Elastic collisions — UPSC Physics Optional CSE 1993 Paper I

Find the fractional decrease of kinetic energy of a particle of mass m_1 when a head-on elastic collision takes place with another particle of mass m_2 initially at rest. In this context show why hydrogen would be best to be used for slowing down neutrons. Actually \text{D}_2\text{O}, not \text{H}_2\text{O} is used why?