Characterisation of a planar germanium detector using a gamma scanner

A 300kBq Na source is placed between the front surface of planar detector and the position sensitive detector (PSD) as shown in figure 1(a). This source emits pairs of 511 keV positron annihilation photons in opposite directions. The germanium detector is placed in the focal plane of the system at a distance of 15 cm from the source whereas the PSD is at 5 cm from it in order to cover the full surface area of the PSD. Both PSD and Na are mounted on a common aluminium frame which can be rotated around the HPGe detector via a double disk ensemble. The disk can be rotated by any desired angle around the central rotating axis of the whole system. The acquistion is triggered by the 511 keV coincident events in the PSD and the germanium detector. A flash ADC(FADC) manufactured by SIS GmbH ( mod. SIS3301) is used to digitise the preamplified charge pulses from the germanium detector. The Multi Branch System (MBS) of GSI, is used to read out the FADC and the readout electronics of the PSD. As described in [1], [2] pulse shape comparison (PSC) technique is employed to investigate the spatial response of the germanium detector at specified positions inside its volume. After the installation of both the HPGe and position sensitive detector, the pulses for all the trajectories which are inside the coincident cone of two detectors are recorded. The (x,y) position coordinates of these trajectories are provided by the PSD. The source and PSD are then moved by 90◦ via the rotating ensemble to shine the planar detector from sideways as shown in panel (b) of figure 1. The pulse shapes are again recorded for this configuration of the system. These two datasets are used to obtain the position coordinates of a specific interaction point inside the germanium detector volume. The comparison of pulses along two chosen trajectories in orthogonal view gives the pulse for the intersection point. This is done on the basis of a ∆ test [2]. The distribution of ∆ values for the comparison of all the pair of pulses in two selected trajectories is shown in figure 2(a). The higher values of ∆ refer to the pair of pulses which are farther from the crossing point of the selected γ-beam directions. By finding a reasonable compromise between the statistics of full energy events and the confinement of the intersection point, the upper ∆ value is selected. The average pulse is calculated from the ones which are below the threshold. Figure 2(b) shows the average pulse shapes obtained for 7 different positions namely at 1, 4, 7, 10, 13, 16, 19 mm from the p contact of the detector. They agree well with the pulses obtained for the same detector using a conventional scanning system at IPHC Strassbourg [1] and also with the theoretical predictions [3] of the pulse shapes in a fully depleted planar germanium detector.