Static magnetic field $B$ is present in a region between two coaxial cylindrical conductors. An electron of mass $m$ and charge $e$ start moving with an initial velocity $u_0$ in the radial direction from the surface of the inner cylinder towards the outer cylinder. The electron just miss to reach the surface of outer cylinder and return to inner cylinder in time interval $\text{'}t\text{'}$. If radius of inner cylinder is $\text{'}a\text{'}$ and outer cylinder is $\text{'}a\sqrt{3}\text{'}$, then