ASNT Industrial Radiography Radiation Safety Practice Test

If a radiographer has an exposure rate of 100 mR/h at 25 ft, what would be the exposure rate at 50 ft with one half-value of shielding?

1. 5 mR/h

2. 12.5 mR/h

3. 25 mR/h

4. 50 mR/h

The correct answer is: 12.5 mR/h

To determine the exposure rate at 50 ft given an initial exposure rate of 100 mR/h at 25 ft with one half-value of shielding, we need to consider both the effects of distance and the additional reduction in exposure due to the shielding. The inverse square law tells us that radiation intensity decreases with the square of the distance from the source. When doubling the distance from 25 ft to 50 ft, the exposure rate will decrease by a factor of four (since the distance increases by a factor of two). Thus: 1. **Calculating the impact of distance:** - At 25 ft, the exposure is 100 mR/h. - At 50 ft, without considering shielding, it would be: \[ \frac{100 \text{ mR/h}}{2^2} = \frac{100 \text{ mR/h}}{4} = 25 \text{ mR/h} \] 2. **Considering one half-value of shielding:** - A half-value layer (HVL) reduces the radiation to half of its original intensity. Thus, applying one half-value layer to the already decreased exposure rate of 25 mR/h will further reduce