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Radioactive Dating

The technique of comparing the abundance ratio of a radioactive isotope to a reference isotope to determine the age of a material is called radioactive dating. Many isotopes have been studied, probing a wide range of time scales.

The isotope 14C, a radioactive form of carbon, is produced in the upper atmosphere by neutrons striking 14N nuclei. The neutron is captured by the 14N nucleus and knocks out a proton. Thus, we have a different element, 14C. The isotope, 14C, is transported as 14CO2, absorbed by plants, and eaten by animals. If we were to measure the ratio of 14C to 12C today, we would find a value of about one 14C atom for each one-trillion 12C atoms. This ratio is the same for all living things–the same for humans as for trees or algae.

Once living things die, they no longer can exchange carbon with the environment. The isotope 14C is radioactive, and beta-decays with a half-life of 5,730 years. This means that in 5,730 years, only half of the 14C will remain, and after 11,460 years, only one quarter of the 14C remains. Thus, the ratio of 14C to 12C will change from one in one-trillion at the time of death to one in two trillion 5,730 years later and one in four-trillion 11,460 years later. Very accurate measurements of the amount of 14C remaining, either by observing the beta decay of 14C or by accelerator mass spectroscopy (using a particle accelerator to separate 12C from 14C and counting the amount of each) allows one to date the death of the once-living things.

Perhaps you have heard of Ice Man, a man living in the Alps who died and was entombed in glacial ice until recently when the ice moved and melted. The man's body was recovered and pieces of tissue were studied for their 14C content by accelerator mass spectroscopy. The best estimate from this dating technique says the man lived between 3350 and 3300 BC.

The boat of a pharaoh was discovered in a sealed crypt and reassembled in a museum near the pyramids (see Fig. 13-4). Its wood was dated using 14C to be about 4,500 years old.

    Fig. 13-4. The pharaoh’s funerary boat. © National Geographic Society

Other methods of dating are used for non-living things. 40K decays with a half-life of 1.3 ´ 109 years to 40Ar which can be trapped in rocks. A potassium-argon method of dating, developed in 1966, measures the amount of 40Ar arising from the 40K decay and is compared to the amount of 40K remaining in the rock. From the ratio, the time since the formation of the rock can be calculated.

The age of our galaxy and earth also can be estimated using radioactive dating. Using the decays of uranium and thorium, our galaxy has been found to be between 10 and 20 billion years old and the earth has been found to be 4.6 billion years old. The Universe must be older than our galaxy. Within experimental error, this estimate agrees with the 15 billion year estimate of the age of the Universe.

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