$7.9\text{ MeV}$ $\alpha$-particle scatters from a target material of atomic number $79$. From the given data the estimated diameter of nuclei of the target material is (approximately) _________ $\text{m}$.
$\left[ \frac{1}{4\pi\epsilon_0} = 9 \times 10^9\text{ Nm}^2/\text{C}^2 \text{ and electron charge} = 1.6 \times 10^{-19}\text{ C} \right]$
