We present measured dielectron production cross sections for Ca + Ca,
C + C, He + Ca, and d + Ca reactions at 1.0 AGeV. Statistical uncertai
nties and systematic effects are smaller than in previous dilepton spe
ctrometer (DLS) nucleus-nucleus data. For pair mass M less than or equ
al to 0.35 GeV/c(2) we obtain (1) the Ca + Ca cross section is larger
than the previous DLS measurement and current model results, (2) the m
ass spectra suggest large contributions from pi(0) and eta Dalitz deca
ys, and (3) d sigma/dM proportional to A(p)A(T). For M > 0.5 GeV/c(2)
the Ca + Ca to C + C cross section ratio is significantly larger than
the ratio of A(p)A(T) values.