  We report on a measurement of hard photons (Eg>30 MeV) in the reaction Ar+Ca
at 180A MeV at an energy in which photons from the decay of pi0 mesons are
dominating. Simultaneous measurement with the TAPS spectrometer of the photon
spectrum and photon-photon coincidences used for the identification of pi0
enabled the subtraction of pi0 contribution. The resulting photon spectrum
exhibits an exponential shape with an inverse slope of
E0=(53+-0.03(stat)-5+8(syst)) MeV. The photon multiplicity, equal to
(1.21+-0.03(stat)+0.3-0.2(syst))10E0-2, is roughly one order of magnitude
larger than the value extrapolated from existing systematics. This enhancement
of the hard photon production is attributed to a strong increase in the
contribution of secondary np collisions to the total photon yield. We conclude
that, on average, the number of np collisions which contribute to the hard
photon production is 7 times larger than the number of first chance np
collisions in the reaction Ar+Ca at 180A MeV.
