  The transverse momentum distribution of prompt photons coming from the very
early phase of ultrarelativistic heavy ion collisions for the RHIC and LHC
energies is calculated by means of perturbative QCD. We calculate the single
photon cross section ($A+B\to \gamma +X$) by taking into account the partonic
sub processes $q+ \bar{q}\to \gamma +g$ and $q+g\to\gamma +q$ as well as the
Bremsstrahlung corrections to those processes. We choose a lower momentum
cut-off $k_0 =2$ GeV separating the soft physics from perturbative QCD. We
compare the results for those primary collision with the photons produced in
reactions of the thermalized secondary particles, which are calculated within
scaling hydrodynamics. The QCD processes are taken in leading order. Nuclear
shadowing corrections, which alter the involved nuclear structure functions are
explicitly taken into account and compared to unshadowed results. Employing the
GRV parton distribution parametrizations we find that at RHIC prompt
QCD-photons dominate over the thermal radiation down to transverse momenta $k_T
\approx 2$ GeV. At LHC, however, thermal radiation from the QGP dominates for
photon transverse momenta $k_T \leq 5$ GeV, if nuclear shadowing effects on
prompt photon production are taken into account.
