. In our experiment we will artificially produce radioactive silver of two types and measure their half-lives. The heavily-shielded drum in the center of the room contains americium, a man-made radioactive element. The americium spontaneously emits alpha (a) particles - helium nuclei consisting of two protons and two neutrons-into a surrounding coating of beryllium. The beryllium a-particles react to form carbon and free neutrons. (This is not quite the reaction which takes place in red giant stars - a different isotope of beryllium is involved there.) The neutrons, which are slowed down by a thick layer of paraffin, bombard our silver targets.
Silver occurs naturally in about equal amounts in two isotopes. Each isotope of a chemical element contains a certain number of neutrons (neutral charges). All silver atoms contain 47 protons; the two isotopes contain either 60 or 62 neutrons. When one of these atoms absorbs a neutron (n) it becomes excited and eventually decays into the element cadmium (with 48 protons) and emits an electron (b-particle) and a neutrino (n). In effect, one neutron is converted to a proton and an electron. The chemical symbol for silver is Ag; for cadmium, Cd. Symbolically we write
Ag107 + n ® Ag108 ® Cd108 + e + n
Ag109 + n ® Ag110 ® Cd110 + e + n
It is the decay of Ag108 and Ag110 which we will investigate. Incidentally, this is a very low energy and therefore perfectly harmless decay.
In this experiment our counters will record not only the decay products of the radioactive silver, but also the chance passing of cosmic ray particles. This near constant background flux of radioactivity must be subtracted from our data before meaningful analysis can proceed. Being a random phenomenon, the background count will also be of the form .
The activity of a sample of radioactive substance is directly proportional to the number of atoms in that sample, as we will see, and allows us to compute that number. Uranium, for one, leaves behind decay products that can be identified, and so the total original amount of uranium in a sample is therefore computable. Determination of the approximate original amount of a radioactive substance is not usually too difficult, because most theories indicate that heavy elements are produced in roughly equal amounts. From the ratio of the present amount to the original amount the age is easily found.
Not only can we determine age by measuring the abundances of uranium isotopes, but the decay products of uranium and thorium include helium (a-particles) and those of potassium-40 include argon. Helium and argon are gases which escape rapidly from fluids, but are generally trapped in solids. The amount of these gases trapped in a mineral then gives an indication of the time the mineral last was molten. These and similar techniques are used for the only reliable age determinations of earth and lunar rocks, meteorites, fossils, and ancient artifacts.
For a further discussion of the role of radioactive dating in cosmochronology and the origin of the elements, see J. A. Wood, Meteorites and the Origin of the Planets, or W. K. Hartmann, Moon and Planets.
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