Coulomb Excitation Cross Sections Research Articles

Multistep Coulomb excitation of Ni64 : Shape coexistence and nature of low-spin excitations

The structure of $^{64}\mathrm{Ni}$, 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 $^{66}\mathrm{Ni}$ and $^{70}\mathrm{Ni}$ as well as in doubly magic, $N=40, ^{68}\mathrm{Ni}$. 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 $^{62,64}\mathrm{Ni}$ 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 $^{64}\mathrm{Ni}$ 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.

Investigation of simultaneous variation of surface diffuseness and central depression in the estimation of absorption effects

In this paper, we investigate the effect of simultaneous variation of surface diffuseness and central depression, in case of model-dependent Fermi matter density distributions, on the estimation of survival probability, Coulomb excitation cross-section and absorption effects for [Formula: see text] Au system. Further, the role of model-dependent Fermi matter density distribution and model independent [Formula: see text] matter density distribution has also been investigated in the estimation of the above-mentioned quantities. The simultaneous variation of the parameters of the model-dependent density distribution and choice of the density distribution are found to be affecting all the above-mentioned quantities significantly and hence plays a crucial role in the analysis of the Coulomb excitation experiments also.

Role of central depression in the estimation of Coulomb excitation cross-section and absorption effects

We have investigated the role of central depression parameter on the estimation of survival probability, the Coulomb excitation cross-sections and absorption effects of [Formula: see text] system. The variation in central depression is found to be affecting all the above-mentioned quantities significantly. The survival probability and the Coulomb excitation cross-section are found to be decreasing with increase in [Formula: see text] while the absorption effects are found to be increasing.

Comment on “From Coulomb excitation cross sections to nonresonant astrophysical rates in three-body systems: The Ne17 case”

Parfenova et al. state in [Phys. Rev. C 98, 034608 (2018)] that the results presented by Casal et al. [Phys. Rev. C 94, 054622 (2016)] concerning the radiative capture for $^{17}\mathrm{Ne}$ formation are incorrect and their conclusions erroneous mainly because of two reasons: (1) it is ``expressed'' that the resonant rate is not important for $^{15}\mathrm{O}(2p,\ensuremath{\gamma})^{17}\mathrm{Ne}$ since it is negligibly small compared with the nonresonant contribution to the rate, and (2) the electromagnetic dipole cross section predicted is dramatically different from the available experimental data for $^{17}\mathrm{Ne}+^{208}\mathrm{Pb}$ at 500 MeV/u [Phys. Lett. B 759, 200 (2016)]. We demonstrate here that these conclusions are incorrectly extracted from our work.

Measurement of the 21+ level lifetime in Sn120 by the Doppler shift attenuation method: Evidence of enhanced collectivity

Background: There has been a considerable interest focused on the study of enhancement or suppression in collectivity of the excited ${2}_{1}^{+}$ states in stable Sn isotopes. Independent measurements of Coulomb excitation cross sections and ${2}_{1}^{+}$ level lifetimes report discrepant transition probabilities. Existing estimates for ${2}_{1}^{+}$ lifetime indicate reduced collectivity.Purpose: A reexamination of lifetime of the ${2}_{1}^{+}$ state in the most abundant $^{120}\mathrm{Sn}$ isotope is thus warranted. The same has been carried out in the present work and the result has been used to determine the transition probability as an indicative of the underlying collectivity.Methods: Low-lying levels in the vibrational $^{120}\mathrm{Sn}$ nucleus have been excited by inelastic scattering with $^{32}\mathrm{S}$ beam at ${E}_{\mathrm{lab}}=120$ MeV. Level lifetime measurements have been carried out using the Doppler shift attenuation method, wherein the Doppler affected $\ensuremath{\gamma}$-ray peaks have been analyzed using updated methodologies.Results: From the measured lifetime of the ${2}_{1}^{+}$ state (${E}_{x}=1171$ keV) in $^{120}\mathrm{Sn}, {\ensuremath{\tau}}_{{2}_{1}^{+}}=0.{863}_{\ensuremath{-}0.036}^{+0.029}$ ps, a value of $B(E2;{0}_{\mathrm{g}.\mathrm{s}.}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})=0.{215}_{+0.009}^{\ensuremath{-}0.007}$ is deduced. An estimate of the ${4}_{1}^{+}$ (feeder) level lifetime, ${\ensuremath{\tau}}_{{4}_{1}^{+}}=1.{77}_{\ensuremath{-}0.089}^{+0.084}$ ps, is also reported from lineshape analysis of $\ensuremath{\gamma}$ rays in cascade.Conclusions: An enhancement in collectivity for the ${2}_{1}^{+}$ state is confirmed, following an improved determination of the level lifetime, with reduced uncertainties. The excited ${2}_{1}^{+}$ state is also found to have a nonvanishing moment of inertia, suggesting anharmonic nature of quadrupole vibrations.

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.

First measurement of the B(E2;3/2−→1/2− ) transition strength in Be7 : Testing abinitio predictions for A=7 nuclei

Electromagnetic observables are able to give insight into collective and emergent features in nuclei, including nuclear clustering. These observables also provide strong constraints for ab initio theory, but comparison of these observables between theory and experiment can be difficult due to the lack of convergence for relevant calculated values, such as $E2$ transition strengths. By comparing the ratios of $E2$ transition strengths for mirror transitions, we find that a wide range of ab initio calculations give robust and consistent predictions for this ratio. To experimentally test the validity of these ab initio predictions, we performed a Coulomb excitation experiment to measure the $B(E2; 3/2^- \rightarrow 1/2^-)$ transition strength in $^7$Be for the first time. A $B(E2; 3/2^- \rightarrow 1/2^-)$ value of $26(6)(3) \, e^2 \mathrm{fm}^4$ was deduced from the measured Coulomb excitation cross section. This result is used with the experimentally known $^7$Li $B(E2; 3/2^- \rightarrow 1/2^-)$ value to provide an experimental ratio to compare with the ab initio predictions. Our experimental value is consistent with the theoretical ratios within $1 \sigma$ uncertainty, giving experimental support for the value of these ratios. Further work in both theory and experiment can give insight into the robustness of these ratios and their physical meaning.

Isospin symmetry in B(E2) values: Coulomb excitation study of Mg21

The $T_z$~=~$-\frac{3}{2}$ nucleus ${}^{21}$Mg has been studied by Coulomb excitation on ${}^{196}$Pt and ${}^{110}$Pd targets. A 205.6(1)-keV $\gamma$-ray transition resulting from the Coulomb excitation of the $\frac{5}{2}^+$ 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 $B(E2;\frac{5}{2}^+\rightarrow\frac{1}{2}^+)$~=~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 large difference of the $B(E2;\frac{5}{2}^+\rightarrow\frac{1}{2}^+)$ values between $T$~=~$\frac{3}{2}$, $A$~=~21 mirror nuclei. The difference is investigated in the shell-model framework employing both isospin conserving and breaking USD interactions and using modern \textsl{ab initio} nuclear structure calculations, which have recently become applicable in the $sd$ shell.

From Coulomb excitation cross sections to nonresonant astrophysical rates in three-body systems: The Ne17 case

Coulomb and nuclear dissociation of $^{17}$Ne on light and heavy targets are studied theoretically. The dipole E1 strength function is determined in a broad energy range including energies of astrophysical interest. Dependence of the strength function on different parameters of the $^{17}$Ne ground state structure and continuum dynamics is analyzed in a three-body model. The discovered dependence plays an important role for studies of the strength functions for the three-body E1 dissociation and radiative capture. The constraints on the $[s^2]/[d^2]$ configuration mixing in $^{17}$Ne and on $p$-wave interaction in the $^{15}$O+$p$ channel are imposed based on experimental data for $^{17}$Ne Coulomb dissociation on heavy target.

First measurement with a new setup for low-energy Coulomb excitation studies at INFN LNL

A new segmented particle detector, SPIDER, has been designed to be used as an ancillary device with the GALILEO γ-ray spectrometer, as well as with other multi-detector γ-ray arrays that will be available at LNL in the future (e.g. AGATA). To commission the SPIDER-GALILEO experimental setup, a multi-step Coulomb excitation experiment was carried out with a 240 MeV beam of 66Zn produced by the Tandem-XTU accelerator at INFN Laboratori Nazionali di Legnaro. The measured particle and γ-ray spectra are compared with the results of detailed GEANT4 simulations which used the Coulomb excitation cross sections, estimated with the computer code GOSIA, as an input. The preliminary results indicate that precise transition probabilities will be obtained which are essential for solving discrepancies reported in the literature for this nucleus.

Low-energy Coulomb excitation of Sr96,98 beams

The structure of neutron-rich 96,98Sr nuclei was investigated by low-energy safe Coulomb excitation of radioactive beams at the REX-ISOLDE facility, CERN, with the MINIBALL spectrometer. A rich set of transitional and diagonal E2 matrix elements, including those for non-yrast structures, has been extracted from the differential Coulomb-excitation cross sections. The results support the scenario of a shape transition at N = 60, giving rise to the coexistence of a highly deformed prolate and a spherical configuration in 98Sr, and are compared to predictions from several theoretical calculations. The experimental data suggest a significant contribution of the triaxal degree of freedom in the ground state of both isotopes. In addition, experimental information on low-lying states in 98Rb has been obtained.

Structure of low-lying states inSm140studied by Coulomb excitation

The electromagnetic structure of $^{140}$Sm was studied in a low-energy Coulomb excitation experiment with a radioactive ion beam from the REX-ISOLDE facility at CERN. The $2^+$ and $4^+$ states of the ground-state band and a second $2^+$ state were populated by multi-step excitation. The analysis of the differential Coulomb excitation cross sections yielded reduced transition probabilities between all observed states and the spectroscopic quadrupole moment for the $2_1^+$ state. The experimental results are compared to large-scale shell model calculations and beyond-mean-field calculations based on the Gogny D1S interaction with a five-dimensional collective Hamiltonian formalism. Simpler geometric and algebraic models are also employed to interpret the experimental data. The results indicate that $^{140}$Sm shows considerable $\gamma$ softness, but in contrast to earlier speculation no signs of shape coexistence at low excitation energy. This work sheds more light on the onset of deformation and collectivity in this mass region.

Highlights of the ISOLDE facility and the HIE-ISOLDE project

The ISOLDE facility is an ISOL-based radioactive beam facility at CERN. It is dedicated to the production and research of nuclei far from stability. Exotic nuclei of variety of chemical elements are available for the study of nuclear structure, nuclear astrophysics, fundamental symmetries and atomic physics, as well as for applications in condensed-matter and life sciences. Since longer than a decade it has offered the largest variety of post-accelerated radioactive beams in the world. In order to broaden the scientific opportunities beyond the present ISOLDE facility, the on-going HIE-ISOLDE (High Intensity and Energy) project will provide major improvements in energy range, beam intensity and beam quality. The first phase will boost the beam energy of the current REX LINAC to 5.5MeV/u resulting in larger cross sections for Coulomb excitation compared to the previous maximum energy of 3MeV/u. Higher energies will also open up many transfer reaction channels. Physics with post-accelerated beams starts in autumn 2015. The second phase of the project is already approved and is expected to be completed in 2018 allowing beam energies up to 10MeV/u for A/q=4.5. In this contribution the present status of the ISOLDE facility including some highlights will be discussed. The HIE-ISOLDE project will be described together with a panorama of the physics cases to be addressed.

Spectroscopic Quadrupole Moments in {96,98}Sr: Evidence for Shape Coexistence in Neutron-Rich Strontium Isotopes at N=60.

Neutron-rich {96,98}Sr isotopes have been investigated by safe Coulomb excitation of radioactive beams at the REX-ISOLDE facility. Reduced transition probabilities and spectroscopic quadrupole moments have been extracted from the differential Coulomb excitation cross sections. These results allow, for the first time, the drawing of definite conclusions about the shape coexistence of highly deformed prolate and spherical configurations. In particular, a very small mixing between the coexisting states is observed, contrary to other mass regions where strong mixing is present. Experimental results have been compared to beyond-mean-field calculations using the Gogny D1S interaction in a five-dimensional collective Hamiltonian formalism, which reproduce the shape change at N=60.

Highlights of the ISOLDE facility and the HIE-ISOLDE Project

The ISOLDE Radioactive Beam Facility is the dedicated CERN installation for the production and acceleration of radioactive nuclei far from stability. Exotic nuclei of most chemical elements are available for the study of nuclear structure, nuclear astrophysics, fundamental symmetries and atomic physics, as well as for applications in condensed-matter and life sciences. Since more than a decade it offers the largest variety of post-accelerated radioactive beams in the world today. In order to broaden the scientific opportunities beyond the reach of the present facility, the on-going HIE-ISOLDE (High Intensity and Energy) project will provide major improvements in energy range, beam intensity and beam quality. Post-accelerated beams will be available already this year boosting the beam energy of the current REX LINAC to 4.3 MeV/u reaching 5.5 MeV/u next spring. In this new energy regime the Coulomb excitation cross sections are strongly increased with respect to the previous energy of bearly 3 MeV/u and many transfer reaction channels will open. The second stage of the energy upgrade will allow energies of the beam up to 10 MeV/u for the worst scenario of A/q = 4.5. The funds are already secured and it is expected to be completed in 2017. In this contribution the present status of the ISOLDE facility will be discussed, some highlights will be briefly described to illustrate the advances of the facility. The HIEISOLDE project will be described together with a panorama of the physics cases to be addressed in the near future with emphasis in the day-one experiment to be done this year.

Highlights of the ISOLDE Facility and the HIE-ISOLDE Project

The ISOLDE Radioactive Beam Facility is the dedicated CERN installation for the production and acceleration of radioactive nuclei far from stability. Exotic nuclei of most chemical elements are available for the study of nuclear structure, nuclear astrophysics, fundamental symmetries and atomic physics, as well as for applications in condensed-matter and life sciences. Since more than a decade it offers the largest variety of post-accelerated radioactive beams in the world today. In order to broaden the scientific opportunities beyond the reach of the present facility, the on-going HIE-ISOLDE (High Intensity and Energy) project will provide major improvements in energy range, beam intensity and beam quality. Post-accelerated beams will be available already this year boosting the beam energy of the current REX LINAC to 4.3 MeV/u reaching 5.5 MeV/u next spring. In this new energy regime the Coulomb excitation cross sections are strongly increased with respect to the previous energy of bearly 3 MeV/u and many transfer reaction channels will open. The second stage of the energy upgrade will allow energies of the beam up to 10 MeV/u for the worst scenario of A/q = 4.5. The funds are already secured and it is expected to be completed in 2017. In this contribution the present status of the ISOLDE facility will be discussed, some highlights will be briefly described to illustrate the advances of the facility. The HIEISOLDE project will be described together with a panorama of the physics cases to be addressed in the near future with emphasis in the day-one experiment to be done this year.

Testing refined shell-model interactions in thesdshell: Coulomb excitation ofNa26

Background: Shell-model calculations crucially depend on the residual interaction used to approximate the nucleon-nucleon interaction. Recent improvements to the empirical universal sd interaction (USD) describing nuclei within the sd shell yielded two new interactions—USDA and USDB—causing changes in the theoretical description of these nuclei. Purpose: Transition matrix elements between excited states provide an excellent probe to examine the underlying shell structure. These observables provide a stringent test for the newly derived interactions. The nucleus Na26 with 7 valence neutrons and 3 valence protons outside the doubly-magic 16O core is used as a test case. Method: A radioactive beam experiment with Na26 (T1/2=1,07s) was performed at the REX-ISOLDE facility (CERN) using Coulomb excitation at safe energies below the Coulomb barrier. Scattered particles were detected with an annular Si detector in coincidence with γ rays observed by the segmented MINIBALL array. Coulomb excitation cross sections of the beam have been obtained by normalization to the well known Coulomb excitation cross sections of the Pd104 target. Results: The observation of three γ-ray transitions in Na26 together with available spectroscopic data allows us to determine E2- and M1-transitional matrix elements. Results are compared to theoretical predictions. Conclusion: The improved theoretical description of Na26 could be validated. Remaining discrepancies between experimental data and theoretical predictions indicate the need for future experiments and possibly further theoretical improvements.Received 28 October 2014Revised 12 December 2014DOI:https://doi.org/10.1103/PhysRevC.91.014311This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical Society

Coulex-multipolarimetry with relativistic heavy-ion beams

We present a method suitable to measure E2/M1 multipole mixing ratios of nuclear ground-state transitions by comparison of relativistic Coulomb-excitation cross-sections at different ion velocities. The observation of the Coulomb-excitation at different ion velocities can be performed with high-resolution γ-ray detectors in one single experiment by using two targets mounted at a few centimeters distance along the beam axis. Excitation in either of the targets is distinguished by different observed Doppler-shifts.

Onset of collectivity in96,98Sr studied via Coulomb excitation

A rapid onset of quadrupole deformation is known to occur around the neu- tron number 60 in the neutron-rich Zr and Sr isotopes. This shape change has made the neutron-rich A = 100 region an active area of experimental and theoretical stud- ies for many decades now. We report in this contribution new experimental results in the neutron rich 96,98 Sr investigated by safe Coulomb excitation of radioactive beams at the REX-ISOLDE facility, CERN. Reduced transition probabilities and spectroscopic quadrupole moments have been extracted from the differential Coulomb excitation cross section supporting the scenario of shape coexistence/change at N=60. Future perspectives are presented including the recent experimental campaign performed at ILL-Grenoble.

Collectivity of light Ge and As isotopes

Background: The self-conjugate nuclei of the $A\ensuremath{\sim}70$ mass region display rapid shape evolution over isotopic or isotonic chains. Shape coexistence has been observed in Se and Kr isotopes reflecting the existence of deformed subshell gaps corresponding to different shell configurations. As and Ge isotopes are located halfway between such deformed nuclei and the $Z=28$ shell closure.Purpose: The present work aims at clarifying the low-lying spectroscopy of ${}^{66}$Ge and ${}^{67}$As, and providing a better insight into the evolution of collectivity in light even-even Ge and even-odd As isotopes.Methods: We investigate the low-lying levels and collectivity of the neutron deficient ${}^{67}$As and ${}^{66}$Ge through intermediate-energy Coulomb excitation, inelastic scattering, and proton knockout measurements. The experiment was performed using a cocktail beam of ${}^{68}$Se, ${}^{67}$As, and ${}^{66}$Ge nuclei at an energy of 70--80 MeV/nucleon. Spectroscopic properties of the low-lying states are compared to those calculated via shell model with the JUN45 interaction and beyond-mean-field calculations with the five-dimensional collective Hamiltonian method implemented using the Gogny D1S interaction. The structure evolution of the lower-mass Ge and As isotopes is discussed.Results: Reduced electric quadrupole transition probabilities $B(E2)$ have been extracted from the Coulomb-excitation cross sections measured in ${}^{66}$Ge and ${}^{67}$As. The value obtained for the $B(E2;{0}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})$ in ${}^{66}$Ge is in agreement with a recent measurement, ruling out the existence of a minimum at $N=34$ in the $B(E2)$ systematics as previously observed. New transitions have been found in ${}^{67}$As and were assigned to the decay of low-lying negative-parity states.