On the cover of George Tilton’s 1951 doctoral thesis, there’s a marking most dissertations don’t get: a SECRET stamp. Tilton’s graduate work at the University of Chicago dealt with using radioactive elements to determine the ages of rocks. Specifically, he was looking at how much natural uranium transmuted into lead. He helped develop a method to use that ratio to determine precisely how much time had passed between a rock’s formation and the present.
Of course, rocks’ ages aren’t state secrets. But some of the World War II–era commodities on which Tilton’s geological dating relied were perhaps more sensitive. The batches of the uranium-235 isotope Tilton used were fruit of the Manhattan Project, and his research was done in connection with the Atomic Energy Commission, the civilian organization that was created after the war to oversee nuclear weapons, science, and technology.
Today, Tilton’s since-declassified work still undergirds geochronology, the field focused on determining the ages of natural material and, from that, the ways geologic events have played out over time. The U–Pb method he developed, called isotope dilution, is one of the most precise radiometric dating systems for rocks that formed about 1 million to 4.5 billion years ago. (Other radioactive isotopes, such as carbon-14, work on younger rocks.) And its development wouldn’t have been possible—at least not on the same timeline—without the nuclear weapons program. Neither, it turns out, would other geologic age–determining systems, which often rely on enriched isotopes. “The weapons program in the US was absolutely essential to allow this to happen,” says Ryan Ickert, an isotope geochemist at Purdue University.
It’s all about the uranium
Radiometric dating didn’t begin in the postwar era. By the turn of the 20th century, scientists had already determined that radioactive decay occurred at a constant rate, a rate that was different for each element. (They hadn’t yet realized that elements have different isotopes.) It was likely Ernest Rutherford who first suggested that decay rate could be used to determine the age of rocks. By measuring the relative amounts of a parent element, such as uranium, and its daughter product, in this case lead, researchers could essentially wind back the clock to tell how long those decays had been taking place.
Before mass spectrometry and the discovery of isotopes, scientists simply chemically separated the total amounts of uranium and lead to do the calculation. But even after the development of mass spectrometers, dating rocks required relatively large samples, on the scale of milligrams, and was somewhat inaccurate.
It was during the Manhattan Project that scientists built the instruments that could identify, whip up, and separate isotopes at scale—a feat that the program achieved primarily so it could produce uranium for weapons.
Ernest Lawrence and colleagues developed a mass spectrometer called a calutron. Calutrons ionize atoms and then accelerate and deflect the ions with a magnetic field. Different isotopes, with different masses, will have different momentums, so the paths of heavier ones will bend less than those of lighter ones. The ions smash into a plate at different points, based on their different masses, where they can be collected. Calutrons were useful for isolating the uranium that weapons scientists wanted from the mixture of isotopes that occurs in nature. A uranium-bearing rock will have more than 99% of its uranium in the form of 238U and less than 1% in the form of 235U, the isotope of interest to weapons engineers.
Manhattan Project–founded labs, including the ones in Tennessee later known as Oak Ridge National Laboratory and the Y-12 complex, housed calutrons and other equipment to get the bomb-grade material from its naturally mixed form. But once the war was over, the labs and those machines needed a peacetime role to maintain their relevance. “And they basically decided, ‘Hey, why don’t we just separate every isotope of every element in the periodic table, just in case somebody needs it one day,’ ” says Ickert. According to a paper by former Oak Ridge director Clarence Larson, in 1947 Eugene Wigner—then head of the lab—wrote to the Atomic Energy Commission about producing isotopes for physics experiments. “Eventually nearly all of the elements of the periodic table were separated to collect the desired isotope,” Larson wrote in the Bulletin for the History of Chemistry.
A useful application for weapons-grade uranium
When the war ended, those enriched isotopes, apparently including some 235U, were made available to the medical establishment and some researchers. Tilton, who would later work at the Carnegie Institution and the University of California, Santa Barbara, was one of them (although those interviewed for this article were not aware of how he obtained his). For his classified 1951 thesis, he used his highly enriched 235U to develop a radiometric dating method called isotope dilution. His findings were eventually published in a 1955 paper in the Geological Society of America Bulletin. Although there have been many refinements to the technique since then, “a graduate student learning things today would find a lot that they would recognize in that original 1951 publication,” Ickert says.
“It’s very, very powerful,” says Don Davis, a researcher with the University of Toronto’s Jack Satterly Geochronology Laboratory. “Very, very precise. But you need to have these enriched isotopes”—in the case of the U–Pb method, isotopes of both uranium and lead.
In the isotope dilution method used today, scientists take highly enriched 235U and dissolve it, calibrating the liquid so they know the precise concentration. They add that solution to a similar mixture of dissolved rock, homogenizing the two. By then measuring the ratio of the two uranium isotopes in the mixture, they can learn how many atoms of natural uranium—with its known ratio of 235 to 238—are in the sample. “It’s some very simple math,” says Ickert. They use the same method for the lead, whose isotopes were also among Oak Ridge’s harvest.
A major advantage of the approach is that it allows scientists to start with a trace of material from the field, on the order of micrograms, and still get an accurate and precise measurement. Moreover, each isotope of uranium has a different, known decay rate into a different isotope of lead, giving two independent ages with each measurement. “The advantage of the uranium–lead method is that there’s 238U and 235U, and both decay,” says Davis. “And so you’ve got essentially two clocks in one.”
Before isotope dilution, rock dates were considered accurate to within around 5%, according to a paper in Reviews in Mineralogy and Geochemistry that Davis coauthored. U–Pb isotope dilution can get the accuracy within 1% or under.
Other dating methods also rely on World War II–era synthesized isotopes. One such clock uses the decay of rubidium into strontium; another uses samarium’s decay into neodymium. Researchers have employed both approaches to date old igneous rocks, like granite.
Implications and legacy
Today the U-Pb system shows up all over the place. Davis has recently been using geochronology to help a nuclear waste management organization investigate which rock is geologically stable enough to house radioactive discards. Uranium’s decay has also revealed the history of volcanic eruptions and mass-extinction events and even the evolution of the solar system—helping to determine, for instance, when planets formed and when meteorites condensed.
That research is clearly important from a scientific perspective. But also, adds Davis, “it’s kind of a cultural thing to know where you stand in the universe.” If the nuclear weapons program hadn’t existed and hadn’t produced the separated, enriched isotopes that allowed scientists to do isotope dilution using purified substances, humans’ sense of that standing might be different, delayed, or nonexistent, because geochronology as we know it today might not exist. “It would have been way, way, way too expensive to do this,” says Ickert.
Many isotope geochemists don’t realize that the substances they’re using were enriched decades ago with equipment the Manhattan Project no longer needed. If you open an isotope geochemistry textbook, the weapons connection likely won’t appear. And there doesn’t seem to have been a reckoning within the field about its connection to nuclear bomb development. “Actually, I never thought about it that way,” says Davis, for instance. “Yes, we have a solution of weapons-grade 235U in our stock bottle but only a few micrograms of it, which is far less than critical mass.”
“Maybe,” he adds, “it’s a good example of beating swords into plowshares.”
Much of the material, now, is an unspoken legacy of the early weapons program. Emphasis on early: Some isotopes key to geochronology haven’t been produced at scale for decades. They’re finite resources, and the field won’t be able to continue in the same vein indefinitely unless someone whips up more. Ickert is worried about his supply of 205Pb, along with 202Pb, both of which he needs for isotope dilution and neither of which occurs naturally. There’s enough, perhaps, for a few more decades of research. In fact, it was Ickert’s concern about the future scarcity of the isotopes he relies on that spurred him to research their provenance.
The more today’s scientists learn about how their predecessors made this material in the old days, the easier it would be to spin infrastructure and production back up again—although there’s no guarantee that will happen. “The problem is that this is geochronology,” Ickert says. “It’s not a national defense thing.”