In Experiment N-4 we studied a short lived nucleus. We found that in a certain period of time called the half life the intensity of radiation decreased to half. The radioactive material seems to have had no memory of when it started. So in every identical period of time, the intensity of radiation drops in half, no matter when that half life started.
For a short lived substance, it is not difficult to wait around and measure the half life. But for more stable substances, one's life could end before the intensity decreased in half. So an alternate procedure will be necessary. At the end of the previous experiment it was noted that the rate radiation is emitted is given by
where N is the number of nuclei of that type present, and R is the rate of decay (using the same time unites as the half life, t1/2). That can be reversed to solved for the half life:
So if we carefully measure (or calculate) all radiation coming for a known number of atoms, we can calculate very long half lives.
The purpose of this experiment is to determine the half life of Potassium-40.
clickscaused by the background radiation (use aiff file, mp3 file, or visual graph) for the two minutes and calculate the counts per minute. 100KB
Finally, a word about worries over the hazards of radiation. In earlier experiments it was noted that radiation is know to be hazardous to living tissue. That hazard can be minimized by increasing the distance from the source, or in the case of short-lived isotopes, storing them away from living cells until the isotope has naturally decayed over time to less hazardous material. But isotopes such as K-40 have very long half-lives. Some people worry about the radiation hazards from such materials. It should be remembered that when a material has a very long half-life, the rate of radiation emitted in very low. For example, the K-40 sample measured in this experiment was only slightly above background radiation intensity. So such long-lived isotopes pose only a trivial risk to living creatures because the radiation intensity is so tiny. Much greater radiation exposures occur when we choose to spend a few hours outdoors or take a single airline flight. Thus there is no reason for concern for the Potassium-40 which exists naturally in our food and in our own bodies. Based on this understanding, consider the radiation risks to society from the portion of nuclear reactor wastes which also have extremely long half-lives.
Communicating technical information such as observations and findings is a skill used by scientists but useful for most others. If you need course credit, use your observations in your journal to construct a formal report.