Texas Industrial Radiography Gamma Practice Test

What is the maximum permissible dose rate at the perimeter of the Restricted Area if exposure time will not exceed 20 minutes in any one hour?

1. 2 mr/hr

2. 5 mr/hr

3. 6 mr/hr

4. 12 mr/hr

The correct answer is: 2 mr/hr

The maximum permissible dose rate at the perimeter of a Restricted Area is determined by considering both the dose rate and the allowable exposure time within a given period. In this scenario, the specified exposure time is limited to a maximum of 20 minutes within any one-hour period. To calculate the maximum permissible dose rate, the total permissible dose for 20 minutes must be distributed over that hour. If we assume that the exposure limit, based on regulatory standards, is typically set at 100 millirem (mr) in a year for unrestricted areas, then during an hour (60 minutes), this translates to a dose that should not exceed a certain threshold. With the exposure rate defined as 2 mr/hr, if someone remains in the area for 20 minutes, they would receive: \[ \text{Dose} = \text{Rate} \times \text{Time} = 2 \, \text{mr/hr} \times \frac{20}{60} \, \text{hr} = \frac{40}{60} \, \text{mr} = \frac{2}{3} \, \text{mr} \] This dose falls well within an acceptable limit for restricted areas and complies with safety regulations