CBSE Class 12 · 2018 · Modern PhysicsShort Answer

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(a) Explain the processes of nuclear fission and nuclear fusion by using the plot of binding energy per nucleon (BE/A) versus the mass number

A

.
(b) A radioactive isotope has a half-life of 10 years. How long will it take for the activity to reduce to 3.12%?

Answer: Both move toward the BE/A peak, releasing energy; $t = 50$ years.

Solution

(a) The BE/A curve rises steeply, peaks (

\approx 8.8 MeV

) near

A = 56

(iron) and then falls slowly for heavy nuclei (see figure).
Fission: a heavy nucleus (low BE/A) splits into middle-mass fragments (higher BE/A); the increase in binding energy is released.
Fusion: very light nuclei (low BE/A) combine into a heavier nucleus of higher BE/A; again energy is released.
(b)

3.12% \approx \frac{1}{32} = (\frac{1}{2})^{5}

, so 5 half-lives:

t = 5 \times 10 = 50 years .

t = 50 years

CBSE Class 12 Board · 2018 · Subjective

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CBSE Class 12 · 2018 · Modern PhysicsShort Answer

Solution

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