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Plutonium-239 undergoes alpha decay into uranium-235 and helium-4. Given rest masses of 239.052157 u for Pu-239, 235.043924 u for U-235, and 4.002602 u for He-4, and using 1 u = 931.5 MeV, what energy is released?
A0.56 MeV
B2.63 MeV
C5.25 MeV
D10.50 MeV
Answer & Solution
Correct answer: C. 5.25 MeV
1. The mass lost in the decay is the parent mass minus the combined product masses: 239.052157 minus (235.043924 + 4.002602) u.
2. Adding the products gives 239.046526 u, so the mass lost is 239.052157 minus 239.046526, which is 0.005631 u.
3. Convert to energy using 1 u = 931.5 MeV: 0.005631 x 931.5.
4. Multiplying gives about 5.25 MeV.
5. 2.63 MeV is half this value, and 10.50 MeV is double it, while 0.56 MeV is roughly a tenth of it.
6. So 5.25 MeV is correct.
_Source: OpenStax Physics (CC BY 4.0), Ch 22 "The Atom", section 22.2 Nuclear Forces and Radioactivity_
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