  In slow collisions of two bare nuclei with the total charge larger than the
critical value $Z_{\rm cr} \approx 173$, the initially neutral vacuum can
spontaneously decay into the charged vacuum and two positrons. Detection of the
spontaneous emission of positrons would be the direct evidence of this
fundamental phenomenon. However, the spontaneously produced particles are
indistinguishable from the dynamical background in the positron spectra. We
show that the vacuum decay can nevertheless be observed via impact-sensitive
measurements of pair-production probabilities. Possibility of such observation
is demonstrated using numerical calculations of pair production in low-energy
collisions of heavy nuclei.
