  Generic spherically symmetric self-similar collapse results in strong
naked-singularity formation. In this paper we are concerned with particle
creation during a naked-singularity formation in spherically symmetric
self-similar collapse without specifying the collapsing matter. In the generic
case, the power of particle emission is found to be proportional to the inverse
square of the remaining time to the Cauchy horizon (CH). The constant of
proportion can be arbitrarily large in the limit to marginally naked
singularity. Therefore, the unbounded power is especially striking in the case
that an event horizon is very close to the CH because the emitted energy can be
arbitrarily large in spite of a cutoff expected from quantum gravity. Above
results suggest the instability of the CH in spherically symmetric self-similar
spacetime from quantum field theory and seem to support the existence of a
semiclassical cosmic censor. The divergence of redshifts and blueshifts of
emitted particles is found to cause the divergence of power to positive or
negative infinity, depending on the coupling manner of scalar fields to
gravity. On the other hand, it is found that there is a special class of
self-similar spacetimes in which the semiclassical instability of the CH is not
efficient. The analyses in this paper are based on the geometric optics
approximation, which is justified in two dimensions but needs justification in
four dimensions.
