Universal Effective Charges in the sd and fp Shells

Area of Science:

• Nuclear Physics
• Atomic and Molecular Physics
• Nuclear and Chemical, Radiation and Radioisotope Application

Background:

• The study of neutron-rich nuclei is crucial for understanding nuclear structure and the evolution of shell closures.
• Previous measurements of electromagnetic transitions in the fp-shell region have provided insights into nuclear properties, but ambiguities remain.
• Accurate determination of nuclear lifetimes and transition strengths is essential for testing nuclear models.

Purpose of the Study:

• To characterize the 247-keV state in ^{54}Sc, populated via beta decay of ^{54}Ca, by measuring its half-life.
• To determine the B(E2) value for the transition from the 247-keV state to the ground state in ^{54}Sc.
• To investigate the implications of the measured properties for nuclear shell structure and effective charges in the fp-shell region.

Main Methods:

• Utilized beta decay of ^{54}Ca to populate the 247-keV state in ^{54}Sc.
• Measured the half-life of the 247-keV isomeric state using timing techniques.
• Calculated the B(E2) value from the measured half-life and known transition energy.
• Compared experimental results with semi-empirical and ab initio shell-model calculations.

Main Results:

• Identified a nanosecond isomer at 247-keV in ^{54}Sc with a half-life of 26.0(22) ns.
• Determined a pure E2 transition strength of 1.93(16) W.u., representing the most precise B(E2) value in the neutron-rich fp region for nuclei with Z>20.
• Observed an E2 enhancement in ^{54}Sc approximately four times larger than in ^{55}Ca.
• Derived empirical effective proton (e_{π}=1.30(8)e) and neutron (e_{ν}=0.452(7)e) charges.

Conclusions:

• The results indicate a weak N=34 subshell gap relative to N=32.
• A significant E2 enhancement in Sc nuclei is attributed to 1p-1h proton excitations across the Z=28 shell closure.
• The derived effective charges suggest a universal set applicable across the sd and fp shells, contradicting previous reports for fp-shell nuclei near N=Z.