Particle accelerators are usually associated with breakthroughs in quantum physics. Normally they're used to elucidate the mysteries of matter, to identify sub-atomic particles and that sort of thing.
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But the new particle accelerator that began operating three months ago at the Weizmann Institute of Science isn't being used to peer into matter. It is helping archaeologists peer into the past through cutting-edge carbon-14 testing of artifacts, a first in the annals of Israeli archaeology.
Carbon-14 is considered the most reliable means of dating artifacts and matter unearthed in archaeological excavations.
It's not a new technology. Carbon-14 dating was invented in 1947 by Willard Libby, which won him a Nobel Prize. The method was improved in the 1980s, which is when researchers began using particle accelerators to determine the age of archaeological findings.
The method relies on a radioactive isotope of carbon, known as carbon-14. All organic matter has carbon and a given proportion of that is carbon-14.
Like all radioactive material, carbon-14 degrades over the years. The age of the organic matter is determined by calculating the ratio between carbon-14 and two other carbon atoms: carbon-12 and carbon-13. The higher the proportion of carbon-14 in relation to that of other two carbons, the newer the material from which the isotope was taken.
Carbon-14 dating became famous after the method was used in the 1980s to test the age of what has been called the Turin Shroud, a linen cloth that, according to Christian belief, enveloped the dead body of Jesus Christ. But three different particle accelerators told a different story and determined that the shroud had been woven in the Middle Ages, more than a thousand years after Jesus’ crucifixion.
Finding the olive pit
Dr. Elisabetta Boaretto, director of the Radiocarbon Dating and Cosmogenic Isotopes Lab, wrote her doctoral dissertation on the concentration of radioactive isotopes in the icebergs of Greenland. For her postdoctorate she used carbon-14 to research the age of the groundwater in Denmark.
Many institutes use particle accelerators for carbon-14 dating, but Boaretto is the only physicist in the world who personally combines work in the lab with work at archaeological digs.
“I don’t receive samples by mail,” she says. “There has always been a disconnect between the exact sciences and the humanities. Science always stayed in the lab, without understanding what was happening in the field, and the archaeologist excavates and sends the findings to the lab. If the data the test finds match his assumption, he publishes something about it. If they don’t, then there was evidently a mistake in the lab.
“I take a different approach. I go out to excavations not only to find the olive pit, but also to understand the connection between the olive and what we want to date ... You can’t rely on examining the wooden beams used in the construction, which would tell you when the house was built. You need to find the last meal and date that.”
The instructions for preparing the olive pit for the accelerator sound like they come from a recipe from an especially wild cookbook.
“Once we’ve cleaned it and made sure there is nothing left but the material we want to date, we crush the pit and heat it to 900 degrees Celsius for two hours to break it down into carbon dioxide. Then we cool it to -30 degrees, and then heat it to 700 degrees for 12 hours,” Boaretto says, describing the process of the $2-million device, which was funded by a contribution from the Dangoor family of England and an anonymous donor.
What you get at the end is a 1-milligram crumb of material called graphite, which contains the carbon atoms belonging to the ancient olive pit. The graphite is fed into the particle accelerator, which sends the carbon atoms through its maze of pipes. While they are flying about inside the accelerator, the atoms of carbon and carbon-14 are separated from the rest. At the end, the accelerator tallies the various particles. All that remains is to plug the numbers into an equation. The result of this calculation is the estimated year when the olive was removed from its tree.
Worldwide, experts in the exact sciences have increasingly joined archaeological excavations in recent years. At some digs there are mobile laboratories that allow immediate tests to be conducted, in order to identify the minerals located in archaeological strata. The identity of the minerals hints at the various uses of vessels or structures unearthed at the site.
Another branch of science, known as microarchaeology, deals with identifying and mapping phytoliths − microscopic particles created by a certain kind of plant.
Dating human migration
Israel is considered a powerhouse when it comes to breaching the divide between archaeology and the sciences. Advances in the field are being spearheaded by another researcher from the Weizmann Institute, structural biologist Prof. Steve Weiner, who directs the Kimmel Center for Archaeological Sciences.
And another sphere of collaborative archaeology research is developing around the modern accelerator. Dr. Lior Regev, a researcher in Boaretto’s lab, sits beside the accelerator, feeding it slices from different layers of tree trunks. In addition to dating the period in which the tree grew, the accelerator enables Regev to assess the climatic conditions at the relevant period. The conclusions gleaned from data relating to modern trees will also, for example, help him draw conclusions from lumps of coal that originated in ancient acacia trees.
The accelerator will also play an important role in a joint excavation project of Boaretto’s lab and Dr. Omry Barzilai of the Israel Antiquities Authority, at the site dubbed Boker Tachtit. Located at the bottom of the Zin River, not far from Sde Boker, this a prehistoric site that contains flint tools, remnants of ash and coal from campfires, and also a few animal bones. Researchers say the information gleaned from the remains found at Boker Tachtit, which will be inserted into the particle accelerator, will contribute to ascertaining the chronology of modern human migration from Africa to every other part of the world.
The classic theory holds that our ancestors, the first group of Homo sapiens, left Africa 60,000 years ago, migrated eastward and northward through the Sinai Desert and the Land of Israel, and continued on from there to Europe and the rest of the world. Dating at sites across Europe largely supports the theory regarding the migration from east to west. The sites in the Middle East predate those in Eastern Europe, while those in Eastern Europe predate the ones in Western Europe.
In recent years, however, two more theories have been put forward about the direction of human migration. According to one, the earliest Homo sapiens groups crossed the Red Sea to the Arabian Peninsula, from which the population spread through the Levant to Europe. Supporters of an even more radical theory maintain that these human beings crossed the Strait of Gibraltar and moved into the Iberian Peninsula.
The site at Zin is along the migration route from Africa leading north, up to the Levant − hence the importance of dating it accurately. But this is precisely where carbon-14 dating hits a wall. The method is accurate only as far back as 50,000 years − it is very hard to date samples older than that because of the isotope’s degradation. However, Boaretto and Barzilai believe that through rigorous fieldwork − which will locate the best samples for dating − and meticulous research in the lab, they will succeed in stretching the 50,000-year limit, date the site accurately and find an important piece of the puzzle.
The frequent use of carbon-14 dating has already led to changes in the accepted thinking about the Late Bronze Age at sites throughout Israel. The prevailing approach, which was based on dating according to the shape of shards at a given site, was that this period came to an end in approximately 2300 B.C.E. The reason for the collapse of the Late Bronze Age communities, according to this perception, was a climatic crisis in the region. But new dating with carbon-14, carried out as part of the doctoral work of another of Boaretto’s students, Johanna Regev (Lior’s wife), showed that the period ended 200 years earlier.
Now the question being asked is why, and what crisis was involved.
“Maybe it was a social crisis. Maybe people decided to mount protests and demanded social justice? Now we need to search for an answer,” Boaretto says. “Every archaeologist comes up with his own clock, but all over the world [the unit of] time is the same one, the year is the same year and carbon-14 synchronizes all the watches. The archaeologists look at history through a window which is generally closed. Today we enable it to be open a little more.”