Loss of alignment gain in the πh 9/2[541 1/2−] ⊗ (νi 13/2)2 band in 167Tm and neighbouring odd-Z N = 98 nuclei

Theπh_9/2[541 1/2^?] band in ¹⁶⁷Tm has been studied through the ¹²⁴Sn(⁴⁸Ca, p4n)¹⁶⁷Tm reaction at E _beam = 210 MeV. The favoured signature of the πh_9/2[541 1/2^?] band is observed to spin 61/2h?. A remarkable small gain in aligned angular momentum at the AB-band crossing has been found. The crossing frequency and the gain in aligned angular momentum for ¹⁶⁷Tm are compared with data for the N = 98 isotones of Lu, Ta and Re, and contrasted with Cranked Shell Model predictions, based on frequency diabatic configurations. Large deviations are found for the gain in aligned angular momentum.     

Anomalous shift in crossing frequency for the proton 1/2[541] band in175Ta

High-spin states in¹⁷⁵Ta have been populated in the¹⁶⁰Gd (¹⁹F,4n)¹⁷⁵Ta reaction with beams provided by the HI-13 tandem accelerator at the Institute of Atomic Energy in Beijing. A level scheme was constructed from γ-γ coincidence experiments. Seven decay sequences built on 5/2⁺[402], 1/2^?[541], 7/2⁺[404] and 9/2^? [514] proton Nilsson configuration have been extended to higher spin. An anomalously large delay of the neutron AB crossing built on the h_9/2 proton Nilsson State 1/2^?[541] is discussed.     

Study of band structures and crossings in179Re

High-spin states in¹⁷⁹Re have been populated by the¹⁶⁵Ho(¹⁸O, 4n) and¹⁷⁰Er(¹⁴N,5n) reactions. A level scheme is constructed from the studies ofγγ-coincidences using anti-Compton spectrometer arrays. The previously known 5/2⁺[402], 1/2^? [541] and 9/2^? [514] bands have been extended to 35/2⁺, 45/2^? and 39/2^?, respectively. In addition a 1/2⁺ [660] band, a three-quasiparticle band and two isomers were established. Furthermore several level sequences were identified decaying into the 5/2⁺ [402] and 1/2^? [541] bands. This allowed to determine the excitation energy of the 9/2^? member of the latter band. The band crossing features of the bands have been explained as the rotation alignment of ani _13/2 quasineutron pair taking into account shape changes.     

Study of low-lying states in151Eu via (n,n′γ) reaction

The levels of¹⁵¹Eu have been investigated in the (n,n′γ) reaction using nuclear reactor fast neutrons. The energies, intensities and angular distributions of theγ-rays have been measured with the Ge(Li) spectrometer. Four rotational bands with the following band heads and Nilsson configurations have been identified: ground state band, 5/2⁺ [402]; 21.5 keV, 7/2⁺[404]; 196.5 keV, 3/2⁺[411]; 260.5 keV, 5/2⁺[413]. The low spin states at 332.2 and 336.2 keV have been tentatively assigned to the l/2⁺[411] Nilsson orbital, but 522.8, 580.0 and 587.0 keV states to the 1/2⁺[420] Nilsson orbital. The negative parity levels at 353.7, 522.1 and probably 546.2 keV have been proposed basing on theh _11/2 proton state.     

On the level scheme of the159Tm nucleus

Level in¹⁵⁹Tm have been excited in the¹⁵⁰Sm(¹⁴N,5n reaction. The yrast band 7/2^?[523] observed up to spin 45/2^? is built on a isomeric state at 166 keV (T _1/2=37±5ns). Connections between the 7/2^?[523] and 7/2⁺ [404] bands fix the band-head energies.     

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. Received: 15 June 1998 / Revised version: 11 January 1999     

Experimental Study of High-Spin States in Deformed Odd-Odd 180 Ir

High spin states in deformed odd-odd ¹⁸⁰Ir have been investigated using the ¹⁵⁴Sm (³¹P, 5nγ)¹⁸⁰Ir reaction through the measurements of excitation functions at 150, 155, 160, 165 and 170 MeV beam energies, K X-γ and γ-γ coincidences at 160 MeV. A new level scheme composed of 5 rotational bands has been established. According to the band structure characteristics and the deduced in band B(M1)/B(E2) ratios, the quasiparticle configurations and spin and parity have been proposed to the observed bands. The neutron AB crossing is observed at hω_c≈0.26 MeV for the π1/2^－[541]⊙ν1/2^－[521] and π1/2^－[541]⊙ν5/2^－[512] bands. This AB crossing frequency is close to that in the ν5/2^－[512] band of ¹⁷⁹Os indicating the loss of intruder nature of the π1/2 - orbit. Gradual alignment gain in both the π9/2^－[514]⊙ν7/2⁺[633] and π9/2^－ν5/2^－[512] bands is observed which is similar to the low spin anomaly in alignment in the πh_11/2,πd_5/2 and πi_13/2 bands of neighboring Ir and Re isotopes. Different alignment properties have been discussed in the framework of cranked shell model, and a larger quadrupole deformation is suggested for the bands with π9/2^－[514] orbit involved.     

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.     

Study of the 176Lu(n, γ)177Lu reaction using a gamma band-filter spectrometer

The γ-ray spectrum from the ¹⁷⁶Lu(n, γ)¹⁷⁷Lu reaction has been investigated in the energy range 450 to 1420 keV using a newly developed gamma band-filter spectrometer. A total of 153 transitions have been observed. Improvements to the ¹⁷⁷Lu scheme are suggested, and in particular the γ-decay of several levels of the $\text{1}\text{2}$^?[530↑] and $\text{3}\text{2}$^?[532↓] rotational bands is proposed. Special attention has been given to the Coriolis coupling between the two latter bands and the $\text{1}\text{2}$^?[541↓] band. Results on a complementary investigation of the ¹⁷⁵Lu(n, γ)¹⁷⁶Lu reaction are presented.     

The decays of 21.55 min 162gTm and 24.3 s 162Tm (a new isomer)

The decay of the 21.55 min ground state and of the 24.3 s isomeric state of ¹⁶²Tm was investigated with semiconductor detectors. The γ-ray spectrum was investigated with a Compton-suppression Ge(Li)-NaI(Tl) arrangement. A Si (Li) detector, mounted in an electron transport solenoid, was used to investigate the conversion electron spectrum. Three-dimensional coincidence measurements were performed with large-volume Ge(Li) detectors. The ¹⁶²Tm ground state has spin-parity 1^? and Nilsson assignment p[411]↓?n[521]↑. An allowed β-transition (log ft ≈ 6.4) was observed to a 2^?, 2 octupole vibrational level at 1572.84 keV. The Q-value determined from positon-gamma coincidence measurements is 4705 ± 70keV. The discrepancy of the experimental K /β⁺ ratio with theoretical predictions might possibly be explained by a large number of unobserved weak γ-rays besides the total of 315 stronger ones observed in this study. The average β-strength function was calculated to be 1.2 × 10^?5. Among the 50 levels observed in the decay, the 2⁺, 4⁺ and 6⁺ members of the ground-state band, the 2⁺, 3⁺ and 4⁺ members of the γ-band, several 0⁺ and 2⁺ members of the K = 0 β-bands and 1^?, 2^? and 3^? octupole vibrational levels were identified. Parameter values Z_γ(0) and Z_γ(2) determining the mixing between the γ-band and the ground-state band, allow no conclusive evidence about unequalness of the intrinsic quadrupole moments of the ground states and the γ-band. The Z(0) parameters, determining the mixing between the β-bands and the ground-state band, and X parameters determining the ratio of E0 to E2 transition probabilities, were deduced. A previously unreported 24.3 sec isomer in ¹⁶²Tm was observed to decay in 10% of the cases by an allowed unhindered (log ft = 4.7) β-ray transition to a level at 1712.20 keV in ¹⁶²Er. The Nilsson configurations assigned to the isomeric and 1712.20 keV levels are p[523]↑ + n[521]↑₅₊ and n[523]↓ + n[521]↑₄₊ respectively. The isomeric level decays in 90% of the cases by an E3 transition (E_IT < 125 keV) to a p[404]↓ ?n[521]↑_2? level at 66.90 keV in ¹⁶²Tm, which decays by an (M1+ < 40 % E2) to the 21.55 min ¹⁶²Tm 1^? ground state.     

Level structure of proton-richN=83 Nuclei150Ho and152Tm

The yrast levels of theN=83 odd-odd nuclei¹⁵⁰Ho and¹⁵²Tm have been studied byγ-ray spectroscopy following reactions induced by 240–250 MeV⁶⁰Ni ions. Isomers have been identified in¹⁵⁰Ho at 1.096, 2.625 and ～8.0 MeV and in¹⁵²Tm at 2.555 and ～ 6.3 MeV, and the level schemes of both nuclei have been established up to 2.6 MeV. The level spectra are interpreted using the shell model in terms of the coupling of anf _7/2,h _9/2 or i_13/2 valence neutron to the yrast excitations of the neighboring N=82 nuclei.     

Magnetic moment of the 5/2−1/2[541] ground state of 14h 185Ir

The magnetic and electric hyperfine splitting frequencies ¦gμ _N B _HF/h¦ ande ² qQ/h of the 5/2^?1/2[541] ground state of 14h ¹⁸⁵Ir in Ni were measured with nuclear magnetic resonance on oriented nuclei to be 360.8(7) MHz and +6.7(2.0) MHz, respectively. The ground state magnetic dipole moment and electric quadrupole moment of¹⁸⁵Ir are deduced to be ¦μ¦=2.601 (14)μ _N andQ=?1.9(5)b, taking values for the hyperfine field and electric field gradient of B_HF=?454.9 (2.3) kG and eq=?0.151(4) × 10¹⁷ V/cm², respectively. The negative quadrupole moment is in agreement with nuclear-orientation data and proves again theI ^π K=5/2^? 1/2 ground state configuration.     

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.     

The decay of 4 min 158Tm

The decay of 4 min ¹⁵⁸Tm has been investigated with on-line mass-separated samples obtained from the Orsay ISOCELE separator. Measurements of γ-rays, conversion electron lines and γ-γ bi-dimensional coincidences were performed. About 180 transitions were ascribed to the decay and two thirds of them were placed in a decay scheme. The β-band and the γ-band were identified with bandheads situated at 806.40 and 820.13 keV respectively. In addition, a number of other vibrational bands (β-γ, β-β, K^π = 0^? and 1^?) are proposed. The decay properties of those bands are discussed in the framework of current nuclear models. The log ft values suggest a 2^? assignment for ¹⁵⁸Tm with the possible configuration (p404J↓-n521↑).     

Gammaübergänge und Energieniveaus in Dy 165

220 neutron capture Gamma rays of Dy 165 were measured with the Risø curved crystal spectrometer. The intense transitions permitted the construction of a level scheme for this oddN nucleus. The rotational bandsK=7/2⁺[633],K=1/2^?[521] andK=5/2^?[512] were precisely localized, including the spin 11/2 states. Gammavibrational bands built on these Nilsson orbits were observed with their band heads at 538.6, 573.5 and 570.2 keV. TheI=5/2 term at 533 keV can be considered as the 5/2^?[523] Nilsson state. The branching ratios allowed the determination of (g _K?g^R)²-factors for the [633], [521] and [512] bands and yieldedE 1-hindrance factors.     

169Tm NMR study of the high-Tc superconductor TmBa2Cu3O6.92

The ground state of Tm³⁺ ion (4t²,³H₆) in a crystal electric field (CEF) of TmBaCuO is a singlet, separated from the lowest excited states by an energy gap of 100÷130 cm^?1. The pulsed NMR of¹⁶⁹Tm nuclei (spinI=1/2) in a magnetically-oriented TmBa₂Cu₃O_6.92 power is studied at temperatures 1.5÷4.2 K. Two sorts of Tm nuclei are observed: rapidly relaxing Tm1 (≥80% of total amount, having relaxation timesT ₁ ⁽¹⁾ ≈35 ms,T ₂ ⁽¹⁾ =80÷250 μs) and slowly relaxing Tm2 nuclei (≦20%,T ₁ ⁽²⁾ =300÷1000 μs). The¹⁶⁹Tm NMR spectra are described by the spin-Hamiltonian ?₁ = ??Σγ _i H _i I _i (i = x,y,z), where |γ _x ⁽¹⁾/2π| = 5.3(1), |γ _y ⁽¹⁾/2π| = 6.6(1), |γ _x ⁽²⁾/2π| = |γ _y ⁽²⁾/2π| = 5.1(1), |γ _z ⁽¹⁾/2π| = |γ _z ⁽²⁾/2π| = 2.3(1) kHz/Oe, and axesx, y, z coincide with the crystal axesa, b, c. The Tm1 and Tm2 nuclei are identified as those belonging to orthorhombic (CEF of D_2h symmetry) and tetragonal (D_4h) phases of the TmBaCuO compound, respectively. Two-exponential and nearly temperature-independent nuclear relaxation is observed at temperatures 1.5÷4.2 K. The smallestT ₂ ⁽¹⁾ -values (≈80 μs) found for the orthorhombic (superconducting) phase in an external field H perpendicular to the crystalc-axis agree fairly well with those one could expect due to the dipole-dipole interaction of thulium nuclei. However, the slowing down of this rapid relaxation of the nuclear transverse magnetization in a field H‖c as well as the origin of unusually largeT ₂ ⁽²⁾ -values remain unclear.     

Nuclear levels in228Th populated in the decay of228Pa

The electron-capture decay of²²⁸Pa to levels in²²⁸Th has been studied using mass-separated sources and high-resolutionγ-ray and conversion-electron spectroscopy. A level at 979.5 keV is assigned as 2⁺ member of a second excited K_π=0⁺ band, with the 0⁺ band head at 938.6 keV. The 2⁺ and 3⁺ members of a second excited K_π=2⁺ band at 1153.5 and 1200.5 keV, which decay by strongE0 transitions to the 969 keVγ-vibrational band, are confirmed. In addition we tentatively propose a K_π=1⁺ band at 944 keV. The K_π=0^?, 1^? and 2^? members of the octupole quadruplet are confirmed, and theγ decay of these levels is analysed in an approach, in which the mixing of the octupole bands by the Coriolis interaction is taken into account. It is suggested that octupole correlations might be important for theE1 transition moments. A total of 29 levels is observed between ～1.4 and ～2.0 MeV, for which the nuclear structure, and the possible assignment to rotational bands, is unclear.     

Multipole mixing ratios of transitions in168Er

Time-integral and time-differentialγ-γ directional correlation measurements were performed for cascades in¹⁶⁸Er populated from the electron-capture decay of¹⁶⁸Tm(87d). The¹⁶⁸Tm source was dissolved in HF acid to minimize extranuclear perturbations. The results of the time-differential experiment for the 448-99.3 keV and 448-198 keV cascades showed agreement with time dependent interaction and gaveλ ₂(1094)=(0.79±0.09) ×10⁷ sec^?1. The attenuation factors for the 80 keV level were determined asG ₂₂ (80)=0.90±0.01 and G₄₄(80)=0.94±0.01. OnlyI=4 spin value for the 1094 keV level is consistent with our measurements. The 632, 731, 741, and 816 keV transitions connecting theK=2⁺ andK = 0⁺ rotational bands were found to be of almost pureE2 character. The 99.3, 198, 547, 720, 830, and 1277 keV transitions are of practically pureE1 multipolarity. The 448 keVMl transition has (0.8 _?0.4 ^+2.0 )%E2 admixture. The results for the 1014?80keV cascade give (23±4)%E3 component in the 1014 keV transition.     

Alpha-decay of 39.3 h 254mEs isomer

Mass-separated samples of ^254mEs were used to investigate its α-decay scheme. Twelve α-groups were identified in a spectrum measured with the Argonne magnetic α-spectrometer. The favored α-transition populates a 211.8 keV level in ²⁵⁰Bk and four members of its rotational band were observed. A three-parameter α-γ time coincidence experiment showed that the 211.8 keV level decayed by three routes: two were prompt and one was delayed. The delayed γ-rays (50.07, 71.30, 90.7, 104.0, 126.0 and 175.7 keV) decayed with a 42±2ns half-life. The energies of the prompt γ-rays were 79.90, 96.3, 177.3 and 211.8 keV. From an α-ce^? coincidence experiment the multipolarity of the 211.8 keV transition was deduced to be E1. The α-branching was determined from an α-singles spectrum and was found to be 0.33±0.01%. From the results of the present investigation and the known properties of ²⁵⁰Bk and ^254mEs, the 211.8 keV state in ²⁵⁰Bk and the ^254mEs ground state were given $\text{{}\text{n}\text{[622]}\text{3}\text{2}{}^{\mathrm{+}}\text{;}\text{p}\text{[633]}\text{7}\text{2}{}^{\mathrm{+}}\text{2}{}^{\mathrm{+}}$ assignment. Other levels in ²⁵⁰Bk were postulated at 104.0 (and its rotational member at 125.3), 131.9 and 175.3 keV and these were given tentative assignments of $\text{n}\text{[620]}\text{1}\text{2}{}^{\mathrm{+}}\text{;}\text{p}\text{[521]}\text{3}\text{2}{}^{\mathrm{?}}\text{1}{}^{\mathrm{?}}\text{,}\text{n}\text{[620]}\text{1}\text{2}{}^{\mathrm{+}}\text{;}\text{p}\text{[633]}\text{7}\text{2}{}^{\mathrm{+}}\text{3}{}^{\mathrm{+}}$ and $\text{n}\text{[613]}\text{7}\text{2}{}^{\mathrm{+}}\text{;}\text{p}\text{[633]}\text{7}\text{2}\text{O}{}^{\mathrm{+}}$ (with I = 1), respectively. The splitting energies between the parallel and antiparallel coupled states were calculated with a Gaussian potential for the residual neutron-proton interaction and were found to be in agreement with the experimental values. A precise measurement of the energies and intensities of γ-rays and K X-rays associated with the ^254mEs β^? decay was also made. The Fm K_α2 and K_α1 energies were found to be 115.280±0.015 and 121.065±0.015 keV, respectively.