Electric Monopole Transition from the Superdeformed Band in ^{40}Ca

Area of Science:

• Nuclear Physics
• Atomic Physics

Background:

• Electric monopole (E0) transitions probe nuclear structure and shape coexistence.
• Doubly magic ^{40}Ca provides a unique system to study nuclear deformation and mixing.

Purpose of the Study:

• To determine the E0 transition strength for the 0_{3}^{+}→0_{1}^{+} transition in ^{40}Ca.
• To investigate the microscopic structure and configuration mixing of excited states in ^{40}Ca.
• To understand the unusually small E0 transition strength observed.

Main Methods:

• e^{+}e^{-} pair-conversion spectroscopy was employed to measure E0 transition strengths.
• Large-scale shell-model (LSSM) calculations were performed to interpret the experimental findings.

Main Results:

• The E0 transition strength ρ^{2}(E0;0_{3}^{+}→0_{1}^{+}) in ^{40}Ca was measured as 2.3(5)×10^{-3}, the smallest observed in A<50 nuclei.
• A significantly larger E0 transition strength, ρ^{2}(E0;0_{2}^{+}→0_{1}^{+})=25.9(16)×10^{-3}, was observed for the transition from the second 0^{+} state, indicating substantial mixing.
• LSSM calculations successfully reproduced the experimental ρ^{2} values, supporting the interpretation of shape-coexisting structures.

Conclusions:

• The unusually small E0 transition strength is attributed to destructive interference in the mixing of shape-coexisting structures.
• This finding challenges conventional models of E0 strengths, as two-state mixing typically does not lead to destructive interference.
• The study highlights the complex interplay of multiparticle-multihole excitations in shaping nuclear properties.