Answer:
The speed, as they crash together, is the same than before the crash, but with opposite directions. In this context, we recur to the collision theory in physics, which says that the amount of movement of each particle before the collision is the same than after the collision, due to the conservation of the amount of movement, which can be described like this:
. This expression means that in a isolated environment (which is present in a neutron's collision), the total amount of movement doesn't change.
Therefore, the speed of neutrons is the same before and after the collision. Specifically the number of this speed depends on the experiment.