# Classification of Elastic and Inelastic scattering

1. Sep 3, 2013

### abotiz

Hi,

I am slightly confused regarding the termenology elastic and inelastic. My focus is on the interactions, Rayleigh, photoelectric, comptonscattering and pair production. I have read around the internet and have some question I did not fully got answered.

1) Is Elastic = Coherent and Ineleastic = Inchoherent?

2) Elastic is a process where the kinetic energy is conserved? This confuses me when one is dealing with photons because they have no mass so a kinetic energy is non existant?

3) Inelastic, neither the momentum nor the kinetic energy is preserved?

4) Inelastic is usually a process that needs a threshold energy?

So if the above is true, then;
Rayleigh and Thomson scattering is the only coherent/elastic scattering, and photoelectric + compton + pairproduction are incoherent/inelastic scattering?

Thank you!

2. Sep 3, 2013

### dauto

Yes, the terminology can be confusing since these terms are defined in slightly different way depending on context. The simplest definition is that an elastic collision is one that preserves total kinetic energy. Note that all of a photon's energy is considered kinetic energy since none of it is due to it's mass. Kinetic energy = Total energy - mass energy - potential energy (if any).

3. Sep 3, 2013

### jeppetrost

Comparing this with classical terminology can be a bit confusing.

Elastic scattering is when the initial and final states are identical. This means the kinetic energy is conserved. (Photons have kinetic energy. In fact, they have only kinetic energy. Take this question to the SR/GR forum if you're still confused.)

Inelastic scattering is when initial and final states are different. In these processes kinetic energy is not conserved.

So eg. e+e- -> 2 photons is inelastic, and e + photon -> e + photon is elastic.

So yea, basically, you are right.