Coulomb Excitation Measurements Research Articles

Exploring shape coexistence in A < 190 nuclei through EM transition rate measurements

The nuclei having mass A ~ 190 exhibit some interesting structural phenomenon like shape coexistence and shape transition. The present work reports on some of the experimental evidence of shape coexistence in this mass region for some of the nuclei. To establish experimentally the shape of a nucleus and hence the presence of shape coexistence demands use of experimental tools such as coulomb excitation or lifetime measurements. Various possible measurable quantities and associated phenomena are discussed, together with details on experiments performed. Experiments performed to populate different isotopes in Tl, Pt and Hg nuclei. Several theoretical and experimental calculations suggest the presence of shape coexistence in different isotopes of Tl, Pt and Hg nuclei.

Proton inelastic scattering on 68,70,72Ni

The proton inelastic scattering on 68,70,72Ni isotopes was measured at the NSCL at MSU, employing the S800 spectrometer coupled to the GRETINA γ-ray array. The aim of the experiment was to determine the degree of collectivity in these neutron-rich Z = 28 isotopes. The use of a hadronic probe allows to complement previous Coulomb excitation measurements of the reduced transition probability B(E2; 0+ → 2+) and deduce the neutron-to-proton transition matrix elements ratio. The high resolution in γ-ray energy achievable with GRETINA gives large control on feeding transitions, thus reducing possible systematics errors in the determination of transition strengths.

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.

Advances in nuclear structure via charged particle reactions with AGATA

In recent decades, gamma -ray spectroscopy has undergone a major technological leap forward, namely the technique of gamma -ray tracking, and has attained a sensitivity that is two orders of magnitude larger than that provided by the former generation of Compton-shielded arrays. Indeed the gain is comparable with the achievements since the dawn of gamma -ray spectroscopy. Such sensitivity can be further heightened by coupling gamma -ray spectrometers to other detectors that record complementary reaction products such as light-charged particles for transfer reactions and scattered ions for Coulomb excitation measurements. Nucleon transfer reactions offer an excellent mean to probe the energies of shell model single-particle orbitals and to study migration in energy of these orbitals as we venture away from stability. Such measurements can also estimate the cross sections of processes relevant to stellar evolution and nucleosynthesis. The measurement of gamma rays in coincidence with particles provides also information on the decay channel for unbound systems, which constitutes a useful input for astrophysics and nuclear structure near the drip-lines. Coulomb-excitation studies make it possible to infer collective structure in nuclei and to extract deformation properties of, in particular, open-shell systems. Here, selected examples will be presented, highlighting the power of these types of experiments when gamma -ray observation is included. The development of the experimental methods is reviewed, showing the results achieved before the advent of gamma -ray tracking. Examples of more recent experiments that have successfully exploited gamma -ray tracking with AGATA are then presented as showcases for the outstanding performance of the composite detection systems. The outlook for experiments using newly developed devices such as GRIT and other detectors such as SPIDER is described.

Triaxiality and the nature of low-energy excitations in Ge76

The deformation properties of the low-lying states in Ge76 have been investigated following a safe-energy Coulomb excitation measurement with the GRETINA tracking array and CHICO2 heavy-ion counter at the ATLAS accelerator facility at Argonne National Laboratory. A comprehensive set of transition and static E2 matrix elements were extracted from the measured differential Coulomb cross sections and compared with results of configuration-interaction shell-model calculations and computations carried out within the framework of the generalized triaxial rotor model. The remarkable agreement between the calculated and experimental data supports a near-maximum triaxial deformation for the ground state of Ge76. In addition, the degree of softness of the asymmetry in Ge76 and Se76 was investigated using rotational invariants generated from configuration-interaction shell-model wave functions computed with the jj44b and JUN45 effective interactions. The resulting invariants are shown to be consistent with a stiff triaxial deformation in Ge76 and a predominantly soft triaxial potential for Se76, in agreement with the conclusions of recent works by this collaboration.5 MoreReceived 27 January 2023Accepted 3 April 2023DOI:https://doi.org/10.1103/PhysRevC.107.044314©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasNuclear reactionsNuclear structure & decaysNucleon distributionProperties59 ≤ A ≤ 89TechniquesCollective modelsShell modelNuclear Physics

Quadrupole and octupole collectivity in the semi-magic nucleus [formula omitted]Hg126

The first low-energy Coulomb-excitation measurement of the radioactive, semi-magic, two proton-hole nucleus 206Hg, was performed at CERN's recently-commissioned HIE-ISOLDE facility. Two γ rays depopulating low-lying states in 206Hg were observed. From the data, a reduced transition strength B(E2;21+→01+)=4.4(6) W.u. was determined, the first such value for an N=126 nucleus south of 208Pb, which is found to be slightly lower than that predicted by shell-model calculations. In addition, a collective octupole state was identified at an excitation energy of 2705 keV, for which a reduced B(E3) transition probability of 30−13+10 W.u. was extracted. These results are crucial for understanding both quadrupole and octupole collectivity in the vicinity of the heaviest doubly-magic nucleus 208Pb, and for benchmarking a number of theoretical approaches in this key region. This is of particular importance given the paucity of data on transition strengths in this region, which could be used, in principle, to test calculations relevant to the astrophysical r-process.

Evolution of collectivity in 126,128Xe studied in Coulomb excitation measurements

The characteristics of 126,128Xe were investigated in Coulomb excitation measurements performed at the National Superconducting Cyclotron Laboratory (NSCL) Re-accelerator facility, ReA3, Michigan State University (MSU). The Xe nuclei were accelerated to sub-barrier energies and were impinged on 196Pt and 208Pb targets in separate experimental runs. The scattered nuclei and the de-excitation γ-rays were detected using the JANUS setup. Electromagnetic matrix elements were extracted from the experimental data with the help of the GOSIA/GOSIA2 codes. The results were compared to schematic Davydov-Filippov γ-rigid rotor theoretical calculations and large-scale calculations within a newly-established microscopic shell model (called PMMU model). The experimental results agree well with the theoretical predictions, except for the quadrupole moments of the second 2+ states in both nuclei, therefore challenging the interpretation of the γ-bands structure.

Structure of Xe126,128 studied in Coulomb excitation measurements

The electromagnetic properties of $^{126,128}\mathrm{Xe}$ were studied in subbarrier Coulomb excitation measurements performed at the National Superconducting Cyclotron Laboratory Re-accelerator facility, ReA3, at Michigan State University (MSU). $^{126}\mathrm{Xe}$ and $^{128}\mathrm{Xe}$ nuclei were accelerated to 3.74 and 3.81 MeV/nucleon, respectively, and were impinged on $^{196}\mathrm{Pt}$ and $^{208}\mathrm{Pb}$ targets. The $\ensuremath{\gamma}$ rays deexciting the populated low-lying states were detected in coincidence with the scattered nuclei using the JANUS setup. Transition and diagonal matrix elements for low-lying states and transitions in $^{126,128}\mathrm{Xe}$ were extracted from the experimental data using the gosia and gosia2 codes. The experimental results were compared with the theoretical calculations by the microscopic shell model and the Davydov-Filippov $\ensuremath{\gamma}$-rigid rotor model. The calculated results from the newly established shell model (called the PMMU model), which is based on the advanced Hartree-Fock Bogoliubov plus generator coordinate method (HFB $+$ gcm) for a large model space, agree well with the measurements in both nuclei, except for the second ${2}^{+}$ state. Interpretation for the experimentally determined nearly vanishing electric-quadrupole moment of this state remains a challenge for theory.

Coulomb excitation of the |Tz|=12, A=23 mirror pair

Background: Electric-quadrupole ($E2$) strengths relate to the underlying quadrupole deformation of a nucleus and present a challenge for many nuclear theories. 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 $E2$ strengths in \textsuperscript{23}Mg are some of the largest in $T_z=\left|\frac{1}{2}\right|$ nuclei in the $sd$-shell. The purpose of the present work is to improve the precision with which these values are known, to enable better comparison with theoretical models. Methods: Coulomb-excitation measurements of $^{23}$Mg and $^{23}$Na were performed at the TRIUMF-ISAC facility using the TIGRESS spectrometer. They were used to determine the $E2$ matrix elements of mixed $E2$/$M1$ transitions. Results: Reduced $E2$ transition strengths, $B(E2)$, were extracted for \textsuperscript{23}Mg and \textsuperscript{23}Na. Their precision was improved by factors of approximately six for both isotopes, while agreeing within uncertainties with previous measurements. Conclusions: A comparison was made with both shell-model and {\it ab initio} valence-space in-medium similarity renormalization group calculations. Valence-space in-medium similarity-renormalization-group calculations were found to underpredict the absolute $E2$ strength - in agreement with previous studies.

Evolution of shape and collectivity along the Ge isotopic chain: The case of Ge80

The shape and collectivity of $^{80}\mathrm{Ge}$ were investigated via a sub-barrier-energy Coulomb excitation measurement using the JANUS setup at the NSCL ReA3 facility. The $^{80}\mathrm{Ge}$ spectroscopic quadrupole moment ${Q}_{s}({2}_{1}^{+})$ of the ${2}_{1}^{+}$ state was measured for the first time, and the precision of the $^{80}\mathrm{Ge}$ $B(E2;{0}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})$ transition strength was increased. The experimental ${Q}_{s}({2}_{1}^{+})$ value indicates a large, prolate deformation for $^{80}\mathrm{Ge}$, which is consistent with large-scale shell-model calculations performed for comparison. These results provide important benchmarks for models that try to describe nuclear shape in neutron-rich nuclei near the magic number $N=50$.

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

Triaxiality near the 110Ru ground state from Coulomb excitation

A multi-step Coulomb excitation measurement with the GRETINA and CHICO2 detector arrays was carried out with a 430-MeV beam of the neutron-rich 110Ru (t1/2=12s) isotope produced at the CARIBU facility. This represents the first successful measurement following the post-acceleration of an unstable isotope of a refractory element. The reduced transition probabilities obtained for levels near the ground state provide strong evidence for a triaxial shape; a conclusion confirmed by comparisons with the results of beyond-mean-field and triaxial rotor model calculations.

Nuclear Data Sheets for A = 70

Spectroscopic data for all nuclei with mass number A = 70 have been evaluated, and the corresponding level schemes from radioactive decay and reaction studies are presented. Since the previous evaluation, the half-life of 70Mn has been measured and excited states in 70Fe observed for the first time. Excited states in 70Ni have been more extensively studied while Coulomb excitation and collinear laser spectroscopy measurements in 70Cu have allowed for firm Jπ assignments. Despite new measurements, there remain some discrepancies in half-lives of low lying states in 70Zn. New measurements have extended the knowledge of high-spin band structures in 70Ge and 70As. This evaluation supersedes the prior A = 70 evaluation of 2004Tu09.

On the equivalence of experimental B(E2) values determined by various techniques

We establish the equivalence of the various techniques for measuring B(E2) values using a statistical analysis. Data used in this work come from the recent compilation by B. Pritychenko et al. (2016) [5]. We consider only those nuclei for which the B(E2) values were measured by at least two different methods, with each method being independently performed at least twice. Our results indicate that most prevalent methods of measuring B(E2) values are equivalent, with some weak evidence that Doppler-shift attenuation method (DSAM) measurements may differ from Coulomb excitation (CE) and nuclear resonance fluorescence (NRF) measurements. However, such an evidence appears to arise from discrepant DSAM measurements of the lifetimes for 60Ni and some Sn nuclei rather than a systematic deviation in the method itself.

Shape coexistence at low spin in the Z = 50 region and its spectroscopic signatures

Nuclei in the Z = 50 region provide excellent examples of shape coexistence, the establishment of which occurred through the use of detailed spectroscopy, based not only on γ-ray spectroscopy but also conversion electron, particle transfer, Coulomb excitation, and lifetime measurements. The evidence to date strongly suggests that the presence of coexisting shapes arises from the promotion of protons across the Z = 50 closed shell and the strong correlations arising from interplay of the pairing and quadrupole interactions. The evidence for the presence of shape coexistence in the Z = 50 region, at low spin and low excitation energies, will be presented and clues for the microscopic origin explored.

Direct Lifetime Measurements of the Excited States in (72)Ni.

The lifetimes of the first excited 2^{+} and 4^{+} states in ^{72}Ni were measured at the National Superconducting Cyclotron Laboratory with the recoil-distance Doppler-shift method, a model-independent probe to obtain the reduced transition probability. Excited states in ^{72}Ni were populated by the one-proton knockout reaction of an intermediate energy ^{73}Cu beam. γ-ray-recoil coincidences were detected with the γ-ray tracking array GRETINA and the S800 spectrograph. Our results provide evidence of enhanced transition probability B(E2;2^{+}→0^{+}) as compared to ^{68}Ni, but do not confirm the trend of large B(E2) values reported in the neighboring isotope ^{70}Ni obtained from Coulomb excitation measurement. The results are compared to shell model calculations. The lifetime obtained for the excited 4_{1}^{+} state is consistent with models showing decay of a seniority ν=4, 4^{+} state, which is consistent with the disappearance of the 8^{+} isomer in ^{72}Ni.

X-ray production with heavy post-accelerated radioactive-ion beams in the lead region of interest for Coulomb-excitation measurements

Characteristic K X-rays have been observed in Coulomb-excitation experiments with heavy radioactive-ion beams in the lead region (Z=82), produced at the REX-ISOLDE facility, and were used to identify the decay of strongly converted transitions as well as monopole 02+→01+ transitions. Different targets were used, and the X-rays were detected by the Miniball γ-ray spectrometer surrounding the target position. A stable mercury isotope, as well as neutron-deficient mercury, lead, polonium, and radon isotopes were studied, and a detailed description of the analysis using the radioactive 182,184,186,188Hg isotopes is presented. Apart from strongly converted transitions originating from the decay of excited states, the heavy-ion induced K-vacancy creation process has been identified as an extra source for K X-ray production. Isolating the atomic component of the observed K X-rays is essential for a correct analysis of the Coulomb-excitation experiment. Cross sections for the atomic reaction have been estimated and are compared to a theoretical approach.

Digital Rise-Time Discrimination of Pulses from the Tigress Integrated Plunger Silicon PIN Diode Wall

Electromagnetic transition rate measurements play an important role in characterizing the evolution of nuclear structure with increasing proton-neutron asymmetry. 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 gamma-ray spectroscopy for Doppler-shift lifetime studies and low-energy Coulomb excitation measurements. Digital pulse-shape analysis of signals from these ancillary detectors for particle identification improves the signal-to-noise ratio of gamma-ray energy spectra. Here, we illustrate the reaction-channel selectivity achieved by utilizing digital rise-time discrimination of waveforms from alpha particles and carbon ions detected with silicon PIN diodes, thereby enhancing gamma-ray line-shape signatures for precision lifetime studies.

Nuclear Data Sheets for A = 239

Spectroscopic data and level schemes from radioactive decay and nuclear reaction studies are presented here for all nuclei with mass number A=239.In general, a relatively small amount of new data on this mass chain has been reported since the previous evaluation in 2003. However, special evaluations, such as “Database of prompt gamma rays from slow neutron–capture from elemental analysis” (2007ChZX), have provided additional precise data for levels in 239U. Also, new Coulomb excitation measurements in 239Pu have extended the knowledge of the 1/2[631] rotational band up to Jπ=(55/2+), and that of the octupole vibrational band up to Jπ=(53/2−).For historical knowledge it is worth mentioning the report on the “Discovery of isotopes of the transuranium elements with 93 <= Z <= 98” (2013Fr02), where the information for elements Np, Pu, Am, and Cf with mass number A=239 is presented.The alpha hindrance factors (HF) presented in this evaluation were calculated using values of the radius parameter (r0) interpolated from those for even–even adjacent nuclei given by 1998Ak04.

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.