Coulomb Excitation Reactions Research Articles

An alternative viewpoint on the nuclear structure towards 100Sn: Lifetime measurements in 105Sn

This work aims at presenting an alternative approach to the long standing problem of the B(E2) values in Sn isotopes in the vicinity of the N=Z double-magic nucleus 100Sn, until now predominantly measured with relativistic and intermediate-energy Coulomb excitation reactions. The direct measurement of the lifetime of low-lying excited states in odd-even Sn isotopes provides a new and precise guidance for the theoretical description of the nuclear structure in this region. Lifetime measurements have been performed in 105Sn for the first time with the coincidence Recoil Distance Doppler Shift technique. The lifetime results for the 7/21+ first excited state and the 11/21+ state, 2+(104Sn) ⊗ν1g7/2 multiplet member, are discussed in comparison with state-of-the-art shell model and mean field calculations, highlighting the crucial contribution of proton excitation across the core of 100Sn. The reduced transition probability B(E2) of the 11/21+ core-coupled state points out an enhanced staggering with respect to the B(E2; 21+→01+) in the even-mass 104Sn and 106Sn isotopes.

Spectroscopic quadrupole moments in Xe124

Background: The Xe isotopic chain with four valence protons above the Z=50 shell closure is an ideal laboratory for the study of the evolution of nuclear deformation. At the N=82 shell closure, Xe136 presents all characteristics of a doubly closed shell nucleus with a spherical shape. In the very neutron-deficient isotopes close to N=50, the α-decay chain of Xe was investigated to probe the radioactive decay properties near the drip-line and the magicity of Sn100. Additionally, the Xe isotopes present higher order symmetries in the nuclear deformation such as the octupole degree of freedom near N=60 and N=90 or O(6) symmetry in stable isotopes. Purpose: The relevance of the O(6) symmetry has been investigated by measuring the spectroscopic quadrupole moment of the first excited states in Xe124. In the O(6) symmetry limit, the spectroscopic quadrupole moment of collective states is expected to be null. Method: A stable Xe124 beam with energies of 4.03A MeV and 4.11A MeV was used to bombard a natW target at the GANIL facility. Excited states were populated via the safe Coulomb excitation reaction. The collision of the heavy ions with a large Z at low energy make this reaction sensitive to the diagonal E2 matrix element of the excited states. The recoils were detected in the VAMOS++ magnetic spectrometer and the γ rays in the AGATA tracking array. The least squares fitting code gosia was used for the analysis to extract both E2 and M1 transitional and E2 diagonal matrix elements. Results: The rotational ground state band was populated up to the 81+ state as well as the 22+ and 42+ states. Using high precision spectroscopic data to constrain the gosia fit, the spectroscopic quadrupole moments of the 21+, 41+, and 61+ states were determined for the first time. Conclusions: The spectroscopic quadrupole moments were found to be negative, large, and constant in the ground state band underlining the prolate axially deformed ground state band of Xe124. The present experimental data confirm that the O(6) symmetry is substantially broken in Xe124.1 MoreReceived 21 October 2022Accepted 23 January 2023DOI:https://doi.org/10.1103/PhysRevC.107.014324©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCollective levelsElectromagnetic transitionsSpectroscopic factors & electromagnetic momentsProperties90 ≤ A ≤ 149Nuclear Physics

Reorientation-effect measurement of the 〈21+∥Ê2∥21+〉 matrix element in Ar36

The spectroscopic quadrupole moment of the first excited 21+ state, QS(21+), at 1.970 MeV in Ar36 was determined at energies well below the Coulomb barrier—where nuclear interference effects are negligible—using the Pt194(Ar36,Ar*36)Pt*194 Coulomb-excitation reaction at 134.2 MeV. Particle-gamma coincidence data were collected using the AFRODITE array—composed of eight high-purity germanium clover detectors—and an upstream double-sided silicon detector at iThemba LABS. A large diagonal matrix element of 〈21+∥E2̂∥21+〉=0.163(42) eb was determined, which yields a more accurate value of QS(21+)=+0.12(3) eb as compared with previous work, QS(21+)=+0.11(6) eb, in agreement with modern beyond mean-field and large-scale shell-model calculations. This value is consistent with the ratio of electric quadrupole moments found for other A=4n self-conjugate nuclei extracted from the reorientation effect and the rotor model, which are surprisingly equivalent to those observed in good rotors in the mass A≈160–180 region.Received 22 September 2021Accepted 28 November 2021DOI:https://doi.org/10.1103/PhysRevC.104.L061305©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCollective levelsCollective modelsElectromagnetic transitionsHydrodynamic modelsNuclear structure & decaysPhotonuclear reactionsShell modelSpectroscopic factors & electromagnetic momentsProperties20 ≤ A ≤ 38TechniquesRadiation detectorsNuclear Physics

Performance of the OEDO beamline

The OEDO (Optimized Energy Degrading Optics for RI beam) beamline has been developed to obtain low-energy (10 – 50 MeV/u) RI beams by slowing down the secondary beams at RIBF, RIKEN. Such beams are useful probes to investigate nuclear structure by means of nucleon transfer and Coulomb excitation reactions. The beamline is designed to produce a well-focused beam of small energy spread with the help of the angle-tuneable wedge degrader and the radio-frequency deflector. The commissioning and first physics experiments using the OEDO beamline were carried out in 2017. OEDO successfully provided the energy-degraded beams of Se, Zr, and Pd. Its performance including the optics and transmission was evaluated using those experimental data and it functions properly as designed. We expect that our novel beamline shed a light on the experimental approaches involving nuclear reactions at several tens of MeV/u to expand our knowledge of nuclei far beyond the line of stability.

Effect of nuclear surface diffuseness on Coulomb excitation and total nuclear reaction cross sections

We investigate the effect of variation of nuclear surface diffuseness, through diffuseness parameter, on the evaluation of survival probability, absorption effects, Coulomb excitation cross section and total nuclear reaction cross section of Kr+80197Au system. The diffuseness parameter is found to be affecting all the above mentioned quantities appreciably. Specifically, the survival probability and Coulomb excitation cross section are found to be decreasing with increase in a while absorption effects and σR are found to be increasing.

Nuclear isovector valence-shell excitation of Hg202

Excited states of $^{202}\mathrm{Hg}$ have been studied via the $^{12}\mathrm{C}(^{202}\mathrm{Hg},^{202}\mathrm{Hg}^{*})$ Coulomb excitation reaction at a beam energy of 890 MeV. The $\ensuremath{\gamma}$-ray transitions from the excited states of $^{202}\mathrm{Hg}$ were detected by the Gammasphere array. The intensities of the observed $\ensuremath{\gamma}$ rays determined the relative populations of the excited states which were used to extract the absolute $M1$ and $E2$ transition strength distributions for excited ${2}^{+}$ states of $^{202}\mathrm{Hg}$ up to 2 MeV. The measured absolute $B(M1;{2}_{7}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})$ strength of $0.18(8){\ensuremath{\mu}}_{\mathrm{N}}^{2}$ indicates that the ${2}_{7}^{+}$ level of $^{202}\mathrm{Hg}$ is the main fragment of the proton-neutron mixed-symmetry ${2}_{1,\mathrm{ms}}^{+}$ state. Upper limits for the $F$-spin mixing matrix elements of ${}^{202,204}\mathrm{Hg}$ are determined as well.

Nuclear-Structure Physics with MINIBALL at HIE-ISOLDE

The MINIBALL spectrometer utilizes successfully a variety of post-accelerated radioactive ion beams provided by the new HIE-ISOLDE accelerator at CERN. In-beam γ-ray spectroscopy after Coulomb excitation (CE) or transfer reactions is performed with optimized setups of ancillary detectors for particle detection. The physics program covers a wide range of shell model investigations. Exotic heavy ion beams will enable unique studies of collective properties up to the actinide region. First data taking with HIE-ISOLDE beams started recently. The higher energies and intensities of the new post-accelerator provides a promising perspective for a new generation of MINIBALL experiments. Intriguing first results were obtained by employing beams of 74,76,78Zn, 110,132Sn, 144Xe with beam energies in the range of 4.0 - 5.5 MeV/u for CE experiments at ‘safe’ energies. In all cases first results for various B(Eλ) values for these isotopes were obtained.

Isomeric Level of the 229Th Nucleus and Its Population in the Coulomb Excitation Reaction

Electromagnetical properties of the deformed neutron-odd nucleus of 229Th are considered on the basis of a unified nuclear model. Particular attention is given to the properties of the low-lying isomeric 3/2+ state. The possibility of its population in the Coulomb excitation reaction is studied.

Populating Low-spin States in Radioactive Nuclei to Measure Magnetic Moments Using the Transient Field Technique

The experimental study of magnetic moments for nuclear states near the ground state, $I \ge 2$, provides a powerful tool to test nuclear structure models. The study of magnetic moments in nuclei far away from the stability line is the next frontier in such studies. Two techniques have been utilized to populated low-spin states in radioactive nuclei: coulomb excitation reactions using radioactive nuclei, and the transfer of $\alpha$ particles to stable beams to populate low spin states in radioactive nuclei. A presentations of these two techniques, along with the experimental challenges presented for future uses with nuclei far away from the stability line, will be presented.

Search for mixed-symmetry states of nuclei in the vicinity of the double-magic nucleus208Pb

In this work we present the results from two experiments dedicated to search for quadrupolecollective isovector valence-shell excitation, the states with so-called mixed proton-neutron symmetry (MSS), in nuclei around the doubly magic nucleus 208Pb. 212Po was studied in an α-transfer reaction. 204Hg was studied in an inverse kinematics Coulomb excitation reaction on a carbon target. Both experiments provide indications for existence of one-phonon MSSs. Those are the first experimentally identified MSSs in the mass A ≈ 208 region.

Development of a measurement system for the determination of ([formula omitted]) cross-sections using multi-nucleon transfer reactions

We have installed a new experimental apparatus to measure γ-rays from highly excited states populated by the multi-nucleon transfer reactions with heavy-ion projectiles to determine the (n, γ) cross-sections by means of the surrogate reaction method. An apparatus consists of two anti-Compton LaBr3(Ce) spectrometers to measure the γ-rays and a Si ΔE–E detector system to detect outgoing projectile-like particles. Reactions of 153-MeV 18O beams with 155Gd and 157Gd targets were used to study the performance of apparatus. By using the LaBr3(Ce) scintillators with relatively large volume (101.6mm in diameter and 127mm in length), we have successfully measured γ-rays from the compound nuclei, which have excitation energy above neutron separation energy, populated by 155Gd(18O, 16O)157Gd and 157Gd(18O, 16O)159Gd two-neutron transfer reactions. To measure in-beam γ-rays through heavy-ion-induced transfer reaction, it is important to assign the reaction channel clearly, since the cross-sections of the transfer reactions are much smaller than those of competing reactions such as Coulomb excitation and fusion reactions. The Si ΔE–E detector system was used to separate outgoing particles. The present study has demonstrated high capability of apparatus to measure the de-excitation γ-rays in the compound nuclei produced by the multi-nucleon transfer reactions for determination of the (n, γ) cross-sections by using the surrogate reaction method.

The Miniball spectrometer

The Miniball germanium detector array has been operational at the REX (Radioactive ion beam EXperiment) post accelerator at the Isotope Separator On-Line facility ISOLDE at CERN since 2001. During the last decade, a series of successful Coulomb excitation and transfer reaction studies have been performed with this array, utilizing the unique and high-quality radioactive ion beams which are available at ISOLDE. In this article, an overview is given of the technical details of the full Miniball setup, including a description of the \(\gamma\)-ray and particle detectors, beam monitoring devices and methods to deal with beam contamination. The specific timing properties of the REX-ISOLDE facility are highlighted to indicate the sensitivity that can be achieved with the full Miniball setup. The article is finalized with a summary of some physics highlights at REX-ISOLDE and the utilization of the Miniball germanium detectors at other facilities.

Absoluteβ-to-ground band transition strengths in154Sm

Low-lying collective states of ${}^{154}$Sm are studied via the ${}^{12}$C(${}^{154}$Sm,${}^{154}$Sm${}^{*}$) Coulomb excitation reaction at 85$%$ of the Coulomb barrier (570 MeV) using the Gammasphere Ge-detector array. Absolute transition strengths are obtained from the Coulomb excitation cross sections deduced from the relative $\ensuremath{\gamma}$-ray yields. The results include transition strengths for $E2$ decays of energy levels in the low-spin part of the first excited ${K}^{\ensuremath{\pi}}={0}^{+}$ rotational band of ${}^{154}$Sm. The agreement of these results with the confined $\ensuremath{\beta}$-soft rotor model predictions establishes the assignment of this band as a $\ensuremath{\beta}$ band.

Projectile Coulomb Excitation of the nucleus 194Pt

Low-lying collective excited states of 194Pt have been studied via the 12C(194Pt,194Pt*) projectile Coulomb excitation reaction at 85% of the Coulomb barrier (850 MeV) using the Gammasphere Ge-detector array at Argonne National Laboratory. Absolute E2 transition strengths have been obtained from the Coulex cross sections that were deduced from the relative γ-ray yields. They are discussed with respect to the structure suggested by the O(6) symmetry of the Interacting Boson Model.

Measurement of the96Rug(41+)factor and its nuclear structure interpretation

Background: The experimental study of $g(Ig2)$ factors of nuclear states can provide information about the evolution of collectivity in certain regions of the nuclear chart, and assist in obtaining a microscopic description of the nuclear wave functions. The measurements and explanations of $g(Ig2)$ factors are still a challenge for experiments and theory.Purpose: Measurement of the $g({2}_{1}^{+})$ and $g({4}_{1}^{+})$ factors, the latter for the first time, in the ${\phantom{\rule{0.16em}{0ex}}}_{44}^{96}$Ru nucleus. Comparison of the experimental results with calculations using the shell model and collective models.Methods: The experiments made use of the transient field technique, using a Coulomb-excitation reaction in inverse kinematics. Large scale shell model calculations were performed; comparisons with previous theoretical predictions, using the tidal-wave model and the hydrodynamical model, were carried out.Results: The values of $g({2}_{1}^{+})=+0.46(2)$ and $g({4}_{1}^{+})=+0.58(8)$ were experimentally obtained. While the $g({2}_{1}^{+})$ value agrees with the hydrodynamical model prediction of $g=Z/A=+0.46$, the $g({4}_{1}^{+})$ is in agreement with the shell model predictions. The trend of the experimental $g$ factors, as a function of nuclear spin, is not reproduced by the theoretical models discussed.Conclusions: Measurements of $g({2}_{1}^{+})$ and $g({4}_{1}^{+})$ in ${}^{96}$Ru were performed. Further theoretical efforts are necessary to explain the trend of the $g$ factors as a function of nuclear spin for the ${}^{96}$Ru nucleus. Future measurements of $g({4}_{1}^{+})$ should reduce the uncertainty of the result.

O(6)-symmetry breaking in theγ-soft nucleusXe126and its evolution in the light stable xenon isotopes

Low-lying collective states in $^{126}\mathrm{Xe}$ have been investigated via the $^{12}\mathrm{C}$($^{126}\mathrm{Xe}$,$^{126}\mathrm{Xe}$${}^{*}$) projectile Coulomb excitation reaction at 399 MeV. The $\ensuremath{\gamma}$ decays were detected with the Gammasphere array. Coulomb excitation cross sections relative to the ${2}_{1}^{+}$ state were obtained. Twenty-two absolute $E2$ transition strengths have been deduced. An $\mathit{sd}$-- interacting boson model (IBM-1) fit agrees well with the new experimental data. This makes a quantitative test of O(6)-symmetry breaking in $^{126}\mathrm{Xe}$ possible. The measured absolute $B(E2)$ values indicate a preservation of O(5) symmetry, while the O(6) symmetry is broken. The evolution of O(6)-symmetry breaking and of O(5)-symmetry conservation in the $^{124,126,128}\mathrm{Xe}$ isotopic chain is discussed.

Enhanced Quadrupole Collectivity atN=40: The Case of Neutron-Rich Fe Isotopes

The transition rates for the 2(1)+ states in (62,64,66)Fe 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 using 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 trend of collectivity suggested by the experimental data is described by state-of-the-art shell-model calculations with a new effective interaction developed for the fpgd valence space.

The MSU EBIT at NSCL

The National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU) is constructing ReA3 to reaccelerate rare-isotope beams available from gas stopping of fast radio-nuclides produced by projectile fragmentation to energies of ∼ 0.3-3 MeV/nucleon. Such beams are unavailable worldwide, but are in demand for Coulomb excitation and transfer reaction studies as well as for the study of astrophysical reactions. ReA3 consists of four main components: an electron-beam ion trap (EBIT), an achromatic mass separator, a radio-frequency quadrupole (RFQ) pre-accelerator, and a superconducting radio-frequency linear accelerator (SRF-LINAC). By increasing the charge of ions injected into the RFQ and SRF-LINAC, the EBIT charge breeder is a key component to provide a compact and cost-efficient reaccelerator for short-lived isotopes. The MSU EBIT will be equipped with an electron-gun cathode yielding a few amperes. A unique EBIS-EBIT hybrid magnet configuration, composed of Helmholtz coils and a solenoid, will provide a maximum magnetic field strength of 6 T. The combination of a high-current cathode and strong magnetic field will allow the MSU EBIT to reach high electron-beam current densities of 104 A/cm2. This will make it well suited to rapidly increase the charge state of short-lived isotopes within tens of milliseconds or less. The unique extended magnet configuration will guarantee high-beam acceptance. This publication will present an overview and status of the MSU EBIT.

Robust test of E(5) symmetry inXe128

Low-lying collectively excited states of {sup 128}Xe were investigated by {gamma}-ray spectroscopy following the {sup 12}C({sup 128}Xe,{sup 128}Xe*){sup 12}C projectile Coulomb excitation reaction. Nineteen absolute E2 transition strengths were obtained including the first measurement of the critical B(E2) decays from the second and third J{sup TT} = 0{sup +} states. These data are compared with the theoretical predictions of the critical point symmetry E(5) and allow us to conclude that {sup 128}Xe is not an E(5) nucleus as previously suggested, leaving {sup 130}Xe as the most likely candidate among the Xe isotopes.

How close to the [formula omitted] symmetry is the nucleus 124Xe?

Excited states in 124Xe have been studied via the 12C( 124Xe, 124Xe ∗) Coulomb excitation reaction. Their population cross-sections relative to the 2 1 + state have been determined from the γ-ray yields observed with Gammasphere. More than twenty absolute E2 strengths for seven off-yrast, low-spin states of 124Xe have been deduced for the first time. The absolute B ( E 2 ) values indicate pronounced O ( 5 ) symmetry, even for the off-yrast states with high O ( 5 ) quantum number τ, while the O ( 6 ) symmetry is substantially broken.