Rotation and Potential Energy

A stationary point source creating a potential field is at the origin O of a coordinate frame. Potential energy of interaction of this .source and a particle of mass m is given by equation $U=\frac{-k}{r^{4}}$
where k is a positive constant and r is distance of the particle from the source. The particle is projected with an initial velocity u from an infinitely distant point. Distance between initial line of motion of the particle and the source is b as shown in the figure. What should the minimum value of b for this projection velocity be so that the particle will not hit the source?

Answer come in the form of $b≥\alpha (km^{-1}u^{-2})^{0.25}$
Type you answer as $\alpha=?$

Answer is positive.