Three-quasiparticle States Research Articles

Description of the odd 249−253No nuclei using Skyrme functionals with modified spin–spin interaction

The Skyrme energy-density functionals with modified spin–spin interaction and pairing strength are used for description of the [Formula: see text]No, [Formula: see text]No and [Formula: see text]No excited states. The results of the HFB and cranked HFB calculations taking into account the blocking effect are in reasonable agreement with the available data. For many states, including rotational and three-quasiparticle states, the results of self-consistent calculations were obtained for the first time.

Three-quasiparticle isomers in odd-even Pm159,161 : Calling for modified spin-orbit interaction for the neutron-rich region

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.

Core excitations across the neutron shell gap in 207Tl

The single closed-neutron-shell, one proton–hole nucleus 207Tl was populated in deep-inelastic collisions of a 208Pb beam with a 208Pb target. The yrast and near-yrast level scheme has been established up to high excitation energy, comprising an octupole phonon state and a large number of core excited states. Based on shell-model calculations, all observed single core excitations were established to arise from the breaking of the N=126 neutron core. While the shell-model calculations correctly predict the ordering of these states, their energies are compressed at high spins. It is concluded that this compression is an intrinsic feature of shell-model calculations using two-body matrix elements developed for the description of two-body states, and that multiple core excitations need to be considered in order to accurately calculate the energy spacings of the predominantly three-quasiparticle states.

Triaxial projected shell model description of high-spin band-structures in 103,105Rh isotopes

High-spin band structures in odd-proton 103,105Rh are investigated using the microscopic triaxial projected shell model approach. It is demonstrated that the observed band structures built on one- and three-quasiparticle states are reproduced reasonably well in the present work. Further, it is evident from the analysis of the projected wavefunctions that side-band in the low-spin regime is the normal γ-band built on the ground-state configuration. However, in the high-spin regime, the side band is shown to be highly mixed and ceases to be a γ-band. We provide a complete set of electromagnetic transition probabilities for the two bands and the experimental measurements are desirable to test the predictions of the present work.

Level structure in the transitional nucleus195Au

Excited states in ${}^{195}$Au have been studied experimentally via the ${}^{192}$Os(${}^{7}$Li, 4$n$) reaction at a beam energy of 44 MeV. Based on the $\ensuremath{\gamma}$-$\ensuremath{\gamma}$-$t$ coincidence measurement, a level scheme consisting of 15 new transitions and 10 new levels is established for ${}^{195}$Au. The triaxial shape-polarizing effect of the high-$j$ ${h}_{11/2}$ proton hole was studied by total Routhian surface calculations. By comparing with the level structures in the odd-$A$ Au isotopes and the even-even core Hg nuclei, configurations are proposed to the rotational bands and three-quasiparticle states observed in ${}^{195}$Au.

Properties of theπh11/2band in the stable nucleus193Ir

Excited states in the stable nucleus $^{193}\mathrm{Ir}$ have been investigated through an in-beam $\ensuremath{\gamma}$-ray spectroscopic technique following the $^{192}\mathrm{Os}$($^{7}\mathrm{Li}$, $\ensuremath{\alpha}2n$) reaction at a beam energy of 44 MeV. A level scheme built on the $\ensuremath{\pi}{h}_{11/2}$ isomer has been extended to high-spin states using eleven newly observed $\ensuremath{\gamma}$ transitions. The $\ensuremath{\pi}{h}_{11/2}$ band is proposed to be formed by coupling an ${h}_{11/2}$ proton to a core with large triaxial deformation. Three-quasiparticle states are suggested in $^{193}\mathrm{Ir}$ on referring to the similar states in $^{187}\mathrm{Ir}$. The features of signature splitting in the $\ensuremath{\pi}{h}_{11/2}$ bands of ${}^{187\ensuremath{-}193}$Ir are discussed with the help of total Routhian surface calculations.

Table of three-quasiparticle rotational bands in deformed nuclei, 153 ⩽ A ⩽ 187

The experimentally observed level structures based on three-quasiparticle (3qp) states are classified according to their intrinsic structures, and other properties are deduced from measurements such as B( M1)/ B( E2) ratios, | g K − g R| values, lifetimes, etc. The present table lists data for a total of 168 such structures which have been extracted from the literature for 55 nuclides in the mass region A = 153–187 ( Z = 63–78, N = 88–112), with the majority of these bands in the A = 180 region (28 bands in seven Re isotopes alone). Nuclear models used for the interpretation of 3qp structures, generalization of the Gallagher–Moszkowski (GM) rules to 3qp states, and high-spin features such as t-bands, high- K isomers, signature splitting, signature inversion, alignment, etc., are discussed briefly. The literature cutoff date for extraction of data for known 3qp structures is July 15, 2005.

HIGH-SPIN STRUCTURE OF NEUTRON-RICH NUCLEI PRODUCED BY HEAVY-ION INDUCED FISSION

Using the Euroball array, we have recently performed several experiments devoted to the study of high-spin states of nuclei with A~80–130, produced from fission following various fusion reactions. High-spin level schemes of many neutron-rich nuclei have been extended or newly established. We present here the new experimental results obtained in some neutron-rich odd-Z nuclei from 43 Tc to 49 In . All the band-head configurations have been identified from the behaviour of the collective bands built on them. Therefore the single-proton states, located around the Fermi levels for Z=43 to 49, can be discussed as a function of the neutron number (and the deformation). In all these isotopic series, three-quasiparticle states have been measured above 2 MeV excitation energy. The case of 49 In nuclei is discussed in the framework of shell-model calculations. Futhermore the individual identification of each fragment by means of its emitted γ transitions allows us to have a new insight into the mechanism of fission induced by heavy ions. Preliminary results of the total number of emitted neutrons are discussed as a function of the N/Z ratios of the fragments.

Three-quasiparticle rotational bands in 101Rh: IBFBPM description and signature inversion of the πg9/2 orbital

High- and intermediate-spin level structures of the 101Rh nucleus have been studied using the reaction 70Zn+ 36S at 130 MeV bombarding energy. γ-rays were detected with the EUROGAM II detector array. Three new ΔI=1 rotational bands have been observed in this nucleus. The deduced level scheme is compared with results of calculations using Interacting Boson–Fermion plus Broken Pair Model. The model calculations give a consistent description of the one- and three-quasiparticle states providing fair agreement between the observed and calculated level energies and branching ratios. Experimental Routhians of the new high-spin bands and the πg 9/2 νh 11/2 bands in the neighbouring Rh and Ag nuclei have been found to exhibit signature inversion. This signature inversion can be interpreted qualitatively as the inversion of the two signatures of the proton g 9/2 orbital caused by triaxial nuclear shape with large positive γ parameter.

Manifestation of pairing effects in the cross section for 238Pu fission induced by neutrons

The cross section for 238Pu fission induced by neutrons with energies between 1 keV and about 5 MeV is described within the statistical model. It is shown that the stepwise structure observed above the fission threshold (at incident-neutron energies of Es>1 MeV) is due to the step in the level density of the fissile 239Pu nucleus at deformations corresponding to the inner fission barrier. In turn, the step in the level density of the odd nucleus 239Pu is associated with the excitation of internal single-and three-quasiparticle states. The level density is described with allowance for collective, pairing, and shell effects.

Decay properties of ground-state and isomer of 103In

The β-decay properties of ground-state and isomer of 103In were investigated by means of γ-ray spectroscopy. The half-lives of 103g In and 103m In were determined to be 60 ± 1 s and 34 ± 2 s, respectively. Out of 149 γ transitions ascribed to the decay of 103g In, 104 have been placed in a decay scheme including 55 excited states of 103g Cd. The main part of the resulting distribution of the Gamow-Teller strength B(GT) is associated with the feeding of 103Cd levels at excitation energies around 3 MeV. This observation can be interpreted as a sign of dominant feeding of three-quasiparticle states in 103Cd, which correspond to the 1+(πg 9/2 −1 , vg7/2)⊗πg9/2 shell-model configuration spread over many levels. The sum of the B(GT) values deduced from the present γ-ray data amounts to 0. 34, which provides a lower limit to the total Gamow-Teller strength. The B(GT) distribution for 103In decay is compared to the corresponding experimental results for 105,107,109In and to theoretical predictions for 99,101In. The latter were calculated by using an advanced spherical shell model. The β-branching ratio for the 103m In decay is estimated and compared with the relevant values for the neighbouring indium isotopes including 101In whose half-life was determined to be 14. 9 ± 1.2 s.

High spin states in121Te

High spin states of121Te, populated in the114Cd(11B,p3n) reaction, have been studied throughγ-ray spectroscopy. The level scheme has been established up toJπ=51/2−. Three-quasiparticle states, based on the πg7/22⊗νh11/2 andπg7/2d5/2⊗νh11/2 configurations, have been identified. A favoured 39/2− state is suggested to be the fully aligned [πg7/22]6+⊗[νh11/2327/2− yrast non-collective oblate configuration. This assignment is supported by Total Routhian Surface (TRS) calculations which also suggest a similar oblate assignment to the states atJπ=21/2− and 23/2−. A higher 47/2− state is also found and is suggested to be the fully aligned [πg7/22]6+⊗[νh11/25]35/2− configuration.

Level scheme and electromagnetic transition strengths in 177Lu

In the 176 Lu(n th ,γ) 177 Lu reaction, γ-ray and conversion electron spectra were measured using the bent crystal spectrometers GAMS and the spectrometer BILL, respectively. An extension of the low-lying level scheme of 177 Lu is achieved. Further, γγ delayed-coincidence lifetime measurements were performed and the following half-lives of excited states in 177 Lu were determined for the first time: T 1 2 (811.543 keV ) = 1.0(1) ns , T 1 2 (1356.930 keV ) = 11(2) ns and T 1 2 (1544.277 keV ) = 0.8 −0.1 +0.2 ns . The strengths of the observed M1, E2 and E1 transitions are compared to rotor-plus-quasiparticle calculations. The enhancement of the newly established K -forbidden E1 transitions linking the 5 2 + [402] and 9 2 − [514] bands may be explained by the influence of the octupole modes. The quadrupole deformation associated with the 1 2 − [541] band is deduced from the measured B( E2 , 9 2 − 1 2 → 5 2 − 1 2 transition strength and found to be larger than that of the ground state. The intrinsic configurations of isomeric three-quasiparticle states are discussed.

High spin states in 121Te

High spin states of 121Te, populated in the 114Cd (11B, p3n) reaction, have been studied through γ-ray spectroscopy. The level scheme has been established up to Jπ= 51/2−. Three-quasiparticle states, based on the πg 7/22 ⊗ vh11/2 and πg 7/2d5/2⊗ vh11/2 configurations, have been identified. A favoured 39/2− state is suggested to be the fully aligned [πg7/22]6 + ⊗ [vh11/23]27/2− yrast non-collective oblate configuration. This assignment is supported by Total Routhian Surface (TRS) calculations which also suggest a similar oblate assignment to the states at Jπ= 21/2− and 23/2−. A higher 47/2− state is also found and is suggested to be the fully aligned [πg7/22]6+ ⊗ [vh11/25]35/2− configuration.

Level Structure of177Lu Studied through the Beta Decay of177Yb

The level scheme of 177 Lu following the β-decay of 177 Yb ( T 1/2 =1.9 h) has been constructed from results of γ singles and γ-γ coincidence measurements. Angular correlations of γ-rays were measured with a 4-Ge detector system. Mixing ratios (E2/M1) of seven γ-rays were obtained. The spins and parities of the 1049.4, 1150.0 and 1241.7 keV levels have been assigned to be 9/2 - , 7/2 + and 7/2 + , respectively. These assignments for the 1049.4 and 1241.7 keV levels are consistent with the interpretation that these levels are members of three-quasiparticle multiplets. Energy splittings of three-quasiparticle states were calculated. The calculated 1160 keV 7/2 + level may correspond to the observed 1150.0 keV 7/2 + level. A calculation with a rotation-vibration coupling model was performed. The K -2 γ-vibrational state of the 7/2 + [404] state and the K ±2 γ-vibrational states of the 9/2 - [514] state are expected to exist near 1300 keV.

Non-collective oblate states in119Te

High spin states of119Te, populated in110Pd(13C, 4n) and110Pd(12C, 3n) reactions, have been studied throughγ-ray spectroscopy. The level scheme has been established up to jπ=(55/2−). Three-quasiparticle states, based onπg 7 2/2 ⊗vh 11/2 andπg 7/2 d 5/2⊗vh 11/2 configurations, have been identified. A particularly favoured 39/2− state is suggested to be the fully aligned [πg 7 2/2 ]6+⊗[vh 11 2/3 ]27/2− yrast non-collective oblate configuration. This assignment is supported by Total Routhian Surface (TRS) calculations which also suggest a similar oblate assignment to states at 21/2−, 23/2− and 37/2−.

High spin structures in119Te

The high spin states of119Te, populated in110Pd(13C,4n) and110Pd(12C,3n) reactions, have been studied through γ-ray spectroscopy. The level scheme has been established upto a spin of 55/2. Three-quasiparticle states, based on πg2 7/2⊗νh11/2 and πg7/2d5/2⊗νh11/2 configurations, have been identified. The 35/2 and 39/2 states are suggested to be the fully aligned states constituted by five valence νh11/2 3, μg7/2, πd5/2 quasiparticles.

The Gamow-Teller decay of 105Sn to three-particle states in 105In

The EC/ β + decay of 105Sn was reinvestigated by using 58 Ni(5 MeV/u) + 50Cr reactions, chemically selective on-line mass separation and γ-ray spectroscopy. The half-life of 105Sn has been determined as 34 ± 1 s. Out of 104 γ transitions ascribed to the 105Sn decay, 89 have been placed in the decay scheme including 52 excited states of 105In. From the EC/ β + feeding of individual states, the distribution of the Gamow-Teller (GT) strength has been derived. It is shown that the main part of the GT strength is associated with the feeding of 105In levels having excitation energies above 3 MeV. This observation can be interpreted as a sign of dominant feeding of three-quasiparticle states in 105In, which correspond to the π( g 9 2 ) −1νg 7 2 νd 5 2 shell-model configuration spread over many levels. The sum of the GT strength deduced from the present gamma-ray data of B Σ (GT) = 1.46 provides a lower limit to the total GT strength. Many weak transitions, mainly from high-energy levels, may not have been detected in this study and therefore part of the strength may be missed. An indirect support for this conclusion has been obtained from the analysis of the indium KX-rays intensity. This analysis indicates about 50% contribution of the electron-capture to the beta decay of 105Sn, which is interpreted as a sign of a predominant feeding of the high-energy 105In states with B Σ (GT) ⩾ 3. The observed GT distribution and strength will be compared to results obtained from a finite-Fermi-system theory and a large-basis shell-model calculation. The core-polarization and higher-order hindrance factors will be discussed.

Intrinsic states and collective structures in 181Ir

One- and three-quasiparticle states, and associated rotational bands have been identified in the odd-proton nucleus 181Ir. They were populated using the 169Tm( 16O, 4n) 181Ir reaction and established using a variety of time-correlated γ-ray and electron techniques. The 9 2 −[514] and 5 2 +[402] intrinsic states (from the h 11 2 and d 5 2 parents) were found to be metastable with mean of 193 and 430 ns, respectively. A third isomer, with K π = 23 2 + and a meanlife of 42 ns was identified and attributed to a one-quasiproton, two-quasineutron configuration, as were two other high- K band heads. The B( M1) B( E2) and g K − g R values extracted from the in-band decay properties of the observed rotational bands, based on these and other intrinsic states, were used to identify and test the proposed configurations. Interference effects were observed between states of the band assigned to the α = + 1 2 signature sequence from the i 13 2 configuration, with those from a second α = + 1 2 sequence, and the states from one of the high- K bands (leading to complicated branchings in the decay scheme). The α = + 1 2 sequence of the h 9 2 proton band was extended to higher spins than known previously and its (unflavoured) α = − 1 2 sequence identified. Both show alignments and alignment gains which are essentially well-behaved and consistent with a stable quadrupole deformation and small γ-asymmetry. In contrast, the h 11 2 and d 5 2 bands show complex alignment gains at low frequencies. These, and the in-band decay properties can be reproduced in a three-band model. However, essentially all bands, except for the h 9 2 one-quasiparticle cases, exhibit an underlying alignment increase with frequency if reference parameters appropriate to the h 9 2 band are used. Comparisons between the one-, two- and three-quasiparticle bands in which different orbitals are blocked, suggest the presence of alignment gains due to the h 9 2 protons but conflicting evidence remains. Some of the alignment effects may be artefacts of larger deformations, or alternatively a reduction in pairing.

Three-quasiparticle states analysis in odd-mass lead isotopes.

Three quasiparticle excitations for odd-even lead isotopes in the mass region 197[le][ital A][le]203 are calculated using a realistic two-body interaction. The analysis is performed in a correlated basis constructed by the tensor product of one times two quasiparticle states. Good agreement with available experimental data is found. The one phonon approximation of the weak coupling model is also analyzed.