Time resolution of planar GeLli) detectors in γ -γ coincidence experiments

In view of a better knowledge of the effects which cause the observed time dispersions we have made a theoretical study on this subject. In the first part we calculate the shape of the charge and current pulses issued from the single photoelectric interaction of a gammna-ray with the atoms of a Ge(Li) detector. This shape depends on the interaction abscissa and on the range of the secondary electron. The influence of the latter is well seen on the shapes of the pulses presented. The distribution of risetime is deduced from this calculation. In the second part, we calculate the distribution of the time at which the signal crosses a threshold, by means of a suitable mathematical transformation (it is the time spectrum of a leading edge timing). Electronic noise is taken into account and the variations of the calculated distributions versus noise, threshold and range of secondary electrons are presented. Discussions about an improvement of the time resolution is included in this part and it is shown that a very simple filter can ameliorate the accuracy of the measurement. In the third part we describe the experimental set-up. The variations of time resolution versus detector bias voltage are presented. The variations of time resolution versus threshold are also shown and one of these curves is compared with the calculated one. Agreement is not perfect but can explain some irregularities of the experimental distributions.