Neutron Diffraction Device I was just wondering if we have a beam of neutrons, lets say of various energies meeting or colliding with a powder of monoatomic crystallites, what would happen to the neutron beam and the energies of the neutron after the collision with the powder sheet?
If I was to say that my neutron beam has a wavelength of $\lambda$ and collides with the powder at an angle of $\theta$, then I do know what would happen to my neutrons after the collision. They would scatter according to Bragg scattering given by
$$n\lambda =2b\sin\theta $$
I want to find a way to artificially enhance my signal by simply eliminating higher energy neutrons hence eliminating higher order scattering.
 A: Neutrons with the angstrom-scale wavelengths appropriate for diffraction from ordinary crystals have kinetic energies of a few milli-eV. Since neutron detection always involves a nuclear reaction with an energy of a few mega-eV, it's more or less impossible to directly measure small changes in a neutron's energy due to scattering in a crystal. So neutron scattering experiments typically rely on a good determination of the neutron wavelength as an initial condition.
You can extract a single wavelength of neutrons from a broad-spectrum beam by putting a high-quality crystal in the beam; neutrons which satisfy the Bragg condition with the crystal will leave the primary beam and can be sent down another pipe, while neutrons that don't satisfy the Bragg condition will pass through the crystal unchanged. This is the technique used to feed the neutron interferometer at NIST, where a narrow wavelength spectrum is absolutely paramount. However, since crystal diffraction only sends out a very small fraction of the beam, it's not a good technique for measurements that need the statistical power of lots of neutrons.
What's more common for scattering instruments is to use neutron pulses, either directly (like the pulsed accelerators at LANSCE, SNS, SINQ, and the future ESS) or by interrupting the continuous neutron beam from a reactor with a rotating "chopper." The neutron's mass is $h/m = 4\,\mathrm{{Å\,m / ms}}$ —  that is, for a ten-meter flight path, the flight time for a 4 Å neutron is ten milliseconds, the flight time for an 8 Å neutron is twenty milliseconds, and so on. In this case you can improve your wavelength resolution simply by building a longer flight path, and you can separate scatterings due to your high- and low-energy neutrons simply by recording both the angle of the scatter and the time of the neutron's arrival at the detector relative to the pulse.
