Question:

A nucleus with mass number 184 initially at rest emits an \(\alpha\)-particle. If the Q value of the reaction is 5.5 MeV, calculate the kinetic energy of the \(\alpha\)-particle.

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Momentum conservation makes the daughter and alpha momenta equal and opposite, so the kinetic energy splits inversely as their masses.
Updated On: Nov 4, 2026
  • \(5.5\ \mathrm{MeV}\)
  • \(5.0\ \mathrm{MeV}\)
  • \(5.38\ \mathrm{MeV}\)
  • \(0.12\ \mathrm{MeV}\)
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The Correct Option is C

Solution and Explanation

Initially at rest: \(p_\alpha = p_d\), so with \(K = \dfrac{p^2}{2m}\): \(\dfrac{K_\alpha}{K_d} = \dfrac{m_d}{m_\alpha} = \dfrac{180}{4} = 45\).
Energy release: \(K_\alpha + K_d = Q = 5.5\ \mathrm{MeV}\).
\(K_\alpha = Q\,\dfrac{m_d}{m_d + m_\alpha} = 5.5\times\dfrac{180}{184} \approx 5.38\ \mathrm{MeV}\).
Answer: (3) \(5.38\ \mathrm{MeV}\)
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