More KE is lost by the neutron when it bounces off a carbon atom.
Work Step by Step
The moderator material is meant to slow down the energetic neutrons so they will be more effective in stimulating further fission events . When a neutron bounces off a carbon nucleus, the C rebounds and takes some KE from the neutron and slowing it down. This is akin to bouncing a golf ball off a tennis ball - the tennis ball takes away some KE because its mass, though larger, is comparable in magnitude to that of the golf ball. Lead is an ineffective moderator because it is so massive that the neutron hardly loses any KE when it rebounds, such as when a golf ball bounces off a large stone ball and hardly loses any speed. This is discussed on page 642.