Realistic Shell-model Calculations Research Articles

Lifetime measurements in 131Sb at LOHENGRIN

The neutron-rich 131Sb nucleus was studied at Institut Laue-Langevin, with the LOHENGRIN spectrometer, via neutron-induced fission of 235U and lifetime measurements using LaBr3(Ce) detectors. The lifetimes of the and states were measured with the generalized centroid difference and the decay slope methods, yielding T½=3(2) ps and T½=1347(171) ps, respectively. The measured value for the is at the limit of the fast-timing technique. The level scheme of 131Sb and the decay properties of the state were described by realistic shell-model calculations, which reproduce well both the experimental spin and energy sequences of 131Sb and the measured B(E2; ) value, pointing to an almost pure 2+(130Sn) ⊗ πg7/2, core-proton coupled configuration for the state.

Merging of the island of inversion at N = 40 and N = 50

We performed a quantitative study of the nuclei with proton numbers from Z=20 (Ca) to Z=28 (Ni) and neutron numbers ranging from N=38 to 52 using the realistic shell model, which merged the descriptions of nuclei around N=40 and N=50 within an extended model space. The 21+ excitation energy (E(21+)) is calculated for neutron-rich Cr, Fe, and Ni isotopes. The results suggest that the islands of inversion at N=40 and N=50 merge around the Cr chain, and the N=50island of inversion is located below 78Ni. To demonstrate the prediction of the N=50island of inversion, we calculated and compared the E(21+), B(E2;01+→21+), effective single-particle energies (ESPEs), the probability of particle-hole excitation, and the average occupations in the N=40 and N=50 isotones. Similar parabolic trends of E(21+) and B(E2;01+→21+) in the N=40 and 50 isotones are obtained. The calculated ESPEs give that the N=40 and N=50 shell gaps both increase with the variation of proton number from Z=20 (Ca) to Z=28 (Ni). However, our calculations indicate that different particle-hole excitations dominate in the ground states of the N=40 and N=50islands of inversion, with 2p2h and 4p4h configurations being prominent in N=40 isotones, whereas 2p2h excitations are most important in the N=50 isotones. Furthermore, large occupations of the intruder quadrupole partner orbitals are also obtained in N=40 and N=50 isotones in our realistic shell model calculations.

Core-proton coupled nature of the 11/2+ state in 131 Sb probed by lifetime measurements

The structure of the state in 131Sb was investigated at the LOHENGRIN spectrometer of Institut Laue-Langevin via neutron-induced fission of 235U and lifetime measurements, yielding , at the edge of the sensitivity of the experimental method. This first result for the state half-life in neutron-rich Sb isotopes provides a quadrupole reduced transition probability to the ground state of W.u., very close to the in 130Sn. Realistic shell-model calculations reproduce well both the experimental level scheme of 131Sb and the value, indicting a dominant core-proton coupled configuration for the state.

Testing the predictive power of realistic shell model calculations via lifetime measurement of the 11/2+ state in Sb131

The lifetime of the $11/{2}_{1}^{+}$ state in the $^{131}\mathrm{Sb}$ nucleus was measured at the LOHENGRIN spectrometer of the Institut Laue-Langevin via neutron-induced fission of $^{235}\mathrm{U}$ using $\ensuremath{\gamma}$-ray fast-timing techniques. The obtained value of ${T}_{1/2}=3\phantom{\rule{0.16em}{0ex}}$(2) ps, at the edge of the sensitivity of the experimental method, is the first result for the $11/{2}_{1}^{+}$ state half-life in neutron-rich Sb isotopes. The corresponding quadrupole reduced transition probability to the ground state is $B(E2)=1.{4}_{\ensuremath{-}0.6}^{+1.5}$W.u., indicating a noncollective nature of this state. Realistic shell-model calculations performed in a large valence space reproduce well the experimental value and point to a dominant ${2}^{+}(^{130}\mathrm{Sn})\ensuremath{\bigotimes}\ensuremath{\pi}{\mathrm{g}}_{7/2}$ configuration for the $11/{2}_{1}^{+}$ state, as expected in a weak-coupling scenario. At the same time, the sum of the quadrupole strength of the multiplet states is predicted to exceed the one of the $^{130}\mathrm{Sn}$ core as a consequence of the equal contribution of the proton and the proton-neutron quadrupole matrix elements, pointing to possible development of collectivity already in the close neighborhood of $^{132}\mathrm{Sn}$.

Evidence of Partial Seniority Conservation in the πg_{9/2} Shell for the N=50 Isotones.

The reduced transition probabilities for the 4_{1}^{+}→2_{1}^{+} and 2_{1}^{+}→0_{1}^{+} transitions in ^{92}Mo and ^{94}Ru and for the 4_{1}^{+}→2_{1}^{+} and 6_{1}^{+}→4_{1}^{+} transitions in ^{90}Zr have been determined in this experiment making use of a multinucleon transfer reaction. These results have been interpreted on the basis of realistic shell-model calculations in the f_{5/2}, p_{3/2}, p_{1/2}, and g_{9/2} proton valence space. Only the combination of extensive lifetime information and large scale shell-model calculations allowed the extent of the seniority conservation in the N=50 g_{9/2} orbital to be understood. The conclusion is that seniority is largely conserved in the first πg_{9/2} orbital.

A solvable model for octupole phonons

A solvable model is proposed for the description of octupole phonons in closed-shell nuclei, formulated in terms of shell-model par\-ticle--hole excitations. With some simple assumptions concerning single-particle energies and two-body interactions, closed expressions are derived for the energy and wave function of the octupole phonon. In particular, it is shown that the components of the octupole phonon are proportional to Wigner $3j$ coefficients. This analytic wave function is proven to be exactly valid in light nuclei, which have $LS$ shell closures that coincide with those of the three-dimensional harmonic oscillator, and to be valid to a good approximation in heavier nuclei, which have $jj$ shell closures due to the spin--orbit interaction. The properties of the solvable model are compared with the results of a realistic shell-model calculation for $^{208}$Pb.

Neutron-rich calcium isotopes within realistic Gamow shell model calculations with continuum coupling

Based on the realistic nuclear force of the high-precision CD-Bonn potential, we have performed comprehensive calculations for neutron-rich calcium isotopes using the Gamow shell model (GSM) which includes resonance and continuum. The realistic GSM calculations produce well binding energies, one- and two-neutron separation energies, predicting that $^{57}$Ca is the heaviest bound odd isotope and $^{70}$Ca is the dripline nucleus. Resonant states are predicted, which provides useful information for future experiments on particle emissions in neutron-rich calcium isotopes. Shell evolutions in the calcium chain around neutron numbers \textit{N} = 32, 34 and 40 are understood by calculating effective single-particle energies, the excitation energies of the first $2^+$ states and two-neutron separation energies. The calculations support shell closures at $^{52}$Ca (\textit{N} = 32) and $^{54}$Ca (\textit{N} = 34) but show a weakening of shell closure at $^{60}$Ca (\textit{N} = 40). The possible shell closure at $^{70}$Ca (\textit{N} = 50) is predicted.

Evidence of octupole-phonons at high spin in 207Pb

A lifetime measurement of the 19/2− state in 207Pb has been performed using the Recoil Distance Doppler-Shift (RDDS) method. The nuclei of interest were produced in multi-nucleon transfer reactions induced by a 208Pb beam impinging on a 100Mo enriched target. The beam-like nuclei were detected and identified in terms of their atomic mass number in the VAMOS++ spectrometer while the prompt γ rays were detected by the AGATA tracking array. The measured large reduced transition probability B(E3,19/2−→13/2+)=40(8) W.u. is the first indication of the octupole phonon at high spin in 207Pb. An analysis in terms of a particle-octupole-vibration coupling model indicates that the measured B(E3) value in 207Pb is compatible with the contributions from single-phonon and single particle E3 as well as E3 strength arising from the double-octupole-phonon 6+ state, all adding coherently. A crucial aspect of the coupling model, namely the strong mixing between single-hole and the phonon-hole states, is confirmed in a realistic shell-model calculation.

Estimating the flux of the 14.4 keV solar axions

In this paper we present a calculation of the expected flux of the mono-energetic 14.4 keV solar axions emitted by the M1 type nuclear transition of 57Fe in the Sun. These axions can be detected, e.g., by inverse coherent Bragg-Primakoff conversion in single-crystal TeO2 bolometers. The ingredients of this calculation are i) the axion nucleon coupling, estimated in several popular axion models and ii)the nuclear spin matrix elements involving realistic shell model calculations with both proton and neutron excitations. For the benefit of the experiments we have also calculated the branching ratio involving axion and photon emission. We find the solar axion flux on Earth to be Φa = 0.703×109cm-2s-1 (107 GeV/fa)2 and the branching ratio of axion to photon for the same model to be: wa/wγ = 0.229×10-15 ≈ 2×10-16.

One-neutron transfer study ofXe137and systematics of13/21+and13/22+levels inN=83nuclei

Excited states in 137Xe have been studied by using the near-barrier single-neutron transfer reactions 13C(136Xe,12C )137Xe and 9Be(136Xe,8Be )137Xe in inverse kinematics.Particle- and particle- coincidence measurements have been performed with the Phoswich Wall and Digital Gammasphere detector arrays. Evidence is found for a 13/2+2 level (E = 3137 keV) and for additional high-lying 3/2– and 5/2– states. The results are discussed in the framework of realistic shell-model calculations. These calculations are also extended to the 13/2+1 and 13/2+2 levels in the N = 83 isotonic chain. Furthermore, they indicate that there is a need for a value of the neutron 0i13/2 single-particle energy (ESPE = 2366 keV) lower than the one proposed in the literature. It is also demonstrated that the population patterns of the j = l ± 1/2 single-particle states in 137Xe are different for the two targets used in these measurements and the implications of this effect are addressed.

First observation ofγrays emitted from excited states south-east ofSn132: Theπg9/2−1⊗νf7/2multiplet ofIn83132

For the first time, the γ decay of excited states has been observed in a nucleus situated in the quadrant south-east of doubly magic Sn132, a region in which experimental information so far is limited to ground-state properties. Six γ rays with energies of 50, 86, 103, 227, 357, and 602 keV were observed following the β-delayed neutron emission from Cd85133, populated in the projectile fission of a U238 beam at the Radioactive Isotope Beam Factory at RIKEN within the EURICA project. The new experimental information is compared to the results of a modern realistic shell-model calculation, the first one in this region very far from stability, focusing in particular on the π0g9/2−1⊗ν1f7/2 particle-hole multiplet in In83132. In addition, theoretical estimates based on a scaling of the two-body matrix elements for the πh11/2−1⊗νg9/2 analog multiplet in Tl127208, one major proton and one major neutron shell above, are presented.Received 15 December 2015DOI:https://doi.org/10.1103/PhysRevC.93.041301©2016 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBeta decayProperties90 ≤ A ≤ 149TechniquesRadioactive beamsShell modelNuclear Physics

High-spin spectroscopy ofCe139

High-spin states in $^{139}\mathrm{Ce}$ have been populated using the $^{130}\mathrm{Te}(^{14}\mathrm{C},5n)$ reaction. The level scheme has been extended to higher spins, including a new band of dipole transitions. The parity of several states has been changed from negative to positive, mainly based on the comparison with the level structure of the core nucleus $^{140}\mathrm{Ce}$ and the results of a realistic shell-model calculation. The dipole band is interpreted as a magnetic rotation band with $\ensuremath{\pi}{h}_{11/2}^{2}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2}^{\ensuremath{-}1}$ configuration built on small deformation axial shape with $({\ensuremath{\varepsilon}}_{2}=0.12,\ensuremath{\gamma}={0}^{\ensuremath{\circ}})$.

Can Realistic Shell-Model Calculations be Predictive?

A major requirement for a nuclear model is, not only to reproduce and describe accurately the available experimental data, but also to provide reliable predictions for physical quantities not yet measured. Over the past two decades, we have performed various realistic shell-model calculations for nuclei in different mass regions, which have all yielded results in good agreement with the available experimental data. In this paper we present some selected results illustrating their predictive power.

Realistic shell-model calculations and exotic nuclei

This paper presents a short overview of the shell-model approach with realistic effective interactions to the study of exotic nuclei. We first give a sketch of the current state of the art of the theoretical framework of this approach, focusing on the main ingredients and most relevant recent advances. Then, we present some selected results for neutron-rich nuclei in various mass regions, namely oxygen isotopes, N = 40 isotones, and nuclei around 132Sn, to show the merit as well as the limits of these calculations.

From Kuo–Brown to today's realistic shell-model calculations

This paper is an homage to the seminal work of Gerry Brown and Tom Kuo, where shell model calculations were performed for 18O and 18F using an effective interaction derived from the Hamada-Johnston nucleon-nucleon potential. That work has been the first successful attempt to provide a description of nuclear structure properties starting from the free nucleon-nucleon potential. We shall compare the approach employed in the 1966 paper with the derivation of a modern realistic shell-model interaction for sd-shell nuclei, evidencing the progress that has been achieved during the last decades.

Realistic shell-model calculations for isotopic chains “north-east” ofCa48in the (N,Z) plane

We perform realistic shell-model calculations for nuclei with valence nucleons outside 48Ca, employing two different model spaces. The matrix elements of the effective two-body interaction and electromagnetic multipole operators have been calculated within the framework of the many-body perturbation theory, starting from a low-momentum potential derived from the high-precision CD-Bonn free nucleon-nucleon potential. The role played by the neutron orbital 1d5/2 has been investigated by comparing experimental data on yrast quadrupole excitations of isotopic chains north-east of 48Ca with the results of calculations including or not including this single-particle state in the model space.

Fully microscopic shell-model calculations with realistic effective hamiltonians

The advent of nucleon-nucleon potentials derived from chiral perturbation theory, as well as the so-called Vlow-k approach to the renormalization of the strong short-range repulsion contained in the potentials, have brought renewed interest in realistic shell-model calculations. Here we focus on calculations where a fully microscopic approach is adopted. No phenomenological input is needed in these calculations, because single-particle energies, matrix elements of the two-body interaction, and matrix elements of the electromagnetic multipole operators are derived theoretically. This has been done within the framework of the time-dependent degenerate linked-diagram perturbation theory. We present results for some nuclei in different mass regions. These evidence the ability of realistic effective hamiltonians to provide an accurate description of nuclear structure properties.

19/2+ isomer of 125Cd and realistic shell-model calculations of neutron-rich Cd isotopes

The decay of a T1/2 = 19(3) μs isomeric state from 125Cd has been observed using γ-ray spectroscopy at the Lohengrin mass spectrometer of the Institut Laue-Langevin, Grenoble. Two coincident γ rays were observed to be emitted from the decay of this isomeric state, contradicting recently published data on this nucleus. Realistic shell-model calculations have been performed to interpret the decay scheme, allowing a spin and parity of 19/2+ to be assigned to the isomeric state. All configurations contributing to this isomeric state have amplitudes less than 7 %. Experimental data have recently been published on the decays of μs isomeric states in 127,128,130Cd. Realistic shell-model calculations for these nuclei are presented which reproduce the known experimental decay schemes reasonably well.

Shell-model study of exotic Sn isotopes with a realistic effective interaction

We report on a shell-model study of Sn isotopes beyond N = 82 employing a realistic effective interaction derived from the CD-Bonn nucleon-nucleon potential renormalized through use of the Vlow-k approach. At present, the most exotic Sn isotope for which some experimental information exists is 134Sn with an N/Z ratio of 1.68. It is the aim of our study to compare the results of our calculations with the available experimental data and to make predictions for the neighboring heavier isotopes which may be within reach of the next generation of radioactive ion beam facilities. The very good agreement between theory and experiment obtained for 134Sn gives confidence in the predictive power of our realistic shell-model calculations.

Influence of theνg9/2orbital on level structures of neutron-richMn61,6236,37

Level structures in $^{61,62}\mathrm{Mn}{}_{36,37}$ were studied with Gammasphere in the reaction of a $430$-MeV $^{64}\mathrm{Ni}$ beam and a thick $^{238}\mathrm{U}$ target. The newly identified levels decrease in excitation energy compared to the analogous structures in the lighter Mn isotopes and behave similarly to states in the corresponding Fe isotones that involve ${g}_{9/2}$ neutron excitations. This behavior illustrates the importance of the inclusion of the $\ensuremath{\nu}{g}_{9/2}$ orbital in any realistic shell-model calculations in this region.