This is the first sample of plutonium big enough to be seen by the naked eye. Probably. There’s a sticker on the side that claiming it’s the first plutonium sample large enough to be weighed, but the papers documenting the origins of this atomic artifact have long since disappeared. Scientists at Berkeley have had to rely on nuclear forensics to substantiate whether this radioactive fleck was really produced in 1942 by the physicist who first discovered the element, Glenn Seaborg.
“I am 99 percent sure that’s what this is,” said Eric Norman, a nuclear engineering professor at the University of California, Berkeley. “[But] we can’t prove it unless you find Seaborg’s DNA or his fingerprints on it.”
|The plutonium fleck, magnified. Image: Eric Norman|
If this little radioactive fleck is actually the sample it claims to be, which seems likely, its historical significance is huge. Plutonium was one of the very first artificial elements created and a critical component for the first atomic bombs. Synthesizing this little grain was a monumental scientific achievement unto itself, and it left behind an atomic legacy that shaped the course of the 20th century.
And its rediscovery is all the more remarkable because the fleck very nearly wound up in a landfill.
Health physicist Phil Broughton found the sample in 2008 in a bucket at U.C. Berkeley bound for the radioactive disposals at the Nevada Test Site. The school had been inventorying and purging old radioactive materials that had been in storage for years, some dating back to the days of the Manhattan project in the 1940s.
“Whenever we open we open a closet and find an unlabeled box… I get a call,” Broughton said.
This particular bucket contained a number of artifacts from Berkeley’s old Heritage Display at the Lawrence Hall of Science, which exhibited artifacts from the school’s history. Alongside the plutonium was also was one of the first artificial samples of carbon-14, the beryllium backstop to the school’s first particle accelerator and a few other “hot” items. The display was first put together in 1968 and stayed up until 2007 when it was disassembled during a redesign of the exhibit hall.
“[The plutonium] belongs next to the original Moon rocks. This is one of the most important scientific artifacts we have at the Berkeley facility,” Broughton said. “They were on their way to disposal as waste.”
Plutonium is important historically because it is the critical element in most atomic bombs. During World War II, the enormous Hanford Engineer Works in Washington state, was devoted to producing the plutonium used in the nuclear weapon used against Nagasaki.
Image: Department of Energy
It’s also one of the first man-made elements ever produced. The periodic table has 92 naturally occurring elements and it stayed that way until 1940 when scientists Edwin McMillan and Philip Abelson from Berkeley announced they had isolated the first traces of neptunium. They did this by bombarding samples of uranium with neutrons fired at it from a cyclotron. The uranium-238 nuclei absorbed one neutron to become uranium-239, then quickly decayed into element 93, later dubbed neptunium. Later that same year, physicist Glenn Seaborg continued this research and realized that the nuclei decayed once again and created element 94, which he later named plutonium.
This particular specimen isn’t the very first plutonium he isolated. That’s on display at the Smithsonian Institute in Washington D.C., but it’s really not much to look at. There’s only a handful of atoms of the element that make up the entire sample, not nearly enough to see, much less weigh. This is because creating plutonium using a cyclotron is a painstaking and inefficient process and can only produce a little at a time. Seaborg needed many pounds of uranium in order to create this single tiny grain of element 94.
However it was that tedious method that ultimately helped substantiate the provenance of the plutonium fleck. The documentation tying this particular sample to Seaborg had long been lost, even before it was slated for disposal. It seems that between 1942 and when it first went on display in 1968, no one kept any reliable records about where it was stored, who was looking after it or even what it looked like. This meant that there was no real way to be sure that what was encased in that lucite box was actually plutonium. The sticker on the side said it was a plutonium sample that weighed 2.7 micrograms, which matched what Seaborg recorded in his notes about his sample, but that’s hardly definitive.
Image: Eric Norman
“It had been on display at the Lawrence hall of science… but there is no paperwork for how it went into that display,” Broughton said. “I’m missing a giant chunk of the paperwork trail.”
Broughton held onto the sample for a few years, then in the spring of 2014 he brought it to Eric Norman to determine as best as possible what this radioactive fleck was by looking at its radioactive signature.
Almost all plutonium is created by putting uranium-238 in a reactor for it to absorb neutrons. Most of the time the uranium absorbs a a single neutron and transmutes briefly into neptunuim-239, and then the more stable plutonium-239. However because there are so many neutrons flying around in a reactor, sometimes the plutonium absorbs a few extra neutrons in the process and becomes plutonium-241, which decays into a different element, americium-241. That doesn’t really happen when using a cyclotron to turn uranium into plutonium.
“It was very unlikely for uranium to capture one neutron and almost no chance of capturing an extra neutron,” Norman said. “Modern plutonium that’s made in nuclear reactors invariably have other plutonium isotopes in it that decay into other radioactive isotopes that we can observe… If this sample is really 70 years old all of the plutonium-241 would have decayed into americium and we would be able to see it.”
Norman and his teammates Keenan Thomas and Kristina Telhami placed the box in a radiation counter to see if they could see what was really in the lucite box without opening it.
“Almost immediately we could see that it had plutonium in it,” Norman said. “We spent the next week measuring it very very carefully with a number of detectors that we have in our lab.”
At the end of the experiment they looked at the radiation profile of their plutonium fleck; it was clean as a whistle. There was almost no other radioactive isotopes in the sample, something that could only have been done with the cumbersome cyclotron method.
They could even get a pretty accurate measurement of the mass of the sample just from the radiation counters. Scientists know what proportion of the element is likely to decay over a given time period, so by counting the number of alpha particles it emitted, they can extrapolate out the total amount of plutonium in the sample. Based on the decay rate, they calculated that there was between two and three micrograms of plutonium in the fleck, right on target with the 2.7 micrograms listed on the sticker, and the 2.77 micrograms that Seaborg recorded in his notes.
|An NRC warning label.
Image: Eric Norman
Norman and Broughton were convinced that what they had found was the original Seaborg plutonium sample.
“We’ve done as much determining of provenance as we’re ever going to do short of the paperwork magically appearing,” Broughton said.
For now the historic sample is safely in storage at the University of California, Berkeley. Finding a more permanent home could be tricky. Possessing plutonium requires a tremendous amount of licensing and oversight from the Nuclear Regulatory Commission, which is not easy to get. But saving it for posterity is something that Broughton feels strongly about.
“I don’t want this to disappear again. I want this on display, I want people to know this is our heritage,” he said. “This is part of our campus’s history that we should be proud of.”