The average number of prompt neutrons produced per fission of ${U^{235}}$ is

When four hydrogen nuclei combine to form a helium nucleus, then:

Carbon-$11$ decays to boron-$11$ according to the following formula:
${ }_{6}^{11} C \rightarrow{ }_{5}^{11} B +e^{+}+ v _{e}+0.96 \,MeV$
Assume that,positrons $\left(e^{+}\right)$ produced in the decay combine with free electrons in the atmosphere and annihilate each other almost immediately. Also,assume that the neutrinos $\left(v _{e}\right)$ are massless and do not interact with the environment. At $t=0$,we have $1 \,\mu g$ of ${ }_{6}^{11} C$. If the half-life of the decay process is $t _{0}$,the net energy produced between time $t=0$ and $t=2 t _{0}$ will be nearly ........... $MeV$.

The energy released if hydrogen atoms are combined to form $^{4}_{2}He$ is . . . . . . MeV. (Take binding energies per nucleon of $^{2}H$ and $^{4}_{2}He$ as $1.1$ MeV and $7.2$ MeV, respectively)

Fast neutrons can be easily slowed down by:

Nuclear fission was discovered by