Coulomb excitation of the deuteron in peripheral collisions with a heavy ion
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Du, Weijie↗
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JANUS — A setup for low-energy Coulomb excitation at ReA3
We report that a new experimental setup for low-energy Coulomb excitation experiments was constructed in a collaboration between the National Superconducting Cyclotron Laboratory (NSCL), Lawrence Livermore National Laboratory (LLNL), and the University of Rochester and was commissioned at the general purpose beam line of NSCL's ReA3 reaccelerator facility. The so-called JANUS setup combines γ-ray detection with the Segmented Ge Array (SeGA) and scattered particle detection using a pair of segmented double-sided Si detectors (Bambino 2). The low-energy Coulomb excitation program that JANUS enables will complement intermediate-energy Coulomb excitation studies that have long been performed at NSCL by providing access to observables that quantify collectivity beyond the first excited state, including the sign and magnitude of excited-state quadrupole moments. Here, in this work, the setup and its performance will be described based on the commissioning run that used stable 78 Kr impinging onto a 1.09 mg/cm 2 208 Pb target at a beam energy of 3.9 MeV/u.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Determination of proton and neutron contributions to the 0$^+_{𝑔𝑠}$ → 2$^+_{1}$ excitations in 42 Si and 44 S using inelastic proton scattering in inverse kinematics and intermediate-energy Coulomb excitation
Here, we have measured the 0$^+_{𝑔𝑠}$ → 2$^+_1$ transition in the neutron-rich 𝑁 = 28 isotope 42 Si using the probes of intermediate-energy Coulomb excitation and inelastic proton scattering in inverse kinematics at the Facility for Rare Isotope Beams with beam particle rates of ≈ 5 particles/s. The results of these two measurements allowed us to determine 𝑀 𝑛 /𝑀 𝑝 , the ratio of the neutron and proton transition matrix elements for the 0$^+_{𝑔𝑠}$ → 2$^+_1$ transition. In addition, we have measured the 0$^+_{𝑔𝑠}$ → 2$^+_1$ transition in the isotone 44 S using inverse kinematics inelastic proton scattering. By comparing the 44 S proton-scattering result with a recent intermediate-energy Coulomb excitation result on the same transition, we were able to determine 𝑀 𝑛 /𝑀 𝑝 for the 0$^+_{𝑔𝑠}$ → 2$^+_1$ transition in this nucleus as well. This work strengthens the evidence that 42 Si has a stable quadrupole deformation in its ground state and that 44 S does not. Both conclusions are further supported by shell-model calculations carried out with the FSU interaction.
39 ≤ A ≤ 58↗
Isospin symmetry in B ( E 2 ) values: Coulomb excitation study of Mg 21
The T z = -$\frac{3}{2}$ nucleus 21 Mg has been studied by Coulomb excitation on 196 Pt and 110 + ground state to the first excited $\frac{1}{2}$ + state in 21 Mg was observed for the first time. Coulomb excitation cross-section measurements with both targets and a measurement of the half-life of the $\frac{1}{2}$ + state yield an adopted value of B (E2; $\frac{5}{2}$ + → $\frac{1}{2}$ + ) = 13.3 (4) W.u. A new excited state at 1672(1) keV with tentative $\frac{9}{2}$ + assignment was also identified in 21 Mg . This work demonstrates a large difference in the B (E2; $\frac{5}{2}$ + → $\frac{1}{2}$ + ) value between T = $\frac{3}{2}$, A = 21 mirror nuclei. Lastly, the difference is investigated in the shell-model framework employing both isospin conserving and breaking USD interactions and using modern ab initio nuclear structure calculations, which have recently become applicable in the s d shell.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Coulomb excitation of 80,82 Kr and a change in structure approaching N = Z = 40
Nuclei approaching N = Z = 40 are known to exhibit strongly deformed structures and are thought to be candidates for shape coexistence. In the krypton isotopes, 80,82 Kr are poorly characterized, preventing an understanding of evolving deformation approaching N = 40. The present work aims to determine electric quadrupole transition strengths and quadrupole moments of 80,82 Kr in order to better characterize their deformation. Sub-barrier Coulomb excitation was employed, impinging the isotopes of krypton on 196 Pt and 208 Pb targets. Utilizing a semiclassical description of the safe Coulomb-excitation process E 2 matrix elements could then be determined. Eleven new or improved matrix elements are determined in 80 Kr and seven in 82 Kr . The new B(E2;0$^+_1$ → 2$^+_1$ ) value in 82 Kr disagrees with the evaluated value by 3σ , which can be explained in terms of deficiencies in a previous Coulomb-excitation analysis. Conclusions: Comparison of measured Q s ( 2$^+_1$ ) and B(E2; 0$^+_1$ → 2$^+_1$ ) values indicates that neutron-deficient ( N ≤ 42 ) isotopes of krypton are closer to axial deformation than other isotopic chains in the mass region. A continuation of this trend to higher Z may result in Sr and Zr isotopes exhibiting near-axial prolate deformation.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Multistep Coulomb excitation of Ni 64 : Shape coexistence and nature of low-spin excitations
Here, the structure of 64 Ni, the heaviest stable Ni isotope, has been investigated via high-statistics, multistep safe Coulomb excitation to search for shape coexistence, a phenomenon recently observed in neutron-rich 66 Ni and 70 Ni as well as in doubly magic, N = 40, 68 Ni. The study was motivated by recent, state-of-the-art Monte Carlo shell-model calculations (MCSM), where a Hamiltonian with effective interactions incorporating the monopole tensor force predicts the existence of shape coexistence, also in the lower-mass 62,64 Ni isotopes. A set of transition and static E2 matrix elements for both yrast and near-yrast structures was extracted from the differential Coulomb excitation cross sections. From comparisons between the new results and MCSM as well as other shell-model calculations, a clearer picture of the structure of 64 Ni emerges. Specifically, the low-spin states are shown to be dominated by proton and neutron excitations mainly within the fp shell, with minimal contribution from the g 9/2 shape-driving neutron orbital. The agreement between experimental data and MCSM results indicates a small oblate deformation for the $0$$^{+}_{2}$ level and a spherical shape for the $0$$^{+}_{3}$ state. In addition, the small upper limit determined for the B(E2) probability of a transition associated with the decay of the recently observed 3463-keV, $0$$^{+}_{4}$ state agrees with its proposed assignment to a prolate shape, herewith providing first evidence for triple shape coexistence in a stable Ni isotope.
59 ≤ A ≤ 89↗
Structure of Low-spin States in \(^{45}\)Sc Studied via Coulomb Excitation
The electromagnetic structure of 45 Sc at low excitation energy was investigated via low-energy Coulomb excitation at the Heavy Ion Laboratory (HIL) of the University of Warsaw and at the Inter-University Accelerator Centre (IUAC) in New Delhi. A set of reduced E2, E3, and M1 matrix elements was extracted from the collected data using the GOSIA code. The reduced transition probability B(E2; 11/2 – → 7/2 – ) has been determined, allowing us to deduce the lifetime of the 11/2 – state at 1237 keV. In addition, the upper limit on the reduced transition probability B(E3; 7/2 – → 5/2 + ) has been determined for the first time. Finally, new large-scale shellmodel and beyond-mean-field calculations were performed to interpret the structure of this nucleus.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Deformation of the $0^+_{1,2}$ states in 110 Cd from low-energy Coulomb excitation
Electromagnetic properties of 110 Cd were studied via low-energy Coulomb excitation with 32 S and 14 N beams. Magnitudes and relative signs of eight E2 matrix elements, including quadrupole moments of the $2^+_1$ and $2^+_2$ states, were determined using the least-squares code GOSIA. From those, quadrupole deformation parameters of the $0^+_{1,2,3}$ states were inferred, providing for the first time conclusive evidence for the non-axial character of the ground state in 110 Cd. The experimental results were compared with new calculations using the general quadrupole collective Bohr Hamiltonian model with SLy4 and UNEDF0 interactions. The non-axiality of the ground state is reproduced by the present calculations, independently of the interaction used.
Collective models↗
Coulomb excitation of 102 Ru with 12 C and 16 O
Here, the Coulomb excitation of 102 Ru was performed with beams of 12 C and 16 O ions. The beam particles scattered at forward angles were momentum analyzed with a magnetic spectrograph. The resolution achieved enabled the populations of the $2^{+}_{1}$ state, the unresolved $2^{+}_{2}$/ $4^{+}_{1}$, and $2^{+}_{4}$/$3^{-}_{1}$, doublets of states, and the $3^{-}_{2}$ state to be determined as a function of the scattering angle. These populations are compared with gosia calculations, yielding B(E2; $2^{+}_{1}$→$0^{+}_{1}$) = 41.5 ± 2.3 W.u., B(E2; $2^{+}_{2}$→$0^{+}_{1}$) = 1.75 ± 0.11 W.u., B(E3; $3^{-}_{1}$→$0^{+}_{1}$) = 31.5 ± 3.5 W.u., and B(E3; $3^{-}_{2}$→$0^{+}_{1}$) = 6.8 ± 0.5 W.u. The B(E3; $3^{-}_{1}$→$0^{+}_{1}$) value is significantly larger than previously measured. The weakly populated $2^{+}_{3}$ state, presumed to be a member of the band built on the $0^{+}_{2}$ state, was observed clearly for a single angle only, and a fit to its population results in B(E2; $2^{+}_{3}$→$0^{+}_{1}$) = 0.053 ± 0.011 W.u. Using the known γ -ray branching ratios for the $2^{+}_{3}$ level, the B(E2; $2^{+}_{3}$→$0^{+}_{2}$) value is calculated to be 18 ± 4 W.u., substantially less than the B(E2; $2^{+}_{1}$→$0^{+}_{1}$) . This suggests that the deformation of the $0^{+}_{2}$ state is lower than that of the $0^{+}_{1}$ state. The results are compared with beyond-mean-field calculations with the Gogny-D1S interaction using the symmetry-conserving configuration-mixing method.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Isomeric Level of the {sup 229}Th Nucleus and Its Population in the Coulomb Excitation Reaction
Electromagnetical properties of the deformed neutron-odd nucleus of {sup 229}Th are considered on the basis of a unified nuclear model. Particular attention is given to the properties of the low-lying isomeric 3/2{sup +} state. The possibility of its population in the Coulomb excitation reaction is studied.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Coulomb excitation of 80 Sr and the limits of the 𝑁 ≈ 𝑍 ≈ 40 island of deformation
The region of 𝑁 ≈ 𝑍 ≈ 40 has long been associated with strongly deformed nuclear configurations. The presence of this strong deformation was recently confirmed through lifetime measurements in 𝑁 ≈ 𝑍 Sr and Zr nuclei. Theoretically, however, these nuclei present a challenge due to the vast valence space required to incorporate all necessary orbitals. Recent state-of-the-art predictions indicate a near axial prolate deformation for 𝑁 = 𝑍 and 𝑁 = 𝑍+2 nuclei between 𝑁 = 𝑍 = 36 and 𝑁 = 𝑍 = 40. In this work we investigate the shores of this island of deformation through a sub-barrier Coulomb excitation study of the 𝑁 = 𝑍+4 nucleus, 80 Sr . Extracting a spectroscopic quadrupole moment of 𝑄 𝑠 ($2^+_1$)= $0.5^{+0.8}_{−0.9}$𝑒b, we find that 80 Sr is inconsistent with significant axial prolate deformation with a significance of 1.5𝜎. This result, albeit with a large uncertainty, indicates that the predicted region of strong prolate deformation around 𝑁 = 𝑍 = 40 is tightly constrained to the quartet of nuclei: 76,78 Sr and 78,80 Zr .
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Exploring the role of high- j configurations in collective observables through the Coulomb excitation of 106 Cd
In this work, the shape and collectivity of 106 Cd was investigated via a sub-barrier-energy Coulomb excitation experiment performed at the NSCL ReA3 facility using the JANUS setup. Transition matrix elements between low-lying states were found to agree with adopted values, and information on the shape and collectivity of higher-lying states was extracted for the first time. Locally-optimized large-scale shell-model calculations were found to describe well the B(E2) transition strengths but failed to reproduce the spectroscopic quadrupole moments Q s . An analysis of the E2 rotational invariants and the normalized quadrupole moment q s indicates that this may be due to a significant degree of triaxiality in 106 Cd which is not captured by the present shell-model calculations. Analogous calculations for the Fe isotopes (two protons below the Z = 28 magic number) reveal the critical role of high-j neutron configurations for the description of quadrupole moments in the heavy Fe and Cd isotopes (two protons below magic Z = 50), but this effect is insufficient to explain the shape of 106 Cd, posing a puzzle for the understanding of nuclear structure towards N = 50.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Electromagnetic properties of low-lying states in neutron-deficient Hg isotopes: Coulomb excitation of 182 Hg, 184 Hg, 186 Hg and 188 Hg
The neutron-deficient mercury isotopes serve as a classical example of shape coexistence, whereby at low energy near-degenerate nuclear states characterized by different shapes appear. The electromagnetic structure of even-mass 182-188 Hg isotopes was studied using safe-energy Coulomb excitation of neutron-deficient mercury beams delivered by the REX-ISOLDE facility at CERN. The population of 0 + 1,2 , 2 + 1,2 and 4 + 1 states was observed in all nuclei under study. Reduced E2 matrix elements coupling populated yrast and non-yrast states were extracted, including their relative signs. These are a sensitive probe of shape coexistence and may be used to validate nuclear models. The experimental results are discussed in terms of mixing of two different configurations and are compared with three different model calculations: the Beyond Mean Field model, the Interacting Boson Model with configuration mixing and the General Bohr Hamiltonian. Partial agreement with experiment was observed, hinting to missing ingredients in the theoretical descriptions.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Coulomb excitation of the | T z | = 1 2 , A = 23 mirror pair
Background: Electric-quadrupole ( E 2 ) strengths relate to the underlying quadrupole deformation of a nucleus and present a challenge for many nuclear theories. Additionally, mirror nuclei in the vicinity of the line of N = Z represent a convenient laboratory for testing deficiencies in such models, making use of the isospin symmetry of the systems. Purpose: Uncertainties associated with literature E 2 strengths in Mg 23 are some of the largest in T z = | 1 2 | nuclei in the s d shell. The purpose of the present paper is to improve the precision with which these values are known, to enable better comparison with theoretical models. Methods: Coulomb-excitation measurements of Mg 23 and Na 23 were performed at the TRIUMF-ISAC facility using the TIGRESS spectrometer. They were used to determine the E 2 matrix elements of mixed E 2 / M 1 transitions. Results: Reduced E 2 transition strengths, B ( E 2 ) , were extracted for Mg 23 and Na 23 . Their precision was improved by factors of approximately 6 for both isotopes, while agreeing within uncertainties with previous measurements. Conclusions: A comparison was made with both shell-model and ab initio valence-space in-medium similarity renormalization group calculations. Valence-space in-medium similarity renormalization group calculations were found to underpredict the absolute E 2 strength, in agreement with previous studies.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Structure of Xe 126 , 128 studied in Coulomb excitation measurements
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73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Coulomb Excitation of Neutron-Deficient 106,108Sn
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Physics - Nuclear physics and radiation physics↗