  Within an isospin- and momentum-dependent transport model for nuclear
reactions at intermediate energies, we investigate the interplay of the
nucleon-nucleon short-range correlations (SRC) and nuclear symmetry energy
$E_{sym}(\rho)$ on hard photon spectra in collisions of several Ca isotopes on
$^{112}$Sn and $^{124}$Sn targets at a beam energy of 45 MeV/nucleon. It is
found that over the whole spectra of hard photons studied, effects of the SRC
overwhelm those due to the $E_{sym}(\rho)$. The energetic photons come mostly
from the high-momentum tails (HMT) of single-nucleon momentum distributions in
the target and projectile. Within the neutron-proton dominance model of SRC
based on the consideration that the tensor force acts mostly in the isosinglet
and spin-triplet nucleon-nucleon interaction channel, there are equal numbers
of neutrons and protons, thus a zero isospin-asymmetry in the HMTs. Therefore,
experimental measurements of the energetic photons from heavy-ion collisions at
Fermi energies have the great potential to help us better understand the nature
of SRC without any appreciable influence by the uncertain $E_{sym}(\rho)$.
These measurements will be complementary to but also have some advantages over
the ongoing and planned experiments using hadronic messengers from reactions
induced by high-energy electrons or protons. Since the underlying physics of
SRC and $E_{sym}(\rho)$ are closely correlated, a better understanding of the
SRC will in turn help constrain the nuclear symmetry energy more precisely in a
broad density range.
