Actually, no. Neutrons "slow down" by losing momentum in collisions with other matter. The most effective "slowing down" material is one that has a high percentage of hydrogen. If you think of the neutron as a billiard ball hitting 1) another billiard ball (i.e. hydrogen), or 2) a bowling ball (lead). With 1), the neutron transfers its momentum to the hydrogen nucleus (i.e. the neutron stops and the hydrogen nucleus goes flying off with the neutron's energy). With 2), the lead nucleus is so massive that the neutron just bounces off with its direction changed, but little energy lost.
What lead stops effectively is/are gamma rays (due to the large number of inner shell electrons that the gammas can "kick out" of their shells).
This is a fairly common misconception among "non-nuke" folks.
"The most effective "slowing down" material is one that has a high percentage of hydrogen."
So a large quantity of diesel fuel or a big pile of coal would work pretty well? How about boron?
Just trying to figure out where to stand if a neutron bomb goes off in the vicinity. ;)