In Rutherford scattering experiment, what will be the correct angle for $$\alpha $$-scattering for an impact parameter, $$b=0$$  ?

Solution :
$$\eqalign{ & b = \frac{{Z{e^2}\cot \left( {\frac{\theta }{2}} \right)}}{{4\pi { \in _0}{k_i}}} = 0 \cr & \Rightarrow \cot \left( {\frac{\theta }{2}} \right) = 0 \cr & \Rightarrow \left( {\frac{\theta }{2}} \right) = {90^ \circ }\,\,{\text{or}}\,\,\theta = {180^ \circ } \cr} $$