Dielectron Cross Section Measurements in Nucleus-Nucleus Reactions at<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>1.0</mml:mn><mml:mi mathvariant="italic">A</mml:mi></mml:math>GeV

We present measured dielectron production cross sections for $\mathrm{Ca}+\mathrm{Ca}$, $\mathrm{C}+\mathrm{C}$, $\mathrm{He}+\mathrm{Ca}$, and $d+\mathrm{Ca}$ reactions at $1.0\phantom{\rule{0ex}{0ex}}A\mathrm{GeV}$. Statistical uncertainties and systematic effects are smaller than in previous dilepton spectrometer (DLS) nucleus-nucleus data. For pair mass $M\ensuremath{\le}0.35\mathrm{GeV}{/c}^{2}$ we obtain (1) the $\mathrm{Ca}+\mathrm{Ca}$ cross section is larger than the previous DLS measurement and current model results, (2) the mass spectra suggest large contributions from ${\ensuremath{\pi}}^{0}$ and $\ensuremath{\eta}$ Dalitz decays, and (3) $d\ensuremath{\sigma}/\mathrm{dM}\ensuremath{\propto}{A}_{P}{A}_{T}$. For $M&gt;0.5\phantom{\rule{0ex}{0ex}}\mathrm{GeV}{/c}^{2}$ the $\mathrm{Ca}+\mathrm{Ca}$ to $\mathrm{C}+\mathrm{C}$ cross section ratio is significantly larger than the ratio of ${A}_{P}{A}_{T}$ values.