b
(b)Fractional decrease in kinetic energy of neutron
=\(1 - {\left( {\frac{{{m_1} - {m_2}}}{{{m_1} + {m_2}}}} \right)^2}\)[As \(m_1=1\) and \(m_2 = 2\)]
\( = 1 - {\left( {\frac{{1 - 2}}{{1 + 2}}} \right)^2}\)\( = 1 - {\left( {\frac{1}{3}} \right)^2} = 1 - \frac{1}{9} = \frac{8}{9}\)