In the proton-proton cycle, four hydrogen nuclei combine to release energy of ....... $MeV$.

Scientists are working hard to develop a nuclear fusion reactor. Nuclei of heavy hydrogen,${ }_1^2 H$,known as deuteron and denoted by $D$,can be thought of as a candidate for a fusion reactor. The $D-D$ reaction is ${ }_1^2 H+{ }_1^2 H \rightarrow{ }_2^3 He+n+$ energy. In the core of a fusion reactor,a gas of heavy hydrogen is fully ionized into deuteron nuclei and electrons. This collection of ${ }_1^2 H$ nuclei and electrons is known as plasma. The nuclei move randomly in the reactor core and occasionally come close enough for nuclear fusion to take place. Usually,the temperatures in the reactor core are too high and no material wall can be used to confine the plasma. Special techniques are used which confine the plasma for a time $t_0$ before the particles fly away from the core. If $n$ is the density (number/volume) of deuterons,the product $n t_0$ is called the Lawson number. In one of the criteria,a reactor is termed successful if the Lawson number is greater than $5 \times 10^{14} \, s/cm^3$.
It may be helpful to use the following: Boltzmann constant $k=8.6 \times 10^{-5} \, eV/K$; $\frac{e^2}{4 \pi \varepsilon_0}=1.44 \times 10^9 \, eV \cdot m$.
$1.$ In the core of a nuclear fusion reactor,the gas becomes plasma because of
$(A)$ strong nuclear force acting between the deuterons
$(B)$ Coulomb force acting between the deuterons
$(C)$ Coulomb force acting between deuteron-electron pairs
$(D)$ the high temperature maintained inside the reactor core
$2.$ Assume that two deuteron nuclei in the core of a fusion reactor at temperature $T$ are moving towards each other,each with kinetic energy $1.5 kT$,when the separation between them is large enough to neglect Coulomb potential energy. Also,neglect any interaction from other particles in the core. The minimum temperature $T$ required for them to reach a separation of $4 \times 10^{-15} \, m$ is in the range
$(A)$ $1.0 \times 10^9 \, K$ $(B)$ $2.0 \times 10^9 \, K$ $(C)$ $3.0 \times 10^9 \, K$ $(D)$ $4.0 \times 10^9 \, K$
$3.$ Results of calculations for four different designs of a fusion reactor using $D-D$ reaction are given below. Which of these is most promising based on the Lawson criterion?
$(A)$ deuteron density $=2.0 \times 10^{12} \, cm^{-3}$,confinement time $=5.0 \times 10^{-3} \, s$
$(B)$ deuteron density $=8.0 \times 10^{14} \, cm^{-3}$,confinement time $=9.0 \times 10^{-1} \, s$
$(C)$ deuteron density $=4.0 \times 10^{23} \, cm^{-3}$,confinement time $=1.0 \times 10^{-11} \, s$
$(D)$ deuteron density $=1.0 \times 10^{24} \, cm^{-3}$,confinement time $=4.0 \times 10^{-12} \, s$
Give the answer for questions $1, 2,$ and $3.$

The average energy released per fission for the nucleus of $_{92}^{235} U$ is $190 \text{ MeV}$. When all the atoms of $47 \text{ g}$ pure $_{92}^{235} U$ undergo fission process, the energy released is $\alpha \times 10^{23} \text{ MeV}$. The value of $\alpha$ is . . . . . . . . . . . (Avogadro Number $= 6 \times 10^{23} \text{ per mole}$)

Fusion reaction takes place at high temperature because

The binding energy of deuteron $_1^2H$ is $1.112 \, MeV$ per nucleon and an $\alpha$-particle $_2^4He$ has a binding energy of $7.047 \, MeV$ per nucleon. Then in the fusion reaction $_1^2H + _1^2H \to _2^4He + Q$,the energy $Q$ released is ........ $MeV$.

$A$ $\gamma$-ray photon creates an electron-positron pair. If the rest mass energy of an electron is $0.51 \, MeV$ and the total kinetic energy of the electron-positron pair is $0.78 \, MeV$,then the energy of the $\gamma$-ray photon in $MeV$ is: