Radial wave function for 2s orbital of hydrogen – like atoms is given as $\psi =2^{\raisebox{1ex}{$-3$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}{\left[\frac{z}{a_0}\right]}^{\frac{3}{2}}\left[2-\frac{zr}{a_0}\right]e^{-\frac{zr}{2a_0}}$ The radial probability $\left(4\pi r^2{\psi }^2\right)$  for an electron in $H{e}^{+}$ at $r=a_0$ is