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Register a new userQuadrupole collectivity beyond N=28: Intermediate-energy Coulomb excitation of 47,48Ar
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We report on the first experimental study of quadrupole collectivity in the very neutron-rich nuclei uc{47,48}{Ar} using intermediate-energy Coulomb excitation. These nuclei are located along the path from doubly-magic Ca to collective S and Si isotopes, a critical region of shell evolution and structural change. The deduced $B(E2)$ transition strengths are confronted with large-scale shell-model calculations in the $sdpf$ shell using the state-of-the-art SDPF-U and EPQQM effective interactions. The comparison between experiment and theory indicates that a shell-model description of Ar isotopes around N=28 remains a challenge.
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The transition rates for the 2_{1}^{+} states in 62,64,66Fe were studied using the Recoil Distance Doppler-Shift technique applied to projectile Coulomb excitation reactions. The deduced E2 strengths illustrate the enhanced collectivity of the neutron-rich Fe isotopes up to N=40. The results are interpreted by the generalized concept of valence proton symmetry which describes the evolution of nuclear structure around N=40 as governed by the number of valence protons with respect to Z~30. The deformation suggested by the experimental data is reproduced by state-of-the-art shell calculations with a new effective interaction developed for the fpgd valence space.
Many-body nuclear theory utilizing microscopic or chiral potentials has developed to the point that collectivity might be dealt with in an {it ab initio} framework without the use of effective charges; for example with the proper evolution of operators, or alternatively, through the use of an appropriate and manageable subset of particle-hole excitations. We present a precise determination of $E2$ strength in $^{22}$Mg and its mirror $^{22}$Ne by Coulomb excitation, allowing for rigorous comparisons with theory. No-core symplectic shell-model calculations were performed and agree with the new $B(E2)$ values while in-medium similarity-renormalization-group calculations consistently underpredict the absolute strength, with the missing strength found to have both isoscalar and isovector components.
The reduced transition probability B(E2) of the first excited 2+ state in the nucleus 104Sn was measured via Coulomb excitation in inverse kinematics at intermediate energies. A value of 0.163(26) e^2b^2 was extracted from the absolute cross-section on a Pb target, while the method itself was verified with the stable 112Sn isotope. Our result deviates significantly from the earlier reported value of 0.10(4) e^2b^2 and corresponds to a moderate decrease of excitation strength relative to the almost constant values observed in the proton-rich, even-A 106-114Sn isotopes. Present state-of-the-art shell-model predictions, which include proton and neutron excitations across the N=Z=50 shell closures as well as standard polarization charges, underestimate the experimental findings
Background: Shape coexistence in heavy nuclei poses a strong challenge to state-of-the-art nuclear models, where several competing shape minima are found close to the ground state. A classic region for investigating this phenomenon is in the region around $Z=82$ and the neutron mid-shell at $N=104$. Purpose: Evidence for shape coexistence has been inferred from $alpha$-decay measurements, laser spectroscopy and in-beam measurements. While the latter allow the pattern of excited states and rotational band structures to be mapped out, a detailed understanding of shape coexistence can only come from measurements of electromagnetic matrix elements. Method: Secondary, radioactive ion beams of $^{202}$Rn and $^{204}$Rn were studied by means of low-energy Coulomb excitation at the REX-ISOLDE facility in CERN. Results: The electric-quadrupole ($E2$) matrix element connecting the ground state and first-excited $2^{+}_{1}$ state was extracted for both $^{202}$Rn and $^{204}$Rn, corresponding to ${B(E2;2^{+}_{1} to 2^{+}_{1})=29^{+8}_{-8}}$ W.u. and $43^{+17}_{-12}$ W.u., respectively. Additionally, $E2$ matrix elements connecting the $2^{+}_{1}$ state with the $4^{+}_{1}$ and $2^{+}_{2}$ states were determined in $^{202}$Rn. No excited $0^{+}$ states were observed in the current data set, possibly due to a limited population of second-order processes at the currently-available beam energies. Conclusions: The results are discussed in terms of collectivity and the deformation of both nuclei studied is deduced to be weak, as expected from the low-lying level-energy schemes. Comparisons are also made to state-of-the-art beyond-mean-field model calculations and the magnitude of the transitional quadrupole moments are well reproduced.
The method of intermediate-energy Coulomb excitation has been widely used to determine absolute B(E2; 0+ -> 2+) quadrupole excitation strengths in exotic nuclei with even numbers of protons and neutrons. Transition rates measured with intermediate-energy Coulomb excitation are compared to their respective adopted values and for the example of 26Mg to the B(E2; 0+ -> 2+) values obtained with a variety of standard methods. Intermediate-energy Coulomb excitation is found to have an accuracy comparable to those of long-established experimental techniques.
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