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5
MCQ (Single Correct Answer)

Tritium is an isotope of hydrogen whose nucleus triton contains 2 neutrons and 1 proton. Free neutrons decay into $p+\bar{e}+\bar{n}$. If one of the neutrons in Triton decays, it would transform into $\mathrm{He}^3$ nucleus. This does not happen. This is because

A
Triton energy is than that of a $\mathrm{He}^3$ nucleus
B
The electron created in the beta decay process cannot remain in the nucleus
C
both the neutons in triton have to decay simultaneously resulting in a nucleus with 3 protons, which is not $\mathrm{aHe}^3$ nucleus.
D
free neutrons decay due to external perturbations which is absent in triton nucleus
6
MCQ (Single Correct Answer)

Heavy stable nuclei have more neutrons than protons. This is because of the fact that

A
neutrons are heavier than protons
B
electrostatic force between protons are repulsive
C
neutrons decay into protons through beta decay
D
nuclear forces between neutrons are weaker than that between protons
7
MCQ (Single Correct Answer)

In a nuclear reactor, moderators slow down the neutrons which come out in a fission process. The moderator used have light nuclei. Heavy nuclei will not serve the purpose, because

A
they will break up
B
elastic collision of neutrons with heavy nuclei will not slow them down
C
the net weight of the reactor would be unbearably high
D
substances with heavy nuclei do not occur in liquid or gaseous state at room temperature
8
MCQ (Multiple Correct Answer)

Fusion processes, like combining two deuterons to form a He nucleus are impossible at ordinary temperatures and pressure. The reasons for this can be traced to the fact

A
nuclear forces have short range
B
nuclei are positively charged
C
the original nuclei must be completely ionized before fusion can take place
D
the original nuclei must first break up before combining with each other
9
MCQ (Multiple Correct Answer)

Samples of two radioactive nuclides $A$ and $B$ are taken $\lambda_A$ and $\lambda_B$ are the disintegration constants of $A$ and $B$ respectively. In which of the following cases, the two samples can simultaneously have the same decay rate at any time?

A
Initial rate of decay of $A$ is twice the initial rate of decay of $B$ and $\lambda_A=\lambda_B$
B
Initial rate of decay of $A$ is twice the initial rate of decay of $B$ and $\lambda_A>\lambda_B$
C
Initial rate of decay of $B$ is twice the initial rate of decay of $A$ and $\lambda_A>\lambda_B$
D
Initial rate of decay of $B$ is same as the rate of decay of $A$ at $t=2 \mathrm{~h}$ and $\lambda_B<\lambda_A$
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