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How radioactivity helps scientists uncover the past

Radioactivity: A Very Short Introduction

By Claudio Tuniz

Neanderthal was once the only human in Europe. By 40,000 years ago, after surviving through several ice ages, his days (or, at least, his millennia) were numbered.

The environment of the Pleistocene epoch was slightly radioactive, the same way it is today, but this was not Neanderthal’s problem. The straw that broke the camel’s back was the arrival of a new human, during an already stressful period of extreme and rapid environmental change. The new humans were slender, talkative, and had a round head with a straight face and no protruding brow. They rapidly conquered the steppes, tundra and forests, stretching from Gibraltar to Siberia, where the Neanderthal had been happily striving for hundreds of thousands years, moving around to cope with the vagaries of the weather.

The Neanderthals had broadened their carnivorous diet to include fish, particularly seashells and mollusks. A variety of naturally occurring radioactive atoms contaminated the food that Neanderthals ingested — about the same radioisotopes we eat today, except for some bias introduced by recent accidents like that of Chernobyl and Fukushima. They included uranium, thorium, their decay products and potassium-40, naturally present in rocks and soils. Their ingestion would contribute an absorbed dose of about 300 microsievert per year to Neanderthal’s body, similar to the dose you receive from your meals today.

As uranium accumulated in Neanderthal’s bones, scientists are now able to determine their age, back to 500,000 years ago, measuring the residual radioactivity. This can be done in a non-destructive way analyzing the weak flux of gamma rays emitted by the skull or other bone remains, using sophisticated germanium detectors and other nuclear physics tools. Ideally, these measurements should be performed in special laboratories like those under the Gran Sasso Mountain, in Italy, run by the Italian Institute of Nuclear Physics.  The noise from cosmic rays and environmental radioactivity is so low in these underground laboratories that scientists can reveal the neutrinos emitted by the uranium burnt in nuclear reactors thousands of kilometers away, with useful applications. For example, one could detect whether plutonium, a key ingredient for nuclear bombs, is illegally produced somewhere on the globe.

We will resume below the discussion on nuclear bombs and artificial radioactivity. First, let’s complete the inventory of natural radioactivity sources during Neanderthal times. Like in the present, a radiation of cosmic origin was bombarding the atmosphere, creating new natural radioisotopes that would enter the food cycle. One of these products was radiocarbon. After being produced 40,000 years ago, a fraction of the radiocarbon atoms that were originally present in the bone, about 8 per thousand, survived to the present. The fact that carbon-14’s half-life is 5,730 years makes it the perfect clock to measure with high precision the ages of bones during the last 50,000 years. Indeed, it can be used to study not only the history of the Neanderthals, including the length of their overlap with modern humans, but also that of other human species that existed 40,000 years ago, like Homo floresiensis, nicknamed the ‘hobbit’, whose bones were found in 2003 on a small Indonesian island.

Only the round-headed humans, who arrived in Europe from Africa 40-45,000 years ago, eventually survived. These humans become a global species. Their powerful minds allowed them to conceive art, music, new ways of hunting animals, and fighting different humans. Radioactivity-based clocks confirm that their appearance coincided with the demise of other human species and the extinction of the large animals of the Pleistocene, like Diprotodon and Genyornis in Australia and Smilodon in America.

By discovering natural radioactivity at the end of the nineteenth century, the so-called ‘modern humans’ became capable of reconstructing the detailed history of their ancestors, providing exact dates for the rock art of Chauvet in France and the ‘Venus’ of Hohe Fels in Germany 35,000 to 40,000 years ago.

These humans also learned, in the twentieth century, how to create their own form of radioactivity. While my generation was listening to the first songs of the Beatles and the Rolling Stones, the US and USSR were exploding nuclear bombs in the atmosphere, the ultimate expression of human insanity. Many of the techniques, including mass spectrometers and radiation detectors, useful for dating hominids, were developed by the same scientists who built these nuclear bombs.

The radiation produced by the explosions increased the amount of radiocarbon in the terrestrial environment until 1962, when it reached a concentration that was twice that of the pre-nuclear era. This was the time when Kennedy and the other representatives of the nuclear powers of the time signed the Nuclear Test Ban Treaty.

As a teenager, I received this spike of man-made radiocarbon in my bones. Since then, the concentration of the artificial radiocarbon in the environment has been decreasing, with a half-life of 15 years, due to the exchange of carbon with the biosphere and the oceans.

The radiocarbon bomb pulse offers new applications as a chronometer in forensic science, as shown in popular TV series like CSI and Cold Case. The radiocarbon analysis of a bone sample provides the time of death of an individual during the last 50 years with a precision of a few months. In recent years, this method was applied to investigate the mass killings carried out by the Nazis in Ukraine at the end of World War II and the war crimes perpetrated in the former Yugoslavia in the 1990s.

Some believe the destructive attitude of H. sapiens has deep roots.

Claudio Tuniz leads a programme on advanced x-ray analyses for palaeoanthropology at the Abdus Salam International Centre for Theoretical Physics. He was Assistant Director of the Abdus Salam International Centre for Theoretical Physics in Trieste . Previously he was Nuclear Counsellor at the Australian Embassy to the IAEA in Vienna and Director of the Physics Division at the Australian Nuclear Science and Technology Organization in Sydney. He is co-author of the book The Bone Readers (2009), and the recently published Radioactivity: A Very Short Introduction (2012).

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