  The photoproduction of pi0pi0 and pi0pi+/- pairs off 40Ca has been
investigated with the TAPS detector using the Glasgow photon tagging
spectrometer at the Mainz MAMI accelerator. Data have been taken for incident
photon energies in the energy range from 200 - 820 MeV. Total cross sections
have been extracted from threshold up to the maximum photon energy and
invariant mass distributions of the pion pairs have been obtained for incident
photon energies between 400 - 500 MeV and 500 - 550 MeV. The double pi0
invariant mass distributions show some relative enhancement with respect to the
mixed charge channel at small invariant masses. The effects are smaller than
previously observed for lead nuclei and the distributions do not significantly
deviate from carbon data. The data are in good agreement with the results of
recent calculations in the framework of the BUU model, with careful treatment
of final state interaction effects but without an explicit in-medium
modification of scalar, iso-scalar pion pairs. This means that for Ca most of
the experimentally observed effect can be explained by final state
interactions. Only at low incident photon energies there is still a small low
mass enhancement of the double pi0 data over the BUU results.
