SOME CHARACTERISTICS OF A SOLID-STATE DETECTOR IN THE SOFT-X-RAY REGION

We report here the linearity of the Si(Li) solid state detector (SSD) in the soft X-ray (1-2 keV) region, particularly around the Si K-shell absorption edge energy (1840 eV) by using a synchrotron radiation facility. The response function of the SSD shows a big change at this energy. We fitted the pulse height (PH) distributions with an empirical model which is a sum of two Gaussians having different parameters. Using this empirical model, we found the nonlinearity at the Si K-edge to be 8.7 +/- 2.6 eV. We introduce another type of response function in which a charge diffusion process in silicon is taken into account. The primary charge cloud is partly absorbed by the electrode when the photoabsorption occurs near the electrode, resulting in the low PH. We found that this type of response function is more appropriate in reproducing the data than the empirical model. Based on the diffusion model, we found the nonlinearity to be 1.5 +/- 2.6 eV. This implies that the apparent nonlinearity seen in the two Gaussian model can be attributed to the diffusion process. We conclude that there is no intrinsic nonlinearity effect in silicon at the K-edge. The diffusion effect is not applicable in the front illuminated CCD. We expect that there is no nonlinearity in the CCD.