Texas Industrial Radiography Gamma Practice Test

What is the dose rate 10 feet from a 20 Ci Iridium 192 source exposed in a collimator with a shielding equivalent of 4 Half-Value layers?

1. 59 mr/hr

2. 73.8 mr/hr

3. 7.38 R/hr

4. 148 mr/hr

The correct answer is: 73.8 mr/hr

To determine the dose rate at a distance from the source, we first need to consider the initial dose rate from the radioactive source and then apply the half-value layers (HVL) to account for the shielding effect. Iridium-192 is a common radionuclide used in industrial radiography, with a well-defined exposure rate constant. The exposure rate for a 20 Ci Iridium-192 source (which is approximately 740 GBq) can typically be calculated using the exposure rate constant, which is about 4.69 R/hr/Ci at a distance of 1 meter from the source. Given that there are 4 half-value layers of shielding, we can calculate the attenuation of the source intensity by a factor of 16 (since each HVL reduces the intensity by half). Therefore, the collapse of the initial dose rate due to the shielding means we are left with 1/16th of the initial dose rate. Before applying the half-value layers, let’s convert the distance from feet to meters for more precise calculations. Since 10 feet equals approximately 3.048 meters, we need to account for the inverse square law in radiation, which states that the intensity decreases with the square of the distance from the source.