Exponential attenuation of gamma through matter
The graph above shows, for 1000 initial 1 MeV gamma rays; the number of surviving gamma which have not interacted, as the function the distance they travel trough the air. Every 90m (a distance known as the attenuation), the number of surviving gamma rays is halved. After 2 attenuation lengths the number drops to 250, after 3 lengths to 125 and so on. This exponential decay closely resembles the way a substance’s radioactivity decreases with time, with the ‘attenuation length’ playing the role of the ‘half-life’. The figure of 90m is specific to 1 MeV gamma rays - less energetic gamma rays will have shorter attenuation lengths.