A direct probe of the in-medium pn scattering cross section

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→pnγ is cancelled out by using the ratio of hard photon spectra RC12+C12/p+n(γ) 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+C12 and p+H2. A reduction factor σpnmedium/σpnfree of about 0.5-0.7 around saturation density is obtained by comparing with the existing experimental data.