A beam of electrons of energy $E$ scatters from a target having atomic spacing of $1 \AA$. The first maximum intensity occurs at $\theta=60^{\circ}$. Then $E$ (in eV ) is $\_\_\_\_$ . (Planck constant $\mathrm{h}=6.64 \times 10^{-34} \mathrm{Js}, 1 \mathrm{eV}=1.6 \times 10^{-19} \mathrm{~J}$, electron mass $\mathrm{m}=9.1 \times 10^{-31} \mathrm{~kg}$ )