# Homework Help: Compton scattering and Bragg diffraction

1. Jun 18, 2009

### eoghan

Hi! I know that x-rays interact with matter (compton effect). But I also know that when X-rays are incident on a crystal, they are diffracted following the Bragg's law.
My question is: why is the x-ray diffracted when it is incident on a crystal, but it isn't scattered by comtpon scattering?

2. Jun 20, 2009

### Andrew Mason

Bragg diffraction and Compton effect are two very different phenomena.

If you are wondering why the x-ray does not experience the Compton effect when undergoing Bragg diffraction, it is because the x-ray does not transfer much energy to the crystal. If it strikes an electron, it will impart energy to the electron and this will necessarily reduce the energy of the x-ray as a result. But the crystal has too much mass to absorb much of the x-ray's energy via the compton effect. It is kind of like a ball hitting a brick wall. Compton effect is observed when you have a photon hitting a particle.

AM

3. Jun 20, 2009

### eoghan

But I've studied that Compton found the effect by the measurements of the x-ray frequencies scattered by graphite block, which is not a particle

4. Jun 20, 2009

### Andrew Mason

Well, the x-rays are not scattered by the block or graphite crystals in the block. They are scattered by the electrons in the graphite.

AM

5. Jun 20, 2009

### eoghan

but then... why in the bragg diffraction aren't the x-rays scattered by the electrons of the cristal?

6. Jun 20, 2009

### Andrew Mason

I suspect that there is some Compton scattering. Whether Compton scattering occurs for a particular electron may depend on the way the electron is bound to the nucleus. If the energy imparted to the electron by the x-ray photon is less than the energy required to take it to the next energy level, the electron cannot take on the x-ray's energy, so there would not be any change in x-ray energy.

AM