What are Chopper Spectrometers and how are they different from Triple-Axis Spectrometers? I've been reading about inelastic neutron scattering experiments. Recently encountered these two techniques. Can anybody please tell me why/when are these two techniques used?
 A: If I recall correctly, triple access spectrometers use the angles between the incoming neutron beam, sample, and outgoing electron beam to look at the scattering cross-section at a specific $\mathbf{k},E$. Usually they are set up to count at a specific $\mathbf{k},E$ point at a time (or a few of them). The angles between the incoming, outgoing neutron beams and the sample give you the $\mathbf{k}$, while the outgoing analyzer crystal gives you the $E$. These are called triple axis because you can rotate the monochromator, sample, and analyzer. These spectrometers have very good resolution and are well-suited to studying small amounts of phase space. Unlike choppers, these can use the full (monochromated) flux of the incoming beam.

Chopper spectrometers, or time-of-flight systems, work by selecting the velocity of the incoming beam using choppers. The direction of the scattered neutrons tells you the direction of the momentum transfer, and timing how long they take to scatter from the sample tells you the energy transfer. These spectrometers can have very large detectors to give you a lot of $\mathbf{k},E$ information, unlike the few points in triple axis systems. The downside is that you have to use pulsed neutrons, which means your intensity is very low and you need to count for a long time.
