High-spin levels in137,139Ce and139,141Nd evidence for hole-core coupling

Excited states of the¹³⁷Ce,¹³⁹Ce,¹³⁹Nd and¹⁴¹Nd nuclei have been studied using the¹³⁸Ba(α, 5nγ)¹³⁷Ce,¹³⁸Ba(³He, 4nγ)¹³⁷Ce,¹³⁸Ba(α, 3nγ)¹³⁹Ce,¹⁴⁰Ce(α, 5nγ)¹³⁹Nd,¹⁴⁰Ce(³He,4nγ)¹³⁹Nd,¹⁴⁰Ce(α, 3nγ)¹⁴¹Nd and¹⁴²Ce(α, 5nγ)¹⁴¹Nd reactions. Singlesγ-ray spectra,γ —γ coincidence spectra, angular distributions and time distributions ofγ-rays with respect to beam pulses have been measured. Gamma transitions between excited states with spin values up to 21/2, 23/2 or 25/2 have been observed. Isomeric states with T_1/2=70 ns have been observed in¹³⁹Ce at 2631.5 keV (19/2) and in¹⁴¹Nd probably at 2952.0 keV (19/2). The level structure observed in the nuclei studied can be explained if the neutron-holes are coupled to the doubly even core excitations. The coupling of theh _11/2 neutron-hole with the 2⁺, 4⁺ and 3^? collective excitations are calculated in terms of the weak and intermediate coupling models. The intermediate coupling results seem to be in better agreement with the experimental data. The energies of theree-particle states, being the result of the coupling of theh _11/2 neutron-hole with the two-proton excitations in the core, are well reproduced in the calculations when empirical values of the two-body interaction matrix elements were used.     

Quasirotational groundstate bands of even-even nuclei with 50≦N,Z≦82

The quasirotational groundstate bands of¹⁴⁰Sm,¹⁴²Sm,¹⁴⁴Gd and¹³⁶Nd have been determined by studying the prompt γ-decay after (α,xn+yp+zα.) reactions using α-particles of up to 104 MeV and targets of¹⁴⁴Sm,¹⁴²Nd,¹⁴¹Pr,¹⁴⁰Ce,¹³⁶Ce and¹³⁹La. The new levels continue the trends of the neighbouring even-even nuclides smoothly. These systematics of the groundstate bands in the region 50<N,Z<82 are not disturbed by expected subshell closure effects.     

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.     

High-spin states in 14460Nd84 observed in the (α, 2nγ) reaction

Excited states of ¹⁴⁴₆₀Nd₈₄ have been studied using the ${}^{142}\text{Ce}\text{(α,2}\text{n}\text{γ)}{}^{144}\text{Nd}$ reaction. Singles γ-ray, γ-γ coincidence spectra, angular and time distributions of γ-rays with respect to the beam bursts have been measured. Excited states up to 4.986 MeV with spin values up to 13 were populated. The majority of the observed states with spin values up to 12 can be interpreted as two-quasiparticle states. Several of the highest-spin levels can also be explained quantitatively as four-quasiparticle states. No isomeric states with a half-life longer than 1 ns have been found.     

Level structure of 76As

The energy-level structure of ⁷⁶As has been studied by the ⁷⁵As(d, p) reaction with 12.0 MeV incident deuterons. Eighty-seven excited energy levels up to 2.5 MeV have been identified. The angular distributions for thirty-nine of the observed transitions are compared with distorted-wave Born approximation calculations. Angular momentum transfers are determined and spectroscopic strengths are deduced from the data. The results are compared with shell-model sum rules and with (d, p) results on N = 42 isotones. Possible spin values are assigned for some of the levels based on the present results, (n, γ), (p, n) and (p, nγ) studies.     

Structure of even-even138Nd from the decay of138Pm

The structure of¹³⁸Nd is investigated through theβ ⁺-decay of¹³⁸Pm, whose decay half-life is measured to be 3.24±0.05 min. A level structure comprising 27 excited states and 52 transitions is constructed. With the help of deducedft values and internal conversion coefficients, spin-parity assignments are made to many states in¹³⁸Nd and to the ground states of¹³⁸Pm. From the positron end-point energy measurement, the total decay energy is determined to be 5.4±0.2 MeV, which is compared with the predictions of several mass formulae and with the decay energy systematics of theN=77 isotones. The structure of¹³⁸Nd is discussed in the light of nuclear systematics of theN=78 isotones.     

Level structure of 72As studied with the (α, xnp) reaction

States of ⁷²As, excited through the ⁷⁰Ge(α, np) and ⁷²Ge(α, 3np) reactions at E_α = 30 to 55 MeV, were studied. Excitation functions, γ-ray angular distributions, γ-γ coincidences and γ-time distributions with respect to the beam bursts, were determined. Five excited states are identified with levels previously observed with the (p, n) reaction. In addition seven new levels and their decay pattern are incorporated into a level scheme. A half-life of 17 ± 3 nsec was determined for the 309.8 keV state, and evidence for the existence of a long-lived state (>100 nsec) was obtained. A simple scheme based on simple particle shell-model configurations accounts for the gross properties of the low lying levels. While definite spin and parity assignment for the upper excited states require further measurements, tentative spin values up to 7 are obtained.     

Ground state bands in142Gd and140Sm

The ground state bands of theN=78 isotones¹⁴² Gd and¹⁴⁰Sm were observed up to the 8⁺ state by bombarding¹⁴⁴Sm and¹⁴²Nd withα-particles of energies between 80 and 130 MeV. Excitation functions,γ-γ coincidences, lifetimes and angular distributions were measured. The ground state band in¹⁴⁰Sm is partially fed by an isomeric state ofT _1/2?17ns. No such isomerism is observed for¹⁴²Gd. The level energies are very similar in both cases and agree well with the predictions of the VMI model.     

New results from GASP in the A=130 mass region

The GASP array has been used to study the high spin structures of the ¹³⁴Nd and ¹³⁶Nd nuclei populated in the ¹¹⁰Pd(²⁸Si,4n) and ¹¹⁰Pd(³⁰Si,4n) reactions at 130 MeV and 125 MeV, respectively. Several deexcitation pathways have been established for the yrast and excited highly deformed (HD) bands in 134Nd. The decay out process in this nucleus has been understood in terms of mixing between the normal deformed and HD states, which is triggered by the crossing between the vi _13/2 and vd _5/2 orbitals. The study of the high spin excitations in ¹³⁴Nd and ¹³⁶Nd revealed two different mechanisms for generating high angular momentum: the stepwise occupation of the N = 6 i _13/2 intruders in the more rigid nucleus ¹³⁴Nd and the alignment of successive pairs of h _11/2 protons and neutrons coupled to a stable small-deformation triaxial core in the γ-soft nucleus ¹³⁶Nd.     

Level structure of 147Nd2 (iii). The 146Nd(n,e?) reaction

The conversion electrons following thermal neutron capture in ¹⁴⁶Nd have been studied in the energy range 30 < E_e < 125 keV, with a conversion electron spectrometer installed at the Grenoble high-flux reactor. Precise values of the multipole mixing ratio M1/E2 have been obtained for transitions up to 100 keV. Furthermore, the spin and parity for the 215 keV level is verified. The states of ¹⁴⁷Nd are compared with the two other N = 87 isotones; the properties of these nuclei can be understood in the framework of the spherical shell model.     

New high-spin states of 147Nd and 145Ce: Octupole correlation in the N = 87 isotones

High-spin states of the N = 87 nuclei, ¹⁴⁷Nd and ¹⁴⁵Ce, have been populated in the ¹²C + ²³⁸U and ¹⁸O + ²⁰⁸Pb fusion-fission reactions at 90 MeV and 85 MeV bombarding energy, respectively. The emitted γ-radiation was detected using the Euroball III and IV arrays. High-spin states of the ¹⁴⁷Nd isotope have been identified for the first time. The high-spin yrast and near-to-yrast structures of the ¹⁴⁵Ce nucleus have been considerably extended. The newly observed structures, discussed by analogy with the neighbouring isotones, show the coupling of an h_9/2 neutron to the quadrupole and octupole excitations of the core.     

Excitation of isomeric 1h 11/2 states in nuclear reactions induced by γ rays and neutrons and in beta decay

Isomeric ratios were measured for N=81 isotones (¹³⁵Xe, ¹³⁷Ba, ¹³⁹Ce, ¹⁴¹Nd, ¹⁴³Sm). In the experiment reported here, μ _ν ≤3±10^? J ^π=11/2^? isomers were excited in (n, γ) and (γ, n) reactions and in the β ⁺ decay of ¹³⁹Pr and ¹⁴¹Pm. In order to determine the reaction yields, use was made of the activation method involving measurement of the gamma-ray spectra of reaction products. It is found that, in the same reactions, isomeric ratios are different for isotones characterized by different atomic numbers Z. Isomeric ratios were calculated with the spectra of low-lying levels and radiative-transition probabilities established on the basis of the quasiparticle-phonon model. Good agreement between the experimental and calculated values of isomeric ratios is obtained for all isotopes invesigated here. The dependence of isomeric ratios on the atomic number Z of a nucleus is explained by the difference of reaction energies, which leads to different probabilities of excitation of activation levels through which the isomers being considered are populated.     

Radioactive decay of 1.7-h149Nd to levels of transitional149Pm

Sources of¹⁴⁹Nd were produced by thermal-neutron irradiation of enriched¹⁴⁸Nd. High resolution Ge(Li) spectrometers were used to collect gamma-ray singles and coincidence spectra in the study of the decay of 1.7-h¹⁴⁹Nd. A total of 214 gamma rays have been assigned to the decay of¹⁴⁹Nd, and 201 of these transitions have been placed in a level scheme of¹⁴⁹Pm involving 42 excited states. Absolute values for the beta-group intensities were determined for transitions to the¹⁴⁹Pm levels, and spin and parity assignments were made for many of the observed states. The experimentally determined level structure of¹⁴⁹Pm was compared with corresponding levels in other odd-mass Pm and Eu nuclides. Interpretations were made in terms of the core-coupling model. Radioactive decay¹⁴⁸Nd(n, γ)¹⁴⁹Nd; measuredE _γ,I _γ,γ-γ coinc.¹⁴⁹Pm deduced levels,J,π. Compton-suppression. Ge(Li) detector. Enriched targets.     

Untersuchungen zur Struktur des Kerns23Ne

Low-lying states of²³Ne up to about 3 MeV excitation energy have been investigated by studying the reactions²²Ne(d, p)²³Ne (E _d =4–6 MeV) and²³Na(n, p γ)²³Ne(E _n=8–9 MeV). From the (d, p) data,l-values and spectroscopic factors for the transferred neutron have been extracted by DWBA analyses. From the (n, p γ) data,γ-ray branchings and possible spin assignments have been derived. The results are used to discuss the applicability of the Nilsson model and the excited core model to the nucleus²³Ne.     

Investigation of backward α-particle scattering in 24Mg and 28Si through α-γ angular correlations

Alpha-gamma angular correlations have been measured for the reaction ²⁴Mg(α, α'γ) in the energy range 15.50 to 17.00 MeV in steps of 300 keV and for ²⁸Si(α, α'γ) from 15.00 to 16.20 MeV in steps of 200 keV to provide energy averaged results. The scattered α-particles were detected for lab angles θ_α = 115°–178° in a multidetector arrangement. The coincident γ-rays were observed with a Ge(Li) detector placed at 90° in the reaction plane. Data were obtained for the 1st, 2nd and 3rd excited states of ²⁴Mg and for the 1st, 2nd, 3rd and 4th excited states of ²⁸Si. The absolute doubledifferential cross sections were analysed with Hauser-Feshbach calculations. The analyses show that compound nuclear processes are responsible for the rise of backward α-scattering in ²⁴Mg(α, α'γ)²⁴Mg and ²⁸Si(α, α'γ)²⁸Si at the energy studied.     

States inN=79142Eu populated in the decay of142Gd

The structure of¹⁴²Eu is investigated through the beta decay of¹⁴²Gd, whose decay half-life is measured to be (69.1±1.0) s. A level scheme comprising 42 transitions depopulating 18 excited states is constructed. With the help of the deduced ft values and measured internal conversion coefficients, spin-parity assignments are proposed for several states in¹⁴²Eu. The level systematics of the doubly odd,N=79 isotones are discussed.     

Levels of 28Si from the 24Mg(α, α)24Mg and 24Mg(α, γ)28Si reactions

Excitation functions have been measured at six angles for ²⁴Mg(α, α)²⁴Mg up to E_α 4.94 MeV. Multi-level R-matrix analysis was performed for nineteen resonances. More than ha of the determined spin-parities and other resonance parameters are new values. The ²⁴Mg(α, γ)²⁸Si studies of Maas et al. were extended up to E_α = 5.13 MeV. Resonance strengths and branchin ratios were determined. The γ-ray angular distribution measurement at E_α = 3.79 MeV gives J^π = 2⁺ value for the resonance. A comparison of the results obtained in different reaction channel is given and the possibility of clusterisation in some excited states of ²⁸Si is discussed.     

Non-yrast states of 132Ce polutated in β-decay

Non-yrast low spin states of the nucleus ¹³²Ce were studied by means of γ-spectroscopy following the β-decay of the ground and medium spin isomeric states of ¹³²Pr. The activity was produced with the reaction ¹¹⁷Sn(¹⁹F,4n) ¹³²Pr at the Cologne FN TANDEM accelerator. The γγ coincidences and singles spectra were measured with the OSIRIS cube spectrometer. The β-decay of ¹³²Pr populates states with spins up to 6ħ and excitation energies up to 4.4 MeV in ¹³²Ce. Besides ground and quasi-gamma bands, an excited band based on the 0₂⁺ state and many other low-lying states were observed. The γγ angular correlations were analyzed to assign spins and parities to the excited states, and to determine the multipolarities of the γ-transitions. We found dominant E2 transitions in the quasi-gamma band and from the quasi-gamma band to the ground band. The experimental data are compared with calculations using the Interacting Boson Model (IBM). Good agreement is reached in the vicinity of the O(6) limit.     

Etude du noyau 63Cu par la reaction 60Ni(α, pγ)63Cu

The level structure and the decay properties of low-lying levels in ⁶³Cu have been investigated via the ⁶⁰Ni(α, pγ)⁶³Cu reaction at E_α = 11.7MeV. Using a Ge(Li) detector, the correlations of twenty-five primary γ-rays in coincidence with protons, stopped in an annular detector at approximatively 180° with respect to the beam direction, were measured. From these measurements, branching ratios, γ-ray mixing ratios and spin assignments have been obtained for most of the levels up to 2.4 MeV excitation.