Near-yrast States Research Articles

High-spin spectroscopy in At207 : Evidence of a 29/2+ isomeric state

Yrast and near-yrast states above the known 25/2$^{+}$ isomer in $^{207}$At are established for the first time. The level scheme is extended up to 47/2$\hbar$ and 6.5 MeV with the addition of about 60 new $\gamma$-ray transitions. The half-life of the 25/2$^{+}$ isomer is revisited and a value of $T_{1/2}$ = 107.5(9) ns is deduced. Evidence of a hitherto unobserved 29/2$^{+}$ isomer in $^{207}$At is presented. A systematic study of $B(E3)$ values for the transitions de-exciting the 29/2$^{+}$ isomer in the neighboring odd-$A$ At isotopes suggests a half-life in the 2$-$4.5 $\mu$s range for this state in $^{207}$At. The experimental results are compared with large-scale shell-model calculations performed using the KHM3Y effective interaction in the $Z$ = 50$-$126, $N$ = 82$-$184 model space and an overall good agreement is noted between the theory and the experiment. A qualitative comparison of the excited states and the isomers with analogous states in neighboring nuclei provides further insight into the structure of $^{207}$At.

Experimental and shell-model study of excited states in Fe292655 and related notes on Cu262955

The fusion-evaporation reaction $^{32}\mathrm{S}+^{28}\mathrm{Si}$ at 125-MeV beam energy was used to populate excited states in $^{55}\mathrm{Fe}$. Combining the Gammasphere spectrometer with ancillary devices including the Microball CsI(Tl) array and a shell of neutron detectors, a comprehensive level scheme could be derived. The experimental results are compared with theoretical results from shell-model calculations. Taking into account isospin-symmetry breaking terms is found to considerably improve the shell-model description for $^{55}\mathrm{Fe}$. This motivated a predictive case study of near-yrast states in the mirror nucleus $^{55}\mathrm{Cu}$.

Structure of the neutron mid-shell nuclei Ag64,66111,113

$^{111}$Ag and $^{113}$Ag were produced in induced fission reaction, where yrast and near-yrast states were populated. To interpret the new data the Interacting Boson-Fermion model was used. A good agreement with the experimental data is achieved, suggesting that the two Ag nuclei have a well developed collectivity, superimposed on $\pi g_{9/2}^{-3}$ excitations previously observed throughout the entire isotopic chain.

Neutron effective single-particle energies above Ni78 : A hint from lifetime measurements in the N=51 isotones Se85 and Kr87

Background: While the N=50 shell-gap evolution towards Ni78 is presently in the focus of nuclear structure research, experimental information on the neutron effective single-particle energy sequence above the Ni78 core remain scarce. Direct nucleon-exchange reactions are indeed difficult with presently available post-accelerated radioactive-ion beams (especially for high orbital-momentum orbitals) in this exotic region. Purpose: In this study we probe the evolution of the ν1g7/2 effective single-particle energy which is a key to understanding the possible evolution of the spin-orbit splitting due to the proton-neutron interaction in the Ni78 region. To achieve this goal, a method based on lifetime measurements is used for the first time. The obtained lifetimes of the 7/21+ states in Kr87 and Se85 are used to investigate the ν1g7/2 evolution. Method: Yrast and near-yrast states in the light N=51 isotones Se85 and Kr87 were populated via multinucleon transfer reactions, using a Se82 beam and a U238 target at the LNL tandem-ALPI facility. The prompt γ rays were detected by the AGATA Demonstrator and particle identification was performed using the PRISMA spectrometer. Lifetime measurements were performed by using the Cologne plunger device for deep inelastic reactions and the Recoil Distance Doppler Shift technique. Results: We obtain τ(7/21+)=0.4−0.4+1.6 ps for Kr87. In the case of Se85 an upper limit of 3(2) ps is obtained for the τ7/21+ value. Conclusion: For Kr87, the measured (7/21+) lifetime is consistent with a core-coupled 2+⊗ν2d5/2 configuration for this state. This result is consistent with that obtained by direct reaction, which validates our method. For Se85, the measured 7/21+ lifetime limit indicates a very small contribution of the ν1g7/2 configuration to the wave function of this state.8 MoreReceived 13 December 2016Revised 20 June 2017DOI:https://doi.org/10.1103/PhysRevC.96.044320©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasEnergy levels & level densitiesLifetimes & widthsNuclear structure & decaysShell modelProperties59 ≤ A ≤ 89Nuclear Physics

Yrast structure of the shell model nucleusNb89

Yrast and near-yrast states in $\text{odd-}A$ $^{89}\mathrm{Nb}$ were investigated using the $^{65}\mathrm{Cu}(^{28}\mathrm{Si}$,$2p2n)$ reaction at a beam energy of 105 MeV. The $\ensuremath{\gamma}\text{-ray}$ coincidence events were recorded with the Indian National Gamma Array spectrometer. About 30 new transitions have been observed extending the level structure of this nucleus up to spin $45/2\ensuremath{\hbar}$ and excitation energy of 10.5 MeV. Large-scale shell model calculations were performed using the effective interactions JUN45 and jj44b to understand the structure of the observed states. Reasonable agreement between the experimental observation and shell model calculations suggests that no cross-shell excitations are important up to the maximum spin observed in the current experiment.

Cross-shell excitations in 30Al and 30Si at high spin

Yrast and near-yrast states in 30Al and 30Si have been populated to high spin with the O 18 + C 14 fusion–evaporation reaction in inverse kinematics. The level schemes for these two isobars have been extended up to J ∼ 9 ℏ at 9.4 and 15.5 MeV, respectively. Their decay schemes indicate that cross-shell excitations dominate at high spin, where negative-parity structures exist. Positive-parity states are compared to the results of shell-model calculations using the USD, USDA, and USDB effective interactions. The negative-parity levels are compared to predictions of the WBP interaction and the recently-developed WBP-a Hamiltonian, by allowing 1p–1h excitations to fp-shell orbitals. The results suggest that single-neutron excitations to the 0 f 7 / 2 orbital play a significant role at high spin.

Yrast studies ofSe80,82using deep-inelastic reactions

We report the results of an experiment in which we studied the near-yrast states in selenium isotopes approaching N=50 following their population in multinucleon transfer reactions between a Se-82 beam and a Os-192 target. The level schemes for Se-80,Se-82 derived from the current work are compared with restricted-basis shell-model calculations and pair-truncated shell-model calculations. These provide a good description of the yrast sequences in these nuclei using a basis space limited to excitations in the nu(p3/2, p1/2, p1/2) and pi(f5/2, p3/2, p1/2) orbitals.

Spectroscopy of 91Zr51 at medium to high spins

= 3167 keV have been identified for thefirst time in this work, a tentative decay scheme of near-yrast states toexcitation energies in excess of 10 MeV has been constructed. These statesare of relevance to shell model structures formed via limited valence-spaceconfigurations in this semi-magic N = 51 nucleus.PACS numbers: 21.10.–k, 21.60.Cs, 27.60.+j, 29.30.Kv

RDM lifetime measurements in 107Cd

Lifetimes for decays linking near-yrast states in 107Cd have been measured using the recoil distance method (RDM). The nucleus of interest was populated via the 98Mo(12C,3n)107Cd fusion–evaporation reaction at an incident beam energy of 60 MeV. From the measured lifetimes, transition probabilities have been deduced and compared with the theoretical B(E2) values for limiting cases of harmonic vibrational and axially deformed rotational systems. Our initial results suggest a rotor-like behaviour for the structure based on the unnatural-parity, h11/2 orbital in 107Cd, providing further evidence for the role of this ‘shape-polarizing’ orbital in stabilizing the nuclear deformation in the A ∼ 100 transitional region.

High-spin isomer in 93Mo

The high-spin states of 93Mo have been studied by a 82Se( 16O, 5n) 93Mo reaction at a beam energy of 100 MeV using techniques of in-beam γ-ray spectroscopy. Measurements of γ-t, γ-γ-t coincidences, γ-ray angular distributions and γ-ray linear polarizations were performed. The high-spin isomer was found as a (39/2-) state at about 9.7 MeV. The near-yrast states in 93Mo were interpreted using the weak-coupling picture of a d5/2 neutron to a neutron magic core nucleus 92Mo.

Ba136studied via deep-inelastic collisions: Identification of the(νh11∕2)10+−2isomer

A multinucleon transfer reaction between a thin self-supporting Pt 78198 target and an 850MeV Xe 54136 beam has been used to populate and study the structure of the N=80 isotone Ba 56136. Making use of time-correlated γ-ray spectroscopy, evidence for an Iπ=(10+) isomeric state has been found with a measured half-life of 91±2ns. Prompt-delayed correlations have also enabled the tentative measurement of the near-yrast states which lie above the isomer. Shell-model calculations suggest that the isomer has a structure which can be assigned predominantly as (νh11∕2)10+−2. The results are discussed in terms of standard and pair-truncated shell-model calculations, and compared to the even-Z N=80 isotones ranging from Sn 50130 to Er 68148. A qualitative explanation of the observed dramatic decrease in the B(E2:10+→8+) value for the N=80 isotones at Ba136 is given in terms of the increasing single-hole energy of the h11∕2 neutron configuration as the proton subshell is filled. The angular momentum transfer to the binary fragments in the reaction has also been investigated in terms of the average total γ-ray fold versus the scattering angle of the recoils.9 MoreReceived 26 September 2003DOI:https://doi.org/10.1103/PhysRevC.69.024316©2004 American Physical Society

Binary-reaction spectroscopy ofMo99,100: Intruder alignment systematics inN=57andN=58isotones

The near-yrast states of $^{99,100}\mathrm{Mo}$ have been studied following their population via a binary reaction between a $^{136}\mathrm{Xe}$ beam and a thin, self-supporting $^{100}\mathrm{Mo}$ target. The yrast sequence in $^{100}\mathrm{Mo}$ has been extended to a tentative spin∕parity $({20}^{+})$, while the decoupled band built on the ${I}^{\ensuremath{\pi}}={\frac{11}{2}}^{\ensuremath{-}}$ isomeric state in $^{99}\mathrm{Mo}$ has been extended through the first alignment up to a tentative spin∕parity of $({\frac{43}{2}}^{\ensuremath{-}})$. The results are compared with self-consistent, cranked-mean-field calculations using a Woods-Saxon potential. The alignment systematics of the intruder ${h}_{11∕2}$ bands in the $N=57$ isotones from Mo $(Z=42)$ to Cd $(Z=48)$ and the yrast sequences in their $N=58$ even-even neighbors are discussed. An overall picture emerges, where the alignment properties evolve from being due to positive-parity neutrons in the $_{48}^{105}\mathrm{Cd}$ to predominantly ${({g}_{9∕2})}^{2}$ proton crossings closer to the $Z=40$ subshell. Qualitatively, this can be explained by an increase in the quadrupole deformation and a simultaneous lowering of the proton Fermi surface in the ${g}_{9∕2}$ shell with decreasing proton number. These data provide excellent examples of rotational-alignment phenomena in weakly deformed nuclei.

Excitations of two- and three-valence-proton nuclei134Teand135I

Analyses of ${}^{248}\mathrm{Cm}$ fission product \ensuremath{\gamma}-ray coincidence data recorded at Gammasphere have yielded additional information about $\ensuremath{\gamma}$-ray cascades in $N=82$ isotones ${}^{134}\mathrm{Te}$ and ${}^{135}\mathrm{I}.$ New-yrast and near-yrast states in both nuclei have been identified, and they are interpreted as specific shell model excitations.

Level statistics of near-yrast states in rapidly rotating nuclei

The nearest neighbour level spacing distribution and the Δ 3 statistic of level fluctuations associated with very high spin states ( I ≳ 30) in rare-earth deformed nuclei are analysed by means of a cranked shell model. The many particle-many hole configurations created in the rotating Nilsson potential are mixed by the surface-delta two-body residual interaction. The levels in the near-yrast region show a Poisson-like level spacing distribution. As the intrinsic excitation energy U increases, the level statistics shows a gradual transition from order to chaos, reaching at U ≳ 2 MeV the Wigner distribution typical-of the Gaussian orthogonal ensemble of random matrices. This transition is caused by the residual two-body interaction. On the other hand, the level spacings between the yrast and the first excited state show a peculiar behaviour, displaying a Wigner-like distribution instead of the Poisson-like distribution seen for the other near-yrast rotational states. The lowest spacings reflect the properties of the single-particle orbits in the mean-field, and are only weakly affected by the residual two-body interaction.

Negative-parity states near the yrast line in 144Nd.

The near-yrast spectrum of the N=84 isotope $^{144}\mathrm{Nd}$ has been studied using the $^{130}\mathrm{Te}$${(}^{18}$O,4n) reaction at a lab energy of 85 MeV and the techniques of \ensuremath{\gamma}-ray spectroscopy. Sixteen new states and 23 new \ensuremath{\gamma} rays were placed in the level scheme. Most of the yrast and near-yrast negative-parity states have been interpreted in terms of empirical shell-model configurations. However, the interpretation of a sequence of states from J=17--24 in terms of the ${\mathit{h}}_{11/2}^{3}$${\mathit{g}}_{7/2}^{\mathrm{\ensuremath{-}}1}$${\mathit{d}}_{5/2}^{\mathrm{\ensuremath{-}}6}$ proton configuration is problematic because the observed energies cannot be satisfactorily reproduced with empirical shell-model calculations.

Electromagnetic properties of 161Tm at high spin

Abstract The decay of the yrast and near-yrast high-spin states in 161Tm has been observed following the reaction 128Te(37Cl,4n). Extension of the known level scheme reveals previously unobserved crossings in the bands built on the 7 2 − [523] , 7 2 + [404] and 1 2 − [541] Nilsson states. The use of both gold- and terbium-backed targets has enabled the measurement by the Doppler shift attenuation method of lifetimes of states in the h 11 2 yrast band in the spin range 41 2 ⩽ I ⩽ 67 2 . The results show little evidence for change in the nuclear deformation over this interval in spin (Qt ≈ 6 eb). From lifetime measurements and B( M1 ) B( E2 ) values in the h 11 2 band, absolute B(M1) values for five transitions (for states with 41 2 ⩽ I ⩽ 51 2 ) were determined. These results provide some evidence for a reduction in the proton spin projection on the deformation axis relative to its value at low rotational frequency. The signature splitting in the Routhians and B( M1 ) B( E2 ) values for the h 11 2 band is discussed in terms of the inclusion of non-axially-symmetric (γ-deformed) components in the nuclear wavefunction.

Yrast level structure of the neutron deficient N=49 isotones 92Tc, 93Ru, 94Rh, and 95Pd up to high angular momentum.

The projectile-target system $^{40}\mathrm{Ca}$${+}^{58}$Ni has been used to produce the neutron deficient N=49 isotones $^{92}\mathrm{Tc}$, $^{93}\mathrm{Ru}$, $^{94}\mathrm{Rh}$, and $^{95}\mathrm{Pd}$. The multidetector array NORDBALL including particle selection was used. The yrast level scheme could in three cases be determined up to and above spin values corresponding to the highest angular momenta attainable in the previously used configuration spaces for shell model calculations. The yrast level schemes of the N=49 isotones bear a striking relationship to those of the corresponding N=50 isotones. By interpreting the experimental high spin yrast structure in shell model configurations we hope to facilitate the choice of a restricted configuration space to be used for calculations. Generally, there is an intrinsic simplicity in most of the observed yrast or near-yrast states.

Shape-driving effects in 193Tl from the spectroscopy of yrast and near-yrast states

The nucleus 193Tl has been populated with the 160Gd( 37Cl,4n) reaction and its level scheme has been extended up to spin 41 2 h ̷ by using an array of 12 Compton-suppressed Ge detectors. Near-oblate band structures arising from a 9 − 2 state and markedly different, single-particle-like, structures above a 13 + 2 state have been established. The two structures are consistent with total routhian surface calculations indicating the existence of two minima in the total routhian surface of the nucleus which survive separately up to the observed spins. Details of the rotational-like part are discussed in terms of the cranked shell model, while the single-particle-like features of the nucleus are compared with non-collective structures in neighboring nuclei, in particular 191Hg.

Lifetimes of yrast and near-yrast states of 159,160,161Yb.

Lifetimes of high-spin states of /sup 159,160,161/Yb, populated by fusion-evaporation reactions, have been measured by the recoil-distance technique. The data were collected in coincidence with a total-energy filter to provide some discrimination on reaction channel. The principal feature of the resulting transition quadrupole moments is a frequency-dependent reduction which is displayed by those bands with aligned quasiparticles. This seems to be mainly due to the influence of the favored i/sub 13/2/ quasineutron. Additional available data are surveyed and support this conclusion. Calculations using the cranked shell model and the cranked Hartree-Fock-Bogoliubov and cranked Nilsson-Strutinsky methods provide a qualitative description of the experimental observations. In addition, a satisfactory account is provided by recent calculations based on a fermion dynamical-symmetry model.

Yrast traps and high-spin states in 210Rn

Yrast and near-yrast states have been investigated in 210Rn to high spin (J > 30) and high energy (E x > 10 MeV). Three different (HI, xn) reactions were used to populate the states of interest and several different γ-ray spectroscopic techniques were utilized. Three high-spin yrast traps were discovered. Two de-excite by strong E3 transitions while the third decays mainly via an extremely inhibited E2 transition. The E3 decays are interpreted as allowed single-particle transitions between proton or neutron states above the 208Pb shell closure while the inhibited E2 transition is interpreted as indicating a substantial change in structure as the decay proceeds down the yrast line. The interpretation has been given in terms of shell-model calculations.