  We examine the photoconductivity of an intrinsic graphene associated with
far- and mid-infrared irradiation at low temperatures. The model under
consideration accounts for the excitation of the electron-hole pairs by
incident radiation, the interband generation-recombination transitions due to
thermal radiation, and the intraband energy relaxation due to acoustic phonon
scattering. The momentum relaxation is assumed to be caused by elastic
scattering. The pertinent collision integrals are adapted for the case of the
massless energy spectrum of carriers that interact with the longitudinal
acoustic mode and the thermal radiation. It is found that the photoconductivity
is determined by an interplay between weak energy relaxation and
generation-recombination processes. Due to this the threshold of nonlinear
response is fairly low.
