# Homework Help: Crystal structure and diffraction

1. Apr 5, 2009

### math_04

1. The problem statement, all variables and given/known data

You wish to study a crystal's structure by diffracting thermal, non relativistic neutrons from the crystal, so you go to a nuclear reactor and set up your diffraction experiment on one of the neutron beam lines. The neutrons emerge from the reactor with a range of wavelengths (ie a range of energies/momenta) so you use a chopper to select neutrons of a certain wavelength.

The simplest chopper consists of two disks made from a highly neutron absorbing material mounted on an axle. The disks are 2m apart. In each disk is a small slot (to let the neutrons through). There is an offset of 10 degs between the two slots and the chopper rotates rapidly.

The distance between the planes of atoms in the diffracting crystal is 1.73 Arnstrongs and you observe strong diffraction when the angle of incidence and diffracted neutron beams is 163 degs. Calculate the minimum rotational speed of the chopper in revolutions/minute.

2. Relevant equations

Braggs law : n*lamda = 2d * sin theta

3. The attempt at a solution

using theta = 180-163/2 = 8.5 and plugging in other values (d= 1.73 X 10^-10 m and n=1)

I got lamda = 5.11*10^-11 m.

Im not sure that my theta value is right and secondly, I have no idea how the experiment works. How do the choppers choose the wavelength of light and any hints on how to proceed?

Thanks.