# Question: a In power reactors using water as a moderator neutron

(a) In power reactors, using water as a moderator (“neutron slower”) works well, because the proton and neutron have nearly the same mass. Explain why this is true. (b) From your reasoning in part (a), it follows that in a head-on elastic collision, we might expect a neutron to lose all of its kinetic energy in one collision, whereas for an “almost miss,” it might be expected to lose essentially none. Let’s therefore assume that, on average, the neutron loses of its kinetic energy during each proton collision. Estimate how many collisions are needed to reduce a 2.0-MeV neutron to a neutron with a kinetic energy of only 0.02 eV (approximately “thermal”).

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