Electromagnetic properties of 21O for benchmarking nuclear Hamiltonians

The structure of exotic nuclei provides valuable tests for state-of-the-art nuclear theory. In particular electromagnetic transition rates are more sensitive to aspects of nuclear forces and many-body physics than excitation energies alone. We report the first lifetime measurement of excited states in $^{21}$O, finding $\tau_{1/2^+}=420^{+35}_{-32}\text{(stat)}^{+34}_{-12}\text{(sys)}$\,ps. This result together with the deduced level scheme and branching ratio of several $\gamma$-ray decays are compared to both phenomenological shell-model and ab initio calculations based on two- and three-nucleon forces derived from chiral effective field theory. We find that the electric quadrupole reduced transition probability of $\rm B(E2;1/2^+ \rightarrow 5/2^+_{g.s.}) = 0.71^{+0.07\ +0.02}_{-0.06\ -0.06}$~e$^2$fm$^4$, derived from the lifetime of the$1/2^+$ state, is smaller than the phenomenological result where standard effective charges are employed, suggesting the need for modifications of the latter in neutron-rich oxygen isotopes. We compare this result to both large-space and valence-space ab initio calculations, and by using multiple input interactions we explore the sensitivity of this observable to underlying details of nuclear forces.Comment: 23 pages, 3 figure

Lifetime measurement of the 21 + state in 74Rb and isospin properties of quadrupole transition strengths at N = Z

Self-conjugate nuclei in the A≈70–80 region have attracted a great deal of attention due to phenomena such as shape coexistence and increasing collectivity along the N=Z line. We investigate the structure of nuclei in this region through lifetime measurements using the GRETINA array. The first implementation of the Differential Recoil Distance Doppler Shift technique with fast radioactive beams is demonstrated and verified through a measurement of the well-known B(E2;21 +→01 +) transition strength in 74Kr. The method is then applied to determine the B(E2;21 +→01 +) transition strength in 74Rb, the heaviest odd–odd N=Z nucleus for which this quantity has been determined. This result and extended systematics along N=Z suggest the dominance of the isoscalar part of the quadrupole transition strengths in self-conjugate nuclei, as well as the possible presence of shape coexistence in 74Rb

Lifetime measurement of the 2₁⁺ state in ⁷⁴Rb and isospin properties of quadrupole transition strengths at N = Z

Self-conjugate nuclei in the A ≈ 70–80 region have attracted a great deal of attention due to phenomena such as shape coexistence and increasing collectivity along the N=Zline. We investigate the structure of nuclei in this region through lifetime measurements using the GRETINA array. The first implementation of the Differential Recoil Distance Doppler Shift technique with fast radioactive beams is demonstrated and verified through a measurement of the well-known B(E2; 2₁⁺→ 0₁⁺)transition strength in ⁷⁴Kr. The method is then applied to determine the B(E2; 2₁⁺→ 0₁⁺) transition strength in ⁷⁴Rb, the heaviest odd–odd N=Z nucleus for which this quantity has been determined. This result and extended systematics along N=Z suggest the dominance of the isoscalar part of the quadrupole transition strengths in self-conjugate nuclei, as well as the possible presence of shape coexistence in ⁷⁴Rb

Evolution of collectivity in Kr 72: Evidence for rapid shape transition

The transition rates from the yrast 2+ and 4+ states in the self-conjugate Kr72 nucleus were studied via lifetime measurements employing the GRETINA array with a novel application of the recoil-distance method. The large collectivity observed for the 4+\u21922+ transition suggests a prolate character of the excited states. The reduced collectivity previously reported for the 2+\u21920+ transition was confirmed. The irregular behavior of collectivity points to the occurrence of a rapid oblate-prolate shape transition in Kr72, providing stringent tests for advanced theories to describe the shape coexistence and its evolution. \ua9 2014 American Physical Society

Enhanced collectivity in12Be

Electromagnetic quadrupole transition strength is a sensitive probe of the evolution of the structure of nuclei, particularly the competition between collectivity and magicity. We have performed a new lifetime measurement of the 21+ state of 12Be to study the interplay of these phenomena. The lifetime was measured with the Doppler Shift Attenuation Method using the γ-ray detector GRETINA. Excited states of 12Be were produced via inelastic scattering at 55 MeV/nucleon, using several different targets to control for systematic uncertainties in the stopping powers. The lifetime is determined to be τ=1.38±0.10(stat)±0.19(sys) ps, which is about half the previously reported value at twice the precision. The reduced transition strength deduced from this result is B(E2;21+→01+)=14.2±1.0(stat)±2.0(sys) e2fm4, which supports the quenching of the N=8 shell gap in 12Be. Keywords: Lifetimes, Nuclear transition probabilities, Collectivit

Lifetime Measurements and Triple Coexisting Band Structure in S-43

Lifetime measurements of excited states in the neutron-rich nucleus S-43 were performed by applying the recoil-distance method on fast rare-isotope beams in conjunction with the Gamma-Ray Energy Tracking In-beam Nuclear Array. The new data based on gamma gamma coincidences and lifetime measurements resolve a doublet of (3/2(-)) and (5/2(-)) states at low excitation energies. Results were compared to the posdthorn pi(sd) - nu(pf) shell model and antisymmetrized molecular dynamics calculations. The consistency with the theoretical calculations identifies a possible appearance of three coexisting bands near the ground state of S-43: the K-pi =1/2(-) band built on a prolate-deformed ground state, a band built on an isomer with a 1f(7/2)(-1) character, and a suggested excited band built on a newly discovered doublet state. The latter further confirms the collapse of the N = 28 shell closure in the neutron-rich region

Evolution of Collectivity in Kr-72: Evidence for Rapid Shape Transition

The transition rates from the yrast 2(+) and 4(+) states in the self-conjugate Kr-72 nucleus were studied via lifetime measurements employing the GRETINA array with a novel application of the recoil-distance method. The large collectivity observed for the 4(+) -> 2(+) transition suggests a prolate character of the excited states. The reduced collectivity previously reported for the 2(+) -> 0(+) transition was confirmed. The irregular behavior of collectivity points to the occurrence of a rapid oblate-prolate shape transition in Kr-72, providing stringent tests for advanced theories to describe the shape coexistence and its evolution

Lifetime measurement of the 21+ state in 74Rb and isospin properties of quadrupole transition strengths at N = Z

Self-conjugate nuclei in the A≈70–80 region have attracted a great deal of attention due to phenomena such as shape coexistence and increasing collectivity along the N=Z line. We investigate the structure of nuclei in this region through lifetime measurements using the GRETINA array. The first implementation of the Differential Recoil Distance Doppler Shift technique with fast radioactive beams is demonstrated and verified through a measurement of the well-known B(E2;21+→01+) transition strength in 74Kr. The method is then applied to determine the B(E2;21+→01+) transition strength in 74Rb, the heaviest odd–odd N=Z nucleus for which this quantity has been determined. This result and extended systematics along N=Z suggest the dominance of the isoscalar part of the quadrupole transition strengths in self-conjugate nuclei, as well as the possible presence of shape coexistence in 74Rb