Two point charges $A$ and $B$ of magnitude $+8 \times 10^{-6} \mathrm{C}$ and $-8 \times 10^{-6} \mathrm{C}$ respectively are placed at a distance $d$ apart. The electric field at the middle point O between the charges is $6.4 \times 10^4 \mathrm{NC}^{-1}$. The distance ' $d$ ' between the point charges $A$ and $B$ is: