Radioactive Dating
Before the twentieth century, determining the age of ancient artifacts was considered the job of archaeologists, not nuclear physicists. By comparing the placement of objects with the age of the rock and silt layers in which they were found, archaeologists could usually make a general estimate as to their age. However, many objects were found in caves, frozen in ice, or in other areas whose ages were not known; in these cases, it was clear that a method for dating the actual object was necessary.
In 1907, the American chemist Bertram Boltwood proposed that rocks containing radioactive uranium could be dated by measuring the amount of lead in the sample. This was because uranium, as it underwent radioactive decay, would transmute into lead over a long span of time. Thus, the greater the amount of lead, the older the rock. Boltwood used this method, called radioactive dating, to obtain a very accurate measurement for the age of the Earth. While the uranium-lead dating method was limited (being only applicable to samples containing uranium), it proved to scientists that radioactive dating was both possible and reliable.
The first method for dating organic objects (such as the remains of plants and animals) was developed by another American, Willard Libby. He had become intrigued by carbon-14, a radioactive isotope that is created in the atmosphere by cosmic rays. When carbon-14 falls to Earth, it is absorbed by plants during photosynthesis; these plants are eaten by animals who are, in turn, eaten by larger animals, so that the entire ecosystem is eventually suffused with carbon-14. As long as these organisms are alive, the supply of carbon-14 is continuously replenished; when they die, however, the supply stops, and the radioactive carbon begins to decay. By determining the extent of decay, Libby proposed, the exact date of the organism's death could be pinpointed.
Libby began testing his procedure by dating objects whose ages were already known, such as samples from Egyptian tombs. He found that his methods, while not as accurate as he had hoped, were fairly reliable. He continued his research and, through improvements to his equipment and procedures, was eventually able to determine the age of an object up to 50,000 years old with a precision of plus-or-minus ten percent. Libby's method, called radiocarbon dating, gave new impetus to the science of radioactive dating. Using the carbon-14 method, scientists determined the ages of artifacts from many ancient civilizations. Still, even with the help of laboratories worldwide, radiocarbon dating was only accurate up to 70,000 years, since objects older than this contained far too little carbon-14 for the equipment to detect.
Starting where Boltwood and Libby had left off, scientists began to search for other long-lived isotopes. They developed the uranium- thorium method, the potassium- argon method, and the rubidium- strontium method, all of which were based on the transformation of one element into another. They also improved the equipment used to detect these elements, and in 1939 scientists first used a cyclotron particle accelerator as a mass spectrometor. Using the cyclotron, carbon-14 dating could be used for objects as old as 100,000 years, while samples containing radioactive beryllium could be dated as far back as 10-30 million years. A new method of radioactive tracing involves the use of a new clock, based on the radioactive decay of uranium-235 to protactinium-231.
A mass spectrometer was used for one of the most famous radioactive dating experiments, the dating of the Shroud of Turin, the supposed burial cloth of Jesus, which was determined to have been made during the 1300s and not during the time of Christ. Because not all radioactive dating methods are applicable to all situations, non-radioactive methods have also been developed. Perhaps the most famous is dendrochronology, better known as tree-ring dating. Contrary to popular belief, scientists do not count the number of rings to determine a tree's age; rather, they try to align regular patterns of climatic change with changes in the rings. For example, regular seasons of wet or dry weather would be reflected as wide and thin rings, respectively. This method has been especially successful in the American Southwest, where the weather patterns have been fairly constant for centuries. Due to the unpredictable weather, dendrochronology has been far less successful in Europe.
Another botanical method is the analysis of pollen. Because pollen is nearly indestructible it can be found centuries after its creation. If a certain kind of pollen is found in an archaeological site, scientists can check when the plant that produced that pollen lived to determine the relative age of the site. If pottery is found, archaeologists can use the method called thermoluminescence. Simply put, thermoluminescence means that pottery, when heated, will glow; the older the object, the brighter it will glow. Using thermoluminescence, pottery pieces as old as 100,000 years can be dated with precision.
Two newer methods for dating non-radioactive objects are through obsidian hydration and amino acid racemization. Scientists have recently found that obsidian (a type of volcanic glass) will absorb water very slowly, forming a hydrated layer. By knowing the rate at which obsidian absorbs water and then measuring the thickness of this hydrated layer, the approximate age of the obsidian can be determined. This method is particularly useful in dating civilizations that used obsidian tools. In 1978 two scientists discovered the chemical process called racemization, by which the L-amino acids present in living organisms are converted over time into D-amino acids. This process can take tens of thousands of years, and can be used to determine the age of once-living things. Racemization is much different from the other processes because it pinpoints not the organism's overall age but its age when it died. However, the sample must be very well preserved, as was the case of the 1,600-year-old Innuit woman found frozen in Alaska in 1972; using amino acid racemization, it was determined that she was 53 (plus or minus 5) years old when she died.
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