ASNT Industrial Radiography Radiation Safety Practice Test

During a single 30-minute exposure with a cobalt source of 61 Ci (2257 GBq), what will the total exposure be from a distance of 100 ft?

1. 10 mR (100 μSv)

2. 2.7 mR (27 μSv)

3. 5.3 mR (53 μSv)

4. 3.0 mR (30 μSv)

The correct answer is: 5.3 mR (53 μSv)

To determine the total exposure from a cobalt source of 61 Ci at a distance of 100 ft over a period of 30 minutes, it is essential to apply the principles of radiation exposure and the inverse square law. Cobalt-60 is a common radioactive source used in industrial radiography, and its exposure can be calculated using appropriate radiation exposure formulas. The exposure rate from a radioactive source generally decreases with distance due to the inverse square law, which states that the intensity of radiation is inversely proportional to the square of the distance from the source. This means that as you double the distance from the source, the exposure rate decreases by a factor of four. Additionally, the exposure rate must be calculated based on the source's activity and the specific radiation energy emitted. The calculation for this scenario, which combines the source activity, exposure time, and distance, leads to the conclusion that at 100 ft away from a 61 Ci cobalt source, the total exposure during a 30-minute period amounts to 5.3 mR (53 μSv). This value considers the decay characteristics of cobalt-60 and the known exposure levels associated with such a source at the given distance. Thus, the selection of 5.3 mR (53 μ