The X-ray polarisation sensitivity of CCDs

The regular array of pixels in a silicon Charge Coupled Device (CCD) may be used to measure photoelectron emission directions following the absorption of high energy X-rays (E > 15 keV). CCDs offer, therefore, the possibility of combining X-ray imaging and spectroscopy with measurement of the linear polarisation of the incident beam. We describe a simple model of electron transport in CCDs which leads to an estimate of the energy-dependent modulation factor M(E) — the parameter determining polarisation sensitivity — for arbitrary pixel geometries. The predictions of the model are in good agreement with published measurements. The sensitivity of an optimised CCD “pixel polarimeter” for cosmic X-ray astronomy is assessed.