Nuclei are mesoscopic systems that are unique in their expression of both single-particle and collective degrees of freedom. Characterizing transitions between single-particle and collective behavior is critical in order to understand the nuclear forces that are responsible for the onset of nuclear deformation, which is the changing of the nuclear shape away from a perfect sphere. One tool to observe such transitions is the nuclear g-factor, which quantifies magnetic strength and has acute sensitivity to contributions from both protons and neutrons. When all the protons and neutrons in the nucleus are working together (collective behavior), the value of the g-factor is expected to be ~ 0.5. However, when the protons and neutrons work individually (single-particle behavior), the g factor is either positive and large due to protons, or negative and large due to neutrons.
An onset of deformation has been observed in the sulfur isotopes: Sulfur-36 is spherical, while Sulfur-42 is deformed. In order to characterize the shape transition in the sulfur isotopes, an experiment to measure the g-factors of Sulfur-38 and Sulfur-40 was performed. Production of the neutron-rich sulfur isotopes requires fragmentation reactions, but g-factor measurement techniques performed in the past are not well suited for the high beam energies of fragmentation products. Therefore, the High Velocity Transient Field (HVTF) method was developed in collaboration with researchers from the Australian National University, and had its first successful application to Sulfur-38 and Sulfur-40.
The g-factor results for Sulfur-38 and Sulfur-40 are shown in the figure. It is evident that the g-factor values are dominated by neither protons nor neutrons. In addition, the near zero g-factors do not agree with the value of ~0.5 expected for pure collective behavior. What is observed is a near cancellation of the proton and neutron contributions to the magnetic strength of Sulfur-38 and Sulfur-40 that suggest several, but not all, of the nucleons contribute to the overall nuclear structure. In essence, a middle ground between one nucleon and all nucleon participation in nuclear structure is seen. The new results will provide an improved understanding of the driving force behind the surprising appearance of nuclear deformation that has been proposed for the most neutron-rich sulfur isotopes.
Reference A.D. Davies et al., Phys. Rev. Lett. (2006), in print.
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