Intermediate-energy Coulomb Excitation Research Articles

The 46Ar(3He, d)47K direct reaction as a probe of the 46Ar proton wavefunction

The discrepancy between shell-model calculations and intermediate-energy Coulomb excitation measurements in 46Ar still stands as an unsolved puzzle in understanding the N = 28 shell evolution. This phenomenon has significant relevance considering the remarkable achievements of the shell model and the SDPF-U interaction in the region which is able to predict the fading of the N = 28 shell gap in neutron-rich 44S. Recent measurements narrowed down this discrepancy to an overestimation of the proton amplitude to the quadrupole transition matrix element. The current work aims to propose a different perspective on the puzzle, by studying a direct proton-transfer reaction on 46Ar as a means to directly probe the proton wavefunction of the ground state this isotope. By measuring the amount of l = 0 transfer to the ground state (1/2+) of 47K with respect to the l = 2 to the first excited state (3/2+), we aim to gain insight into the ground state proton wavefunction of 46Ar. We will present a brief description of the experiment performed at the SPIRAL1 facility in GANIL (France). The experimental apparatus allowed a full reconstruction of the two-body reaction thanks to the combination of AGATA, VAMOS, MUGAST, CATS2, and HECTOR.

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.

Quadrupole collectivity in the neutron-rich sulfur isotopes S38,40,42,44

Electromagnetic transition strengths in the even-even neutron-rich sulfur isotopes $^{38,40,42,44}\mathrm{S}$ were measured using intermediate-energy Coulomb excitation at the National Superconducting Cyclotron Laboratory. By utilizing the sensitivity of the experimental technique to $E2$ excitations from the ground state, the evolution of the pattern of $B(E2)$ strengths to several low-lying ${2}^{+}$ states was investigated at $Z=16$ from near stability to the $N=28$ island of inversion. The experimental results allowed a detailed comparison with predictions from shell-model calculations using the SDPF-MU Hamiltonian, which was designed to describe collectivity in this region of the nuclear chart. While the shell-model calculations succeeded in modeling transition strengths at $N=22,24,26$, the experimental $B(E2;{0}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})$ at $N=28$ was smaller than the predicted value by about a factor of 2, similar to previous observations for chlorine and argon isotopes around $N=28$. The dependence of this overprediction by theory on the choice of effective charges was explored.

Jacque Raynal, ECIS, and $$M_n/M_p$$ values in radioactive nuclei

Jacque Raynal’s coupled channels code ECIS has played an important role in the effort to search for shell effects in exotic isotopes using both intermediate energy Coulomb excitation and inverse kinematics proton scattering to probe their level structures and to extract neutron and proton excitation modes through their $$M_n/M_p$$ values. It will continue to be an important tool as nuclei very far from the stable ones are produced in new exotic beam accelerator centers.

Transition strengths in the neutron-rich Ni73,74,75 isotopes

Reduced transition probabilities have been measured for the neutron-rich Ni73,74,75 nuclei with relativistic Coulomb excitation performed at the RIKEN Nishina Center. For the even-even Ni isotope the determined B(E2;0+→2+) value is compatible within errors with the result of a previous intermediate energy Coulomb excitation experiment, although being somewhat larger. For the odd Ni isotopes B(E2) values have been determined for the first time. In the middle νg9/2 shell nucleus Ni73 three peak candidates have been observed and tentatively assigned to the (5/2+), (13/2+), and (11/2+) excitations. Two peak candidates have been observed in Ni75 and tentatively assigned to the (13/2+) and (11/2+) levels. The measured B(E2) values are in line with the large B(E2) reported in Ni70 and at variance with the small strength measured in Ni72. Excitation energies compare well with the result of large-scale shell-model calculations, while B(E2) values are larger than predicted. This could indicate an increasing contribution of proton excitations across the Z=28 shell closure when approaching Ni78.Received 29 November 2019Accepted 1 July 2020DOI:https://doi.org/10.1103/PhysRevC.102.014323©2020 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsNuclear structure & decaysShell modelProperties59 ≤ A ≤ 89Nuclear Physics

JANUS — A setup for low-energy Coulomb excitation at ReA3

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. In this work, the setup and its performance will be described based on the commissioning run that used stable 78Kr impinging onto a 1.09 mg/cm2208Pb target at a beam energy of 3.9 MeV/u.

Quadrupole collectivity beyond N=50 in neutron-rich Se and Kr isotopes

We report $B(E2;{0}_{1}^{+}\ensuremath{\rightarrow}{2}_{n}^{+})$ strengths for the neutron-rich $^{88,90}\mathrm{Kr}$ and $^{86}\mathrm{Se}$ isotopes from intermediate-energy Coulomb excitation. The electric quadrupole transition strengths to the first ${2}^{+}$ states complete, with considerably improved uncertainties, the evolution of quadrupole collectivity in the Kr and Se isotopes approaching $N=60$, for which $^{90}\mathrm{Kr}$ and $^{86}\mathrm{Se}$ had previously been the most uncertain. We also report significant excitation strength to several higher-lying ${2}^{+}$ states in the krypton isotopes. This fragmentation of $B(E2)$ strength in $^{88,90}\mathrm{Kr}$ confirms shell-model calculations in the $\ensuremath{\pi}(fpg)\text{\ensuremath{-}}\ensuremath{\nu}(sdgh)$ shell with an only minimally tuned shell-model setup that is based on a nucleon-nucleon interaction derived from effective field theory and effective charges adjusted to $^{86}\mathrm{Kr}$.

Doppler-shift attenuation lifetime measurement of the Ar3621+ level

At TRIUMF, the TIGRESS Integrated Plunger device and its suite of ancillary detector systems have been implemented for charged-particle tagging and light-ion identification in coincidence with γ-ray spectroscopy for Doppler-shift lifetime studies and low-energy Coulomb excitation measurements. As a test of the device, the lifetime of the first 2+ excited state in Ar36 was measured from the γ-ray line shape of the 21+→0g.s.+ transition using the Doppler-shift attenuation technique following Coulomb excitation. The line-shape signatures, vital for precision lifetime measurements, were significantly improved by enhanced reaction-channel selectivity using a complementary approach of kinematic gating and digital rise-time discrimination of recoiling charged particles in a silicon PIN diode array. The lifetime was determined by comparisons between the data and simulated line shapes generated using our TIGRESS Coulomb excitation code as an input to the Lindhard method, which was then extended and included as a class in geant4. The model-independent lifetime result of 490±50 fs corresponds to a reduced quadrupole transition strength of B(E2;21+→0g.s.+)=56±6e2fm4 and agrees well with previous intermediate energy Coulomb excitation measurements, thereby resolving reported discrepancies in the 21+ level lifetime in this self-conjugate nucleus.5 MoreReceived 13 April 2017Revised 28 June 2017DOI:https://doi.org/10.1103/PhysRevC.96.024305©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsLifetimes & widthsNuclear structure & decaysProperties20 ≤ A ≤ 38TechniquesRadiation detectorsShell modelNuclear Physics

Optimum forward scattering zone for intermediate-energy Coulomb excitation experiments

Here we present a comparative study of various schemes commonly used for the determination of the safe minimum value of the impact parameter, which decides the maximum value of forward laboratory scattering angle, in intermediate-energy Coulomb excitation experiments. We have found that these are special cases of the recently proposed parameterization scheme in Kumar Rajiv et al., Phys. Rev. C, 81 (2010) 037602. The scheme may be used to demarcate the absorption-free as well as no-flux loss zone for intermediate-energy Coulomb excitation experiments.

Relativistic Coulomb excitation inMg32near 200 MeV/nucleon with a thick target

Intermediate-energy Coulomb excitation of $^{32}\mathrm{Mg}$ using in-beam $\ensuremath{\gamma}$-ray spectroscopy with a relatively thick lead target was studied at a beam energy of 195 MeV/nucleon at the RIKEN Radioactive Isotope Beam Factory (RIBF). The angular distribution of the inelastic scattering was analyzed with distorted-wave calculations taking into account both the Coulomb and the nuclear excitation contributions. A $B(E2)$ value of 432(51) ${e}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{fm}}^{4}$ was extracted, which agrees with most of the previously reported results obtained at several tens of MeV/nucleon. Our investigation demonstrates that the electromagnetic properties of exotic nuclei can be studied with Coulomb excitation at beam energies of a few hundreds of MeV/nucleon, where a thicker target can be used to increase the excitation yields.

Quadrupole transition strength in the (74)Ni nucleus and core polarization effects in the neutron-rich Ni isotopes.

The reduced transition probability B(E2;0(+)→2(+)) has been measured for the neutron-rich nucleus (74)Ni in an intermediate energy Coulomb excitation experiment performed at the National Superconducting Cyclotron Laboratory at Michigan State University. The obtained B(E2;0(+)→2(+))=642(-226)(+216) e(2) fm(4) value defines a trend which is unexpectedly small if referred to (70)Ni and to a previous indirect determination of the transition strength in (74)Ni. This indicates a reduced polarization of the Z=28 core by the valence neutrons. Calculations in the pfgd model space reproduce well the experimental result indicating that the B(E2) strength predominantly corresponds to neutron excitations. The ratio of the neutron and proton multipole matrix elements supports such an interpretation.

Precision measurement of the electromagnetic dipole strengths in [formula omitted

The electromagnetic dipole strength in Be11 between the bound states has been measured using low-energy projectile Coulomb excitation at bombarding energies of 1.73 and 2.09 MeV/nucleon on a Pt196 target. An electric dipole transition probability B(E1;1/2−→1/2+)=0.102(2) e2fm2 was determined using the semi-classical code Gosia, and a value of 0.098(4) e2fm2 was determined using the Extended Continuum Discretized Coupled Channels method with the quantum mechanical code FRESCO. These extracted B(E1) values are consistent with the average value determined by a model-dependent analysis of intermediate energy Coulomb excitation measurements and are approximately 14% lower than that determined by a lifetime measurement. The much-improved precisions of 2% and 4% in the measured B(E1) values between the bound states deduced using Gosia and the Extended Continuum Discretized Coupled Channels method, respectively, compared to the previous accuracy of ∼10% will help in our understanding of and better improve the realistic inter-nucleon interactions.

Evolution of collectivity in the78Ni region: Coulomb excitation of74Ni at intermediate energies.

The study of the collective properties of nuclear excitations far from stability provides information about the shell structure at extreme conditions. Spectroscopic observables such as the energy or the transition probabilities of the lowest states, in nuclei with large neutron excess, allow to probe the density and isospin dependence of the effective interaction. Indeed, it was recently shown that tensor and three-body forces play an important role in breaking and creating magic numbers. Emblematic is the case of the evolution of the Ni isotopic chain where several features showed up moving from the most neutron rich stable isotope (64Ni) towards the 78Ni nucleus where the large neutron excess coincides with a double shell closure. In this framework, we have recently performed an experiment with the goal to extract the B(E2; 0+ → 2+) value for the 74Ni nucleus in an intermediate-energy Coulomb excitation experiment: preliminary results are discussed.

Quadrupole collectivity in neutron-deficient Sn nuclei:104Sn and the role of proton excitations

We report on the experimental study of quadrupole collectivity in the neutron-deficient nucleus \nuc{104}{Sn} using intermediate-energy Coulomb excitation. The $B(E2; 0^+_1 \rightarrow 2^+_1)$ value for the excitation of the first $2^+$ state in \nuc{104}{Sn} has been measured to be $0.180(37)~e^2$b$^2$ relative to the well-known $B(E2)$ value of \nuc{102}{Cd}. This result disagrees by more than one sigma with a recently published measurement \cite{Gua13}. Our result indicates that the most modern many-body calculations remain unable to describe the enhanced collectivity below mid-shell in Sn approaching $N=Z=50$. We attribute the enhanced collectivity to proton particle-hole configurations beyond the necessarily limited shell-model spaces and suggest the asymmetry of the $B(E2)$-value trend around mid-shell to originate from enhanced proton excitations across $Z=50$ as $N=Z$ is approached.

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.

Quadrupole collectivity in neutron-rich Fe and Cr isotopes.

Intermediate-energy Coulomb excitation measurements are performed on the N ≥ 40 neutron-rich nuclei (66,68)Fe and (64)Cr. The reduced transition matrix elements providing a direct measure of the quadrupole collectivity B(E2;2(1)(+) → 0(1)(+)) are determined for the first time in (68)Fe(42) and (64)Cr(40) and confirm a previous recoil distance method lifetime measurement in (66)Fe(40). The results are compared to state-of-the-art large-scale shell-model calculations within the full fpgd neutron orbital model space using the Lenzi-Nowacki-Poves-Sieja effective interaction and confirm the results of the calculations that show these nuclei are well deformed.

Study of the neutron rich sulfure isotope 43S through intermediate energy Coulomb excitation

The reduced transition probability B(E2: 3/2− 7/2−2) has been measured in 43S using Coulomb excitation at intermediate energy. The nucleus of interest was produced by fragmentation of a 48Ca beam at GANIL. The reaction products were separated in the LISE spectrometer. After Coulomb-excitation of 43S in a 208Pb target, the γ rays emitted inflight were detected by 64 BaF2 detectors of the Chǎteau de Cristal array. The preliminary value deduced for the reduced transition probability B(E2: 3/2−7/2−2) is in agreement with the predictions of the shell model calculations and supports a prolate-spherical shape coexistence in the 43S nucleus.

Probing core polarization around78Ni: intermediate energy Coulomb excitation of74Ni

The study of the evolution of nuclear shells far from stability provides fundamental information about the shape and symmetry of the nuclear mean field. Nuclei with large neutron/proton ratio allow to probe the density dependence of the effective interaction. Indeed, it was recently shown that tensor and three-body forces play an important role in breaking and creating magic numbers. Of particular interest is the region of 78 Ni where the large neutron excess coincides with a double shell closure.We have recently measured the B(E 2; 0+ → 2+ ) of the 74 Ni nucleus in an intermediate-energy Coulomb excitation experiment performed at the National Superconducting Cyclotron Laboratory of the Michigan State University. The 74 Ni secondary beam has been produced by fragmentation of 86 Kr at 140 AMeV on a thick Be target. Selected radioactive fragments impinged on a secondary 197 Au target where the measurement of the emitted γ -rays allows to extract the Coulomb excitation cross section and related structure information. Preliminary B(E2) values do not point towards an enhancement of the transition matrix element and the comparison to what was already measured by Aoi and co-workers in [1] opens new scenarios in the interpretation of the shell evolution of the Z=28 isotopes.

Intermediate-energy Coulomb excitation of58,60,62Cr: The onset of collectivity towardN=40

Intermediate-energy Coulomb excitation measurements were performed on the neutron-rich isotopes ${}^{58,60,62}$Cr. The electric quadrupole excitation strengths, $B$($E$2; 0${}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+}$), of ${}^{60,62}$Cr are determined for the first time. The results quantify the trend of increasing quadrupole collectivity in the Cr isotopes approaching neutron number $N=40$. The results are confronted with large-scale shell-model calculations in the $fpgd$ shell using the state-of-the-art LNPS effective interaction. Different sets of effective charges are discussed that provide an improved and robust description of the $B(E2)$ values of the neutron-rich Fe and Cr isotopes in this region of rapid shell evolution. The ratio of the neutron and proton transition matrix elements, $|{M}_{n}/{M}_{p}|$, is proposed as an effective tool to discriminate between the various choices of effective charges.

Quadrupole Collectivity beyondN=28: Intermediate-Energy Coulomb Excitation ofAr47,48

We report on the first experimental study of quadrupole collectivity in the very neutron-rich nuclei (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-Uand EPQQM effective interactions. The comparison between experiment and theory indicates that a shell-model description of Ar isotopes around N=28 remains a challenge.