ISBN 978-3-86853-965-3

978-3-86853-965-3, Reihe Physik

Jean-Marc Régis
Fast Timing with LaBr₃(Ce) Scintillators and the Mirror Symmetric Centroid Difference Method

111 Seiten, Dissertation Universität Köln (2011), Softcover, A5

Zusammenfassung / Abstract

The measurement and determination of the lifetime of an excited state, or a radioactive isotope, respectively, is fundamental for the analysis of nuclear structure. This work is dedicated to the electronic “delayed coincidence technique” introduced in the 1950s, which is often called the fast timing technique. This technique practices a renaissance as a result of the rapid progress in the developement of new scintillation (fluorescent) materials during the last decade. The time resolution of scintillator detectors is two to three orders of magnitude better than the widely used semi-conductor detectors for high-resolution energy spectroscopy.

After an introduction with historical background, the theoretical basis of the electronic lifetime measurement is reviewed. This includes descriptions of the typical components of a fast timing setup. The exceptional property of the Cerium doped Lanthanum-Tribromide LaBr3(Ce) scintillators is the very good energy resolution of 2%-4% for γ-ray energies larger than 300 keV. An important contribution for the understanding of the fast timing technique is given. Therein, the timing principle of the constant fraction discriminator (CFD) is discussed, and an energy dependent function of the CFD time marker is derived for the first time.

The known methods to analyze the experimentally obtained time distributions are reviewed. The use of the centroid shift methode is sensitive to time difference measurements in the few picosecond region. Most important for this method is the accurate calibration of the time response of the electronics setup which is often called the “prompt curve”. In the ideal case, the prompt curve is defined by the energy dependent CFD time marker. Within this work, the mirror symmetric centroid difference (MSCD) method was developed as an extension of the centroid shift method. This MSCD method is discussed in detail.

The theoretical predictions made are verified experimentally. Using standard γ-ray sources and the γ-γ fast timing technique, the theories are successfully confirmed and a complete charachterization of the LaBr3(Ce) scintillation detector is presented. It is shown that the MSCD method is more sensitive than the centroid shift method due to imperative mirror symmetry of the method. Also, it is shown for the first time that the time response of a standard fast timing setup is defined by the CFD timing principle. Beside the introduction of a highly reliable and accurate procedure to calibrate the linearly combined time response of the whole setup, the “prompt response difference”, attention is payed to possible systematic errors. The background correction procedure of the MSCD method is also presented. In different experiments, lifetimes of certain nuclear excited states were measured consistently within an error of only 5-10 ps. The results are discussed using different nuclear models. The new results on the heavy 214Po and 214Bi confirm previously published assumptions on their nuclear structure.