  Hard photon production from neutron-proton bremsstrahlung in intermediate
energy heavy-ion reactions is examined as a probe of the in-medium pn
scattering cross section within a transport model. Uncertainty of photon
production probability pn-pngamma is cancelled out by using the ratio of hard
photon spectra R_{12C+12C/p+n}(gamma) from two reactions. The in medium pn
scattering cross section is constrained by using the ratio of hard photon
production cross sections of proton-induced reactions p+12C and p+2H. A
reduction factor sigma_pn^medium/sigma_pn^free of about 0.5 ~ 0.7 around
saturation density is obtained by comparing with the existing experimental
data.
