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Neutron Activation Analysis

There are at least 50 elements occurring in nature that have radioactive isotopes with one neutron more than their stable isotopes. This means that the radioactive species can be made by neutron bombardment. This procedure is typically done in a nuclear reactor, although other neutron sources can sometimes be used. The stable nucleus absorbs one neutron and becomes a radioactive nucleus. By detecting the decay of these nuclei, which can be done with great sensitivity, one can measure the concentration of the stable element of interest in the sample.

A simple example of neutron activation analysis involves the measurement of iridium in soils. This is the measurement that led to the theory that the extinction of the dinosaurs, 65 million years ago, was caused by the impact of an asteroid or comet somewhere on Earth. An impact would produce so much debris in the air that earth would receive a large reduction in sunlight and thus fewer plants would grow. With less food available, there would be a devastating reduction in number of animals. This reduction was so severe that most species, including the dinosaurs, became extinct.

In 1979, a group of scientists reported that neutron activation analysis had shown unusual amounts of the element iridium in Italian Cretaceous-Tertiary boundary sediments. Since iridium is a metal which has very low abundance on earth, they attributed the excess iridium to a impact of a 10-kilometer diameter asteroid. This work, which melded the disciplines of physics (Nobel Laureate Luis Alvarez), geology (his son, Walter Alvarez) and nuclear chemistry (Isadore Perlman, Frank Asaro and Helen Michel), galvanized the scientific world because the concepts presented could be tested by many diverse techniques.

Since the initial report, anomalous amounts of iridium have been found at the Cretaceous-Tertiary boundary in over 100 sites worldwide. Many experiments have confirmed its impact origin.

  last updated: August 9, 2000 webmaster