Single-particle and collective excitations in the N = 28 isotones Fe 54 and Mn 53
Not Available
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
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Metastable states from multinucleon excitations in 202 Tl and 203 Pb
The excited level structures of 202 Tl and 203 Pb, above the 7 + and 29/2 – isomers, respectively, have been studied. An isomer with I π = 20 + and T 1/2 = 215(10) μs has been established in 202 Tl, and the level scheme extended from I = 10 to 20ℏ with the placement of fifteen new transitions. In 203 Pb, the I π = 37/2 + state is established to be metastable, with T 1/2 = 2.5(3) ns. Levels in both nuclei arise from intrinsic excitations, with likely particle-hole character for the higher-lying states in 203 Pb. The 20 + isomer in 202 Tl is most likely associated with a $πh$ $^{–1}_{11/2}$ Ⓧ ν($i$ $^{–2}_{13/2}$, $f$ $^{–1}_{5/2}$) configuration, while the 37/2 + state in 203 Pb results from the excitation of five neutrons. Furthermore, calculations, using both an empirical approach and the oxbash code, have been performed to aid in the description of the excited level structure.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Structure of even-even Sr isotopes with 50 ≤ N ≤ 58 neutrons
Excited levels in Sr 90 , Sr 92 , Sr 94 , and Sr 96 nuclei were reinvestigated using high-statistics multiple- γ coincidence data from neutron-induced fission of U 235 and spontaneous fission of Cf 252 , measured using Exogam at Institut Laue Langevin and Gammasphere arrays, respectively. The experimental goal was the search for new excited levels and firm spin-parity assignments to known levels. A total of 23 new levels with 30 new or corrected decays and 39 new or improved spin-parity assignments were obtained in the four nuclei. Negative-parity structures on top of 3 - excitation were firmly identified and extended to higher spins. New positive-parity structures in Sr 94 and Sr 96 were observed with 3 + excitations characteristic of γ collectivity. The 277.7-keV, E 2 decay from the 1507.0-keV level to the second 0 + level in Sr 96 , found in this paper, completes the coexisting deformed band in this nucleus. To learn about the microscopic structure of levels in the Sr 88 - 96 nuclei, we performed large-scale shell-model calculations. The calculations compared to the experiment, helped the discussion of the evolution of collectivity in strontium isotopes, highlighting the important role of various single-particle excitations in phase transitions and shape coexistence in the region. The special role of the neutron 9 / 2 + [ 404 ] extruder as a catalyst of the deformation change in the region is highlighted.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Complete set of bound negative-parity states in the neutron-rich nucleus 18 N
High-resolution γ -ray spectroscopy of N 18 is performed with the Advanced GAmma Tracking Array, following deep-inelastic processes induced by an O 18 beam on a Ta 181 target. Six states are newly identified, which together with the three known excitations exhaust all negative-parity excited states expected in N 18 below the neutron threshold. Spin and parities are proposed for all located states on the basis of decay branchings and comparison with large-scale shell-model calculations performed in the p-sd space, with the YSOX interaction. Of particular interest is the location of the 0 1 - and 1 2 - excitations, which provide strong constrains for cross-shell p-sd matrix elements based on realistic interactions and help to simultaneously reproduce the ground and first-excited states in N 16 and N 18 , for the first time. Overall, understanding the N 18 structure may also have significant impact on neutron-capture cross-section calculations in r-process modeling including light neutron-rich nuclei.
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↗
Systematics of E 2 strength in the s d shell with the valence-space in-medium similarity renormalization group
Recent developments in ab initio nuclear theory demonstrate promising results in medium- to heavy-mass nuclei. A particular challenge for many of the many-body methodologies, however, is an accurate treatment of the electric-quadrupole, E2, strength associated with collectivity. The valence-space in-medium similarity renormalization group (VS-IMSRG) is a particularly powerful method for accessing medium- and high-mass nuclei but has been found to underpredict E2 strengths. The purpose of this work is to evaluate the isospin dependence of this underprediction. We perform a systematic comparison of VS-IMSRG calculations with available literature. We make use of isoscalar and isovector contributions to the E2 matrix elements to assess isoscalar and isovector contributions to the missing strength. It is found that the E2 strength is consistent throughout T z =|12|, T z =|1|, T z =|32|, and T z =2 pairs within the sd shell. Furthermore, no isovector contribution to the deficiency is identified. A comparison with toy-models and coupled-cluster calculations is used to discuss potential origins of the missing strength, which arises from missing many-particle, many-hole excitations out of the model space. The absence of any significant isovector contribution to the missing E2 strength indicates that the E2 strength discrepancy, and therefore any correction, is largely independent of the isospin of the nuclei in question.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Isomers in Tl 203 and core excitations built on a five-nucleon-hole structure
Isomers with three- and five-nucleon-hole configurations have been established in 203 Tl These include newly identified levels with a three-nucleon-hole structure: I π = (15/2 – ) with T 1/2 = 7.9(5) ns and I π = (35/2 – ) with T 1/2 = 4.0(5) ns. In addition, five-nucleon-hole states have also been established: I π = (39/2 – ) with T 1/2 = 1.9(2) ns and I π = (49/2 + ) with T 1/2 = 3.4(4) ns. The previously determined long-lived decay, T 1/2 = 6.6(3) μs from this work, is associated with isomerism of the I π = (29/2 + ) state. Levels above this long-lived isomer have been identified through a delayed-prompt γ–γ coincidence measurement. Five-nucleon-hole states with excitation energies E x ≈ 7 MeV have been established as well as possible octupole excitations of the 208Pb core built on these levels. Furthermore, the level scheme of 203 Tl is extended up to E x ≈ 11 MeV with the inclusion of 25 new transitions. Empirical and shell-model calculations have been performed to aid in the description of the observed states which are found to be of intrinsic character.
190 ≤ A ≤ 219↗
Nuclear structure investigations of Es 253 − 255 by laser spectroscopy
Not Available
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Investigation of B 11 and Ca 40 levels at 8–9 MeV by nuclear resonance fluorescence
Here, we report on the measurement of 11 B and 40 Ca levels between excitation energies of 8 and 9 MeV using nuclear resonance fluorescence (NRF). The experiment was carried out with nearly monoenergetic and linearly polarized photon beams provided by the High-Intensity γ-ray Source (HIγS) facility at the Triangle Universities Nuclear Laboratory (TUNL). States in 11 B are important for calibrations of NRF measurements, while the properties of 40 Ca levels impact potassium nucleosynthesis in globular clusters. For 40 Ca, we report on improved excitation energies and an unambiguous 2¯ assignment for the state at 8425 keV. For 11 B, we obtained improved values for γ-ray multipolarity mixing ratios and branching ratios of the 8920-keV level.
39 ≤ A ≤ 58↗
Reinvestigation of the level structures of the N=49 isotones 89 Zr and 91 Mo
Here, excited states of 89 Zr and 91 Mo have been investigated using the fusion reaction 6 Li+ 89 Y at beam energy of 34 MeV. Based on new experimental data, about 16 (13) new transitions have been added to the level scheme of 89 Zr ( 91 Mo). The level structures are investigated in the framework of the spherical-basis shell model using the effective interactions JUN45 and GWBXG. Reasonable agreement is obtained between the experimental results and the calculations. The calculated results indicate the newly proposed states are characterized by proton excitations from the 1f 5/2 , 2p 3/2 , 2p 1/2 orbits to the high-j 1g 9/2 orbit. The systematic features of low lying states in the N = 49 isotones are discussed.
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↗
Model-independent determination of the dipole response of 66 Zn using quasimonoenergetic and linearly polarized photon beams
Background: Photon strength functions are an important ingredient in calculations relevant for the nucleosynthesis of heavy elements. The relation to the photoabsorption cross section allows to experimentally constrain photon strength functions by investigating the photoresponse of atomic nuclei. Purpose: Here, we determine the photoresponse of 66 Zn in the energy region of 5.6 MeV to 9.9 MeV and analyze the contribution of the 'elastic' decay channel back to the ground state. In addition, for the elastic channel electric and magnetic dipole transitions were separated. Methods: Nuclear resonance fluorescence experiments were performed using a linearly polarized quasi-monoenergetic photon beam at the High Intensity γ-ray Source. Photon beam energies from 5.6 to 9.9 MeV with an energy spread of about 3% were selected in steps of 200–300 keV. Two high purity germanium detectors were used for the subsequent γ-ray spectroscopy. Results: Full photoabsorption cross sections are extracted from the data making use of the monoenergetic character of the photon beam. For the ground-state decay channel, the average contribution of electric and magnetic dipole strengths is disentangled. The average branching ratio back to the ground state is determined as well. Conclusions: The new results indicate lower cross sections when compared to the values extracted from a former experiment using bremsstrahlung on 66 Zn. In the latter, the average branching ratio to the ground state is estimated from statistical-model calculations in order to analyze the data. Corresponding estimates from statistical-model calculations underestimate this branching ratio compared to the values extracted from the present analysis, which would partly explain the high cross sections determined from the bremsstrahlung data.
59 ≤ A ≤ 89↗
Measurement of the $\textit{B}(\textit{E}2 ↑)$ strengths of 36 Ca and 38 Ca
In this work, the $B(E2, 0^+_1 → 2^+_1)$ strengths of 36 Ca and 38 Ca are measured to be 131(20) $e^2\text{fm}^4$ and 101(11) $e^2\text{fm}^4$, respectively. The $\textit{B}(\textit{E}2)$ value for 36 Ca required a measurement of the p/γ branching ratio because the 2 + state is proton unbound. This branching ratio is $B_p$=0.087(8). These $\textit{B}(\textit{E}2)$ and branching-ratio values can be reproduced in the shell-model with the ZMB2 interaction, an interaction that predicts the $\textit{Z} = 20 \textit{sd}$-shell closure is incomplete with large proton pf-shell occupancies in the ground state. These occupancies are at odds with other shell-model and energy-density-functional calculations of 36 Ca. New data are used to provide an update on constraints of the density dependence of the symmetry energy through mirror charge-radii differences as well as to help reduce uncertainties of the astrophysical important 35 K(p,γ) reaction.
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Experimental study of the S 38 excited level scheme
Information on the 38 S level scheme was expanded through experimental work utilizing a fusion-evaporation reaction and in-beam γ-ray spectroscopy. Prompt γ-ray transitions were detected by the Gamma-Ray Energy Tracking Array (GRETINA) and recoiling 38 S residues were selected by the Fragment Mass Analyzer (FMA). Tools based on machine-learning techniques were developed and deployed for the first time in order to enhance the unique selection of 38S residues and identify any associated γ-ray transitions. The new level information, including the extension of the even-spin yrast sequence through J π =8( + ), was interpreted in terms of a basic single-particle picture as well shell-model calculations which incorporated the empirically derived FSU interaction. A comparison between the properties of the yrast states in the even-Z, N = 22 isotones from Z = 14 to 20, and for 36 Si– 38 S in particular, was also presented with an emphasis on the role and influence of the neutron 1 p 3/2 orbital on the structure in the region.
20 ≤ A ≤ 38↗
Lifetime measurements in 206 Po with a shell-model interpretation
The lifetimes of the first excited 2⁺ and 4⁺ states in 206 Po were measured using the recoil-distance Dopplershift method. The experimental results were compared to large-scale shell-model calculations that describe the deduced transition probabilities well. Those calculations were extended to the neighboring 204,208 Po isotopes giving a good overall description of the yrast states. However, the calculations underpredict the energies of the 6+1 and 8+1 states, which suggests that further improvement of the proton-neutron interaction is required.
190 ≤ A ≤ 219↗
Proton and neutron contributions to the quadrupole transition strengths in 39 Ca and 39 K studied by lifetime measurements of mirror transitions
The E2 transition matrix elements of isobaric multiplets are expected to follow a linear trend as a function of isospin projection. However, measurements of the 2 + → 0 + transitions in the A = 38 triplet of Ca, K, and Ar show a deviation from this trend with an enhanced transition strength in 38 Ca with respect to its mirror 38 Ar. We have studied analogue 11/2 – → 7/2 – E2 transitions in 39 Ca and its mirror partner 39 K to determine if this enhancement persists in neighboring Ca isotopes. Recoil-distance lifetime measurements of 39 Ca and 39 K were performed utilizing a 42 Sc secondary beam, the TRIPLEX plunger, the GRETINA array, and the S800 spectrograph. Our data provide a lifetime measurement of the (11/2 – ) state in 39 Ca as well as an improved lifetime result for the (9/2 – ) state, while the 39 K data are used to validate the present analysis. Furthermore, a comparison of the present data to shell-model calculations suggests an enhanced transition strength in 39 Ca, pointing to both proton and neutron contributions to core excitations across the Z = N = 20 shell gaps in close proximity to 40 Ca.
39 ≤ A ≤ 58↗
𝐵(𝐸2) measurements in the yrast band of 28 Mg: Implications for the 𝑁 = 20 island of inversion
High-precision lifetime measurements in 28 Mg were performed to study neutron shell evolution in Mg isotopes and the onset of the N = 20 island of inversion. Here, using both the recoil distance and Doppler shift attenuation methods, five lifetimes were measured in addition to six upper limits. The observation of two long-lived, negative-parity states demonstrate the importance of studying Mg isotopes for the contribution of intruder configurations to sd-shell nuclei. Lifetimes of the $2^+_1$ and $4^+_1$ states of 1.81(5) ps and 172($^{+11}_{–10}$) stat. (4) stop. (8) feed. (4) targ. fs, respectively, demonstrate a loss of collectivity with increasing spin in the yrast band, permitting for distinguishing between current theoretical models. These measurements also highlight the progression of yrast structure across the Mg isotopic chain from rotational at N = 12 to large shape mixing at N = 16 and back to collective behavior at N = 20 but with dominating intruder configurations.
20 ≤ A ≤ 38↗
Magnetic dipole γ-ray strength functions in the crossover from spherical to deformed neodymium isotopes
We calculate the magnetic dipole $\gamma$-ray strength functions in a chain of even-mass neodymium isotopes $^{144-152}$Nd in the framework of the configuration-interaction (CI) shell model. We infer the strength function by applying the maximum entropy method (MEM) to the exact imaginary-time response function calculated with the shell-model Monte Carlo (SMMC) method. The success of the MEM depends on the choice of a good strength function as a prior distribution. We investigate two choices for the prior strength function: the static path approximation (SPA) and the quasiparticle random-phase approximation (QRPA). We find that the QRPA is a better approximation at low temperatures (i.e., near the ground state), while the SPA is a better choice at finite temperatures. We identify a low-energy enhancement (LEE) in the MEM deexcitation $M1$ strength functions of the even-mass neodymium isotopes and compare with recent experimental results for the total deexcitation $\gamma$-ray strength functions. The LEE is already seen in the SPA strength function but not in the QRPA strength function, indicating the importance of large-amplitude static fluctuations around the mean field in reproducing the LEE. Our method is currently the only one which can reproduce LEE in heavy open-shell nuclei where conventional CI shell model calculations are prohibited. With the onset of deformation as number of neutrons increases along the chain of neodymium isotopes, we observe that some of the LEE strength transfers to a low-energy excitation, which we interpret as a finite-temperature ``scissors'' mode. Here, we also observe a finite-temperature spin-flip mode.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗