  Theory of superfluid $^4$He shows that, due to strong correlations and
backflow effects, the density profile of a vortex line has the character of a
density modulation and it is not a simple rarefaction region as found in clouds
of cold bosonic atoms. We find that the basic features of this density
modulation are represented by a wave--packet of cylindrical symmetry in which
rotons with positive group velocity have a dominant role: The vortex density
modulation can be viewed as a cloud of virtual excitations, mainly rotons,
sustained by the phase of the vortex wave function. This suggests that in a
vortex reconnection some of these rotons become real so that a vortex tangle is
predicted to be a source of non-thermal rotons. The presence of such vorticity
induced rotons can be verified by measurements at low temperature of quantum
evaporation of $^4$He atoms. We estimate the rate of evaporation and this turns
out to be detectable by current instrumentation. Additional information on the
microscopic processes in the decay of quantum turbulence will be obtained if
quantum evaporation by high energy phonons should be detected.
