Jonathan Fahey

Jonathan Fahey, Contributor

5/21/2010 @ 12:00PM

The Radioactive Clock In Your Teeth

Hikers on a secluded trail in Newfoundland, Canada, found a bag containing a human skull in 2001. An investigative team examined the scene and the material recovered, and tried to figure out what had happened.

“We had no idea who this person was,” says Inspector John House of the Royal Newfoundland Constabulary. “The case went cold very quickly.” All investigators could determine, with the help of dentists and forensic anthropologists, was that the victim was a long-haired male that had been murdered.

But House picked up the case in 2007 to see if advances in forensic science might help him get the case going again. He discovered a paper in a 2006 issue of the journal Nature co-authored by Bruce Buchholz, a physicist at Lawrence Livermore National Laboratory and Kirsty L. Spalding of the Karolinska Institute in Stockholm.

They had discovered that a bygone era of world political history had made it possible to date unidentified human remains more precisely than other forensic methods. Aboveground testing of nuclear bombs in the 1950s and early 1960s created elevated levels of radioactive carbon that soon became incorporated into all living things. Those elevated levels of radiocarbon were tracked carefully, and they began to drop off quickly after a test ban treaty was signed in 1963.

The effect was to create a clock that investigators can now use to determine the age of victims of crimes or natural disasters, for example. In a paper published this month in the journal Molecular and Cellular Proteomics, Buchholz, Spalding and their colleagues showed that after examining 44 teeth from 41 people they could determine ages within about two years–far better than more common methods like looking at patterns of wear and the changes that proteins in the teeth undergo as people age.

The element carbon usually has six protons and six neutrons, and when it does it is called carbon 12. But cosmic rays streaming into our atmosphere occasionally smash into the abundant nitrogen in our atmosphere and create a form of carbon with two extra neutrons called carbon 14. (Over time this carbon undergoes radioactive decay and turns back into nitrogen.) This happens at low rates; there’s about one carbon 14 for every trillion carbon 12s.

But aboveground nuclear testing that started in 1955 released torrents of high-energy neutrons into the atmosphere. The neutrons smashed into nitrogen and created much more carbon 14 than normal. That carbon quickly hooked up with oxygen in the air, creating carbon dioxide, and spread relatively evenly around the globe.

Plants breathe in this now-special carbon dioxide and use it to make leaves, fruits and grains. Animals eat these plants, using the carbon to grow, and people eat the plants and animals. “Everything that is alive has been labeled with this elevated tracer,” says Buchholz.

While our flesh gets replenished as cells die and are replaced, the enamel of our teeth is made only once, and at very specific times in our lives. The carbon Jane Doe used to make her teeth in 1968 looks to Buchholz quite different from the carbon John Doe used to make his in 1972. “It turns out measuring the [carbon 14] is a better chronometer for determining a date of birth than anything used in forensic science,” says Buchholz.

It’s less useful, of course, for people who made their teeth before bomb testing started in 1955, and it will become ever less useful as more of the bomb carbon is absorbed by oceans and plant and animal life.

Buchholz, a physicist and a nuclear engineer, is an expert at something called accelerator mass spectrometry, an extraordinarily precise method of determining the chemical composition of materials. Atoms of the material in question are accelerated to very high speeds, stripped of electrons (rendering them positively charged) and passed by a magnet that separates the nuclei of the atoms by weight. Detectors at the end of the accelerator can then count individual atoms according to how they are sorted.

Buchholz and Spalding first were trying to use this method to see if they could determine whether neurons in the brain were grown once or if they were regenerated throughout a lifetime. They needed a long-lived tissue to compare neurons to, and decided on tooth enamel. That’s when they realized this could be used as forensics tool. Buchholz is still working on the neuron research.

That is, when he’s not playing “CSI: Livermore Labs.” After learning about Buchholz and Spalding’s work, House sent teeth and hair samples from his unknown victim to them. They were able to determine that he was born between 1955 and 1960 by looking at the carbon 14 in the victim’s teeth. Because they had a hair sample, they could also look at the carbon in the last cells the victim created before he died. The roots of his hair suggested he was killed sometime between 1994 and 1997.

In December House made these and other findings public, and he now has some leads. Says House: “We had no place to go with this, now we have information we are working on. It has really advanced the investigation.”

See Also:

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