Uses of radioactivity

In the last lecture we spoke of some of the problems of working with radioactivity. But there are also a number of significant uses. I will just mention a couple here: medical uses and isotope dating.

Medical uses

Cancer is a disease characterized by cells losing control over their normally regulated cell-division process and growing out of control in the body. There are many different kinds of cancers representing many different cell types that can be involved. We mentioned earlier that radiation could cause the kind of damage in the DNA that can lead to this loss of control. But radiation can also be used in the treatment of cancer cells, as a weapon to kill the cells before they can reproduce too much. A variety of techniques are used to deliver radiation dosages to cancerous tissues, from exposure to beams of particles from a radiation source external to the body to the administration of isotopes in a form that are carried directly to the cancerous tissue. The isotopes involved are by their nature unstable and must be prepared artificially, and such preparation requires a nuclear reactor or some other source of particles such as neutrons to produce the new isotopes. For example, Cobalt-60, commonly used in cancer radiation treatment, is made in a reactor by the following sequence of reactions:

Cobalt-60 undergoes beta decay with a half life of 5.27 years.

Isotopes are also used for diagnostic purposes. Low levels of radioactive substances can be used to generate images of various body components, such as blood vessels in the brain. Localization of certain isotope derivatives in certain tissues can be used as an indicator of cancer cell presence.

Dating

I’ve already mentioned the long half-life of uranium-238. It undergoes a series of decay reaction, consisting of alternative beta and alpha decays, producing a series if intermediate isotopes with shorter half-lives. Finally lead-206 is produced as the stable end-product of the decay series. Since normal lead is isotope 208, the presence of lead-206 in a rock sample is an indication that uranium-238 was originally there in its place. By comparing the amount of uranium-238 and the amount of led-206 in any given rock sample, one can tell how long the rock has been solidified by the fraction of decay of the original uranium-238, and thus date the age of the rock.

On a much shorter time frame, we use the radioactive isotope of carbon-14 to date events in the order of a few thousand years. Carbon-14 undergoes beta decay with a half-life of 5730 years. It is continually being formed in the upper atmosphere by reaction of neutrons from the sun with nitrogen-14 as follows:

This production and the normal decay of the isotope produce a steady state level of carbon-14 in the CO₂ of the atmosphere. When CO₂ is incorporated into living matter by plants, the material (wood, for example) will have a carbon-14 content equivalent to that of the atmosphere. Once the plant dies, the carbon is not replenished, and the carbon-14 continues to decay. By measuring the amount of carbon-14 remaining, one can deduce how long ago the wood sample was living, and therefore can date archeological artifacts. This kind of dating was used, for example, to question the authenticity of the shroud of Turin, supposedly having been taken from Christ’s tomb.

End of Chapter Problems

The rest of the period was used to go over in class some of the end-of-chapter problems. We particularly paid attention to problems 3, 8, 9, 10, 12, 14, 24, 26, and 28.