[Paper Review] Fast nuclear rotation and octupole deformation

W. Urban, J. Bacelar+1 more|ArXiv.org|Apr 11, 2001

Nuclear Physics and Applications3 citations

TL;DR

This study investigates fast-rotating 150Sm using γ-ray spectroscopy following the 136Xe(18O,4n) reaction at 76 MeV, observing an alternating-parity s=+1 band up to I=22ℏ. The band terminates abruptly after two alignments, indicating octupole deformation vanishes above I=22, consistent with calculations predicting a transition to a reflection-symmetric shape.

ABSTRACT

The 150Sm nucleus has been studied to high spins in a measurement of gamma radiation following the 136Xe(18O,4n)150Sm, compound-nucleus reaction at beam energy of 76 MeV. The measurement was performed at NBI Riso using the NORDBALL array. Alternating parity, s=+1 band in 150Sm has been observed up to spin I=22. This band is crossed by two aligned bands, corresponding to a reflection-symmetric shape. After the second crossing the s=+1 band ends abruptly, suggesting that the octupole shape vanishes in 150Sm above spin I=22, as predicted by calculations. Other explanations, assuming continuation of the s=+1 band past the two alignments are also discussed.

Motivation & Objective

• To experimentally test theoretical predictions that octupole deformation in 150Sm vanishes at high spins due to rotational alignment.
• To investigate the interplay between octupole correlations and rotational alignment in a nucleus with strong octupole interactions.
• To determine whether the s=+1 alternating-parity band continues beyond spin I=22 or terminates abruptly.
• To identify and characterize the emergence of reflection-symmetric, aligned bands in 150Sm.

Proposed method

• Used the 136Xe(18O,4n) reaction at 76 MeV beam energy to populate high-spin states in 150Sm.
• Employed the NORDBALL array of high-purity germanium detectors with anti-Compton shielding to measure γ-γ coincidences.
• Collected ~2×10⁸ γγ coincidence events to reconstruct the high-spin excitation scheme.
• Analyzed B(E1)/B(E2) branching ratios to probe octupole collectivity and shape evolution.
• Compared experimental transition intensities and alignment patterns with predictions from cranking model calculations.
• Used statistical significance (σ) thresholds (3σ) to assess the observation limit of γ transitions.

Experimental results

Research questions

• RQ1 Does the s=+1 alternating-parity band in 150Sm terminate at I=22ℏ, as predicted by calculations?
• RQ2 What is the role of proton and neutron pair alignments in terminating the octupole-deformed shape in 150Sm?
• RQ3 Is there evidence for a reflection-symmetric, four-quasiparticle configuration emerging after two alignments?
• RQ4 Why do γ-ray intensities in the s=+1 band drop sharply beyond I=22ℏ, despite sufficient spin-carrying states?
• RQ5 Could the s=+1 band continue past I=22ℏ with reduced quadrupole deformation, as seen in similar nuclei?

Key findings

• The s=+1 alternating-parity band in 150Sm was observed up to spin I=22ℏ, with γ-ray intensities dropping below 3σ beyond this point.
• Two aligned bands—corresponding to a reflection-symmetric shape—cross the s=+1 band, with the second alignment occurring near I=20ℏ.
• The B(E1)/B(E2) branching ratios in the s=+1 band stabilize at ~0.01 above I=14ℏ, indicating enhanced octupole collectivity.
• The positive-parity aligned band was observed up to I=28ℏ, and a new negative-parity band was identified up to I=31ℏ.
• The abrupt termination of the s=+1 band suggests that octupole deformation vanishes above I=22ℏ, consistent with theoretical predictions.
• The 4-quasiparticle configuration in 150Sm has negative parity, differing from the predicted positive-parity state in 222Th, due to the sequence of alignments.