Neutrons Choppers

For most neutron experiments it is not enough to simply have an intense neutron beam, we also need to have a well-defined beam. When talking about a well-defined beam, we mean that we often want to control the energy or wavelength of the neutrons, sometimes we want to control precisely the direction the neutrons are traveling in before they hit the sample, and we might want to control exactly where on the sample they hit. The first of these three goals can be achieved by either choppers or analyzers. The other two can be achieved by collimator and slits, respectively. You can read more about these different parts of neutron instruments in the neutron instrumentation theory page. One common trait for all of these tuning or refinements is that we lose the intensity of the neutron beam each time we do it, so it is always a trade-off between how well-defined a beam we need versus how much intensity we need to perform the experiment. The requirement of the neutron beam varies between the different neutron techniques. For example, in neutron imaging we don’t need a well-defined wavelength of the neutrons we just need a lot of intensity, but in Quasi-Elastic Neutron Scattering (QENS) knowing the energy of the neutrons is essential!

Neutron Energy Control with Choppers

To the left is an illustration of a pulse shaping chopper separating a neutron beam into short durations of pulses with a wide range of velocities. To the right is a graph with time of flight for the neutrons on the x-axis and length through instrument on the y-axis. As the length of the instrument increases, more examples of choppers are applied to the neutron beam to separate the neutrons with the wanted velocity from the ones with unwanted velocities. The first chopper applied to the neutron beam is the pulse shaping chopper, which in this case separates the neutron beam into three beams. The next chopper that is applied to the beams is a band width chopper and the last one is a frame overlap chopper. After traveling through the choppers, the neutron beam goes through a target sample and hits a neutron detector. It is also illustrated on the graph, that the neutron beams that hit the detector at the end, are only the neutrons with the exact velocities that were wanted, and that every other neutron that got separated from the beam, had an unwanted velocity.

Figure. View of application of choppers to neutron beams.

A simple and effective way to find or sort the wavelength of a neutron is by Time of Flight (ToF) analysis. Simply put, if we know the time it takes a neutron to travel from point A to B, and we know the distance between point A and B then we can calculate the velocity of the neutron. So how can we know the time the neutron was emitted? If we insert a rotating disc with an opening slit, called a chopper, in the beam after the moderator, we only allow neutrons to pass when the open slit is at the position of the beam. By tuning the rotation speed of the disc we can chop up the beam pulse emitted from the moderator into one or several bunches of neutrons with a well-defined starting point and time. Disc choppers usually have a radius of 0.5-1 m.

Sometimes pairs of closely spaced counter-rotating discs with fairly wide openings are used to produce short duration pulses of neutrons with a wide range of velocities. This kind of chopper-system is sometimes called a pulse-shaping chopper, since it defines the pulse shape of neutrons in our experiment.

We can also use a chopper pair to allow only a narrow range of neutron velocities in our experiment, but in this case the choppers are further apart and with narrow openings. Only neutrons within a specific range of velocities will travel the distance between the two choppers in the same timespan it takes the second chopper to rotate so its opening allows the neutrons through. Those neutrons with the correct velocity will get to the experiment, the rest will be removed by the second chopper. Therefore this setup is called a velocity selector chopper or a monochromatiseing chopper, since it selects the neutron velocities we are using in the experiment, and thus also wavelength and energies.

A third use of choppers is a so-called frame overlap chopper. Very slow neutrons can have a flight time between the pulse-shaping and velocity selecting chopper that enables them to get through the velocity selector with the next pulse in a so-called frame-overlap. This situation will eventually lead to neutrons of undesired or unexpected velocities in the experiment. These neutrons can be removed by inserting a third chopper to cut them away and thus remove the frame-overlap.