$\pi$ mesons can be

The ratio of mass densities of nuclei of $^{40}Ca$ and $^{16}O$ is close to

The density (in $kg m^{-3}$) of nuclear matter is of the order of

The radius of a nucleus of mass number $64$ is $4.8 \text{ fermi}$. Then the mass number of another nucleus having radius of $4 \text{ fermi}$ is $\frac{1000}{x}$,where $x$ is . . . . . . .

The electric potential at the surface of an atomic nucleus $(Z = 50)$ of radius $9.0 \times 10^{-15} \, m$ is

${ }_{92}^{238} U$ and ${ }_{90}^{236} Th$ are $\qquad$ of each other.