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## Homework Statement

Given/Known:

- A photon with Energy E

_{γ}, momentum p

_{γ}hit an atom with mass M.

- The photon is perfectly elastic scattered at angle [itex]\theta[/itex]

Problem: Find recoil energy

## Homework Equations

Relevant Knowledge:

- Due to elastic scattering, the photon's energy remain the same (almost), only direction is changed. To my understanding, the electron in the atom absorbs the initial photon's energy. However, the energy is not enough to excite or knock out the electron, the electron then emits a photon with similar wavelength (same energy), but in random direction.

## The Attempt at a Solution

- Using q

^{2}= k

^{2}+ k'

^{2}- 2kk' cos [itex]\theta[/itex]

I could find the recoil momentum (momentum transfer). But then I don't know how to find energy