I Creating a Neutron Beam: Challenges & Solutions

[Aidan Davis]

When an artificial neutron source is created, from decay (from ground state or from an excited state resulting from fusion or a photon), or spallation, the trajectory of the emitted neutron is random,right? This would mean that there would be a neutron flux field, following the inverse square law, as opposed to a beam. If this is correct, what mechanisms are used to bunch up the neutrons into a single beam, considering that they are easily absorbed and do not have a charge?

 

[Drakkith]

The beam has to be collimated somehow. You can do so by leaving a small aperture in a material which absorbs neutrons. The neutrons traveling in the direction you want will pass through the aperture. The neutrons moving at a larger angle will be absorbed by the collimator. Here's the collimator for the neutron stream of the University of Washington's cyclotron:

 

[Aidan Davis]

The idea of a collaminator came to mind at first, but it seems very inefficient. Is there anything that reflects or reroutes them to make a beam?

 

[Drakkith]

To be honest I'm not sure. Sorry.

 

[mfb]

You can directly create them as beam via spallation. If you shoot high-energetic protons (as beam) on a target, it produces many neutrons, most of them will fly roughly in the same direction as the original protons. A collimator makes the beam more well-defined without too large losses.

In a nuclear reactor, you can use neutron reflectors with a gap - most neutrons don't become part of the beam, but you need most neutrons to keep the chain reaction alive anyway, and the high neutron flux still leads to an interesting flux for experiments.

 

[rare earth]

The idea of a collaminator came to mind at first, but it seems very inefficient. Is there anything that reflects or reroutes them to make a beam?

Yes, neutron supermirrors are used for this purpose.