Search

The experimental gamma-ray strength functions (gamma-SFs) of 142,144-151Nd have been studied for gamma-ray energies up to the neutron separation energy. The results represent a unique set of gamma-SFs for an isotopic chain with increasing nuclear deformation. The data reveal how the low-energy enhancement, the scissors mode and the pygmy dipole resonance evolve with nuclear deformation and mass number. The data indicate that the mechanisms behind the low-energy enhancement and the scissors mode are decoupled from each other., Comment: 14 pages and 10 figures

The validity of the Brink-Axel hypothesis, which is especially important for numerous astrophysical calculations, is addressed for 116,120,124Sn below the neutron separation energy by means of three independent experimental methods. The $\gamma$-ray strength functions (GSFs) extracted from primary $\gamma$-decay spectra following charged-particle reactions with the Oslo method and with the Shape method demonstrate excellent agreement with those deduced from forward-angle inelastic proton scattering at relativistic beam energies. In addition, the GSFs are shown to be independent of excitation energies and spins of the initial and final states. The results provide a critical test of the generalized Brink-Axel hypothesis in heavy nuclei, demonstrating its applicability in the energy region of the pygmy dipole resonance., Comment: 6 pages, 4 figures, Phys. Rev. Lett. (in press)

Neutron-rich Pm ( Z = 61 ) isotopes were studied by delayed γ -ray spectroscopy at RIBF, RIKEN Nishina Center using the in-flight fission of a 345 MeV/nucleon U 238 beam. A cluster-type Ge detector array, EURICA, was used to measure the delayed γ rays from stopped ions. Isomeric γ decays were observed in Pm 159 and Pm 161 with half-lives of 4.97(12) μ s and 0.79(4) μ s , respectively. Level schemes for Pm 159 and Pm 161 were constructed in this study. The isomeric states of Pm 159 and Pm 161 could be interpreted as two quasiparticle excitations of neutrons with the configurations of ν ( 7 / 2 [ 633 ] ⊗ 5 / 2 [ 523 ] ) and ν ( 7 / 2 [ 633 ] ⊗ 1 / 2 [ 521 ] ) , respectively. They are analogous to the isomers that have been observed systematically in other even-mass N = 98 and N = 100 isotones in this region. A projected shell model calculation was performed and it reproduced the order of three-quasiparticle states only if new Nilsson parameters with an N -dependent spin-orbit interaction were used. This work demonstrates that the strength of spin-orbit interactions in standard Nilsson parameters needs to be modified to study the properties of neutron-rich rare-earth nuclei around A = 165 , and provides new evidence supporting the existence of the deformed N = 98 subshell gap in odd-mass nuclei for the first time.

6 pags., 4 figs., 1 tab.-- Open Access funded by Creative Commons Atribution Licence 4.0, Inclusive one- and multi-nucleon removal cross sections have been measured for several Sn, Sb and Te isotopes just beyond the N=82 neutron shell closure. The beams were produced in the projectile fission of a U beam at the Radioactive Isotope Beam Factory at RIKEN. The experimental cross sections are compared to predictions from the most recent version of the Liege intranuclear cascade model. Although the overall agreement is good, severe discrepancies are observed for the cases of one- and two-neutron removal from Sn and Sb projectiles and one-proton knockout from all measured N=84 isotones. These discrepancies, as well as the relevance of quasi-elastic reaction channels to the one-neutron removal cross sections, are discussed. In addition, the measured inclusive one-proton knockout cross section for the semi-magic Sn projectile is compared to eikonal direct reaction theory calculations to assess if the suppression factors to these calculated cross sections, deduced from data on reactions of lighter projectile nuclei, are also applicable to heavy nuclei., We thank the staff of the RIKEN Nishina Center accelerator complex for providing high-intensity beams to the experiment. This work was supported by the Spanish Ministerio de Economía y Competitividad under contracts FPA2014-57196-C5-4-P and FPA2017-84756-C4-2-P. J.A.T. acknowledges the support of the Science and Technology Facilities Council (UK) grant ST/L005314/1 and R.O. that of JSPS KAKENHI Grant No. 26887048. G.M.T. grate-fully acknowledges funding of this research from the Research Council of Norway, Project Grant No. 222287.

Recent measurements of the nuclear level densities and γ-ray strength functions below the neutron thresholds in 180,181,182Ta are used as input in the nuclear reaction code TALYS. These experimental average quantities are utilized in the calculations of the 179,180,181Ta radiative neutron capture cross sections. From the latter, astrophysical Maxwellian-averaged (n,γ) cross sections (MACS) and reaction rates are extracted, which in turn are used in large astrophysical network calculations to probe the production mechanism of 180 Ta. These calculations are performed for two scenarios, the s-process production of 180,181Ta in Asymptotic Giant Branch (AGB) stars and the p-process nucleosynthesis of 180Tam in Type-II supernovae. Based on the results from this work, the s-process in stellar evolution is considered negligible in the production of 180Tam whereas 181Ta is partially produced by AGB stars. The new measurements strongly constrain the production and destruction rates of 180Tam at p-process temperatures and confirm significant production of nature’s rarest stable isotope 180Tam by the p-process., SCOPUS: ar.j, info:eu-repo/semantics/published

The nuclear level densities (NLDs) and the $\ensuremath{\gamma}$-ray strength functions ($\ensuremath{\gamma}\mathrm{SFs}$) of $^{153,155}\mathrm{Sm}$ have been extracted from ($d,p\ensuremath{\gamma}$) coincidences using the Oslo method. The experimental NLD of $^{153}\mathrm{Sm}$ is higher than the NLD of $^{155}\mathrm{Sm}$, in accordance with microscopic calculations. The $\ensuremath{\gamma}\mathrm{SFs}$ of $^{153,155}\mathrm{Sm}$ are in fair agreement with QRPA calculations based on the D1M Gogny interaction. An enhancement is observed in the $\ensuremath{\gamma}\mathrm{SF}$ for both $^{153,155}\mathrm{Sm}$ nuclei around 3 MeV in excitation energy and is attributed to the $M1$ scissors resonance (SR). Their integrated strengths were found to be in the range 1.3--2.1 and 4.4--6.4 ${\ensuremath{\mu}}_{N}^{2}$ for $^{153}\mathrm{Sm}$ and $^{155}\mathrm{Sm}$, respectively. The strength of the SR for $^{155}\mathrm{Sm}$ is comparable to those for deformed even-even Sm isotopes from nuclear resonance fluorescence measurements, while that of $^{153}\mathrm{Sm}$ is lower than expected.

Understanding the evolution of level densities in the crossover from spherical to well-deformed nuclei has been a long-standing problem in nuclear physics. We measure nuclear level densities for a chain of neodymium isotopes $^{142,144-151}$Nd which exhibit such a crossover. These results represent to date the most complete data set of nuclear level densities for an isotopic chain between neutron shell-closure and towards mid-shell. We observe a strong increase of the level densities along the chain with an overall increase by a factor of $\approx 170$ at an excitation energy of 7.5 MeV and saturation around mass 150. Level densities calculated by the shell model Monte Carlo (SMMC) are in excellent agreement with these experimental results. Based on our experimental and theoretical findings, we offer an explanation of the observed mass dependence of the level densities in terms of the intrinsic single-particle level density and the collective enhancement., 7 pages, 4 figures

15 pags., 10 figs., 4 tabs., Newly observed decay schemes of the nuclei Sb137 and Sb138 are reported. The neutron-rich Sb isotopes were produced by the in-flight fragmentation of a U238 primary beam with an energy of 345 MeV/nucleon. Several new excited states of Te137 with tentatively assigned spin-parities of (5/2-), (9/2-), and (7/2) have been established which play an important role in the evolution of neutron levels beyond N=82. The study of the ß decay of Sb138 led to a considerable extension of the level scheme of Te138 including the identification of several nonyrast states. The structure of Te137 and Te138 is discussed on the basis of large-scale shell-model calculations performed using two different effective interactions., This work was carried out at the RIBF operated by RIKEN Nishina Center and CNS, University of Tokyo. Part of the WAS3ABi was supported by the Rare Isotope Science Project (RISP) of the Institute for Basic Science (IBS) funded by the Ministry of Science, ICT and Future Planning (MSIP) and National Research Foundation (NRF) of the Republic of Korea (Grant No. 2013M7A1A1075764). This research was supported by NRF of the Republic of Korea (Grants No. 2019R1A6A3A03031564 and No. 2018R1A5A1025563), IBS of the Republic of Korea (Grants No. IBS-R031-D1 and No. IBS-R031-Y1), the Spanish Ministerio de Economía y Competitividad (Grant No. FPA2017-84756-C4-2-P), and JSPS KAKENHI of Japan (Grant No. 25247045). The support from FR-JP LIA is also acknowledged. C.S.L. acknowledges the support from NRF of the Republic of Korea (Grants No. 2017M2A2A6A02071071 and No. 2016R1D1A1A09917463). Z.V. acknowledges the supports from the National Research, Development and Innovation Fund of Hungary, financed under the K18 funding scheme with Project No. K128947 and by the European Regional Development Fund (Grant No. GINOP-2.3.3-15-2016- 00034), as well as by the National Research, Development and Innovation Office NKFIH (Grant No. PD 124717).

In this paper we present the first systematic analysis of the impact of the populated vs. intrinsic spin distribution on the nuclear level density and γ-ray strength function retrieved through the Oslo Method. We illustrate the effect of the spin distribution on the recently performed 239Pu(d,pγ)240Pu experiment using a 12 MeV deuteron beam performed at the Oslo Cyclotron Lab. In the analysis we couple state-of-the-art calculations for the populated spin-distributions with the Monte-Carlo nuclear decay code RAINIER to compare Oslo Method results to the known input. We find that good knowledge of the populated spin distribution is crucial and show that the populated distribution has a significant impact on the extracted nuclear level density and γ-ray strength function for the 239Pu(d,pγ)240Pu case.

We have done a systematic study of the low-energy behavior of the γ-ray strength function with full-configuration shell-model calculations. We have performed calculations in several isotonic and isotopic chains, spanning several mass regions. There are systematic trends: The strength function shows a low-energy enhancement that is more pronounced near shell closures and less so in the mid-shell regions. We predict that the low-energy enhancement is omnipresent, but that for high-mass nuclei it has so far been out of experimental reach due to detector thresholds, except for the case of 151, 153Sm.

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.

The $\beta$-decay half-lives of 55 neutron-rich nuclei $^{134-139}$Sn, $^{134-142}$Sb, $^{137-144}$Te, $^{140-146}$I, $^{142-148}$Xe, $^{145-151}$Cs, $^{148-153}$Ba, $^{151-155}$La were measured at the Radioactive Isotope Beam Factory (RIBF) employing the projectile fission fragments of $^{238}$U. The nuclear level structure, which relates to deformation, has a large effect on the half-lives. The impact of newly-measured half-lives on modeling the astrophysical origin of the heavy elements is studied in the context of $r$ process nucleosynthesis. For a wide variety of astrophysical conditions, including those in which fission recycling occurs, the half-lives have an important local impact on the second ($A$ $\approx$ 130) peak.

International audience; The low-energy electromagnetic structure of 110Cd was studied using safe-energy Coulomb excitation at the Heavy Ion Laboratory, University of Warsaw. The preliminary results on the quadrupole deformation of lowlying 0+ states in 110Cd are presented and compared to the recent beyondmean-field and General Bohr Hamiltonian calculations.

The new Oslo Scintillator Array (OSCAR) has been commissioned at the Oslo Cyclotron Laboratory (OCL). It consists of 30 large volume (diameter 3.5 x 8 inches) LaBr$_3$(Ce) detectors that are used for $\gamma$-ray spectroscopy. The response functions for incident $\gamma$-rays up to 20 MeV are simulated with $\texttt{Geant4}$. In addition, the resolution, and the total and full-energy peak efficiencies are extracted. The results are in very good agreement with measurements from calibration sources and experimentally obtained mono-energetic in-beam $\gamma$-ray spectra., Comment: Changes to v2 (submitted version): Accepted for NIMA with minor revisions (implemented); Changes to v1: see v2 - major changes in experimental setup and simulations [stalled version]

The cascading 3.21 and 4.44 MeV electric quadrupole transitions have been observed from the Hoyle state at 7.65 MeV excitation energy in $^{12}\mathrm{C}$, excited by the $^{12}\mathrm{C}(p,{p}^{\ensuremath{'}})$ reaction at 10.7 MeV proton energy. From the proton-$\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ triple coincidence data, a value of ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{rad}}/\mathrm{\ensuremath{\Gamma}}=6.2(6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ was obtained for the radiative branching ratio. Using our results, together with ${\mathrm{\ensuremath{\Gamma}}}_{\ensuremath{\pi}}^{E0}/\mathrm{\ensuremath{\Gamma}}$ from Eriksen et al. [Phys. Rev. C 102, 024320 (2020)] and the currently adopted ${\mathrm{\ensuremath{\Gamma}}}_{\ensuremath{\pi}}(E0)$ values, the radiative width of the Hoyle state is determined as ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{rad}}=5.1(6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\text{ }\text{ }\mathrm{eV}$. This value is about 34% higher than the currently adopted value and will impact models of stellar evolution and nucleosynthesis.

The level structures of 70 Co and 70 Ni, populated from the β decay of 70 Fe, have been investigated using β -delayed γ -ray spectroscopy following in-flight fission of a 238 U beam. The experimental results are compared to Monte-Carlo Shell-Model calculations including the pf+g9/2+d5/2 orbitals. The strong population of a (1+) state at 274 keV in 70 Co is at variance with the expected excitation energy of ∼1 MeV from near spherical single-particle estimates. This observation indicates a dominance of prolate-deformed intruder configurations in the low-lying levels, which coexist with the normal near spherical states. It is shown that the β decay of the neutron-rich A=70 isobars from the new island of inversion to the Z=28 closed-shell regime progresses in accordance with a newly reported type of shell evolution, the so-called Type II, which involves many particle-hole excitations across energy gaps. Elsevier

Background: The neutron-deficient osmium isotopic chain provides a great laboratory for the study of shape evolution, with the transition from the soft triaxial rotor in $^{168}\mathrm{Os}$ to the well-deformed prolate rotor in $^{180}\mathrm{Os}$, while shape coexistence appears around $N=96$ in $^{172}\mathrm{Os}$. Therefore, the study of the Os isotopic chain should provide a better understanding of shape changes in nuclei and a detailed scrutiny of nuclear structure calculations. In this paper, the lifetimes of the low-lying yrast states of $^{170}\mathrm{Os}$ have been measured for the first time to investigate the shape evolution with neutron number.Purpose: Lifetimes of excited states in the ground-state band of $^{170}\mathrm{Os}$ are measured to investigate the shape evolution with neutron number in osmium isotopes and compare with state-of-the-art calculations.Methods: The states of interest were populated via the fusion-evaporation reaction $^{142}\mathrm{Nd}(^{32}\mathrm{S},4n)$ at a bombarding energy of 170 MeV at the ALTO facility from IPN (Orsay, France). Lifetimes of the ${2}_{1}^{+}$ and ${4}_{1}^{+}$ states in $^{170}\mathrm{Os}$ were measured with the recoil-distance Doppler-shift method using the Orsay universal plunger system.Results: Lifetimes of the two first excited states in $^{170}\mathrm{Os}$ were measured for the first time. A very small $B(E2;{4}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})/B(E2;{2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})=0.38(11)$ was found, which is very uncharacteristic for collective nuclei. These results were compared to state-of-the-art beyond-mean-field calculations.Conclusions: Although theoretical results give satisfactory results for the energy of the first few excited states in $^{170}\mathrm{Os}$ and the $B(E2;{2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})$ they fail to reproduce the very small $B(E2;{4}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})$, which remains a puzzle.

The Oslo method has been applied to particle-$\ensuremath{\gamma}$ coincidences following the $^{239}\mathrm{Pu}(d,p)$ reaction to obtain the nuclear level density (NLD) and $\ensuremath{\gamma}$-ray strength function ($\ensuremath{\gamma}\mathrm{SF}$) of $^{240}\mathrm{Pu}$. The experiment was conducted with a 12 MeV deuteron beam at the Oslo Cyclotron Laboratory. The low spin transfer of this reaction leads to a spin-parity mismatch between populated and intrinsic levels. This is a challenge for the Oslo method as it can have a significant impact on the extracted NLD and $\ensuremath{\gamma}\mathrm{SF}$. We have developed an iterative approach to ensure consistent results even for cases with a large spin-parity mismatch, in which we couple Green's function transfer calculations of the spin-parity dependent population cross-section to the nuclear decay code rainier. The resulting $\ensuremath{\gamma}\mathrm{SF}$ shows a pronounced enhancement between 2--4 MeV that is consistent with the location of the low-energy orbital $M1$ scissors mode.

The nuclear level density and $\ensuremath{\gamma}$-decay strength of $^{192}\mathrm{Os}$ have been extracted using particle-$\ensuremath{\gamma}$ coincidence data from the $^{192}\mathrm{Os}(\ensuremath{\alpha},{\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{\gamma})^{192}\mathrm{Os}$ reaction by means of the Oslo method. The level density is found to be a rather smooth function of excitation energy, approximately following the constant temperature model. The $\ensuremath{\gamma}$-decay strength is compared to photoneutron cross-section data above the neutron separation energy, and to $E1$ and $M1$ strengths for nuclei in this mass region derived from primary transitions following neutron capture. Our results are in good agreement with these previous data and draw a consistent picture of the $\ensuremath{\gamma}$-strength function in the range ${E}_{\ensuremath{\gamma}}\ensuremath{\approx}1.5--6\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$. Using the measured nuclear level density and $\ensuremath{\gamma}$-decay strength as input to the nuclear-reaction code talys, we provide the first experimentally constrained Maxwellian-averaged cross section (MACS) for the $^{191}\mathrm{Os}(n,\ensuremath{\gamma})^{192}\mathrm{Os}$ reaction relevant to $s$-process nucleosynthesis. The systematic uncertainties introduced by the normalization procedure of the level density and $\ensuremath{\gamma}$-strength function were investigated and propagated to the calculated Maxwellian-averaged cross section. The obtained result of the Maxwellian-averaged cross section at ${k}_{B}T=30\phantom{\rule{0.28em}{0ex}}\mathrm{keV}$, ${\ensuremath{\langle}\ensuremath{\sigma}\ensuremath{\rangle}}_{n,\ensuremath{\gamma}}=1134\ifmmode\pm\else\textpm\fi{}375\phantom{\rule{0.28em}{0ex}}\mathrm{mb}$, is in very good agreement with the theoretical estimate provided by the KADoNiS project, giving experimental support to the adopted KADoNiS value. Good agreement is also found with MACS values obtained from other libraries, such as TENDL-2017, ENDF/B-VII.0, and JEFF.

CITATION: Brits, C. P., et al. 2019. Nuclear level densities and γ-ray strength functions of 180,181,182Ta. Physical Review C, 99(5):054330, doi:10.1103/PhysRevC.99.054330.

The reduced transition probability B(E2;01+→21+) to the first excited 2+ state of the neutron-rich nucleus Te136, with two protons and two neutrons outside the doubly magic Sn132 core, was measured via Coulomb excitation at relativistic energies at the RIKEN Radioactive Isotope Beam Factory. A value of B(E2)=0.191(26) e2b2 was extracted from the measured inelastic scattering cross section on an Au target taking into account the contributions from both Coulomb and nuclear excitations. In addition, an upper limit for the transition strength to a 2+ state of mixed-symmetry character in the excitation energy range of 1.5-2.2 MeV was determined and compared to the predictions of various theoretical calculations. Because of the high statistics gathered in the present experiment the error of the deduced B(E2) value is dominated by the systematic uncertainties involved in the analysis of Coulomb excitation experiments at beam energies around 150 MeV/u. Therefore, the latter are for the first time assessed in detail in the present work.

In this work, we have studied the low-energy behaviour of the \(\gamma \)-ray strength function within the framework of the shell model. We have done calculations on isotopic and isotonic chains, spanning several mass regions. We find systematic trends, where the strength function exhibits a low-energy enhancement that is steeper near shell closures and flatter in the mid-shell regions. Further, we compile strength functions from discrete experimental data and find evidence for a low-energy enhancement.

Numerous scientific fields including astrophysics, nuclear power, and nuclear forensics require a knowledge of basic nuclear properties for large numbers of short-lived, radioactive isotopes far removed from stable nuclei. Neutron-capture cross sections are one such piece of nuclear data where direct measurements are not possible and theoretical predictions can vary by orders of magnitude. A recently developed indirect technique for inferring neutron capture rates, the β -Oslo method, has been introduced but not compared against a known, directly measured neutron capture cross section. To provide this benchmark, two indirect methods based on β decay and charged-particle reactions were used to extract the nuclear level density and γ -ray strength function of 51 Ti . The nuclear level density and γ -ray strength function from the two data sets were found to be equivalent and were used to extract the neutron capture cross section of 50 Ti which agrees with previous direct measurements at high neutron energies. The results demonstrate the validity of the β -Oslo method for extracting neutron capture cross sections of short-lived nuclei and provide a sufficiently small uncertainty to be used in various applications.

The decay of a new four-quasiparticle isomeric state in 160Sm has been observed using γ-ray spectroscopy at the RIBF, RIKEN. The four-quasiparticle state is assigned a 2 π ⊗ 2 ν π 5 2 − [ 532 ] , π 5 2 + [ 413 ] , ν 5 2 − [ 523 ] , ν 7 2 + [ 633 ] configuration. The half-life of this ( 11 + ) state is measured to be 1.8(4) μs. The ( 11 + ) isomer decays into a rotational band structure, based on a ( 6 − ) ν 5 2 − [ 523 ] ⊗ ν 7 2 + [ 633 ] bandhead, consistent with the g K − g R values. This decays to a ( 5 − ) two-proton quasiparticle state, which in turn decays to the ground state band. Potential energy surface and blocked-BCS calculations were performed in the deformed midshell region around 160Sm. They reveal a significant influence from β 6 deformation and that 160Sm is the best candidate for the lightest four-quasiparticle K isomer to exist in this region. The relationship between reduced hindrance and isomer excitation energy for E1 transitions from multiquasiparticle states is considered with the new data from 160Sm. The E1 data are found to agree with the existing relationship for E2 transitions.

We are also grateful for the financial support received from the Research Council of Norway (NFR). S.S. and G.M.T. acknowledge funding under NFR project Grants No. 210007 and No. 262952/F20. A.C.L. acknowledges financial support from the ERC-STG2014 under Grant No. 637686.

Lifetimes of several short-lived excited states in Nd138 were measured with the ROSPHERE array at IFIN-HH, Bucharest, using the recoil-distance Doppler shift technique following the Sb123(F19,4n) reaction. The resulting electromagnetic transition probabilities are compared to large-scale shell model calculations and to constrained Hartree-Fock-Bogoliubov calculations with the Gogny D1S interaction, configuration mixing, and a five-dimensional collective Hamiltonian formalism. The onset of collectivity in Nd isotopes below the N=82 shell closure and the deformation induced by the alignment of protons and neutron holes in the h11/2 orbitals are discussed.

International audience; Excited states in the neutron-rich nucleus 150Ba have been observed via β–γ spectroscopy at the Radioactive Isotope Beam Factory, RIKEN Nishina Center. The 150Ba ions were produced by the in-flight fission of a 238U beam with an energy of 345 MeV/nucleon. The E(2+) energy of 150Ba was identified at 100 keV, which is the lowest known in the neutron-rich Ba isotopes. A γ -ray peak was also observed at 597 keV. A mean-field calculation with a fully 3D real space was performed and a static octupole deformation was obtained for the Ba isotopes. Kπ = 0− and 1− excited states with significant octupole collectivity were newly predicted at around or lower than 1 MeV on the ground state of 150Ba by a random-phase approximation calculation. The 597 keV γ ray can be interpreted as a negative-parity state, showing that 150Ba may possess octupole collectivity.

The low-lying structures of the midshell νg9/2 Ni isotopes 72Ni and 74Ni have been investigated at the RIBF facility in RIKEN within the EURICA collaboration. Previously unobserved low-lying states were accessed for the first time following β decay of the mother nuclei 72Co and 74Co. As a result, we provide a complete picture in terms of the seniority scheme up to the first (8+) levels for both nuclei. The experimental results are compared to shell-model calculations in order to define to what extent the seniority quantum number is preserved in the first neutron g9/2 shell. We find that the disappearance of the seniority isomerism in the (81+) states can be explained by a lowering of the seniority-four (6+) levels as predicted years ago. For 74Ni, the internal de-excitation pattern of the newly observed (62+) state supports a restoration of the normal seniority ordering up to spin J=4. This property, unexplained by the shell-model calculations, is in agreement with a dominance of the single-particle spherical regime near 78Ni. The low-lying structures of the midshell νg9/2 Ni isotopes 72Ni and 74Ni have been investigated at the RIBF facility in RIKEN within the EURICA collaboration. Previously unobserved low-lying states were accessed for the first time following β decay of the mother nuclei 72Co and 74Co. As a result, we provide a complete picture in terms of the seniority scheme up to the first (8+) levels for both nuclei. The experimental results are compared to shell-model calculations in order to define to what extent the seniority quantum number is preserved in the first neutron g9/2 shell. We find that the disappearance of the seniority isomerism in the (81+) states can be explained by a lowering of the seniority-four (6+) levels as predicted years ago. For 74Ni, the internal de-excitation pattern of the newly observed (62+) state supports a restoration of the normal seniority ordering up to spin J=4. This property, unexplained by the shell-model calculations, is in agreement with a dominance of the single-particle spherical regime near 78Ni. ispartof: Physics Letters B vol:781 pages:706-712 status: published

We have made a thorough study of the low-energy behaviour of the $\gamma$-ray strength function within the framework of the shell model. We have performed large-scale calculations spanning isotopic and isotonic chains over several mass regions, with the purpose of studying the systematic behavior of the low-energy enhancement (LEE) for $M1$ transitions. There are clear trends in the calculations: From being all but absent in the lowest mass region, the LEE becomes steeper and more pronounced as the mass number increases, and for a given mass region it further increases towards shell closures. Moreover, the LEE is found to be steeper in regions near doubly-magic nuclei where proton particles couple to neutron holes. These trends enable us to consolidate several previous works on the LEE into a single, consistent concept. We compare the inferred trends to the available experimental data from the Oslo method, and find suppport for the systematic behaviour. Lastly we have compared the calculations to strength functions compiled from discrete, experimental lifetimes, and find excellent agreement; the discrete data are consistent with a LEE, and indicate that the slope varies as function of mass number., Comment: 11 pages, 13 figures

Excited states have been studied in \iso{159}{Sm}, \iso{161}{Sm}, \iso{162}{Sm} (Z~=~62), \iso{163}{Eu} (Z~=~63), and \iso{164}{Gd} (Z~=~64), populated by isomeric decay following 238U projectile fission at RIBF, RIKEN. The isomer half-lives range from 50 ns to 2.6 \mus. In comparison with other published data, revised interpretations are proposed for \iso{159}{Sm} and \iso{163}{Eu}. The first data for excited states in \iso{161}{Sm} are presented, where a 2.6 \mus isomer is assigned a three-quasiparticle, K^\pi = 17/2^- structure. The interpretation is supported by multi-quasiparticle Nilsson-BCS calculations, including the blocking of pairing correlations. A consistent set of reduced E1 hindrance factors is obtained. Limited evidence is also reported for isomeric decay in 163Sm, 164Eu and 165Eu.

We report for the first time the β-decay scheme of Te140 (Z=52) to I140 (Z=53), with a specific focus on the Gamow-Teller strength along N=87 isotones. These results were obtained in an experiment performed at the Radioactive Ion Beam Factory (RIBF), RIKEN, where the parent nuclide, Te140, was produced through the in-flight fission of a U238 beam at 345 MeV per nucleon impinging on a Be9 target. Based on data from the high-efficiency γ-ray spectrometer, EUROBALL-RIKEN Cluster Array (EURICA), we constructed a decay scheme of I140. The half-life of Te140 has been determined to be 350(5) ms. A level at 926 keV has been assigned as a (1+) state based on the logft value of 4.89(6). This (1+) state, commonly observed in odd-odd nuclei, can be interpreted in terms of the πh11/2νh9/2 configuration formed by the Gamow-Teller transition between a neutron in the h9/2 orbital and a proton in the h11/2 orbital. We observe a sharp contrast to this type of β-decay branching to the lower-lying 1+ states between I140 and I136, where we see a large reduction as the number of neutrons increases. This is in contrast to the prediction by large-scale shell model calculations. To investigate this type of the suppression, results of the Nilsson model calculations will be discussed. Along the isotones with N=87, we discuss a characteristic feature of the Gamow-Teller distributions at 1+ states with respect to the isospin difference.

The $\ensuremath{\gamma}$ decay of $^{65}\mathrm{Ni}$ has been studied from particle-$\ensuremath{\gamma}$ coincidence data on the $^{64}\mathrm{Ni}(d,p\ensuremath{\gamma})^{65}\mathrm{Ni}$ reaction. $\ensuremath{\gamma}$-ray spectra at excitation energies below ${E}_{x}\ensuremath{\approx}2\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$ have been studied and compared with previous measurements. Coincidences corresponding to ${E}_{x}\ensuremath{\approx}4.4--6.1\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$ have been used to constrain the shape of the nuclear level density and $\ensuremath{\gamma}$-strength function of $^{65}\mathrm{Ni}$ by means of the Oslo method. The experimental $\ensuremath{\gamma}$-strength function presents an enhancement at $\ensuremath{\gamma}$ energies below ${E}_{\ensuremath{\gamma}}\ensuremath{\approx}3\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$. In addition, a resonance-like structure centered at ${E}_{\ensuremath{\gamma}}\ensuremath{\approx}4.6\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$ is seen together with accumulated strength at ${E}_{\ensuremath{\gamma}}\ensuremath{\approx}2.6--3.6\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$. The obtained results contribute to the systematic study of $\ensuremath{\gamma}$ decay in the Ni isotopes, which is of great interest for the understanding of both single-particle and collective nuclear structure phenomena.

The level structure of the neutron-rich ^{77}Cu nucleus is investigated through β-delayed γ-ray spectroscopy at the Radioactive Isotope Beam Factory of the RIKEN Nishina Center. Ions of ^{77}Ni are produced by in-flight fission, separated and identified in the BigRIPS fragment separator, and implanted in the WAS3ABi silicon detector array, surrounded by Ge cluster detectors of the EURICA array. A large number of excited states in ^{77}Cu are identified for the first time by correlating γ rays with the β decay of ^{77}Ni, and a level scheme is constructed by utilizing their coincidence relationships. The good agreement between large-scale Monte Carlo shell model calculations and experimental results allows for the evaluation of the single-particle structure near ^{78}Ni and suggests a single-particle nature for both the 5/2_{1}^{-} and 3/2_{1}^{-} states in ^{77}Cu, leading to doubly magic ^{78}Ni.

Excited states in the nucleus 133Sn, with one neutron outside the doubly-magic 132Sn core, were populated following one-neutron knockout from a 134Sn beam on a carbon target at relativistic energies at the Radioactive Isotope Beam Factory at RIKEN. Besides the rays emitted in the decay of the known neutron single-particle states in 133Sn additional strength in the energy range 3.5-5.5 MeV was observed for the fi rst time. Since the neutron-separation energy of 133Sn is low, Sn=2.402(4) MeV, this observation provides direct evidence for the radiative decay of neutron- unbound states in this nucleus. The ability of electromagnetic decay to compete successfully with neutron emission at energies as high as 3 MeV above threshold is attributed to a mismatch between the wave functions of the initial and final states in the latter case. These fi ndings suggest that in the region south-east of 132Sn nuclear structure effects may play a signifi cant role in the neutron vs. competition in the decay of unbound states. As a consequence, the common neglect of such effects in the evaluation of the neutron-emission probabilities in calculations of global b -decay properties for astrophysical simulations may have to be reconsidered.

A particularly challenging part of the question of how elements heavier than iron are created in extreme, astrophysical environments is the creation of the so-called p-nuclei, which are believed to be mainly produced in the O-Ne layer of type II supernovae or type Ia supernovae. The lack of needed nuclear data presents an obstacle in nailing down the precise site and astrophysical conditions for the production of these isotopes. Astrophysical model calculations are able to reproduce abundance patterns of most p-isotopes reasonably well, with some pivotal exceptions. In particular, p-isotopes of mass 92 ≤ A ≤ 98 are underproduced in calculations compared to the actual abundance of these isotopes. The p-isotope 92Mo represents one of the most severe cases of underproduction. State-of-the-art p-process calculations systematically underestimate the observed solar abundance of this isotope. The main destruction mechanism of this isotope in the standard description of the p-process is through the 92Mo(γ, p)91Nb reaction. Measurements on the nuclear level density and average gamma strength function of 92Mo have been carried out at the Oslo Cyclotron Laboratory. TALYS cross section and reaction rate calculations where the experimental results are used as input will be presented. The data provide stringent constraints on the 91Nb(p, γ)92Mo (and consequently the inverse) reaction rate. The reaction rate extracted in this work was used in reaction network calculations for the scenario of a p-process taking place in a type II supernova explosion as the shock front passes through the O-Ne layer of a 25 solar mass star.

We study for the first time the internal structure of Te140 through the β-delayed γ-ray spectroscopy of Sb140. The very neutron-rich Sb140 nuclei with Z=51 and N=89 were produced by the in-flight fission of U238 beams at the Radioactive Isotope Beam Factory, RIKEN. The half-life and spin-parity of Sb140 are reported as 173±12 ms and 3−, respectively. In addition to the excited states of Te140 produced by the β-decay branch, the β-delayed one-neutron and two-neutron emission branches were also established. By identifying the first 2+ and 4+ excited states of Te140, we found that Te isotopes persist in their vibrator character with E(4+)/E(2+)=2. We discuss the distinctive features manifest in this region revealed in pairs of isotopes with the same neutron holes and particles with respect to N=82.

CITATION: Kheswa, B. V., et al. 2017. ¹³⁷,¹³⁸,¹³⁹La(n,γ) cross sections constrained with statistical decay properties of ¹³⁸,¹³⁹,¹⁴⁰La nuclei. Physical Review C, 95(4):1-9, doi:10.1103/PhysRevC.95.045805.

M. Klintefjord et al. -- 11 pags., 8 figs., 1 tab. -- Open Access funded by Creative Commons Atribution Licence 4.0