Γ-ray Spectroscopic Techniques Research Articles

High-Spin States in93Nb

The high-spin states of 93 Nb are studied by the 82 Se( 16 O, p4n) 93 Nb reaction at a beam energy of 100 MeV. The in-beam γ-ray spectroscopic techniques are used. The γt-, γγt-coincidence, γ-ray a...

Single-particle States in 23F

The proton single-particle states have been investigated using the in-beam γ-ray spectroscopy technique via the following reactions: 4He(22O,23F); 4He(23F,23F); 4He(24F,23F); and 4He(25Ne,23F). The characters of excited states in 23F were deduced by comparison of the population strengths in these reactions. We found that the 2268- and 4059-keV levels have strong proton single-particle nature of the s1/2 and d3/2 orbitals, respectively. These singleparticle states are located at higher excitation energies than the shell model predictions for the USD and SDPF-M interactions, which indicates that the centroid energy gaps for πd5/2−πs1/2 and πd5/2−πd3/2 are wider than predicted.

Proton single-particle states in the neutron-rich 23F nucleus

Abstract The proton shell structure in neutron-rich fluorine 23F was investigated using the in-beam γ-ray spectroscopy technique via the proton transfer reaction onto the 22O nucleus. We also measured the α inelastic scattering on 23F, the neutron knockout from 24F, and the two-nucleon knockout from 25Ne using a cocktail secondary beam. By comparison of the population strengths for excited states in these reactions, we deduced the character of the excited states. The levels and γ-decay scheme in 23F were proposed based on de-excitation γ-rays and the products of these reactions. We found that the 2268- and 4059-keV levels have strong proton single-particle nature of the s 1 / 2 and d 3 / 2 orbitals, respectively, by analysis of the population strengths and the angular distributions of the states. These single-particle states are located at higher excitation energies than the shell model predictions for the USD and SDPF-M interactions, which indicates that the centroid energy gaps for π d 5 / 2 – π s 1 / 2 and π d 5 / 2 – π d 3 / 2 are wider than predicted.

Rotational structures in the 125Cs nucleus

The collective band structures of the 125Cs nucleus have been investigated by in-beam γ-ray spectroscopic techniques following the 110Pd ( 19F, 4n) reaction at 75MeV. The previously known level scheme, with rotational bands built on πg7/2, πg9/2 and πh11/2 orbitals, has been extended and evolves into bands involving rotationally aligned ν(h11/2)2 and π(h11/2)2 quasiparticles. A strongly coupled band has been reassigned a high-K πh11/2 ⊗ νg7/2 ⊗ νh11/2 three-quasiparticle configuration and a new side band likely to be its chiral partner has been identified. Configurations assigned to various bands are discussed in the framework of Principal/Tilted Axis Cranking (PAC/TAC) model calculations.

Band structures in 123I

Excited states of 123I were populated via the 116Cd(14N, α3n) reaction at 65 MeV. The resultant γ-rays were detected using standard γ-ray spectroscopic techniques with the NORDBALL detector array. Two previously known positive-parity ΔI = 2 sequences have been extended up to 31/2+ and 41/2+. In addition, a number of ΔI = 1 transitions linking the two ΔI = 2 sequences have been observed. It is suggested that both ΔI = 2 sequences are based on a common configuration. This ΔI = 1 band is proposed to be built predominantly on the g7/2[404]7/2+ oblate configuration, based on the energy-level spectra, B(M1)/B(E2) ratios and the theoretical predictions from the particle-rotor model. The previously identified ΔI = 1 rotational band built on the prolate g9/2[404]9/2+ orbital has also been extended to higher spins. Another previously identified but weakly populated ΔI = 1 band is confirmed and is proposed to be built on the d5/2[413]5/2+ configuration with the ground state of 123I as the bandhead.

Search for neutron excitations across theN=20shell gap inNe25−29

Nuclear structure of the neutron rich Ne25−29 nuclei has been investigated through the in-beam γ-ray spectroscopy technique using fragmentation reactions of both stable and radioactive beams. Level schemes have been deduced for these Ne isotopes. In order to examine the importance of intruder fp configurations, they are compared to shell model calculations performed either in the restricted sd or in the larger sdpf valence space. The Ne25,26 and Ne27 nuclei were found to be in agreement with the sd shell model calculations, whereas Ne28 exhibits signatures of the intruder fp shell contribution.4 MoreReceived 27 October 2004DOI:https://doi.org/10.1103/PhysRevC.72.054316©2005 American Physical Society

Rotational structures in 123Cs

High-spin states in 123Cs, populated via the 100Mo ( 28Si, p4n) fusion-evaporation reaction at Elab = 130 MeV, have been investigated employing in-beam γ-ray spectroscopic techniques. Rotational bands, built on πg7/2, πg9/2 and the unique-parity πh11/2 orbitals, have been extended and evolve into bands involving rotationally aligned ν(h11/2)2 and π(h11/2)2 quasiparticles. A three-quasiparticle band based on the high-K πh11/2 ⊗ νg7/2 ⊗ νh11/2 configuration has also been observed. Total Routhian Surface (TRS) calculations have been used to predict the nuclear shape parameters ( β2, β4, γ) for the various assigned configurations. The assigned configurations have been discussed in the framework of a microscopic theory based on the deformed Hartree-Fock (HF) and angular-momentum projection techniques.

Study of single-particle states in 23F using proton transfer reaction

The proton shell structure in neutron-rich fluorine 23F was investigated using the in-beam γ-ray spectroscopy technique via the proton transfer reaction onto the unstable nucleus 22O, in addition to α inelastic scattering on 23F, and the neutron-knockout reaction from 24F. The level and γ-decay scheme in 23F was deduced from de-excitation γ-ray-particle coincidence events. We found that a single-particle state at 4.061MeV has a large contribution from the d shell by the analysis of population strengths and the angular distribution for the state. We reported here the present experiment and the preliminary results.

Collective structures of the 131Cs nucleus

The collective structures of 131Cs have been investigated by in-beam γ-ray spectroscopic techniques following the 124Sn (^11B , 4n) reaction at a beam energy Elab = 57MeV. The previously established rotational bands, built on πg7/2, πd5/2 and the unique-parity πh11/2 orbitals, have been extended and evolve into new bands involving rotationally aligned ν(h11/2)2 and π(h11/2)2 quasiparticles. In addition, a new multiquasiparticle band based on the πg7/2 ⊗ νg7/2 ⊗ νh11/2 configuration has also been observed. Theoretical interpretations for the assigned configurations are discussed in the framework of Total Routhian Surface (TRS) and Tilted Axis Cranking (TAC) model calculations. TAC model calculations predict a decrease in the B(M1) values with increasing rotational frequency for the πg7/2/πd5/2 ⊗ ν(h11/2)2 and πh11/2 ⊗ ν(h11/2)2 bands, thus indicating a magnetic rotation character for these bands.

Multi-quasiparticle excitations in 145Tb

High-spin states in 145Tb have been investigated by means of in-beam γ-ray spectroscopy techniques with the 118Sn(32S, 1p4n) reaction. Excitation functions, X–γ–t and γ–γ–t coincidences and γ-ray anisotropies were measured. A level scheme of 145Tb was established up to Ex≈ 7 MeV. The level structure shows characteristics of a spherical nucleus. Based on the systematics of level structure in the odd-A N = 80 isotones, the level structure below 2 MeV excitation is interpreted by coupling an h11/2 proton to the excitations in the even–even 144Gd core. Above 2 MeV excitation, most of the yrast levels are interpreted with multi-quasiparticle shell-model configurations.

Matter with strangeness

A recent experimental progress of strangeness nuclear physics is reviewed from viewpoint of hyperon-nucleon ( YN) and hyperon-hyperon ( YY) interactions. Detailed level structure of Λ hypernuclei has been studied by γ-ray spectroscopy technique using a germanium detector array as well as an NaI counter array, which determined the strengths of the ΛN spin-spin and spin-orbit interactions. In the emulsion-counter hybrid experiment, a double Λ hypernucleus ΛΛ 6 He was observed and the strength of the ΛΛ interaction was unambiguously derived. As for the ΣN interaction, a recent 28Si( π −, K +) spectrum suggests a strongly repulsive Σ-nucleus potential, while the spin-orbit force of Σ is being studied by a Σp scattering experiment.

Particle-core coupling in 141Nd

High-spin states in141Nd have been studied using in-beam γ-ray spectroscopic techniques via the130Te(16O, 5nγ)141Nd reaction. The level scheme of141Nd has been extended up to an excitation energy of 7614.5 keV including 12 new γ rays deexciting 15 new levels. According to particle-vibrator coupling and semi-empirical shell model calculations, the level structure of141Nd can be well interpreted by coupling an h11/2 neutron-hole to the respective excited states in142Nd core.

Rotational bands in 169Re

High-spin states in 169Re have been investigated by means of in-beam γ-ray spectroscopy techniques with the 144Sm(28Si, 1p2nγ)169Re reaction. X-γ and γ-γ-t coincidences, γ-ray anisotropies, and DCO ratios were measured. A strongly coupled band based on the 9/2-[514] Nilsson state and a decoupled band built on the h9/2 intruder proton orbital (nominally 1/2-[541]) have been established. The AB neutron crossings are observed at ω = 0.23 and 0.27 MeV for the 9/2-[514] and 1/2-[541] bands, respectively. The 9/2-[514] band in 169Re shows the largest signature splitting at low spin among the odd-mass Re isotopes. Band properties of the AB neutron crossing frequencies, alignment gains, and signature splittings are discussed, and compared with those in the heavier odd-A Re isotopes. Additionally, a three-quasipariticle band is observed, and the π9/2 -[514] ⊗ νAE configuration is proposed tentatively.

High-Spin States in Odd–Odd168Lu

High-spin states in the odd-odd 168Lu nucleus, populated in the 154Sm(19F,5n) reaction at a beam energy of 96 MeV, were investigated using in-beam γ-ray spectroscopy techniques. The BC neutron crossing in the yrast band, based on πg7/2[404]7/2+⊗νi13/2[642]5/2+ configuration, occurs at ħω= 0.31 MeV. The two side bands, based on πh11/2[514]9/2−⊗νi13/2[642]5/2+ and πh9/2[541]1/2−⊗νi13/2[642]5/2+ configurations, show anomalous signature-splitting and signature-inversion in the first one, to occur at ħω= 0.24 MeV. A moderately delayed BC-crossing is anticipated in the second one.

Signature inversion in πi13/2⊗νi13/2 structure in 178Ir

High-spin states in 178Ir were investigated by means of in-beam γ-ray spectroscopy techniques using the multidetector array GASP. Excited states of 178Ir were populated through the 159Tb(24Mg, 5n) fusion-evaporation reaction at E(24Mg) = 131-141 MeV. Several rotational bands were observed. Among them, the πi13/2⊗νi13/2 structure has been identified up to spin 36 ?. This band exhibits an anomalous signature splitting and a signature inversion around spin 25 ?.

Study of the Mo-Ba partition in 252Cf spontaneous fission

Measurements of fission fragment yields and neutron multiplicities have been carried out for the Mo-Ba fragment pairs in the spontaneous fission of 252Cf, using the γ-ray spectroscopy technique to analyze γ-γ-γ coincidence data. Prompt γ -ray multiplicities were also measured as a function of the number of neutrons emitted in the fission process leading to the Mo-Ba partition. We do not observe the enhancement in the yields of events with high neutron emission multiplicity (νn > 7) that has been associated to a second fission mode leading to the production of hyperdeformed Ba fragments, as reported in some earlier studies. The average γ-ray multiplicity is found to be rather weakly dependent on the number of neutrons emitted in the fission process.

High-spin states in odd-odd 168Lu

High-spin states in the odd-odd 168Lu nucleus, populated in the 154Sm(19F,5n) reaction at a beam energy of 96 MeV, were investigated using in-beam γ-ray spectroscopy techniques. The BC neutron crossing in the yrast band, based on πg 7/2[404]7/2+⊗νi 13/2[642]5/2+ configuration, occurs at ħω= 0.31 MeV. The two side bands, based on πh 11/2[514]9/2−⊗νi 13/2[642]5/2+ and πh 9/2[541]1/2−⊗νi 13/2[642]5/2+ configurations, show anomalous signature-splitting and signature-inversion in the first one, to occur at ħω= 0.24 MeV. A moderately delayed BC-crossing is anticipated in the second one.

Intrinsic states and rotational bands in 176Ta and 178Ta

High-spin states in the deformed, doubly odd nuclei, 176Ta and 178Ta have been studied by time-correlated γ-ray and electron spectroscopy techniques following (HI, xn) reactions. The new results independently confirm and extend in spin the previously known two- and four-quasiparticle states, albeit with some differences. Many high-seniority structures, most of them with associated rotational bands, have been identified for the first time. Several high- K isomers with half-lives ranging from a few nanoseconds up to hundreds of milliseconds have been discovered. Observation of rotational bands, built upon the intrinsic states has allowed characterization of the configurations, through both the in-band decay properties and alignments. The excitation energies of the observed multi-quasiparticle structures are generally reproduced using calculations based on the Lipkin-Nogami pairing model with inclusion of the effects of blocking and empirical nucleon-nucleon interactions. Some discrepancies in the energies, such as for the 14 − and 15 − isomers in 176Ta and 178Ta, respectively, are attributed to specific configuration mixing. Several anomalously fast K-forbidden transitions are discussed.

High-spin states in odd-odd 168Lu

High-spin states in the odd-odd 168Lu nucleus, populated in the 154Sm(19F,5n) reaction at a beam energy of 96 MeV, were investigated using in-beam γ-ray spectroscopy techniques. The BC neutron crossing in the yrast band, based on πg7/2[404]7/2+⊗νi13/2[642]5/2+ configuration, occurs at ħω= 0.31 MeV. The two side bands, based on πh11/2[514]9/2−⊗νi13/2[642]5/2+ and πh9/2[541]1/2−⊗νi13/2[642]5/2+ configurations, show anomalous signature-splitting and signature-inversion in the first one, to occur at ħω= 0.24 MeV. A moderately delayed BC-crossing is anticipated in the second one.

High spin states and evidence for octupole correlations in 117Xe

High-spin states of 117Xe were investigated by means of in-beam γ-ray spectroscopic techniques via the reaction of 28Si bombarding a 92Mo target at beam energies of 100-120 MeV. A positive-parity rotational band decaying into the yrast negative-parity band by a series of enhanced E1 transitions was observed for the first time, implying the existence of octupole correlations in 117Xe. The B(E1) values increase with spin. The νd5/2 band was firmly established up to 27/2+ and the B(M1)/B(E2) ratios were extracted from the relative intensities of γ-rays in this band. The previously known νh11/2 and νg7/2 [413]5/2+α=−1/2 bands were confirmed and extended up to high spins and two bandcrossings are observed in the latter at ħω=0.33 and 0.44 MeV, respectively. The bandcrossings and configurations of these bands are discussed by TRS and CSM calculations. In a γ–γ–t measurement, the 11/2− and 7/2− levels were identified as two isomers with half-lives of 59.4±20 ns and 16.5±8.0 ns, respectively.