E0 Measurements in 154Sm Using SAGE: An Investigation of Collectivity in Rare-Earth Nuclei

Using the Silicon And Germanium array (SAGE) at the University of Jyvaskyla (JYFL), E0 transition strengths in 154Sm were measured as a test case for further measurements in the rare-earth region. A large degree of collectivity is expected in nuclear excitations of even-even rare-earth nuclei and the nature of excited $0^+$ states in these nuclei is of particular interest. SAGE was employed to measure the E0 transition strength of the 0_2^+ -> 0_1^+ and 2_2^+ -> 2_1^+ transitions in $^{154}$Sm. Such transition strengths provide a key observable to differentiate between nuclear models. To facilitate this measurement, peak-fitting techniques were developed to improve the analysis of the particular peak shapes observed in electron spectra from SAGE. A method of background subtraction was developed to remove time-random coincidences using the timing information available from the JYFL total data readout system. An isotopically enriched samarium target was bombarded by a 65 MeV beam of 16O, populating states of interest via Coulomb excitation, after which de-excitation via internal conversion and gamma-ray emission were observed. The 0_2^+ -> 0_1^+ E0 transition could not be measured in this work due to the background found in the current experimental setup. The equally important 2_2^+ -> 2_1^+ E0 transition strength could be studied by implementation of background subtraction to remove false-coincidences. The experiment was normalised by measurement of conversion coefficients for known transitions in 154Sm and in the contaminants 152Sm, 166Yb and 167Yb. An electron peak associated with the 2_2^+ -> 2_1^+ transition in 154Sm was not observable, but a stringent upper-limit was placed on the E0 transition strength. The measurement was compared to the interaction boson model and Bohr and Mottleson collective models and was found to be smaller than predicted by both collective interpretations. The usefulness of SAGE and this measurement technique was proved and the direction for future research in this area is discussed.