Testing of the SPEDE conversion electron spectrometer at ISOLDE

The aim of this work was to test the performance of the SPEDE detector in the MINIBALL setup at CERN’s ISOLDE laboratory. The main research objective of MINIBALL is to study properties of atomic nuclei employing radioactive ion beams. Radioactive Bi-207 and Hg-191 were used in this experiment. SPEDE detects internal conversion electrons which are created in transitions between states in atomic nucleus. The internal conversion is competing process to more common γ-ray emission. This way it is possible to measure different properties of nuclear structure for example the E0-transitions. The simultaneous γ and electron measurements are possible when SPEDE is used in conjunction with the MINIBALL spectrometer. The GEANT4 simulation results were used to help interpretation of experimental results. As a result, αK/L-ratio was determined for Bi-207 conversion electrons, for the 5^2− -> 1^2− transition αK/L = 3.29±0.06 and for the 13^2+-> 5^2− transition αK/L = 3.11±0.05 were obtained. Also, the partial internal conversion coefficient αKmeas = 0.30 ± 0.13 was determined for Au-191 for the 5^2+ ->3^2+ transition , which is well in-line with the theoretical value αKtheor = 0:28 ± 0:01. Some of the results are inconsistent with theory and previous results, which can be explained by the electronics used in tests. One can still say that SPEDE is capable to measure in future experiments in MINIBALL-experimental setup. SPEDE will bring new possibility to explore nuclear structure in MINIBALL.