AQA A Level Physics复习笔记8.1.4 Inverse-Square Law of Gamma Radiation

• As an electromagnetic wave, gamma radiation shares many of the same wave properties as light
• Light sources which are further away appear fainter because the light they emit is spread out over a greater area than a light source which is closer by
• The moment the light leaves the source, it begins to spread out uniformly as a sphere, according to an inverse square law

When an EM wave spreads out, the area over which it spreads is proportional to the radius squared

• This applies to gamma radiation too, and can be calculated using the equation:

• Where:
  • I = intensity of the gamma radiation (W m–2)
  • k = constant of proportionality
  • x = the distance from the source (m)

   

• Since k is a constant, this equation can be written for radiation at two different points as follows:

• Where:
  • I1 = intensity of the gamma radiation at x1 (W m–2)
  • I2 = intensity of the gamma radiation at x2 (W m–2)
  • x1 = the initial distance from the source (m)
  • x2 = the subsequent distance from the source (m)

   

Step 1: List the known quantities

Initial count rate, I1 = 200 Bq

Initial distance, x1 = 0.2 m

Final count rate = I2

Final distance, x2 = 0.5 m

Step 2: Write down the inverse square law equation

Step 3: Rearrange and calculate the count rate at 0.5 m