Generally, the (average) translational kinetic energy of free particles, including neutrons, is
E = (3/2)kBT
Where kB is the Boltzmann constant. This is the same expression you would use for mono-atomic gas.
This is not the same as the most probably kinetic energy in the distribution.
This expression is for a collection of particles able to move in three mutually perpendicular directions. A question that I would have is whether a "beam" restricts the motion to one direction, in which case the number of degrees of freedom changes. Because I wasn't sure about this, I looked around for the kinetic energy of a neutron beam and there seem to be a few different answers, although some places refer to the mean, and some the mode of the distribution. So tbh I am not completely sure of the right way to treat this system.
Bottom line though is that the temperature relates to the kinetic energy through the Boltzmann distribution. The exact relation depends on how the motion is restricted and what statistical parameter you are interested in (mean, mode, median, etc.) since there will always be a distribution of energies.