Position Reconstruction by Pulse Shape Analysis with the Ge-detector VEGA

An optimum algorithm to find interaction positions of a γ-ray in a Ge-detector is being investigated by analyzing pulse shapes of the Ge-detector VEGA [1, 2]. The VEGA detector consists of four Ge crystals forming like a four leaf clover. Each crystal has four electric segmentations in radial quarters. A collection time of electron and hole (eh) depends on their drift length, therefore the slope of the signal pulse depends on the distance of interaction position from the inner contact. For a segmented Ge-detector, an additional position information can be extracted by analyzing pulse shapes of neighboring segments. The drift of e-h affects the electric field in the neighboring segments and causes small transient pulses from image charges in the neighboring segments. The amplitude of the image charge signal depends on the position of the interaction in the irradiated segment. An analysis of pulse shapes gives more precise position information than a position sensitivity defined by the geometry of segments. Signal pulses from the preamplifier of the VEGA detector (SSC No.2) were recorded by using a Flash ADC. A γ-ray source which was collimated to a 2 mm diameter beam was used to scan the detector surface by 4 mm steps. Details of the measurement have been reported [1]. An example of the pulse shapes is shown in fig. 1. For the radial