  The center-of-mass energy can be arbitrarily high in Schwarzschild spacetime
if one considers the front collision of two particles, one of which moves along
so-called white hole geodesics and another one along a black hole geodesic.
This process can take place if one considers the gravitational collapse model.
In this paper, we consider the well-known naked singularity formation in Vaidya
spacetime and investigate the question about two particle collision near the
boundary of the collapsing cloud. The center-of-mass energy of the front
collision is considered. One particle moves away from the naked singularity and
another one falls onto a collapsing cloud. We show that the center-of-mass
energy grows unboundly if the collision takes place in the vicinity of the
conformal Killing horizon.
