  A new model for the thermal spike produced by the nuclear energy loss, as
source of transient processes, is derived analytically, for power law
dependences of the diffusivity on temperature, as solution of the heat
equation. The contribution of the ionizing energy loss to the spike is not
included. The range of validity of the model is analysed, and the results are
compared with numerical solutions obtained in the frame of the previous model
of the authors, which takes into account both nuclear and ionization energy
losses, as well as the coupling between the two subsystems in crystalline
semiconductors. Particular solutions are discussed and the errors induced by
these approximations are analysed.
