[Paper Review] Exotic Collective Excitations at High Spin: Triaxial Rotation and Octupole Condensation

Xiaofeng Wang|ArXiv.org|Oct 28, 2008

Nuclear physics research studies3 citations

TL;DR

This thesis investigates exotic nuclear excitations at high spin, identifying triaxial strongly deformed (TSD) bands in 163Tm and octupole correlations in 238-242Pu isotopes. Using lifetime measurements and Coulomb excitation, it demonstrates that TSD bands arise from particle-hole excitations in triaxial minima, not wobbling, and that strong octupole correlations in 240Pu lead to phonon condensation, explaining unique deexcitation patterns and band crossings predicted by theory.

ABSTRACT

In this thesis work, two topics, triaxiality and reflection asymmetry, have been discussed. Band structures in $^{163}$Tm were studied in a "thin" target experiment as well as in a DSAM lifetime measurement. Two new excited bands were shown to be characterized by a deformation larger than that of the yrast sequence. These structures have been interpreted as Triaxial Strongly Deformed bands associated with particle-hole excitations, rather than with wobbling. Moreover, the Tilted-Axis Cranking calculations provide a natural explanation for the presence of wobbling bands in the Lu isotopes and their absence in the neighboring Tm, Hf and Ta nuclei. A series of so-called "unsafe" Coulomb excitation experiments as well as one-neutron transfer measurements was carried out to investigate the role of octupole correlations in the $^{238,240,242}$Pu isotopes. Some striking differences exist between the level scheme and deexcitation patterns seen in $^{240}$Pu, and to a lesser extent in $^{238}$Pu, and those observed in $^{242}$Pu and in many other actinide nuclei such as $^{232}$Th and $^{238}$U, for example. The differences can be linked to the strength of octupole correlations, which are strongest in $^{240}$Pu. Further, all the data find a natural explanation within the recently proposed theoretical framework of octupole condensation.

Motivation & Objective

• To identify the nature of newly observed excited bands in 163Tm and determine whether they arise from triaxial rotation or wobbling motion.
• To investigate the role of octupole correlations in the Pu isotopes, particularly in 240Pu, where strong reflection-asymmetric deformations are expected.
• To test the theoretical framework of octupole condensation in explaining experimental data on band structures and deexcitation patterns.
• To resolve discrepancies between measured and calculated quadrupole transition moments in triaxial and wobbling bands.
• To extend the understanding of collective excitations in actinide nuclei by comparing data across the Pu isotopic chain and neighboring nuclei.

Proposed method

• Performed 'thin' target experiments and Doppler-shift attenuation measurements (DSAM) to determine lifetimes of excited states in 163Tm.
• Conducted 'unsafe' Coulomb excitation and one-neutron transfer experiments on 238,240,242Pu to probe octupole collectivity.
• Applied Tilted-Axis Cranking (TAC) calculations to model triaxial shape and rotational dynamics in 163Tm and Lu isotopes.
• Used the Gammasphere detector array with Compton suppression and BGO shielding to detect weak gamma-ray transitions.
• Employed Blue database and Radware software for data calibration, background subtraction, and level scheme reconstruction.
• Tested the octupole condensation model by comparing experimental band structures and transition probabilities with theoretical predictions.

Experimental results

Research questions

• RQ1 Are the newly observed bands in 163Tm best described as triaxial strongly deformed bands or wobbling bands?
• RQ2 Why are wobbling bands observed in Lu isotopes but not in neighboring Tm, Hf, or Ta nuclei?
• RQ3 What is the strength and nature of octupole correlations in 240Pu compared to 238Pu and 242Pu?
• RQ4 Can the observed deexcitation patterns in 240Pu, including decay to negative-parity bands, be explained by octupole phonon condensation?
• RQ5 How do the experimental data on band crossings and transition strengths compare with predictions from the octupole condensation model?

Key findings

• Two new excited bands in 163Tm are identified as triaxial strongly deformed (TSD) bands, with deformations larger than the yrast band, and are not associated with wobbling motion.
• Lifetime measurements in 163Tm show that feeding to the TSD bands originates from states with even larger deformation, suggesting complex alignment and alignment dynamics.
• Tilted-Axis Cranking (TAC) calculations successfully explain the presence of wobbling bands in Lu isotopes and their absence in Tm, Hf, and Ta nuclei.
• In 240Pu, a unique deexcitation pattern is observed where a band built on the first excited 0+ state decays exclusively to an octupole band, indicating strong octupole correlations.
• The data for 240Pu are best explained by the octupole condensation model, where phonons align with the rotational axis, forming a co-rotating condensate that leads to oscillations in energy differences between positive- and negative-parity bands.
• The level structure in 242Pu is consistent with weak octupole correlations, while 238Pu shows intermediate behavior, confirming a systematic trend in octupole strength across the Pu isotopes.