Quantum numbers of26Al levels above 6 MeV excitation energy

Measurements of resonance strengths and ofγ-ray angular distributions or anisotropies have been performed on selected resonances of the²⁵Mg(p, γ) reaction in the rangeE _p=2–4 MeV,E _x=8.2?10.1 MeV with an emphasis on high-spin andT=1 analog resonances. EightT=1 states are identified, among them high-spin states at 8747 keV (I=6), 9286 KeV (I=5), and 9986 keV (I ^π = 7⁺, 6⁺). Shell model calculations in thes-d basis space reproduce the branching ratios of these states and clarify the nature of final states. New high-spinT=0 states are observed at 9720 keV (I ^π = 7⁺), 8602 keV (I = 6), and 6695 keV (I ^π = 7⁺). TheI ^π assignments to severalE _x = 6–8 MeV states are revised and the role of two-particle excitations into thef-p shell is elucidated. A revised spectrum of 73 positive-parity,T = 0 states is compared to the predictions of shell-model calculations in thes-d basis space using the universals-d shell Hamiltonian.     

Collective excitations in148Gd populated in thep p′-reaction on a radioactive target

Collective excitations in the two-particle nucleus¹⁴⁸Gd up to 2.9 MeV have been investigated by thepp′-reaction at 25 MeV beam energy, and angular distributions have been analyzed with standard DWBA calculations. It is found that¹⁴⁸Gd has larger octupole- than quadrupole collectivity in the energy range investigated. The 3^? strength distribution is in quantitative agreement with predicted results for the levels formed by the coupling of two valence particles to the¹⁴⁶Gd core octupole phonon.     

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.     

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.     

In-beamγ-ray spectroscopy of the neutron deficient100Ag

The neutron deficient nucleus¹⁰⁰Ag, three proton holes below and three neutron particles above the N=Z=50 shell closure at¹⁰⁰Sn, has been identified and studied by in-beam spectroscopy. The reactions⁴⁶Ti(⁵⁸Ni,3pn) at 231 MeV and⁶⁴Zn(⁴⁰Ca,3pn) at 167 MeV of the respective⁵⁸Ni and⁴⁰Ca beams were used, and states up to 8.7 MeV excitation energy and spin I?20 were found in two mainγ-ray cascades forming band-like structures of even and odd parity. Large scale shell model calculations suggest predominantπg ₉ ^2/?3 ν(d_5/2,g_7/2)³ andπg ₉ ^2/?3 ν(d_5/2,g_7/2)²h_11/2 structures with maximum spins I ^π = 19⁺ and I ^π =22^?, respectively, for the two level sequences. The influence ofπp_1/2 vs.νh_11/2 excitations is discussed for low lying odd-parity levels.     

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.     

Negative parity states in144Nd and multipolarity of gamma-transitions from 1− to 2+ states by beta-gamma-gamma angular correlation method

Some of the low-lying states in many isotopes¹⁴⁴Nd,¹⁴⁸Sm,¹⁵²Gd and¹⁵⁶Gd show a similar typical behaviour. The first 2⁺ is regarded as a single quadrupole phonon state and 3⁻ as a single octupole phonon state. The levels with the spins and parities 1⁻, 5⁻, 3⁻, 4⁻, etc. are considered due to the simultaneous excitation of quadrupole and octupole phonons. If this consideration is correct, then the transition fromJ ⁻ to 2⁺ states must contain an appreciableE3 content. Theβ-γ-γ angular correlation coefficients for the cascade ofβ-rays ofE _max 800 keV→γ-rays of 1489 keV→γ-rays of 696 keV are used to estimateE3 content inE1 transition in¹⁴⁴Nd.     

Density of high-spin states in38Ar and42Ca

Theγ-decay modes of³⁸Ar levels withE _x ≦11,630keV and of⁴²Ca levels withE _x ≦10,036keV have been studied using the³⁵Cl(α, pγ) reaction at 16MeV and the³⁹K(α, pγ) reaction at 15.14 MeV, respectively. In both nuclei the number of states withJ≧6 exceeds fifty. Weak coupling calculations of the Bansal and French type reproduce the density of high-spin states. The success of the model implies that the excitations of up to four particles from thed _3/2 into thef _7/2 shell play a role in both nuclei. The structure of deformed states was found to be predominantly 4p/s 6h in³⁸Ar and 4p/s 2h in⁴²Ca, respectively.     

Two proton high-spin excitations and dipole bands in192Hg

The¹⁹²Hg nucleus was populated in the¹⁶⁰Gd(³⁶S, 4n) reaction at a beam energy of E=159 MeV. Emittedγ-rays were detected with the EUROGAM array comprising 43 Compton-suppressed large volume Ge detectors. The level scheme of¹⁹²Hg has been extended up to an excitation energy of E=10.4 MeV and spin I=34 ?. Two new structures, made of competing ΔI=1 and ΔI=2 transitions have been observed and their connexions with the low-lying levels established. Their lowest levels are located at 6.304 MeV and 6.879 MeV excitation energy. The experimental results are compared with mean-field HF+BCS calculations. It is proposed that the new structures originate from deformation-aligned quasi-proton excitations π(i_13/2 * h_9/2)_K=11 andπ (h_9/2) _K=8 ² , coupled to rotation-aligned quasi-neutron ν(i_13/2)ⁿ and quasi-proton π(h_11/2)² excitations.     

In-beam study of74Br and a first cascade in75Kr

An in-beam study of⁷⁴Br has been carried out using γ-ray spectroscopy in connection with the reactions (³He,p2in) and (d,2n) on⁷⁴Se targets. Separate level schemes on top of the two β-decaying isomers of⁷⁴Br are proposed. Two cascades connecting highspin states are assigned to populate theI ^π=4⁽⁺⁾ isomer. In this level scheme, the excited states at 72.1, 188.3 and 472.0 keV are found to have half-lives of 13.3, 0.7 and 0.7 ns, respectively. The low-spin isomer of⁷⁴Br is tentatively assigned asI ^π=(0^?). This state is populated by two other cascades. For the low-spin levels two-particle configurations of proton and neutron Nilsson orbitals are proposed. The high-spin states are tentatively interpreted as arising from the configurations (πg _9/2,νg _9/2) and (πg _3/2,νg _9/2). As a by-product, a cascade of four transitions has been observed during the³He bombardment of⁷⁴Se and tentatively assigned to negative-parity states in⁷⁵Kr.     

High-spin states in109Sn and their decay to the ground state

An extended level scheme of¹⁰⁹Sn is presented showing high-spin states up to E_x≈ 8 MeV and spins up toJπ=(41/2+). Their decay to the 5/2+ ground state has been observed identifying a 12.8 keV 7/2⁺ → 5/2⁺ transition. A half-life of T_1/2=7(1) ns has been measured for the 17/2⁺ state atE _x =2114 keV. The experimental data are compared with the predictions of shell-model calculations.     

Dipole excitations in 122Te, 126Te and 130Te

Excited states of the nuclei ¹²²Te, ¹²⁶Te and ¹³⁰Te were populated via the (γ, γ') reaction at endpoint energies of the bremsstrahlung between 4.5 and 5.5 MeV. Gamma rays were detected with a EUROBALL-CLUSTER detector and a single detector. In all nuclei several dipole transitions were identified at energies around 3 MeV. The lowest corresponding J = 1 states are interpreted as two-phonon excitations. Quasiparticle-phonon-model calculations predict one 1^? state arising from the coupling of the first quadrupole and the first octupole phonon and one 1⁺ state arising from the coupling of the first and the isovector second quadrupole phonon at about 3 MeV. The calculated transition strengths are compatible with experimental ones.     

Structure of28Si from the spectroscopy of high-spin states

A search for high-spin states in²⁸Si has been performed byn?y coincidence measurements in the²⁵Mg(α,nγy) reaction atE _α=14 and 15.5 MeV. Spin-parity assignments of the observed levels were obtained fromn?γ angular correlation and lifetime measurements atE _α=14.5 MeV. Theγ-decay of the 9,164 keV level was investigated separately with the²⁷Al(p, γ) reaction at theE _p=2,160 and 2,312 keV resonances. Rotational bands withK ^π=3^? (comprising levels atE _x=6,879, 8,413, 10,188 and 12,204 keV),K ^π =5^? (comprising levels atE _x=9,702, 11,577 and 13,741 keV) andK ^π=0⁺ (comprising levels atE _x=6,691, 7,381, 9,164 and 11,509 keV) were observed. The finding of the latter band supports the idea of coexisting oblate and prolate shapes in²⁸Si. A level at 14,643 keV excitation energy has the properties of theI ^π=8⁺ member of the ground state band. There are additional positive-parity high-spin states which do not fit into rotational bands. All types of positive-parity states are well accounted for by shell model calculations.     

Alpha particle scattering to negative parity states of14N

Inelastic⁴He scattering on melamine targets is studied for negative parity excitations of¹⁴N. ElevenΔL=3 excitations are presented, for states up to 15.41 MeV; threeΔL=1 transitions are also studied. A microscopic DWBA calculation accounts for several of the octupole excitations, requiring an isoscalar effective strength about twice that expected for free alpha-nucleon scattering. Great purity of the structure of the three lowest 2^? states is noted, with the 5.11 MeV and 9.13 MeV levels excited byΔL=3, but with the 7.97 MeV state excited purely byΔL=1.     

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.     

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.     

The structure of25Mg

Gamma-decay modes and spin(-parity) assignments of levels in²⁵Mg have been systematically investigated up to 10 MeV excitation energy by particle-γ-ray angularcorrelation measurements with the²⁴Mg(d, pγ) reaction at 6.5 MeV bombarding energy and with the²²Ne(α,nγ) reaction at 11.8, 12.5, 14.4 and 15.5 MeV bombarding energy. A level scheme has been established which is comprehensive up to 8.3 MeV excitation energy forI≦9/2 and up to 10 MeV for 9/2O d _5/2 — 1s _1/2-O d _3/2 shell and the unifieds-d shell Hamiltonian. The agreement is good to excellent. The first intruder states are located near 6.8 MeV excitation energy. The collective properties of²⁵Mg beyond the well established rotational bands are investigated using both the new experimental information and theB(E2)'s obtained from the shell model. The spectrum of²⁵Mg is completely rotational for the first five to six MeV above the yrast line. Shell modelB (M 1)'s reflect the Nilsson model structure of²⁵Mg in great detail. The prospectiveI ^π=9/2^?, 13/2^?, and 15/2^? members of the established negative-parity,K=1/2 band are found in levels atE _x=7801, 9410, and 8896 keV.     

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.     

Triple differential cross section for angle,atomic number and energy (or angular momentum transfer) calculated for the 280MeV40Ar+58Ni (or 365MeV63Cu+197Au) system in a simple model

Highly excited levels of³³S populated by α-particle capture in²⁹Si have been investigated forE _α=1.962 MeV to 4.287 MeV. Excitation curves measured with Ge(Li) and BF₃ detectors are reported. More than fifty resonances can be identified with levels in³³S. (α, γ) angular distributions measured on five strong resonances have yieldedJ ^π values 1/2+, 3/2 +, 5/2?, 5/2?, 5/2? and 3/2+ respectively, for theE _x =10.054, 10.466, 10.523, 10.721, 10.758 and 10.776 MeV levels in³³S. Elastic scattering experiments have been performed and theJ ^π assignments are found to be consistent with thel-values inferred from the elastic scattering data. Decay schemes from the above³³S levels have been proposed. A new level at 9.245 MeV is also suggested and theJ ^π values for the 4.425 and 2.87 MeV states are shown to be consistent with 7/2+ and 3/2+ assignment, respectively. Nuclear Reactions²⁹Si(α, γ) and²⁹Si(α, α),E _α=1.962-4.287 MeV. Measured relative σ(E). DeducedJ ^π andE _γ of³³S levels. New³³S level atE _x =9.245 MeV. Enriched targets.