Polarity, in context, means direction of travel.

There are five laws that are obeyed in all physical circumstances. They are the balance of:

- Mass

- Linear Momentum

- Angular Momentum

- Kinetic Energy

- Entropy

If you want to change the direction of travel of the beam of neutrons, you must obey those laws. That means you need to put in enough Work so that their kinetic energy goes the opposite direction.

If they were electrons, then they would respond to the application of a magnetic field. They are not.

The best that modern physics might allow is some sort of barrier that, in general, diffuses them back in the direction they came from. Its not efficient.

If you neutron beam is non-thermal, that is, if they have typical nuclear energy densities (MeV) then reversing the path of travel of the beam is going to be VERY energy expensive.