SOURCE-DOSE RATE RELATIONSHIP

When considering an unshielded source which emits gamma radiation isotropically, and treating the surrounding air as a vacuum, the resulting analysis shows that the gamma-ray flux will vary inversely as the square of the distance from the source. If the source strength is S photons per second emitted, the expression for the gamma-ray flux at a distance R cm is given by the expression:

The flux, N has units of photons per cm² per second, and describes the number of gamma rays that pass through a (hypothetical) 1 square centimeter window at the point at R cm from the point source. The window is oriented perpendicular to the line from the source to the point as shown:

To acquaint the student with the use of survey meters and to demonstrate the variation of gamma-ray flux with distance from the source.

1. Place the source in the open, away (as far as possible) from all structure.
2. Using the portable survey meter, take a set of readings at 10 (or less) cm intervals from the source.
3. Record the distance from the source to the center of the sensitive volume of the meter vs the reading.
4. Maintain the axis of the detector perpendicular to the source-to-detector line.
5. Repeat, keeping the detector axis coincident with the line.
6. Place the source adjacent to a wall of dense material, and repeat the readings.
7. Record clearly the orientation of the source and detector and wall for this step.

• Plot the dose rate recorded vs distance on a log-log plot, showing the three runs on the same plot.
• Does the data support
• Discuss the behavior of each run.
• Explain the differences.