The particle emitted in the decay of ${ }_{92}^{238} \mathrm{U}$ to ${ }_{92}^{234} \mathrm{U}$

The mass defect of ${ }_2^4 \mathrm{He}$ is 0.03 u . The binding energy per nucleon of helium (in MeV) is

The energy (in eV ) required to excite an electron from $n=2$ to $n=4$ state in hydrogen atom is

In a nuclear reactor, the function of the moderator is to decreases