For the nuclear fusion reaction ${ }_1^2 H +{ }_1^3 H \rightarrow{ }_2^4 He +{ }_0^1 n$,the temperature to which the gases must be heated is $3.7 \times 10^9 \, K$. The potential energy between two nuclei is closest to ........ $J$ (Boltzmann's constant $k = 1.38 \times 10^{-23} \, J/K$).

$A$ $\pi^0$ meson at rest decays into $2$ $\gamma$ rays: $\pi^0 \to \gamma + \gamma$. Which of the following must happen?

The source of the Sun's energy is

Which of the following is a fusion reaction?

$A$ nucleus with mass number $A = 240$ and binding energy per nucleon $= 7.6 \,MeV$ breaks into two fragments each of $A = 120$ with binding energy per nucleon $= 8.5 \,MeV$. Calculate the released energy.

Given that the mass of ${ }_{3}^{7} Li = 7.0160 \, u$,the mass of ${ }_{2}^{4} He = 4.0026 \, u$,and the mass of ${ }_{1}^{1} H = 1.0079 \, u$. When $20 \, g$ of ${ }_{3}^{7} Li$ is converted into ${ }_{2}^{4} He$ by proton capture,the energy liberated (in $kWh$) is: [Take $1 \, u = 931.5 \, MeV/c^2$ and $1 \, kWh = 3.6 \times 10^6 \, J$]