Silicon drift detector
A silicon drift detector (SDD) is a relatively new radiation detector for the measurement of ionizing radiation. They are used among other things in X-ray spectrometers for the detection of X-rays. The semiconductor detector was presented in 1984 by E. Gatti and P. Rehak and since then further developed for various areas of high energy physics and x-ray spectroscopy.
The basic principle of a silicon drift detector, substantially corresponds to a PN diode or a photo-diode of silicon having a highly doped p-type (p ) and a moderately doped n -type region. Is due to an externally applied electric voltage ( mainly) is depleted, the n- doped region at the transition to the p -doped region, i.e., the field of the applied voltage leads to a displacement of the Majoritätsladungstäger ( in the n- doped region of the electrons). This already existing without voltage space charge region widens. Incident X- radiation is absorbed and generates electron-hole pairs that can be separated in this charge -carrier- field due to the applied voltage without recombining. Thus, the electrons drift to the n- doped region and reach the anode. The holes (holes, electrons) drift, however, the p- doped region and move to the cathode of the detector crystal. This basic structure may be (for example, a wafer ) easily realized vertically in a thin n-doped silicon substrate, the anode and the cathode are each mounted on a respective side surface of the substrate. However, this structure has disadvantages, because for one put the anode surface a large electric capacity there, resulting in a large signal shaping time, on the other hand there is only a relatively small space charge region.
A significant improvement was achieved with the 1984 presented by E. Gatti and P. Rehak drift chamber. In contrast to the structure described above, a thin n-doped silicon substrate ( the a wafer ) is provided on both sides with a p -doped region and contacting ( the cathode). The n-type bulk silicon is contacted only over a relatively small contact at one of the sides. In spite of the small dimension of the anode, it is possible to deplete the whole wafer by externally applied electric voltage. Initially growing even without voltage present space-charge regions (both substrate sides ) with magnitude of the voltage until they touch both space charge zones, so that forms an impoverished area between the two p - doped regions. Increasing the power further to the space charge region continues to spread laterally outside the p -type regions in the direction of the anode.
Of incident X-ray radiation, which is absorbed in the charge -carrier-added area, produced electron-hole pairs are separated due to the applied voltage. The holes (holes, electrons) drift to the p -type contacts and the electrons in the opposite direction to the substrate midway between the two P contacts. Through superposition of the space charge region having a second voltage across the wafer surface, the electrons drift to the anode can be controlled, where they are fed to an amplifier circuit or evaluation electronics. In this way we obtain the basic structure of the presented by E. Gatti and P. Rehak semiconductor drift chamber.
Today, the typical structure of this basic concept differs more or less. So modern SDD are usually made in a cylindrical shape on a high purity silicon wafer. To increase the efficiency several p -doped regions are arranged in a ring around a cylindrical n-type anode in the wafer center. This standard method of semiconductor technology are used, for example, photolithographic patterning, ion implantation for doping or deposition of silicon dioxide and aluminum. In addition to the original two-sided (structured) version of alternative variants were presented with only one-sided structure in the literature in which additional transistors have been integrated as a preamp on the detector crystal, see, Scholze et al., Pieolli et al. and Friedbacher and Bubert.
Pros and Cons
Due to the small thickness and thus of the lower volume detector with respect to the Si (Li) detectors have an SSD above about 10 keV a lower efficiency. However, this is hardly interfere with the RFA, since the radiation intensity is high enough here usually. The ( volume-dependent ) leakage currents are also significantly lower, which reduces the noise of the output signal. Therefore, it is enough to cool to about -20 ° C with small Peltier coolers. This (and because of more efficient production on wafers ) they are smaller and cheaper than Si ( Li) s Since the electrical signals are collected in the middle of the silicon drift detector on a small anode, their electric capacity of the anode is lower than that of Si ( Li) detector, which allows a faster measurement time by a factor of ten. In addition, permits the production with standard methods of semiconductor technology has typically 4 -inch or 6- inch wafers a simple integration of one or more transistors that can be used as a preamplifier. Thus, the preamplifier is still sitting closer to the detector material than in the Si ( Li) detectors, which in turn allows for better electronic analysis. For these reasons they are becoming increasingly popular, the Si ( Li) detectors from.