Monte Carlo simulation of the X-ray response of a germanium microstrip detector with energy and position resolution

Abstract

We present Monte Carlo computer simulations of the X-ray response of a micro-strip germanium detector over the energy range 30–100 keV. The detector consists of a linear array of lithographically defined 150 μm wide strips on a high purity monolithic germanium crystal of 6 mm thickness. The simulation code is divided into two parts. We first consider a 10 μm wide X-ray beam striking the detector surface at normal incidence and compute the interaction processes possible for each photon. Photon scattering and absorption inside the detector crystal are simulated using the EGS4 code with the LSCAT extension for low energies. A history of events is created of the deposited energies which is read by the second part of the code which computes the energy histogram for each detector strip. Appropriate algorithms are introduced to account for lateral charge spreading occurring during charge carrier drift to the detector surface, and Fano and preamplifier electronic noise contributions. Computed spectra for different energies and beam positions are presented and compared with the experimental results obtained at the European Synchrotron Radiation Facility (ESRF), where the detector was scanned with a micro-beam of monochromatic photons. The remarkable agreement between the computed and measured data is discussed, and we show that consideration of charge diffusion and the resultant signal splitting between adjacent strips is essential to explain the observed results. We also comment upon the relevance of phenomena neglected in our model, such as secondary electron transport and charge trapping.