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Nuclear Detectives

In California some gumshoe physicists are using a particle accelerator to nab polluters.

By Deborah Blum
Apr 1, 1993 6:00 AMNov 12, 2019 5:17 AM


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For a nuclear physicist to include a stroll through an almond grove as part of his research may seem a bit peculiar. Unless, of course, the physicist works at the Crocker Nuclear Laboratory at the University of California at Davis, where unusual is the norm. In the name of physics--and in the pursuit of paying customers--Crocker scientists have done everything from trudging through dusty lake beds to poring over ancient manuscripts. They have studied the ink in the famed Gutenberg Bible, tracked the source of air pollution in the Grand Canyon, verified the handwriting of Johann Sebastian Bach, and analyzed the smoke from burning Kuwaiti oil wells. For two decades now, as long as the project has called for good, solid everyday science, the Crocker group has been happy to fire up its small particle accelerator for a cost that’s now just $384 an hour (volume discounts available).

We’re the McDonald’s hamburger of elemental analysis, jokes Davis nuclear physicist Thomas Cahill.

Cahill and colleagues have made the Crocker lab internationally recognized as a place where a standard cyclotron is used in nonstandard ways. We attract people who are bored by doing repetitive, traditional work, says Crocker director Robert Flocchini, the physicist who’s been analyzing the blowing dust clouds raised by California almond growers; he hopes to settle a furious dispute over the extent to which agricultural dust contributes to the often gritty air of the state’s heavily farmed Central Valley.

While its choices have made the Crocker somewhat low-profile in the high-powered community of traditional particle physicists (Do they have a cyclotron at UC Davis? asked one surprised physicist from Stanford’s powerful linear accelerator program), outside that inner circle the laboratory shines brightly. Cahill, who heads the Crocker’s atmospheric research program, visited 13 countries last year, ranging from Australia to Chile, helping establish environmental analysis programs at particle accelerator centers. Besides making themselves accessible, the Crocker scientists hope to leave a legacy of accelerators around the planet, all joined into a network of sophisticated monitoring stations. Already the idea is catching on: the National Park Service relies on the Crocker to keep watch over pollution in its parks, and the World Meteorological Organization has adopted the Crocker’s analytic techniques for its pollution monitoring network. Cahill, at 56, has visions of recruiting a whole generation of physicists to keep tabs on Planet Earth: What do I want out of this? he asks slowly and then grins. I want to be the Pied Piper of environmental physics.

That kind of ambition has always characterized the Crocker. The laboratory came to Davis in 1965, three years after university physicist John Jungerman discovered that the Lawrence Berkeley Laboratory had a 220- ton magnet going to waste. The Berkeley group, busy building a new accelerator, regarded the magnet as a historic relic, an aging five-foot- wide chunk of (slightly) radioactive metal. The magnet had served as the core of the pioneering cyclotron built by physicist Ernest O. Lawrence in 1939, famed as one of the machines used to help develop the atomic bomb.

Jungerman knew this magnet well. In the early 1950s he’d worked with Lawrence himself, and he’d completed his master’s thesis on the old cyclotron. That machine deserved great respect, he thought; in its heyday it had been used to discover seven of the heavy atomic elements, including plutonium. When he heard that it would be replaced by a newer edition, he hastily put in a bid for the magnet. With help from Glenn Seaborg, a Nobel laureate and a codiscoverer of plutonium, he got $2.5 million from the Atomic Energy Commission (now the Nuclear Regulatory Commission) to make a new cyclotron. The machine was up and running in 27 months.

Cyclotrons are the grandfathers of all circular accelerators, the first small step on the way toward the planned Superconducting Supercollider, with its 54-mile main accelerator ring in the rolling prairies of Texas. Hard to believe, almost, when one considers that the first cyclotron, invented by Lawrence in 1931, was not even a foot in diameter and could boost a handful of protons to only about 80,000 electron volts. The SSC, when completed, is expected to generate two beams, each with an energy of 20 trillion electron volts. The Davis cyclotron is a piker by comparison, with a power range between 4.5 and 67.5 million electron volts.

But in essence the two are the same: the job of any accelerator is to put a rocketlike boost behind a charged particle and send it screaming off to wreak subatomic havoc. Any such particle--a negatively charged electron, a positively charged proton--can be turned into a missile if given enough speed. As accelerators push ions to nearly the speed of light and let them crash, the particles can literally crack each other to bits. Such atom smashing is indeed the goal of the big accelerators--to break everything down to primordial energy and discover the hidden particles of the subatomic world. With the enormous SSC, physicists hope to re-create, in part at least, the boiling, energetic aftermath of the Big Bang.

Jungerman wasn’t thinking Big Bang when he and the crew at Davis started putting together the Crocker cyclotron. He was thinking about putting his 60-inch magnet into a dependable piece of equipment. At its simplest, a cyclotron starts with two semicircular electrodes--powerful conductors of electric current called dees (because each is shaped like the letter D). The electrodes go into a vacuum chamber that fits between the poles of an electromagnet, the more powerful the better. (Jungerman souped up the old Berkeley magnet by running an 8-inch ring of steel around it, giving it the power of a 76-inch magnet.) When a cyclotron is powered up, voltage crackles through each dee, producing an electric field in the gap between them. A stream of low-energy ions--protons, in this case--is then introduced into the vacuum chamber. As the ions swirl lazily through the chamber, the charged field gives them a swift kick, propelling them forcefully ahead in a direction set by their own charge.

Circular accelerators, like the Davis cyclotron, use the force of their magnetic field--which tugs at right angles to the particles’ motion-- to bend the path of the moving particles into an orbit between the electrodes. On each orbit the particles cross the charged gap twice, and they gain a rush of acceleration each time. With each boost the radius of their orbit increases a bit. Eventually, when the ions are really sizzling, they are slung through a vacuum pipe. They emerge in a tightly focused beam that is aimed at a target. The Crocker, at full power, can whip particles up to about one-third the speed of light. Such highly charged beams can produce heady consequences in target materials--they can change the form of elements, make them radioactive, or--significantly, for Cahill’s purposes-- reveal the structure and composition of the target material.

In the early 1970s, just as Cahill was becoming interested in air pollution analysis, he learned that Swedish researchers had used cyclotrons to induce certain target materials, including air pollution samples, to emit X-rays. The technique is called proton-induced X-ray emission, or PIXE. Cahill jumped on the idea immediately; he realized it would allow him to take a black fleck of pollution and read the elements out of it like words on a page.

To do a PIXE analysis, the Davis cyclotron is run at low power, with the proton beam held near its low end of 4.5 million electron volts. Operators aim the beam at a paper-thin Teflon air filter, held in the frame of a photographic slide, that has been used to collect pollution samples. The beam whispers through the smoggy grit and filter without doing any damage; it’s just strong enough to nudge the electrons of the various elements in the target. A big-charge beam would blast the electrons out of place, spraying them about like bird shot. But this light push merely elbows the electrons out of their customary atomic orbits for a moment. When the electrons then fall back into place, they release energy in the form of X-rays.

What’s important here is that the energy necessary to push an electron from its orbit--and the energy released when that electron returns--is different for each element. Thus, when the target material is hit by the proton beam, the X-rays emitted can be used to identify the elements from which they came. The identification is done with the help of a silicon diode, which converts each discrete packet, or photon, of X-ray energy into an electrical pulse. (The trick is to get just one photon to hit the diode at a time, and the physicists accomplish this by lowering the number of protons in the beam until the odds of more than one proton hitting an electron in the target at any one instant are infinitesimal.) The resulting signal is fed into a computer and analyzed; the specific charge of the pulse is like a signature for the origin of the X-rays.

The Crocker’s expertise with this revolutionary technique was first acknowledged in 1977 when the EPA asked Cahill to do air pollution monitoring in Zion, Bryce Canyon, and Canyonlands national parks. Cahill installed air-filtering devices in each one, collected the dirty filters, then took them back to the cyclotron for analysis. Today the laboratory earns over $2 million a year through its PIXE work, largely from environmental researchers. Atmospheric scientists say the extreme sensitivity of the analysis, the ability to analyze a few specks of dust, a sprinkle of grime, has made even the barest shimmer of pollution identifiable.

Not everything the lab looks at is as subtle as a shimmer, though. In 1991, for example, the Crocker was asked to develop an elemental fingerprint of the smoke spiraling up from Kuwaiti oil wells that had been torched by departing Iraqi soldiers in the Gulf War. Atmospheric scientists knew from experience that oil fires often release a bewildering mixture of chemicals, which vary according to local geology. But, says Cahill, there were elements about these fires that were unique. First, there was the sheer size of the disaster, which was unprecedented. Second, these wells were under tremendous geologic pressure, so the oil was squirting into the air as if from a syringe, burning in a plume. Third, some of the plumes were burning white instead of black, and there was much speculation about what they were.

Cahill notes that at the time there was also much speculation about the fires’ possible environmental impact--speculation, he says, that bordered on the hysterical, with phrases like ecological holocaust and Armageddon being used by scientists who perhaps should have known better. Still, the situation was potentially threatening, and facts were needed to understand what specifically was pouring into the air and whether it could become a regional or global problem.

When the National Oceanic and Atmospheric Administration (NOAA) extended an invitation, the Crocker team was eager to go. By obtaining samples of Kuwaiti crude oil, testing them in the laboratory, then comparing them with the atmospheric contamination, they were able to identify the toxic metals coming from the burning wells: showering the region was as much as 1,000 tons of fine vanadium particles and 500 tons of nickel. They also found that the white plumes were ancient salt water that was being ejected with the oil. It turns out that sodium chloride from seawater was being put into a spray and burned and then forming very fine salt particles, says Cahill.

Furthermore, he found, natural conditions in Kuwait were keeping the situation from becoming truly disastrous. Kuwait City has some of the dirtiest air in the world--the air is filled with fine particles of alkaline dust blowing up from the desert floor. We found that most of the mass of the smoke plumes was desert soil, Cahill says. And this fine desert dust was neutralizing the acids released by the burning oil. Basically you had a scrubber forming in the sky. Cahill says the result was that the amount of sulfates in the air was almost exactly the same as it is during a typical summer over Los Angeles--worrisome, but hardly Armageddon.

With all the talk about Kuwait being a disaster, these were not really toxic levels, Cahill says. Our analysis showed too that as the acids combined with the dust they became heavier, so they didn’t travel as far as they might have or go as high in the atmosphere; they tended to settle locally, making it more of a local problem than a regional one, and not at all a global problem. So our work brought facts to what was up to then largely a speculative situation.

The situation had been much the same--speculative and murky-- several years earlier, when the Crocker team was asked to take a look at the Grand Canyon. And that time the politics was as complex as the chemistry. Cahill was part of a Park Service study of pollution in the canyon. Park administrators had become increasingly dismayed about dirty air blurring the canyon views. They suspected that the culprit was the massive coal-burning Navajo Generating Station, located just 18 miles from the northeastern portion of the canyon, which had been fired up in the late 1970s. The plant, operating without scrubbers, was estimated to be releasing some 300 tons of sulfur dioxide daily, more than twice the sulfur pollution load of the entire Los Angeles basin. Nevertheless, the operators of the plant, which included the U.S. Bureau of Reclamation, blamed the park’s clouded visibility on either pollution blown in from western urban areas or from copper smelters located south of the park.

The problem, of course, was that no one could actually see pollution coming from any of these sources. The plume, consisting largely of sulfur dioxide, is largely invisible, says Cahill. The SO2 has to do two things before it can be seen: it must convert from a gas to a particle, and it must pick up water. That allows it to scatter light. Even with nonvisual techniques, though, the pollution was hard to pick up. Both the Park Service and the utility had been carefully checking for pollution drift, perching their air-sampling equipment like sentries along the canyon’s 7,000-foot rim. But park rangers had been noticing that even on days when the view was clear at the rim, it was murky below. They suggested the canyon might be acting as a sink that collected and concentrated the dirty air.

With a colleague, John Molenar of Air Resource Specialists, an environmental consulting company, Cahill began monitoring deep in the canyon itself. In the winter of 1990-91 the work paid off. First of all, Cahill and Molenar determined that the plume from the Navajo Generating Station drifted over nearby Lake Powell, where it picked up water and cooled; when the colder air reached the canyon, it sloshed over the rim. They were even able to show part of this sequence of events on videotape. During an exceptionally cold winter night Molenar set up a video camera and recorded a flow of gritty air, like a tumble of water, falling over the rim of the canyon and almost splashing into the bottom.

How did they know the air was from the generating plant? It was an indirect process, Cahill says. They couldn’t follow the plume from the plant on video, but with PIXE analysis they could track tiny amounts of telltale elements. The plume included trace metals from the burning of coal, such as selenium, Cahill explains. The plume would mix with other pollutants in the air, but the selenium came along as a trace element. And during the times we saw a lot of haze in the canyon, we also saw high levels of selenium. It took the cyclotron to pick it up--these ‘high’ levels were still very low to anybody else.

In March 1991 hearings were scheduled on the Grand Canyon pollution. But the Park Service was barred from testifying. The decision had come from then secretary of the interior Manuel Lujan. Lujan was presiding over two agencies with competing interests--the Park Service, which wanted the plant cleaned up, and the Bureau of Reclamation, which had a financial interest in leaving it alone. He didn’t want them slugging it out in public, and so he ordered them to submit written statements only.

Cahill was extremely frustrated. In his view, the whole point of having good data is to spread the word. He had the cyclotron data, and Molenar had his videotape. Both men decided to attend the hearing on their own.

The two flew to Phoenix, where the hearing was set to begin, and met with environmental groups that were scheduled to speak. The Grand Canyon Trust, a regional conservation organization, was impressed enough to give up an hour of its slotted time to the scientists. Cahill made his presentation as a private citizen. After a second hearing, representatives from the utility and the environmental groups decided to negotiate a cleanup plan: No one wanted to be the one who trashed the Grand Canyon, Cahill says. The final agreement calls for the plant to be equipped by 1999 with $430 million worth of scrubbers, which will reduce the pollution load by some 94 percent.

The delicacy of the Crocker’s analytic techniques has moved the lab into other surprising areas for nuclear physics, most notably the analysis of old documents. That venture really began some 15 years ago in a conversation between Cahill and Davis historian Richard Schwab. Schwab is an expert on the eighteenth-century Encyclopédie of Diderot, the world’s first great encyclopedia and a work so popular that it was immediately followed by a wild number of forgeries. Schwab had learned to identify the fakes by various peculiarities in the text, which he admits did not endear him to universities that owned forged editions. He was cheerfully complaining about this to Cahill at a dinner party. Then, right in the middle of dessert, we both had this brilliant idea, says Schwab. We realized that a piece of paper would look just like an air filter to the cyclotron beam, and the ink on it would look like the pollution particles.

Schwab promptly donated a not very valuable eighteenth-century book from his library so the idea could be tested. Cahill and fellow physicist Bruce Kusko set about slicing small rectangles from the edges of several pages to fit them into the cyclotron’s beam-analysis chamber. After the first run, Cahill called Schwab in puzzlement to report that the paper, which the beam had also struck, seemed to change every eight pages. Schwab was struck with a shiver of recognition. In the eighteenth century, books were made by putting together quires of paper (a quire is one large sheet folded to make smaller pages). The cyclotron was picking up the slight elemental changes from one quire to another.

But the scientists realized that slicing up valuable books was unlikely to appeal to the owners of rare old documents. Like every conventional cyclotron, the Crocker’s proton beam was enclosed in a vacuum pipe. In open air the beams fall apart as the protons collide with air molecules and begin losing energy. About four inches out of the pipeline, the beam simply wears out. The Crocker physicists started wondering whether those four inches could be used. By the time an opportunity came along in 1982 to analyze a Gutenberg Bible, the Crocker group had put in place a proton milliprobe. The milliprobe is a proton beam left open at one end, with an X-ray detector attached about an inch from the opening. The uncut pages of a book, a letter, or a map can easily be fit between the beam line and the detector and analyzed in serious detail: the proton beam focuses to a tenth the size of a single period on a printed page.

Analyzing hundreds of pages of the Gutenberg was a major production, taking 42 hours of cyclotron time. The analysis was further slowed by the security anxieties of the Bible’s lender, St. John’s Seminary in California, which was terrified that someone would steal the valuable object. They insisted that only the scientists and technicians directly involved know that the book was in Davis. Cahill, a devoted fan of spy novelist John Le Carré, was frankly thrilled by both the secrecy and the security. (The researchers promptly dubbed their experiment the fishing expedition and code-named the Bible the whale.) But the scientists were even more excited by their results. One of the mysteries about Gutenberg’s printing had been the ink he used--after some 540 years, it was still as black and glossy as a newly printed page. The X-ray analysis showed that Gutenberg had mixed an ink that was nearly a slurry of metal, almost all copper and lead. We know more about Gutenberg’s ink than he did, Schwab says. If we had a few million dollars, we could probably do a Gutenberg forgery.

Since that time the scientists have looked through a variety of artifacts. They may be proudest of their work with the personal Bible of Johann Sebastian Bach. The composer’s signature was clearly scrawled across the flyleaf. But scholars were baffled by underlinings and exclamation points throughout the book--were they Bach’s or someone else’s? If they were Bach’s, they might offer new insights into the religious influences on his music. After analysis, the Crocker scientists are convinced that Bach himself marked up the book. The ink was the same, exactly, as that in the signature.

For all his fascination with history, however, Cahill’s first love has remained environmental studies, and it is here that the Crocker influence has been, and will continue to be, felt most strongly. As an example, Cahill cites the Crocker’s role in a court battle in the early 1980s over water from Mono Lake--an ancient body of water set in dense volcanic rocks along the east side of the Sierra Nevada. Since World War II the city of Los Angeles had been diverting tributaries from the lake to provide water for its growing population. But as the lake dried, and as its wildlife began to suffocate and great clouds of dust began to blow off the now uncovered lake bed, Mono Lake became the focus of a furious environmental battle.

Los Angeles sought to portray it as a tug-of-war between helping people and helping wildlife. That changed after the state air resources board asked Cahill to analyze the dust clouds. He discovered they were full of arsenic, a known human carcinogen. The arsenic was natural, part of the volcanic bedrock, and was being released as the lake bed dried and blew away. With the sensitivity of the cyclotron, Cahill could track the dust, even as the clouds broke apart, into a near-invisible, hazardous haze drifting east toward Nevada. In 1989, when California state courts restricted water diversions from lake tributaries, they cited the arsenic risk as a major factor.

It’s an issue that Cahill’s group has not let go of; they are now deeply involved in an investigation of the dust blowing off the bed of Owens Lake, about 125 miles south of Mono and a body of water also eliminated by L.A. water diversions. At the turn of the century Owens Lake covered 110 square miles in the Owens Valley, a spread of water so deep in waterfowl that residents ten miles away claimed they could hear the thunder of beating wings as ducks rose off the lake. Now only a pool of salty brine, unfit to drink, remains. In a high wind the dust storms off the lake bed blow like the bitterest northern blizzard. The NOAA has calculated that the lake contributes 6 percent of the breathable dust particles in the air over the continental United States. It’s a serious problem, says Cahill, whose group has, not surprisingly, found arsenic in the Owens Lake dust. Again, they painstakingly tracked its creeping spread 80 miles; this time winds being funneled through the Owens Valley were pushing the dust south toward Los Angeles.

Crocker scientists are now studying the polluting effects of power plants in the Pacific Northwest and of agricultural burning in California’s Central Valley and near Florida’s Everglades. Cahill dreams of a whole new generation of physicists that will tackle such problems: I want to get all these young physicists who are frustrated and bored and want to do something that can make a difference and say, ‘Look at what you can do.’

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