The difference in energy between the nucleus before and after the decay is given to the particles created in the decay to increase their speed , or kinetic energy . The decay energy is initially released as the energy of emitted photons plus the kinetic energy of massive emitted particles ( that is , particles that have rest mass ) . If these particles come to thermal equilibrium with their surroundings and photons are absorbed , then the decay energy is transformed to thermal energy , which retains its mass.