Compton Effect -- Why is the electron considered to initially be at rest?

[Viona]

Hi every one,

In Compton effect I understand that the electron is considered free electron because the energy of the incident photon is very much larger than the binding energy of the atomic electron. But iam not sure why the electron is considered to be at rest (v=0). please help.
Thanks.

 

[vanhees71]

A typical setup for the Compton effect is to scatter X-rays (or gamma-rays) on a fixed target. The thermal energy of the electrons in this target (of the order $k_{\text{B}} T$) is so much smaller than the energy of the incoming gamma quanta that you can neglect the thermal motion of the electrons, i.e., you can assume they are at rest.

 

[Viona]

A typical setup for the Compton effect is to scatter X-rays (or gamma-rays) on a fixed target. The thermal energy of the electrons in this target (of the order $k_{\text{B}} T$) is so much smaller than the energy of the incoming gamma quanta that you can neglect the thermal motion of the electrons, i.e., you can assume they are at rest.

Thanks for your explanation. what about the orbital velocity of the electrons around the nucleus?

 

[Nugatory]

what about the orbital velocity of the electrons around the nucleus?

There is no such thing - electrons don't really move in circular orbits around the nucleus.

 

[vanhees71]

Of course there's always the probability that a photon interacts with an electron and does not lead to compton scattering but to other processes. For a bound electron of course you can also have a photoeffect, i.e., the absorption of the photon or it may kick the electron out of the bound state and producing one or more new photons (bremsstrahlung). That's all covered in quantum theory, providing the probability for the various processes to occur.