High-spin spectroscopy of168Hf

The nucleus¹⁶⁸Hf was studied up to spin (38⁺) in the yrast band and to spins (41^?) and (38^?) in the lowest two negative-parity bands. The onset of a proton alignment (h_9/2 or i_13/2 quasiparticles) is observed in these three bands for the highest transitions. A new band with even spins and negative parity was found. The interaction strength between the ground-state band and theAB band is measured.     

First observation of yrast band in odd-odd162Lu

High spin states of the odd-odd¹⁶²Lu nucleus have been studied via¹⁴⁷Sm(¹⁹F,4nγ)¹⁶²Lu reaction at 95 MeV beam energy. Level scheme for yrast band based onπ[h_11/2 v[i_13/2] quasiparticle configuration was established up to I ^π = (23^?) for the first time. This band shows the signature inversion in energy before backbending generally appeared in this mass region. It is stressed that the signature splitting in¹⁶²Lu is larger than that in the¹⁶⁰Tm nucleus.     

Influence of particle-number projection on potential energy surfaces of fissioning nuclei

The structure of high-spin states in the transitional nuclei^191,193Hg is described in the framework of the interacting boson fermion model with one broken pair. The results of model calculation reproduce in detail the structure of yrast states up toJ=49/2⁺ ?, including the first backbending, and the two lowest three-neutron negative parity bands.     

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 toJ ^π=51/2^?. Three-quasiparticle states, based on the πg _7/2 ²?νh _11/2 andπg _7/2 d _5/2?νh _11/2 configurations, have been identified. A favoured 39/2^? state is suggested to be the fully aligned [πg _7/2 ²]₆₊?[νh _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 the states atJ ^π=21/2^? and 23/2^?. A higher 47/2^? state is also found and is suggested to be the fully aligned [πg _7/2 ²]₆₊?[νh _11/2 ⁵]_35/2? configuration.     

First observation of yrast band in odd-odd 162Lu

High spin states of the odd-odd ¹⁶²Lu nucleus have been studied via ¹⁴⁷Sm(¹⁹F,4nγ)¹⁶²Lu reaction at 95 MeV beam energy. Level scheme for yrast band based on π[h_11/2]υ[i_13/2] quasiparticle configuration was established up to I^π=(23^?) for the first time. This band shows the signature inversion in energy before backbending generally appeared in this mass region. It is stressed that the signature splitting in ¹⁶²Lu is larger than that in the ¹⁶⁰Tm nucleus.     

Quasiparticle structures in118Te

The structure of¹¹⁸Te has been investigated following the¹¹⁰Pd(¹³C,5n) reaction using gamma-ray spectroscopic techniques and a substantially extended level scheme is proposed. A negative parity quasirotational band based on πg_7/2h_11/2 configuration has been established upto J^π=19. The level structure, above the fully aligned πg²7/2?vh²11/2 J^π=16⁺ yrast state, is found to evolve into several non-collective states.     

Yrast band spectroscopy of132Sm and130Nd

The states in the yrast band of¹³² Sm have been identified for the first time up to J^π=16⁺. The yrast band of¹³⁰Nd has also been extended from J^π=16⁺ to 24⁺. The energy of the first 2⁺ state in¹³²Sm indicates that the quadrupole deformation ε₂ ～ 0.3 and is still increasing as the neutron number N decreases across the samarium isotopes. The results for¹³⁰Nd allow the (h_2/11)² proton crossing frequency to be determined as ω=0.325 ± 0.005 MeV/?. This result is compared with cranked shell model predictions.     

Structure of high spin states in83Y

We report on the first in-beam study of high spin levels in⁸³Y which were established up to 9.0 MeV excitation energy and probable spin of 41/2⁺ resp. 27/2^? by means of the reaction⁵⁸Ni(²⁸Si, 3p). Ten lifetimes and six lifetime limits in the 10^?12–10^?9 s range were determined in two recoil distance Doppler shift experiments. The positive parity yrast states form ag _9/2 Coriolis decoupled band with partial alignment, near-rigid rotor moment of inertia and deformationβ ₂=0.29. The negative parity yrast band has very similar collective properties; it shows a pronounced band crossing at rotational frequency ?ω≈0.40 MeV which we associate with twoquasiparticleg _9/2 proton alignment. At 2.56 MeV excitation, a second ΔI=1 band starting with 17/2^? was found. On the basis of the similarity to⁸⁵Y and the very weakE2 decay of this state, we suggest that this band has (3qp) configuration with an alignedg _9/2 neutron pair. The lifetimes of the lowest 2⁺ and 4⁺ state in⁸⁴Zr populated in the reaction⁵⁸Ni (²⁸Si, 2p) were determined to be τ(2)=17.8(11) ps resp. τ(4)=3.5(4) ps.     

In-beam study of high-spin states in199,200,201Po and systematical features of Z=84 polonium isotopes

Excited states in^199,200,201Po were populated in the reactions¹²C+¹⁹⁴Pt and¹²C+¹⁹⁵Pt. The subsequentγ-radiation was studied using conventional in-beam spectroscopic methods. States with spins up to (29/2), 18 and ≧35/2?, respectively, were populated in the three nuclides. Three isomeric states with the following half-lives were observed in²⁰⁰Po:T _1/2(8⁺)=90(15) ns,T _1/2(11^?)=120(20) ns andT _1/2(12⁺)=267(4) ns. The structure of the excited states was interpreted within the framework of the spherical shell model. The three isomers are suggested to be two-quasiparticle states with configurationsπ(h _{2 2})_8J, π(h₉i_13/2)₁₁- and v(i _{1 3/2 ?2})₁₂₊. A self-consistent Hartree-Fock calculation was performed to obtain ground-state deformations of the neighbouring Pb cores. From the results of the experiments and the calculations it was concluded that no appreciable deformations of the cores are manifested in the yrast states of these three Po nuclei.     

Yrast states in the π- particle ν- hole nucleus 65 146 Tb81

The nucleus ¹⁴⁶Tb was studied from in beam γγ-and conversion electron measurements. The level scheme was established up to ～5MeV above the (πh_11/2 vd _3/2 ^?1 )_5? β-isomer. In addition to the known (πh_11/2 vh _11/2 ^?1 )₁₀₊ E3-isomer, the 8⁺ and 11⁺ members of this configuration were located. The levels at the yrast line are dominated by the couplings of the πh_11/2 vh _11/2 ^?1 valence nucleons to the collective 3^? octupole state and to the πh_11/2d _5/2 ^?1 and πh_11/2g _7/2 ^?1 particle-hole excitations of ¹⁴⁶Gd.     

Microscopic study of positive-parity yrast bands of 224−234Th isotopes

The positive-parity bands in ^224???234Th are studied using the projected shell model (PSM) approach. The energy levels, deformation systematics, B(E2) transition probabilities and nuclear g-factors are calculated and compared with the experimental data. The calculation reproduces the observed positive-parity yrast bands and B(E2) transition probabilities. Measurement of B(E2) transition probabilities for higher spins and g-factors would be a stringent test for our predictions. The results of theoretical calculations indicate that the deformation systematics in ^224???234Th isotopes depend on the occupation of low k components of high j orbits in the valence space and the deformation producing tendency of the neutron–proton interaction operating between spin orbit partner (SOP) orbits, the [(2g_9/2) _π –(2g_7/2) _ν ] and [(1i_13/2) _π –(1i_11/2) _ν ] SOP orbits in the present context. In addition, the deformation systematics also depend on the polarization of (1h_11/2) _π orbit. The low-lying states of yrast spectra are found to arise from 0-quasiparticle (qp) intrinsic states whereas the high-spin states turn out to possess composite structure.     

Protonh 2 11/2 and octupole excitations in148 66Dy82 and149 66Dy83

The yrast states of¹⁴⁸Dy and¹⁴⁹Dy have been studied by γ-ray and conversion electron measurements in (α, xn) and (¹⁶O,xn) reactions on enriched¹⁵²Gd and¹³⁵Ce targets. Level schemes to above 4 MeV for the two nuclei are reported. The πh ² _11/2 spectrum identified in¹⁴⁸Dy and the πh _11/2 effective chargee _eff=1.52±0.05e, derived from the measuredE2 transition rate between the (πh ² _11/2) 10⁺ and 8⁺ states, are discussed and compared with results for other two-particle nuclei. The yrast cascades in¹⁴⁸Dy and¹⁴⁹Dy continue above the (πh ² _11/2) 10⁺ and πh ² _11/2 vf _7/2) 27/2^? states by ～ 1 MeVE1 transitions de-exciting the lowest members of octupole multiplets built on these states. The energy shifts for the observed members of the πh ² _11/2 × 3^? multiplet are analyzed in terms of twoparticle-phonon exchange coupling using an empirical coupling strength extracted from the one valence particle nucleus¹⁴⁷Tb. The dominantvf _7/2×3^? character of low-lying 13/2⁺ isomers in¹⁴⁹Dy and otherN=83 nuclei is emphasized.     

Level schemes of163, 165Yb and backbending behaviour of neutron deficient Yb-isotopes

High spin states in^{163, 165}Yb have been populated using (¹⁶O,xn) reactions with^{152, 154}Sm targets. Several rotational bands are observed and the lifetime of highly converted transitions have been measured utilizing a solenoid-Si(Li) spectrometer and Ge(Li) detectors. Thei _13/2 and 3/2^? [521] bands do not backbend, whereas the 5/2^? [523] bands do, indicating additional processes besides the rotational alignment of one i_13/2 neutron pair that are responsible for the backbending.     

On the yrast levels in204Pb

High-spin states in²⁰⁴Pb were populated in the²⁰⁴Hg(α,4n) reaction using α-particles in the energy region 42–51 MeV. Prompt and delayedγ-rays as well as conversion electrons were studied in addition to excitation functions, angular distributions andγ-γ coincidences. In this way a stretched cascade ofγ-rays from a level at 8125.9 keV was found to feed the previously known isomeric 9^? level at 2185.7 keV. Spins and parities were established for levels up to and including a 19^? level at 6098.0 keV. The levels with c= 17^2212; and 19^? at excitation energies of 5664.3 and 6098.0 keV are likely to be due to the simplep ₁ ^2/s-1 i ₁₃ ^2/?3 andf ₅ ^2/?1 i ₁₃ ^2/?3 configurations. The agreement between calculated and experimental energies for all observed levels in the regionJ=9–19 is very good in cases where the empirical two-particle interactions used are satisfactorily well known. Above the 19^? level there are three weakly populated levels at 7402.1, 7849.2 and 8125.9 keV, which are likely to haveJ≥20. None of these energies agrees with the calculated value 7695±20 keV for the 20⁺ state of thei ₁₃ ^2/?4 configuration which has the highest angular momentum produced by the four valence neutron holes. This apparent anomaly can be understood if the yrast levels withJ≥20 have angular momentum contribution from the core. It seems likely that the states at 7402.1, 7849.2 and 8125.9 keV are due to proton core excited states of the typeπh _9/2 h ₁₁ ^2/?1 ×νp ₁ ^2/?2 i ₁₃ ^2/?2 withJ ^π=20⁺ andJ ^π=21⁺ andπh _9/2 h ₁₁ ^2/?1 ×νp ₁ ^2/?1 f ₅ ^2?1 i ₁₃ ^2/?2 withJ ^π=22⁺ or 23⁺, respectively. The state at 8126 keV has the highest energy so far directly observed in a stretched cascade ofγ-rays from the decay of a heavy nucleus produced in (α, xn) reactions.     

ON THE BACKBENDING MECHANISM OF MUITIBAND STRUCTURE IN NUCLEUS 185Pt

The observation of multi-band structure in nucleus ¹⁸⁵Pt reported recently,once again raises the open question about whether the alignment of proton h_9/2 pair or neutron i_13/2 pair should be responsible to the backbending in five band.Blocking experiments prefer the proton h_9/2 pair while the Total Rothian Surface calculation supports neutron i_13/2 pair.Based on the analysis of assumptions implied in both approaches,a new theoretical calculation which includes the dynamic effect of band crossing is presented.Such a calculation indicates that the alignment of proton h pair should be the main cause for the observed backbendings.     

Collective structures and smooth band termination in 109Sn

Six rotational bands up to energies E _x = 24.7 MeV and spins J^π=(79/2^?) have been identified in ¹⁰⁹Sn using the GAMMASPHERE γ-detector array. Four of the bands show smoothly decreasing dynamic moments of inertia at rotational frequencies ?ω > 0.6 MeV. The bands arise at medium spins from a coupling of a valence d_5/2, g_7/2 or h_11/2 neutron to the deformed 2p2h proton excitation of the Z=50 core ¹⁰⁸Sn. At very high ?ω these bands show the typical behaviour of smoothly terminating bands, i.e. a gradual alignment of the angular momenta of the valence particles and holes corresponding to a transition from high collectivity to noncollective states.     

On the nature of the superband in156Dy

Superbands, responsible for the backbending in¹⁵⁶Dy and other N=90 and N=88 nuclei, are proposed to be alignedn(i_13/2)² bands, whereas it is argued that the recently discovered positive parity band in¹⁵⁶Dy withI ^π= (2⁺) up to 10⁺ members does not constitute the low spin extension of the superband.     

Two-quasiparticle bands in126Ba

High spin states of¹²⁶Ba have been populated via the¹¹⁶Sn (¹³C, 3n) reaction and were investigated by inbeam gamma-ray spectroscopy. Partially NaJ (Tl)-BGO anti-compton shields were used. Two positive parity band crossings were established. The yrast band was observed up to a tentative 18⁺ level. The gamma-bands and two band structures of negative parity were found. In addition a tentative 3^? state has been seen. Two-quasiparticle Routhians are constructed from the one-quasiparticle Routhians of the bands in the odd neighbouring isotopes and isotones and are compared to the ones observed in¹²⁶Ba. This leads to ah _11/2 g _7/2-neutron structure of the strongly coupled negative parity bands and a two-quasiproton structure of the decoupled bands of negative parity. The branching and mixing ratios for the neutron bands are analyzed.     

In-beam gamma-ray spectroscopy of 116Te

High-spin states of the even-even ¹¹⁶Te were studied by in-beam g-ray spectroscopy using the ¹⁰³Rh (¹⁶O, p2n) fusion-evaporation reaction at a beam energy of 80 MeV. γ-γ coincidence and γ-γ angular correlation analyses were employed for determining the level scheme of ¹¹⁶Te. Levels up to I^π = 27^? and several new states were established. We identified two rotational bands with negative parity, one of which was newly established in the present work. We suggest that such two bands be associated with two proton [(g_7/2) ? (h_11/2)] and two neutron [(d_5/2) ? (h_11/2)] configurations, respectively.     

High-spin states in206Bi populated in the205Tl(α, 3n) reaction

Using alpha-particles in the energy range 35–51MeV and in-beam gamma ray and conversion electron spectroscopy techniques the reaction²⁰⁵Tl(α, 3n)²⁰⁶Bi was studied. A 15±1ns isomeric 15⁺ state was found at an excitation energy of 3147keV in²⁰⁶Bi. The main configuration of the isomeric state is suggested to beπh _9/2 vp ₁ ^2/?1 i ₁₃ ^2/?2 . The isomeric state decays mainly through a stretched cascade of five gamma rays to the previously known 0.88ms 10^? state of theπh _9/2 vi ₁₃ ^2/?1 (j ^?2)₀₊ configuration at an excitation energy of 1045 keV. A shell model calculation of the yrast states has been performed and it is found that the calculation agrees fairly well with the experiments. The average deviation between experimental and calculated energies for the yrast states with angular momenta in the region 6–18 is +4keV and the root mean square deviation is 22 keV.