Untersuchung der Verbesserung des Zeitauflösungsvermögens im ps-Bereich mit Hilfe einer Pulsformanalyse

The subject of this thesis is the study of the possibility to improve the time resolution of X-ray scintillation detectors in the picosecond range by a pulse shape analysis. In a system built for positron lifetime measurement in material at the university of Siegen the time resolution was around 180ps. Here 5cm thick BaF 2 crystals are used.
The time resolution is limited by the propogation time difference of the light photons coming from different conversion locations of the X-rays in the BaF 2 crystal.
A Monte Carlo program for the simulation of the signal pulse shape was written to study this phenomenon in detail. In principle, the pulse shape contains information about the location of gamma conversions. But the studies showed that gamma conversion locations nearby or far away from the PMT, respectively, only can be distinguished by averaging over corresponding pulse shapes of more than 10 events and by using an amplifier with a minimal bandwidth of 300MHz. The main difficulties in recognizing the signals from single gamma conversions arise from the very high fluctuations of light photons which are due to a low light production in the scintillator, a limited bandwidth and a relative low quantum efficiency of the photomultiplier.
The consistence of the simulations results was proved by the measurement in a dedicated experiment with a BaF 2 Scintillator and a fast PMT. In order to compare the results of the simulation with the measurement very precisely a very fast LeCroy sampling system with a sampling rate of 10GSPS was used.
A novel fast Flash Analog to Digital Converter capable of digitizing in special mode up to a sampling frequency of 350MHz has been developed and tested. The potential for using this FADC to build a transient recorder with a sampling frequency of 700MHz and higher is discussed.