Polarization effects in a Si(Li) detector

In previous work time- and position-dependent artifacts and a time-dependent increase in the background radiation of x-ray spectra was observed in planar Si(Li) detectors for x-rays in the keV region. In the present experiments the time-dependent background was studied by use of a thin x-ray beam. The time-dependent background is considered to be due to incomplete charge collection. The amount of background is greater when the detector is left unexposed to x-rays during the bias time than when the detector is constantly irradiated. The time-dependent background was observed to decay with half-lives of the order of 30 s, 10 min and 1-4 h. The results are interpreted such that polarization of the detector material occurs by traps that are formed under the influence of the bias voltage. During irradiation some of these traps are filled by charges from the hole-electron pairs from the x-ray beam. By using Ramo's theorem and converting the energy scale in the x-ray spectra to detector depth, the polarization effect in the detector can be estimated to proceed with a speed of the order of 1 μm min-1 in an electric field of about 3000 V cm-1. (author)