failure, may one day be a common nuisance in everyday life, and will be prevented only by heroic new methods of chemical analysis, of which Hurstís resonance technique is a precursor.

At bottom the problem is one of geometry. A single atom resembles a point, which mathematicians call zero-dimensional. A stream of atoms, like a stream of water from a hose, looks like a line, and is one- dimensional, while a surface is two-dimensional. But the bulk sample in which an impurity is to be detected is intrinsically three-dimensional. With each dimension the number of atoms, and hence the difficulty of sorting, increases dramatically.

Consider a cube of metal with a width of one micron, a millionth of a meter, the size of certain current commercial microchip structures. There are about ten thousand atoms along each edge, a number that is large but manageable. On each surface the number of atoms is ten thousand squared, or a hundred million--a more formidable quantity. But the entire block consists of ten thousand cubed, or a trillion, atoms. Finding one atom in a trillion is the kind of challenge the atom counters are up against. The problem is not so much in handling single atoms but in discarding myriads of others. In his search for a needle in a haystack, Hurst has discarded the conventional approach of sorting through the filaments of hay and brought in a magnet that ignores them.

The atoms found by Hurst's method are not necessarily unwelcome as impurities. Consider this possible scenario: In a secluded valley of northern China two bright-faced young men clad in colorful Western hiking clothes, each carrying a small nylon backpack, scramble down a slope to the bank of a bubbling mountain stream. Pushing his sunglasses into his black hair, the first one kneels to scoop up some sand in a little vial, while the other one records an identifying number in his notebook. They confer briefly, stow away their equipment, and set off again upstream. Although their sporty appearance belies it, they are prospectors, and they are engaged in the worldís most sophisticated method of panning for gold.

Later, in a hamlet farther down the valley, in a panel truck converted into a portable laboratory, they test the sand for the presence of this precious metal. But they are not looking for whole nuggets, or even flecks--they are counting single atoms. The reasoning behind this approach is that although gold is impervious to corrosion and most normal chemical interactions, it is not indestructible on the atomic level. If there is a clump of gold somewhere upstream, individual atoms are continuously worn off it by the incessant action of water and the surrounding stones and sand. Billions of atoms are removed in this way and below the clump a plume of gold atoms usually spreads out for miles downstream. The high-tech prospectors map out the extent of this plume by measuring concentrations of gold that are far too puny to be of any use whatever. Since traces of gold are present everywhere on Earth, the actual density of the plume does not tell them much either. Changes in the concentration of gold atoms are far more informative: increases point the way to deposits, decreases away from them.

After learning about the technique from Hurst when he visited them several years ago, a group of physicists at Qinghua University in Beijing developed a spectacularly sensitive analytical system for the detection of gold and other metals in minerals. In their published papers they report their findings in terms of the abbreviation ppt, for parts per trillion--a pretty discouraging amount for a conventional gold hunter. However, they have succeeded in demonstrating the feasibility of their novel technique and will soon be able to put it to use to enhance the urgently needed economic development of their country.

In the United States, where gold mining is lower on the national agenda, atom counting has been applied to other societal problems. One intriguing possibility is what might be the beginning of a technique known as single atom medicine. It has been known for many decades that when the nucleus of the boron atom is hit by a neutron from a nuclear reactor, it emits an alpha particle, the projectile used by Ernest Rutherford in his historic demonstration that atoms have nuclei. If this alpha particle were released within living tissue, it would travel no farther than the edge of the cell it happened to inhabit before it expanded its energy and came to a stop. Then it would attract two stray electrons and turn into a harmless, inert helium atom. But the energy it imparted to the cell in the process of slowing down would kill the cell, making neutron irradiation of boron atoms a promising candidate for cancer therapy.

The idea is appealing. Instead of drowning cancer in a flood of noxious chemicals, or blasting it indiscriminately with a volley of powerful radiation, a doctor could pick off the disease, cell by cell, without harming the rest of the body. This kind of microscopic control over organic processes is the dream of the medicine of the future.

At the Idaho National Engineering Laboratory in Idaho Falls, there are plans to adapt a nuclear reactor for the development of this promising therapeutic technique. But before the first patient can benefit from it, much remains to be done. For example, a reliable method of introducing boron atoms into cancer cells must be perfected, and the movement of minuscule numbers of boron atoms through the human body must be closely monitored and controlled. When Sam Hurst heard about the problem he knew that his counting technique had found yet another significant application.

Other kinds of medical uses of atom counting include the study of the effects of extremely small traces of various elements in the human body, and the potential for reducing the sizes of the samples required for various laboratory tests. Both of these applications came together in a recent discovery in neonatology. In view of the size and delicacy of a newborn baby, blood samples must obviously be kept to an absolute minimum-- but small concentrations of elements in minuscule samples are undetectable by conventional techniques. For this reason, Atom Sciences has been collaborating with a team of pediatricians and chemists to examine the problem. What they have found is that traces of metals like chromium, copper, and zinc that are required for normal development are transferred from the mother to the fetus rather late in pregnancy, and that a very premature baby may therefore lack these elements and suffer various ailments and birth defects. The researchers therefore aim to learn how to supply the proper amounts of necessary trace elements to premature babies. The amount of blood required for this purpose is small, measured in minute drops, but still represents boundless oceans on the atomic scale.

Yet another application of atom counting, as significant as safeguarding the purity of electronic materials and following the movement of elements through the human body, is found in the field of ecology. Even as our understanding of the atmosphere and oceans grows more global, it is also becoming increasingly microscopic. The large and the small meet not only in the realm of cosmology, where quarks and leptons are considered to be the stuff of the Big Bang, but here at home too, where the details of atomic interactions ultimately determine the future of the planet.

Atom Sciences has become instrumental in unraveling the history of the water reservoirs of the world. The method is similar to the time- honored technique of carbon dating: if you know how many radioactive atoms of some type were present in a given sample of water at some specific starting time, and you know how fast they decay, and you then measure how many of them are left today, you can deduce the time that has elapsed since the initial moment. Certain atomic species, such as krypton-81, are continuously replenished in the atmosphere by the action of cosmic rays, so the concentration of krypton-81 in air at sea level is assumed to be a constant that has not varied for millions of years. It is furthermore known that as long as water is in contact with the atmosphere, it contains a fixed, minute concentration of krypton-81.

Suppose that at some time long ago a quantity of water was somehow sequestered and protected from the atmosphere, by trickling into an underground reservoir, for example, or by becoming fixed in ice. At that moment the number of krypton-81 atoms in the sample began to diminish at a steady pace, and the concentration of atoms, measured today, serves as a clock to determine the time of sequestration. The numbers, both at the beginning and end of the process, are small: a quart of water contains a thousand krypton-81 atoms in the beginning, and half that number after 200,000 years. When you deal with concentrations at that level, you are not engaging in chemical analysis--you are counting atoms.

In this way the age of groundwater and polar ice--which is to say the time elapsed since they were last exposed to the atmosphere--has been measured. Such information is necessary for understanding the history of the surface of Earth and its trends, such as global warming. For processes that unfold more quickly, other trace elements besides krypton are more appropriate. For example, argon-39, with a half-life of 270 years, is used for monitoring how long ago water accumulated under the Sahara.

The list of present and future applications of Hurstís invention is endless, but beyond that the technique has a subtle philosophical implication. Chemistry has portrayed a world clearly divided into different substances--the ink on this page is primarily carbon, the air we breathe consists of oxygen and nitrogen, my wedding ring is made of gold. Although chemists have always understood that all substances are riddled with impurities, most of the minor constituents were too rare to be measured and could be safely ignored. Consequently every substance was labeled, at least in principle, like a candy bar, with a short list of its most significant ingredients, and a brief appendix of additional trace components at the edge of detectability.

But since 1970, when Hurst first envisioned his scheme, this way of comprehending the world has changed. The limit of detectability for every element has been reduced to its theoretical minimum, the single atom. Today there is no longer such a thing as a concentration that is too small to be measured: either an element is present or it isnít. A chemical assay is now different from the biological survey of a patch of ground, which must inevitably end with a phrase like ìplus an innumerable quantity of invisible microorganisms.î The chemical composition of matter is now, in principle, absolutely known.

With such a dramatic increase in the sensitivity of analytical chemistry, most naturally occurring substances must be considered to contain atoms of every element. A grain of sand, for example, which was once thought of as consisting almost entirely of quartz, a compound of silicon and oxygen, with a minute admixture of trace elements, has been found to contain a few atoms representing practically the entire table of elements. The qualitative classification of the world into different substances now becomes a quantitative classification in which the period at the end of this sentence is no different from the ring on my finger--both contain carbon and gold atoms, only the proportions differ.

Another consequence of this improved analytical technique is the completion of the inventory of the world begun by the ancient Greek atomic philosopher Democritus. As long as many of the ingredients of ordinary things were too scarce to be identified by any known method, the notion of a universe composed of atoms was still an abstraction. But today it is possible, in principle, to identify and list every single atom in any given object. What is the world made of? Atoms, answers Democritus, and Atom Sciences Inc. can tell you which ones.

At the end of my visit, Sam Hurst showed me one of the thick notebooks of overhead transparencies he uses to introduce his method to colleagues all over the world. To my astonishment it was filled with page after page of poetry:

... water gives way to the fish

As it swims, and opens a passage for it to pass,

Because there is a space left behind the fish

Into which the liquid can flow: and this, they say, demonstrates

How other things can change place, although space is full ...

and so on, for a hundred pages or more. It was Lucretius, who described the atomic basis of matter in a poem 2,000 years ago. Hurst, it turned out, is a serious fan of the Roman poet and has culled an ingenious collection of quotations to illustrate every conceivable technical point that can come up in a discussion of atom counting. If Lucretius was a prophet, Sam Hurst is clearly his apostle.

I was delighted to know that Hurst places his own work into its rich historical context, all the way back to the beginning of the Christian Era. The fascination of Lucretius is not only that he had anticipated so many details of modern science, but that through his poetry he tried passionately to bring science into the lives of his contemporaries, no matter how indifferent they seemed to be to it, and perhaps even to learning of any kind. Hurst realizes how desperately urgent this same concern is in the contemporary world, and I applauded his identification with Lucretius as proselytizer of the atomic doctrine.

At the same time I was troubled by this preoccupation with the Roman poet, almost to the exclusion of everyone who came later. The theory of Democritus that Lucretius taught is only the opening scene of the story of the modern atom. The world consists of atoms, yes, but what are they? Why was Hurst not equally fascinated by the ascetic Werner Heisenberg, who believed that we cannot fashion an intuitively appealing image of the interior of the atom, or the single-minded Erwin Schr–dinger, who wanted to reduce all material phenomena to waves, or the radical Max Born, who interpreted atomic reality in terms of probabilities? Perhaps Hurst was wise to stop at a point of certainty, the fact that we are made of atoms, without plunging into the swamps of incomprehensibility that beset those who venture into the meaning of quantum mechanics. But I was worried that by choosing Lucretius as his hero, he might succeed too well in communicating to laymen the exhilarating experiences of his own discoveries, and therefore fail to acknowledge that the most important questions remain unanswered.

Throughout his career Hurst has relied on quantum mechanics to describe laser light and its interactions with atoms, so his experience of both the power and the limitations of the theory is extensive. He is typical of most modern physicists, to whom quantum mechanics at the workaday level is no more mysterious than auto mechanics. The paradoxes of its interpretations hardly concern him; what matters to him is that he knows the rules, and that they work.

During my visit Hurst strayed from this pragmatic position only once. We were discussing recent experiments on the interference of particles. When the conversation turned to Youngís famous double slit experiment, performed with a beam so feeble that only one particle passes through the apparatus at a time but acts as if it is going through both slits simultaneously, Hurst mumbled: "I just donít understand that!" and then, "That one really bothers me." Even thorough familiarity with the nature of atoms cannot dispel their ineffable aura of mystery.

For the next century Lucretius will not suffice as prophet. A new generation of physicists will lead us beyond the mere recognition of the existence of atoms and penetrate their elusive cores.

This article is an excerpt from Hans Christian von Baeyer's book, Taming the Atom, to be published in August by Random House.

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