How does the activity of a cobalt-60 source change over a period of time?

Cobalt-60 is a radioactive isotope that undergoes beta decay, and its activity changes over time according to its half-life, which is approximately 5.27 years. As a radioactive source decays, the number of unstable nuclei decreases, leading to a reduction in the rate of radioactive decay processes. This results in a gradual decrease in the activity, which is the measure of decay events per time unit.

The concept of half-life is integral to understanding this behavior; after one half-life, the activity will have dropped to half its original value. Over successive half-lives, the activity continues to decrease steadily, reflecting the exponential decay of the radioactive material. As such, the correct answer highlights the predictable nature of radioactive decay, where the activity does not remain constant or increase but rather follows a defined decline, aligning with the principles of radiological decay and half-life calculations.