The mass-energy difference between a neutral proton-rich parent atom and its possible daughter atom is 0.93 MeV. Neglect electron-binding energies. Which decay mode is energetically possible?
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Question

The mass-energy difference between a neutral proton-rich parent atom and its possible daughter atom is 0.93 MeV. Neglect electron-binding energies. Which decay mode is energetically possible?

G1 Physics · Accepted Answer

For electron capture, a positive parent-minus-daughter atomic mass-energy difference is sufficient, so 0.93 MeV allows electron capture. For β⁺ emission, the threshold due to positron creation is 2mₑc² = 1.022 MeV. Since 0.93 MeV < 1.022 MeV, β⁺ emission is forbidden. Therefore only electron capture is energetically possible. Correct option: B.