ASNT Industrial Radiography Radiation Safety Practice Test

What would be the restricted area on the cold or collimated side of the collimator if a cobalt source of 73 Ci is exposed for 60 minutes with a 14.0 R/Ci factor through a 3 half-value collimator?

1. 153 ft (46.63 m)

2. 200 ft (60.96 m)

3. 253 ft (77.44 m)

4. 300 ft (91.44 m)

The correct answer is: 253 ft (77.44 m)

To determine the restricted area on the cold or collimated side of the collimator when using a cobalt source of 73 Ci, one must first calculate the exposure rate at a certain distance, and then consider the half-value layers of the collimator. In this situation, the exposure factor is provided as 14.0 R/Ci. First, we calculate the total exposure generated by the cobalt source during the 60 minutes of exposure. This is done by multiplying the activity of the source (73 Ci) by the exposure factor (14.0 R/Ci) and then by the duration (1 hour or 60 minutes): Total exposure = Activity (Ci) × Exposure factor (R/Ci) × Time (hr) Total exposure = 73 Ci × 14.0 R/Ci × 1 hr = 1022 R Next, since the collimator has three half-value layers (HVL), it naturally reduces the exposure rate. Each half-value layer decreases the radiation by half, allowing us to calculate the exposure at an adjusted distance while accounting for the diminished intensity. After passing through three half-value layers, the exposure at the cold side can be calculated to see how far that exposure can reach before