Lời giải:Đáp án : B
Ta có:
\({{\rm{W}}_P} + {{\rm{W}}_n} = \Delta E + {{\rm{W}}_\alpha } = - 2,7 + {{\rm{W}}_\alpha }\)
\(\dfrac{{{{\rm{W}}_P}}}{{{{\rm{W}}_n}}} = \dfrac{{{m_p}}}{{{m_n}}} = \dfrac{{30}}{1} = 30 \Rightarrow 31{{\rm{W}}_n} = - 2,7 + {{\rm{W}}_\alpha }\) (1)
Mặt khác: \({m_\alpha }\overrightarrow {{v_\alpha }} = {m_P}\overrightarrow {{v_P}} + {m_n}\overrightarrow {{v_n}} \Rightarrow {m_\alpha }{v_\alpha } = ({m_P} + {m_n}){v_n} \Leftrightarrow m_\alpha ^2v_\alpha ^2 = {31^2}m_n^2v_n^2\)
\( \Leftrightarrow {m_\alpha }{{\rm{W}}_\alpha } = {31^2}{m_n}{{\rm{W}}_n} \Leftrightarrow {{\rm{W}}_n} = \dfrac{{{m_\alpha }{{\rm{W}}_\alpha }}}{{{{31}^2}{m_n}}} = \dfrac{{4.{{\rm{W}}_\alpha }}}{{{{31}^2}.1}} = \dfrac{{{\rm{4}}{{\rm{W}}_\alpha }}}{{961}}\) (2)
Từ (1) và (2): \(31\dfrac{{{\rm{4}}{{\rm{W}}_\alpha }}}{{961}} = - 2,7 + {{\rm{W}}_\alpha } \Rightarrow \dfrac{{{\rm{27}}{{\rm{W}}_\alpha }}}{{31}} = 2,7 \Rightarrow {{\rm{W}}_\alpha } = 3,1(MeV)\)
