  A prototype Silicon-Tungsten electromagnetic calorimeter (ECAL) for an
International Linear Collider (ILC) detector was installed and tested during
summer and autumn 2006 at CERN. The detector had 6480 silicon pads of dimension
1x1 cm^2. Data were collected with electron beams in the energy range 6 to 45
GeV. The analysis described in this paper focuses on electromagnetic shower
reconstruction and characterises the ECAL response to electrons in terms of
energy resolution and linearity. The detector is linear to within approximately
the 1% level and has a relative energy resolution of (16.6 +- 0.1)/
\sqrt{E(GeV}) + 1.1 +- 0.1 (%). The spatial uniformity and the time stability
of the ECAL are also addressed.
