Hard photons from neutron-proton bremsstrahlung in intermediate energy
heavy-ion reactions are examined as a potential probe of the nuclear symmetry
energy within a transport model. Effects of the symmetry energy on the yields
and spectra of hard photons are found to be generally smaller than those due to
the currently existing uncertainties of both the in-medium nucleon-nucleon
cross sections and the photon production probability in the elementary process
pn→pnγ. Very interestingly, nevertheless, the ratio of hard photon
spectra R1/2(γ) from two reactions using isotopes of the same element
is not only approximately independent of these uncertainties but also quite
sensitive to the symmetry energy. For the head-on reactions of
132Sn+124Sn and 112Sn+112Sn at Ebeam/A=50 MeV, for
example, the R1/2(γ) displays a rise up to 15% when the symmetry
energy is reduced by about 20% at ρ=1.3ρ0 which is the maximum density
reached in these reactions.Comment: Added new results in Fig. 6 and new references [27.28]. Phys. Lett. B
in pres