Radioactivity and radioisotopes

1. Radioactivity and radioisotopes • Gamma rays range in air • Inverse Square Law

2. Inverse Square Law for g-rays Using the same equipment investigate how the distance from the g-source affects the Activity I detected. The limit of activity in the environment where medical staff operate g-rays machinery from is 0.3 Bq. In your investigation you are required to find the minimum distance away from the source to ensure the appropriate protection for medical staff, i.e. no more than 0.3 Bq in the operating room. Use the apparatus drawn underneath and think of a way to account for background radiation. g-source embedded in lead casing Distance from source GM-tube Counter

3. Inverse Square Law for g-rays • Here are some hints: • Plot a graph of 1/√C against the distance from the source . • Your graph should be a straight line if the count rate follows the Inverse Square Law: • Use your graph to find the value of C and the distance from the source required for the limit of activity of 0.3 Bq. • Suggest why the graph doesn’t intercept the x-axis at the origin (hint: a diagram of the g-source and the G-M tube might help you to understand the problem)

4. Inverse Square Law for g-rays • The intensity of radiation is obviously proportional to the count rate C, so a similar equation applies for it: • This equation can be solved to give: • Where I is the intensity at distance x from the source and I0 the intensity for x = 0

5. Inverse Square Law for g-rays • But why does the Intensity follows that pattern? • The radiation spreads out as you move away from the radioactive source • A point source sends radiation in all directions, so the spreading is over the area of a sphere of radius equal to the distance from the source • The area of a sphere is proportional to the radius squared 4A A r r