An examination of the mixing of low-lying excited 0+ states in 116sn

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(Thesis) M.Sc.
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The even-even tin isotopes are known to exhibit shape coexistence, the phenomenon where multiple shapes coexist in a narrow energy region at relatively low-lying levels of the nucleus. These nuclei have a 0+ spherical ground state and multipleexcited 0+ states, one of which is a band head for a deformed rotational band, caused by the promotion of two protons across the Z=50 shell gap. Experimental and theoretical investigations have been performed on 116Sn to describe the nature of the mixing that occurs between the vibrational phonon levels and the deformed rotational band by probing the character of the excited 0+ states. At the time it was thought that the 0+ states showed almost equal mixing of rotational and vibrational character, but this result was based on an indirect observation and fit of the intensity of a weak 85 keV transition. The current work, a high-statistics 116Sn measurement, demonstrates unequal mixing of character between the two excited 0+ states based on a direct measurement of the intensity of the 85 keV transition. These new results might prompt a new interpretation of the structure of 116Sn. The experiment to investigate the low-lying structure of 116Sn was conducted at TRIUMF, Canada’s National Laboratory for Nuclear and Particle Physics. A high-intensity and high-purity beam of 116In was used to populate states in 116Sn via beta decay. The resulting gamma rays were observed with the 8π detector array, which consists of twenty high-purity Compton-suppressed germanium detectors coupled to a suite of ancillary detectors for β particle detection and conversion electron spectroscopy. From this high-statistics measurement 57 gamma-ray transitions were observed,with 4 new transitions that depopulate the 3096 keV level observed for the first time with energies of 101 keV, 296 keV, 447 keV, and 871 keV. Branching ratios were determined for all of the observed transitions. For the 57 transitions observed, a relative intensity had not been reported for 17 of them, and a branching ratio had not been reported for 12 of them. Transition rates were determined for 25 transitions that depopulate levels with previously reported lifetimes, and 2 of these transition rates had not been previously observed.
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Thesis advisor: Andreoiu, Corina
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