  A relativistic petawatt laser light can heat heavy metals over keV
temperature isochorically and ionize them almost fully. Copious hard X-rays are
emitted from the high-Z hot plasma which acts as X-ray sources, while they work
as a cooling process of the plasma. The cooling process can affect on the
creation of high energy density plasma via the interaction, however, the
details are unknown. The X-ray spectrum depends on the plasma temperature, so
that it is worthwhile to investigate the radiation cooling effects. We here
study the isochoric heating of a solid silver foil irradiated by relativistic
laser lights with a help of particle-in-cell simulations including Coulomb
collisions, ionizations, and radiation processes. We have conducted a parameter
survey varying laser intensity, $10^{18-20}\,\rm{W/cm^2}$, to check the cooling
effects while keeping the incident laser energy constant. The silver plasma
heated mainly by the resistive heating dissipates its energy by keV X-ray
emissions in a picosecond time scale. The radiation power from the silver foil
is found to be comparable to the incident laser power when the laser intensity
is less than $10^{19}\,{\rm W/cm^2}$ under the constant energy situation. The
evolution of the plasma energy density inside the target is then suppressed,
due to which a highly compressed collisional shock is formed at the target
surface and propagates into the plasma. The radiation spectra of the keV silver
plasma are also demonstrated.
