• 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