THE X-RAY-ENERGY RESPONSE OF SILICON .A. THEORY

In this, the first part of a two-part study of the interaction of soft X-rays with silicon, motivated by the calibration requirements of CCD imaging spectrometers in astronomy, we describe a Monte Carlo model of X-ray energy loss whose products are the energy- and temperature-dependences of (i) W, the average energy required to create an electron-hole pair, and (ii) the Fano factor F. W and F have invariably been treated as material constants in previous analyses of Si X-ray detector performance. We show that in fact, at constant detector temperature T, W is an increasing function of X-ray energy for E < 0.5 keV while F is predicted to increase slowly with E. The temperature coefficient dW / dT has a calculated value approximately 1 x 10(-4) K-1 at a typical CCD operating temperature of 170 K. We discuss the practical implications of these results. Finally, we describe our separate calculations of the near-edge variation of CCD quantum detection efficiency arising from silicon K-shell Extended X-ray Absorption Fine Structure (EXAFS).