  We argue that the decays of radioactive nuclei related to $^{44}$Ti and
$^{56}$Ni ejected during supernova explosions can provide a vast pool of mildly
relativistic positrons and electrons which are further accelerated to
ultrarelativistic energies by reverse and forward shocks. This interesting link
between two independent processes - the radioactivity and the particle
acceleration - can be a clue for solution of the well known theoretical problem
of electron injection in supernova remnants. In the case of the brightest radio
source Cas A, we demonstrate that the radioactivity can supply adequate number
of energetic electrons and positrons for interpretation of observational data
provided that they are stochastically pre-accelerated in the upstream regions
of the forward and reverse shocks.
