(b) Magnetic field $B_{centre} = \frac{\mu_{0}I}{2r}$
But $\ \ \ \ \ \ \ \ I = ne$
$\therefore\ B_{centre} = \frac{\mu_{0}ne}{2r}$
$\Rightarrow B_{centre} = \frac{4\pi \times 10^{- 7} \times 6.6 \times 10^{15} \times 1.6 \times 10^{- 19}}{2 \times 0.53 \times 10^{- 10}} = 12.6\ T$