Two-proton neutron-hole yrast excitations in 66 147 Dy81

The level structure of theN=81 nucleus¹⁴⁷Dy has been studied byγ-ray spectroscopy following reactions of 230–250 MeV^58,60Ni beams on⁸⁹Y and^90,92Zr targets. Yrast and near-yrast levels in¹⁴⁷Dy above the known 59-s 11/2^? state are established up to ～3.7 MeV; they include isomeric levels at 2,681, 3,407 and 3,650 keV. Guided by the results of shell model calculations, we interpret most of the observed level as senioritythree states arising from the coupling ofs _1/2,d _3/2, andh _11/2 neutron holes withπh _11/2 ².     

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.     

Description of negative parity yrast states in156Er

Positive and negative parity yrast states are studied in ₆₈ ¹⁵⁶ Er₈₈ with a particle number projected Hartree-Fock-Bogoliubov method constrained on an average angular momentum. The theory predicts a second anomaly of the positive parity yrast states due to the alignment of ah _11/2 proton pair. The double backbending in the negative yrast band is understood: AtJ ^π=9^? to 11^? it is due to the intersection of the (πg _7/27/2⁺)×(πh _11/27/2^?) and the (πi _13/21/2⁺)×(πh _9/23/2^?) 2q.p. bands. The second backbending found experimentally fromJ ^π=21^? to 23^? is connected with an alignment of ai _13/2 neutron pair of the core in the proton 2q.p. band.     

Properties of the 69 150 Tm81 6− β-decaying isomer

Decay experiments for¹⁴⁸Ho 6^? and¹⁵⁰Tm 6^? were carried out at the GSI On-line mass separator using 5 MeV/A⁵⁸Ni-beams on⁹⁴Mo and⁹⁶Ru targets. Gammaγ-andγX-coincidences established the¹⁵⁰Tm 6^? decay scheme, and e^? gave firm I ^π assignments for¹⁴⁸Dy and¹⁵⁰Er levels fed in the decays. It was found that the previously unknown 6 ₁ ^? state in¹⁵⁰Er receives significantβ-strength, which strongly suggestsπh _11/2 νs ₁ ^2/?1 character for the¹⁵⁰Tm 6^? isomer. The results are discussed in terms of the expected GT strength function. Shell model calculations for theπh ₁₁ ^2/4 yrast states are presented.     

TheN=83 nucleus149Dy from Gamow-Teller decay of its 11/2− and 1/2+ 149Ho parents

A high-sensitivityγ-spectroscopic study of the¹⁴⁹Hoπh _11/2 andπs _1/2 β-decays using mass separated sources has located dominant 0⁺ → 1⁺ GT decay strength associated with decay of pairedh _11/2 protons, leading to 3п-states in the¹⁴⁹Dy daughter nucleus. In theirγ-decay low-lying¹⁴⁹Dy levels characteristic of anN=83 nucleus are excited. They include theνf _7/2,νp _3/2,νh _9/2 andνp _1/2 single particle- and theνs ₁ ^2/?1 andνd ₃ ^2/?1 two-particle one-hole states, as well as the νf _7/2 × 3^? andνf _7/2 × 2⁺ particle-phonon multiplets. A synopsis is given of these excitations in theN=83 isotones from¹⁴⁹Nd to¹⁵³Yb. The¹⁴⁹Dy GT decay strength is discussed in terms of the¹⁴⁷Tb₈₂ and¹⁴⁸Dy₈₂ decays. The strength function results are also compared with independent¹⁴⁹Ho 11/2^? decay data from the literature based on totalγ-ray absorption measurements.     

Two-nucleon excitations in 66 148 Dy82 from Gamow-Teller decay of the 67 148 Ho81 6− isomer

Two-particle excitations in the two-proton nucleus¹⁴⁸Dy populated inβ-decay of 10-s ¹⁴⁸Ho(6^?) were investigated throughγ-ray measurements following on-line mass separation. Theπh _11/2s_1/2 two-proton doublet and the (vh _9/2 s ₁ ^2/?1 )5^? neutron particle hole configuration are firmly identified, and theπh _11/2 d ₅ ^2/?1 and πh_11/2D_3/2 multiplets are also observed. The results suggest dominantπh _11/2vs ₁ ^2/?1 character for the¹⁴⁸Ho 6^?-isomer.     

A new method to test virtual photon spectra

Millisecond isomers in¹⁴⁶Tb,¹⁴⁸Ho and¹⁵⁰Tm, observed in⁶⁰Ni reactions on³⁹Y,⁹⁰Zr, and⁹²Mo targets, are interpreted as (πh_11/2 vh_11/2 ^?1) 10⁺ isomers predicted by the shell model. Their decays are characterized.     

Gamow-teller strength and particle-phonon states in149Dy from decay of its 11/2− and l/2+ 149Ho parents

Studies of the¹⁴⁹Ho 11/2^? and 1/2⁺ GT-decays using on-line mass separation have established in the N=83¹⁴⁹Dy nucleus the single neutron excitations below 1.8 MeV and members of the νf_7/2×3^? and νf_7/2×2⁺ particle-phonon multiplets. Quantitative analysis of the GT-decay strength to¹⁴⁹Dy 1qp- and 3qp-states is made in terms of the strength values measured in¹⁴⁹Dy₀ and¹⁴⁷Tb_11/2? decay.     

A 0.5 μs 10+ isomer in148Dy82 and the πh11/2 effectiveE 2 charge

In¹⁵²Gd(α,8n) and¹³⁶Ce(¹⁶O,4n) reaction studies a 48O ns 10⁺ isomer in¹⁴⁸Dy has teen identified, and its decay has been characterized. All members of the πh ₁₁ ^2/2 level spectrum have been located, and the 10⁺ → 8⁺ B(E2) value gives the πh ₁₁ ^2/2 effective charge as 1.53 e. The 2⁺ and 3^? level energies in¹⁴⁸Dy₈₂ provide supporting evidence for a shell closure at Z=64.     

A 45 ns 8+ isomer in the doubly odd150Eu nucleus

In the doubly odd ₆₃ ¹⁵⁰ Eu⁸⁷ nucleus a 45 ns 8⁺ isomer at 589 keV excitation has been identified through (p, 3n) in beamγ ande ^? measurements. The low lying negative parity levels populated in its decay show great similarity to analogous states in the lighter odd-odd Eu neighbours suggesting a spherical shape for¹⁵⁰Eu. A spherical shape is also proposed for the isomer which is assigned as (πh _11/2 vf ₇ ^2/?3 )8⁺ state analogous to the 9⁺ isomers known in¹⁴⁸Eu and¹⁴⁶Eu. The spin change is attributed to the crossing of the half-filledf _7/2 neutron shell atN=86.     

Theπ h 11 2/3 spectrum in the three valence proton nucleus 67 149 Ho82

A πh ₁₁ ^2/3 level spectrum and other lowlying yrast states in¹⁴⁹Ho have been identified in reactions of 155–180 MeV³²S with¹²¹Sb and¹²²Te targets. The results agree well with shell model predictions.     

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.     

High-spin levels in138Ce and140Nd Observed in the (α, 4n γ) reactions

Levels of¹³⁸Ce and¹⁴⁰Nd have been studied using the¹³⁸Ba(α,4nγ)¹³⁸Ce and¹⁴⁰Ce(α, 4nγ)¹⁴⁰Nd reactions. Singleγ-ray spectra,γ-γ coincidence spectra, angular and time distributions with respect to the beam bursts have been measured. A number of higher excited states with excitation energies up to about 5 MeV and with spin value up to 12 are populated in both nuclei. The lower states with spins and parities 7^?, 5^?, 6^? and 10⁺ can be explained by two-quasiparticle neutron configurations of the types (h _11/2 ^?1 ,d _3/2 ^?1 ) 7^? , (h _11/2 ^?1 ,S _1/2 ^?1 ) 5^?, 6^? and (h _11/2 ^?2 ) 10⁺. Several high-spin states observed in¹³⁸Ce and¹⁴⁰Nd can be explained qualitatively as four-quasiparticle states with two-proton-two-neutron configurations. The 3^? state at an energy of 2,137.4 keV is observed in¹³⁸Ce. The evidence for the existence of the low-lying 3^? states in¹⁴⁰Nd at 2,124.0 keV is discussed. Beside the known 9.6 ms (7^?) isomeric state in¹³⁸Ce another state at 3,538.5keV (10⁺) with a half life of about 200 ns has been observed. The observed levels in the¹³⁸Ce and¹⁴⁰Nd nuclei are compared with theoretical predictions using delta force interaction.     

Three-nucleon yrast states in145Sm

In-beamγ-ray and conversion electron measurements with (α, xn) reactions have established the¹⁴⁵Sm highspin states up toI ^π=25/2⁺ at 3.5 MeV excitation. A shell model analysis using empirical two- and one-body energies from neighbouring nuclei classifies the low-lying odd-parity levels as 3-quasiparticle states formed by the¹⁴⁴Sm two-proton-hole excitations and thef _7/2 valence neutron. The higher-lying positive-parity states involve particle-hole core excitations with one proton inh _11/2.     

The 235 ns 9+ isomer in 63 148 Eu85

From (p, xn) in-beamγ-ray and electron measurements aT _1/2=235(14) ns isomer at 720 keV was identified in ₆₃ ¹⁴⁸ Eu₈₅ and the levels populated in its decay have been characterized. The results determineI ^π=9⁺ for the isomeric state, and we assign it as the (πh _11/2 j ₀ ^?2 ν f ₇ ^2/3 )9⁺ shell model state, analogous to the configuration of the 235 μs isomer in ₆₃ ¹⁴⁶ Eu₈₃.     

Gamow-teller transitions in β-decay of 66 146 Dy80

The β-decay of¹⁴⁶Dy₈₀ was studied by γ-ray and conversion electron spectroscopy following on-line mass separation. 58 % of the β-decay proceeding to a group of levels close to 2 MeV excitation is attributed to GT-decay of (πh ₁₁ ^2/2 )₀₊ pairs populating (πh_11/2νh_9/2νj ₀ ^?2 )1⁺ states in¹⁴⁶Tb₈₁. The total decay strength is significantly higher than found in¹⁴⁸Dy₈₂ decay, which arises from GT-transitions involving the ν-holes.     

Particle plus octupoleM2/E3 isomers and high-spin particle-hole states in147Tb and148Tb

The level structures of theN=82 andN=83 nuclei¹⁴⁷Tb and¹⁴⁸Tb have been studied by means of (α, 8n) and (α,7n) reactions induced by 68 to 110 MeVα particle bombardments of¹⁵¹Eu targets. In-beam conversion electron measurements have established that isomers withT _1/2=4.8(6)ns in¹⁴⁷Tb andT _1/2=22(1)ns in¹⁴⁸Tb decay byM2+E3 transitions to the ground states. The measuredB(E3) values show that the isomeric states arise from the coupling of the valence nucleon(s) to the¹⁴⁶Gd core octupole. Particlephonon coupling in these nuclei and in the one-neutron nucleus¹⁴⁷Gd is discussed and compared with well known cases involving the²⁰⁸Pb core. The higher lying yrast states in the two Tb nuclei are described as shell-model particle-hole excitations using empirical single particle energies and nucleon-nucleon interactions.     

High-spin bands in117,119I and118Xe

Excited states in¹¹⁷I,¹¹⁸Xe and¹¹⁹I were populated in the reactions¹⁰⁶Pd+¹⁶O and^{108, 110}Cd+¹²C. The groundstate band in¹¹⁸Xe was observed to its (18⁺) member and the 11/2^? bands in¹¹⁷I and¹¹⁹I to their (35/2^?) and (43/2^?) members, respectively. The structure of the bands in the I isotopes is interpreted as aπ h _11/2 proton coupled to an even-even band structure in Xe, and the band crossings observed in¹¹⁸Xe and¹¹⁹I are interpreted in the Cranked Shell Model framework asAB _n ofv h _11/2.     

Evidence for a 3.6 minute isomeric 12− state of the πh 11 2/−1 vi 13 2/−1 configuration in206Tl and effective two-particle interactions

Using the²⁰⁴Hg(α, pn)-reaction andα-particles of energies 39–55 MeV, we have found an isomeric 3.6 min 12^? state in²⁰⁶Tl at 2,642.9 keV which has the two-hole configurationπh ₁₁ ^2/?1 vi ₁₃ ^2/?1 The 12^? state decays mainly by anE5 transition of energy 1,021.4 keV to a 7⁺ state at 1,621.5 keV whose main configuration isπs ₁ ^2/?1 vi ₁₃ ^2/?1 There is, in addition, evidence for a weak 565 keVM 4 branch to an 8⁺ state at 2,078 keV whose main configuration should beπh ₁₁ ^2/?1 vf ₅ ^2/?1 . The 7⁺ state decays by a stretched cascade ofγ-rays to states of the following values ofJ ^π and excitation energy: 5 _? ⁺ , 1,405.4 keV; 4^?, 952.1 keV; 2^?, 265.8 keV and 0^?, 0 keV. The main configurations of these states areπh ₁₁ ^2/?1 vp ₁ ^2/?1 ,πd ₃ ^2/?1 vf ₅ ^2/?1 ,πd ₃ ^2/?1 vp ₁ ^2/?1 andπs ₁ ^2/?1 vp ₁ ^2/?1 respectively. From the nuclear masses of²⁰⁸Pb,²⁰⁷Pb,²⁰⁷Tl, and²⁰⁶Tl and the experimental excitation energies it is possible to obtain the proton hole-neutron hole interaction in²⁰⁶Tl. This interaction is compared with the calculations of Kuo and Herling and the discrepancies are discussed. The 12^?→8⁺ M4 transition rate is reduced because of destructive interference between the protonh _11/2→d _3/2 and the neutroni _13/2→f _5/2 contributions. The magnitude of the reduction is accurately reproduced by the wave functions of Kuo and Herling. The 12^?→7⁺ E5 transition rate is about twice as large as the single-holeπh ₁₁ ^2/?1 πs ₁ ^2/?1 transition rate. This deviation is fully explained by the configuration admixtures in the 7⁺ state, given by Kuo and Herling.     

Decays of millisecond isomers in odd-oddN=81 nuclei146Tb,148Ho and150Tm

Following reactions induced by 245 MeV⁶⁰Ni ions, isomers have been identified in the odd-oddN=81 isotones¹⁴⁶Tb,¹⁴⁸Ho and¹⁵⁰Tm with half-lives of 1.18(2), 2.35(4) and 5.2(3) ms, respectively. Their decays have been characterized byγ-ray spectroscopy. The isomers are interpreted as (πh _11/2 νh ₁ ^2/?1 ) 10⁺ states decaying to members ofπh _11/2 νd ₃ ^2/?1 andπh ₁₁ ^2/?1 multiplets. The observed level spectra and M1 branching ratios are found to be in good agreement with shell model predictions based on empirical and Schiffer-Trueπν ^?1 residual interactions.