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4
MCQ (Single Correct Answer)

$M_x$ and $M_y$ denote the atomic masses of the parent and the daughter nuclei respectively in radioactive decay. The $Q$-value for a $\beta^{-}$decay is $Q_1$ and that for a $\beta^{+}$decay is $Q_2$. If $m_e$ denotes the mass of an electron, then which of the following statements is correct?

A
$Q_1=\left(M_x-M_y\right) c^2$ and $Q_2=\left[M_x-M_y-2 m_e\right] c^2$
B
$Q_1=\left(M_x-M_y\right) c^2$ and $Q_2=\left(M_x-M_y\right) c^2$
C
$Q_1=\left(M_x-M_y-2 m_e\right) c^2$ and $Q_2=\left(M_x-M_y+2 c_e\right) c^2$
D
$Q_1=\left(M_x-M_y+2 m_{\mathrm{e}}\right) c^2$ and $Q_2=\left(M_x-m_y+2 m_e\right) c^2$
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
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