Texas Industrial Radiography Gamma Practice Test

If the original activity of an Iridium source is 60 Ci, what will its activity be after 150 days?

1. 6.0 Ci

2. 15 Ci

3. 4.5 Ci

4. 30 Ci

The correct answer is: 15 Ci

To determine the activity of an Iridium source after a certain period, it is essential to understand the concept of radioactive decay and the half-life of the isotope in question. Iridium-192, which is commonly used in industrial radiography, has a half-life of approximately 74 days. The decay of a radioactive isotope can be expressed using the formula: \[ A = A_0 \left( \frac{1}{2} \right)^{\frac{t}{t_{1/2}}} \] where: - \( A \) is the remaining activity, - \( A_0 \) is the original activity, - \( t \) is the elapsed time, - \( t_{1/2} \) is the half-life. In this case, the original activity \( A_0 \) is 60 Ci, and the elapsed time \( t \) is 150 days. To find out how many half-lives have occurred in 150 days, we calculate: \[ \frac{t}{t_{1/2}} = \frac{150 \text{ days}}{74 \text{ days}} \approx 2.02. \] This indicates that a little over 2