Gamma Rays: Finite Range in Water?

[lavster]

i was told that high emergy gamma rays had a finite range in water. how can this be if they are both massless and chargeless. is it because they are absorbed/produce electrons/positrons

thanks

 

[kloptok]

The intensity of the gamma rays decays exponentially, so they don't really have a well-defined range (as opposed to charged particles, for example alpha and beta particles). In the limit of infinity the intensity goes to zero, but for a fixed range there will be a non-zero probability that some photons are still there.

When the photons interact with matter, they will do it in three ways: Photoelectric absorbtion (where the photon is absorbed by an electron), Compton scattering (elastic scattering between a photon and an electron) and pair production (where the photon annihilates into an electron and a positron, can only happen close to a nucleus). For lower energies of the gamma rays photoelectric absorbtion is dominant, for medium energies Compton scattering and for high energies pair production.

 

[Drakkith]

When traveling through a medium the gamma rays will have a finite lifetime and range. In completely empty space a gamma ray photon has effectively an infinite range.