Emergence of nuclear collectivity in Cd and Te isotopes from M1 and E2 observables

Abstract

This thesis describes measurements of M1 and E2 observables near Z=50 and toward Sn-132. Coulomb-excitation measurements of odd-mass Cd isotopes are reported with transition strengths for the low-lying states in agreement with previous measurements. Measurements were performed in inverse kinematics with Cd beams impinged on a C target. A multi-step Coulomb-excitation measurement of Cd-111 is described with a Cd-111 beam on a natural Pb target using the GRETINA and CHICO2 arrays. The observed level scheme is reported with a discussion of the nature of states. A new state is tentatively proposed and several new transitions are observed.

Measured transition strengths are compared to shell-model calculations with good agreement found. Comparison is made of the low-lying structure of the Cd isotopes to the particle-vibration model with poor agreement found. Consistent with previous work, we suggest that the vibrational explanation of collective states in the Cd isotopes is insufficient. A qualitative comparison of results to the particle-rotor model is then given. Neither the particle-vibration nor the particle-rotor model is seen to give specific, accurate descriptions of these weakly collective nuclides.

Transient-field g-factor measurements on the first-excited 4+ states in even-even Te-124 to Te-130 are reported for the first time. Measurements were made with Ni-58 beams on enriched Te targets for each measured isotope. The ratio of g factors of the first-excited 4+ and 2+ states is seen to converge to unity away from the shell closure. The measured g factors are compared to shell-model calculations, and the observed trends are in agreement with shell-model predictions. The reported shell-model calculations show the importance of angular-momentum sharing between protons and neutrons in the development of collectivity.

Inverse-kinematics, multi-step Coulomb-excitation measurements of Te-124,126,130 beams on an enriched Ti-50 target are reported. Transition strengths for states above the first-excited 2+ state are obtained to higher precision than in previous measurements in several cases, and the transition strength between the first-excited 4+ and 2+ states in Te-130 measured for the first time.

Finally, a discussion of developing collectivity in the Sn-132 region is made with reference to shell-model calculations for both the Cd and Te measurements. The pathway to nuclear collectivity is described with nuclear deformation and triaxiality shape invariants calculated using the Kumar-Cline sum rules based on shell-model-calculated E2 matrix elements. Triaxiality is predicted to be an important feature of low-excitation states in the Te isotopes. Deformation of states is also seen to be smaller in higher-spin states than in lower-spin states. A suggestion is made that collectivity builds from low-excitation, low-spin states toward high-spin states.