= Solution
A compact silicon-tungsten sampling <electromagnetic calorimeter> can serve as the small-angle luminosity monitor. Alternate dense tungsten absorber plates with finely segmented silicon sensor planes. Incident <electrons> and <Positrons> create <electromagnetic showers> in the absorber; their charged daughters create <electron>-hole pairs by <ionization> in depleted silicon. An <electric field> collects the charge, and the summed calibrated signal estimates shower <energy>.
Radial and azimuthal sensor segmentation gives precise shower positions and hence polar angles using the measured distance to the interaction point. Multiple layers measure shower development and improve the centroid estimate. Opposite-side coincidence, beam-time synchronization and an <energy> requirement reject beam backgrounds. Mechanical survey, alignment and edge-response calibration define the small-angle fiducial boundary: high position precision matters especially because the <Bhabha scattering> rate changes steeply with angle. The monitor's overall <energy resolution> need not match the central precision crystal detector; it must accurately reconstruct its own fiducial selection and efficiency.
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