Texas Industrial Radiography Gamma Practice Test

If the radiation intensity from an Ir 192 source is 10R/hr at a distance of 2 feet, what would the intensity be at a distance of 1 foot?

1. 500 mr/hr

2. 40 R/hr

3. 400 R/hr

4. 100 R/hr

The correct answer is: 40 R/hr

The correct choice, which indicates that the intensity at a distance of 1 foot from the Ir-192 source would be 40 R/hr, is based on the inverse square law, which states that the intensity of radiation is inversely proportional to the square of the distance from the radiation source. This means that as the distance from the source decreases, the radiation intensity increases significantly. When the intensity is measured at 2 feet and is 10 R/hr, one can calculate the intensity at 1 foot by applying the inverse square law formula: If the intensity at 2 feet is \( I_2 = 10 \) R/hr, the formula can be set up as follows: \[ I_1 = I_2 \times \left( \frac{d_2^2}{d_1^2} \right) \] Here, \( d_1 \) represents the distance at which we want to find the intensity (1 foot), and \( d_2 \) is the distance at which the intensity is known (2 feet). Plugging in the values gives: \[ I_1 = 10 \, R/hr \times \left( \frac{2^2}{1^2} \