Hyperfine interactions in intermetallic compounds between Gd and 3d transition metals

Measurements of hyperfine interactions at ¹⁵⁵Gd nuclei in metallic compounds between Gd and 3d transition metals and at ⁶¹Ni in GdNi compounds by Mössbauer spectroscopy are reported. The results are discussed in terms of various models proposed for the electronic structure of these compounds. The Gd isomer shifts with respect to metallic Gd are at variance with the model of a strong d electron transfer from rare earths to transition metal ions. From the observation of a linear relation between magnetic hyperfine fields at Gd and at Dy nuclei in corresponding compounds it is inferred that crystal-field induced variations of Dy moments are neglible and that the conduction electron polarization induced by 4f moments is directly related to that caused by 3d moments.     

The level structure of the 153Gd nucleus studied in the 150Sm(α, n)153Gd reaction

The level structure of ¹⁵³Gd has been studied by means of the ¹⁵⁰Sm(α, n)¹⁵³Gd reaction. The experiment included measurements of γ-γ coincidences, γ-angular distributions, γ-ray yield at 17 MeV and 19 MeV beam energy, and γ-ray multiplicities. Favoured and unfavoured members of the positive-parity i_13,2, band were identified. States belonging to the h_9,2 and f_7,2 band structures have been located.Surprisingly low multiplicity numbers were deduced for ₁₅₃Gd γ-rays. This may indicate that the (α, n) reaction is not a pure compound reaction. The level structure of ¹⁵³Gd has been compared to the known structure of other odd-mass N = 89 nuclei, and a close similarity is found.The positive-parity band structure has been compared to calculations with the pairing-plus-recoil model. Good agreement is obtained without any ad hoc Coriolis attenuation.     

Band mixing in156Gd and158Gd

A detailed investigation of the energies and intensities of the γ-rays that depopulate the low spin levels of the β- and γ-vibrational bands of¹⁵⁶Gd and the γ-vibrational band of¹⁵⁸Gd has been conducted. Both singles and γ-γ coincidence measurements were made on sources of 15-d¹⁵⁶Eu and 46-min¹⁵⁸Eu by use of large volume, high resolution Ge(Li) detectors. In addition to the γ-band at 1154.09 keV, twoK ^π=0⁺ bands were observed in¹⁵⁶Gd with band heads at 1049.45 and 1168.11 keV, respectively. The 2⁺ and 3⁺ members of the γ-vibrational band in¹⁵⁸Gd were observed at 1187.12 and 1265.43 keV, respectively, as well as a newK ^π=0⁺ band at 1195.98 keV. A first order perturbational treatment of the branching ratios was applied to both nuclei. In addition, the mixing between the ground state, the β-, and the γ-vibrational bands of¹⁵⁶Gd is considered from two approaches, but neither satisfactorily explains all the experimentalB(E2) ratios.     

Atomic masses above146Gd derived from a shell model analysis of high spin states

From a shell model analysis of high-spin states in neutron deficient nuclei above¹⁴⁶Gd we have derived the ground state masses of theN=82 and 83 isotones of Eu, Tb, Dy, Ho, and Er. The results can be used to calculate the energies of aligned multiparticle yrast configurations. They also link ten α-decay chains to the nuclei with known masses, providing many new absolute mass values which are compared with predictions. An examination of the two-proton separation energies atN=82 shows an 0.5 MeV break in the nuclear mass surface atZ=64.     

Levels in152Gd and152Sm populated by the decay of152Eu

The energy level schemes of¹⁵²Gd and¹⁵²Sm have been established on the basis of singlesγ-spectra, andγ- γ coincidence measurements. Ge(Li) detectors were used to study the gamma spectra produced in the EC/β⁺ and β^? decays of¹⁵²Eu to¹⁵²Sm and¹⁵²Gd, respectively. Thirteen new transitions are reported and data from eleven coincidence gates enabled five new levels to be suggested. Relative intensities and logf t values were calculated and spin/parities deduced. Comparisons are made with new predictions of the Interacting Boson Model.     

Particle hole yrast states in146Gd and147Gd and the Z=64 shell closure

The level structures of¹⁴⁶Gd and¹⁴⁷Gd have been investigated by in-beamγ-ray ande ^? spectroscopy with (α, xn) reactions on enriched Sm targets. Detailed level schemes up to ～4 MeV, which differ radically from earlier schemes, are reported. The energy levels are characterized as particle-hole excitations using empirical single particle energies and two nucleon interactions. Analysis of pure 1p 1h proton excitations demonstrates that theZ=64 andN=82 energy gaps are about equally large.     

Allowed β-transitions,weak magnetism and nuclear structure in light nuclei

The general formalism for allowed β-transitions is derived in the framework of the elementary particle as well as in the impulse approximation treatment. Explicit expressions for the shape factor, the asymmetry and anisotropy of electrons emitted from oriented nuclei, the β-γ (CP) correlation and the β-γ angular correlation are given. Emphasis is imputed to the question how model independently these quantities can be evaluated. This problem has been studied in detail for the β-transitions in the nuclei ¹²B¹²N, ²⁰F²⁰Na, ²²Na and ²⁴Na²⁴Al. The computations of the nuclear matrix elements are carried out for the A = 12 system in intermediate coupling and for the sd-shell nuclei in the SU(6)-SU(3) coupling scheme using different residual interactions. Comparison to experiments is made. All results can be explained with ordinary CVC predictions without invoking second class currents.     

Algebraic realization of the quantum rotor — odd-A nuclei

An algebraic realization of the quantum rotor for non-zero spin values (integer as well as half-integer) is established by constructing a model Hamiltonian out of rotationally invariant functions of the generators ofSU(3). The eigenvalues of this Hamiltonian in the leading normal-SU(3) symmetry for²⁵Mg and the so-called leading pseudo-SU(3) symmetries for¹⁵⁹Dy and¹⁶⁵Er are compared with the corresponding rotor results. For spinfree systems the internal symmetry group of the rotor and itsSU(3) realization are known to be D₂, the Vierergruppe. This symmetry extends to integral spin values, while for half-integer spins the rotor and itsSU (3) realization are shown to display an internal quaternion group symmetry. The theory points to a microscopic (many-particle shell-model) picture of nuclear rotational motion with spin degrees of freedom taken fully into account. An algebraic realization of the many-particle Nilsson model for odd-A nuclei, with the orbit-orbit and spin-orbit terms included, is given and applied to²³Na.     

Angular correlation measurements in154Gd

γ-γ angular correlation measurements were performed to determine the mixing parameters δ of some γ-transitions in¹⁵⁴Gd. High energy compton background could easily be eliminated using a two dimensional data aquisition system. The relativeB (E2)-values of (K=2→K=0) interband transitions were determined and compared with collective model predictions. Radioactivity:¹⁵⁴Eu[from¹⁵³Eu(n, γ)], enriched target. \(^{154} Eu\xrightarrow{{\beta ^{ --} }}^{154} Gd.\)      

Space symmetry in light nuclei. I Application to the 2s−1d shell

The Wigner supermultiplet scheme is studied in the 2s?1d shell using the spectral distribution methods of French. Expressions for the average expectation value of H and H² over states belonging to a definite SU(4) symmetry are derived. Numerical results for these averages are given for the 2s?1d shell. These averages are then used to predict ground state energies and low energy spectra for nuclei in this shell, using Ratcliff's procedure; a comparison with shell model and observed spectra is made. Mixing intensities of SU(4) representations near the ground state are also evaluated. These provide us with a measure of symmetry breaking. It appears from our calculations that the symmetry is strongly mixed. Finally a preliminary application to the theory of level densities is made.     

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.     

Magnetic hyperfine fields at Gd and in sites in GdPdIn compound

Perturbed gamma-gamma angular correlation (PAC) technique was used to measure the hyperfine interactions in the intermetallic compound GdPdIn using ¹¹¹In→ ¹¹¹Cd and ¹⁴⁰La→ ¹⁴⁰Ce probe nuclei at the In and Gd sites, respectively. The PAC results for ¹¹¹Cd show two well-defined electric quadrupole frequencies above T _C assigned to probes occupying Gd and In sites, with ~50% of site occupation each. The fraction corresponding to In sites increases with temperature reaching 95% around 500 K. Below T _C the measurements for ¹¹¹Cd probe showed combined electric quadrupole plus magnetic dipole interaction with sharp increase in the magnetic field below around 80 K. A pure magnetic interaction with lower hyperfine field values was observed at the Gd sites occupied by ¹⁴⁰Ce below 100 K.     

Low-spin states in151Gd

The level structure of the 87-neutron nucleus¹⁵¹Gd has been investigated by studying the EC- andβ ⁺-decay of¹⁵¹Tb. Gamma-ray and conversion electron spectra as well as gamma-gamma coincidence spectra were measured by using semiconductor spectrometers and a high-capacity two-parameter recording system. The proposed decay scheme contains several new energy levels in¹⁵¹Gd, among them a 5^?/2 state at 427 keV. To explain the low-energy level structure, a small stable deformation is assumed for¹⁵¹Gd. Using a single-particle-plus-rotor model based on a generalized Woods-Saxon potential, complete mixing of the shells 1h _9/2, 2f _7/2 and separately 1i _13/2 is observed. By these mixings the correct level order and approximately correct excitation energies up to 1 MeV are reproduced.     

The hyperfine interaction of gadolinium and holmium in cubic Ho: (Gd,Ce)Fe2 compounds

The low-temperature hyperfine parameters of the ferrimagnetic C15 intermetallic system Ho_0.03Ce_xGd_0.97-xFe₂ have been measured at Gd using the 87 keV γ-ray of ¹⁵⁵Gd and at Ho using spin echo nmr of ¹⁶⁵Ho. A linear decrease of the hyperfine field B_t with increasing x is observed, the slope being almost the same for both nuclei. The observed slope (dB_t/dx ? -37T) is nearly three times greater than that found for ¹⁵⁵Gd in the analogous H₀:(Y, Gd)Fe₂ system; this is attributed to the influence of the increased conduction electron density when trivalent lanthanide ions are replaced by Ce⁴⁺.     

Atomic masses of rare earth isotopes145Pm,145Sm and151Gd,derived from decay measurements of145Sm and151Gd

The comparison of experimental with calculated K-capture probabilities yielded the decay energies of¹⁴⁵Sm and¹⁵¹Gd,Q _EC=622(5) and 463(3) keV, respectively. Earlier discrepancies in the mass adjustment of these isotopes were removed and adjusted masses for¹⁴⁵Sm,¹⁴⁵Pm and¹⁵¹Gd derived. In the decay of¹⁵¹Gd five newγ-rays were found and the half-life remeasured to be 129(4) d.     

The ground state mass of147Gd from single-neutron-transfer reactions and its impact on derived mass values

Using a¹⁴⁸Gd radioactive target and the (p,d), (d,t), and (³He,α) single neutron pick-up reactions we have measured the¹⁴⁷Gd mass excess as — 75,366(4) keV, which differs by 139(24) keV from the adopted value of the 1983 mass table. From this result, and from recently reported first transfer-reaction mass determinations for¹⁴⁵Eu and¹⁴⁶Eu, we have recalculated the masses of nuclei above¹⁴⁶Gd from a previous shell model analysis of high-spin states.     

Gyromagnetic ratios of 4+ and 6+ states in156Gd and158Gd

The followingg-factors have been derived from time integral measurements of γ-γ angular correlations in the static magnetic hyperfine field of magnetized gadolinium metal probes:¹⁵⁶Gd:g(4 ₁ ⁺ )=+0.310(19)g(6 ₁ ⁺ )=+0.25(21)g(4 ₃ ⁺ , 1511 keV)=+0.809(27)¹⁵⁸Gd:g(4 ₁ ⁺ )=+0.409(15). The 5.35d ¹⁵⁶Tb sources were produced by the reaction¹⁵⁶Gd(d, 2n)¹⁵⁶Tb in our cyclotron. A carrier-free 150y ¹⁵⁸Tb source was obtained from ISOLDE/CERN. In comparison with the precisely knowng-factors of the 2 ₁ ⁺ states,g(2 ₁ ^su+ ,¹⁵⁶Gd) =+0.386(4) andg(2 ₁ ⁺ ,¹⁵⁸Gd)=0.381(4), we observe a large reduction for the¹⁵⁶Gd 4 ₁ ⁺ state whereasg increases slightly for¹⁵⁸Gd. The half-life of the 4 ₁ ⁺ state of¹⁵⁸Gd was remeasured as¹⁵⁸Gd:T _1/2(4 ₁ ⁺ )=148(2) ps. A measurement of the rotation in the 4 ₃ ⁺ state of¹⁵⁶Gd in external magnetic fields of various strengths up toB _ext=9.5 T did not confirm the anomalous dependence of the magnetic hyperfine field in gadolinium metal on the external field, which has been reported by Persson et al. [29].     

Quasi-K quantum number in Gd

Evidence is discussed for the persistence of the K quantum number in a transitional nucleus ¹⁵²Gd in the form of a quasi-K quantum number K_σ. It plays an important role for β-transitions in a similar way to the K quantum number in deformed nuclei.     

The observation of excited states in 142Gd and 142,144Dy

Levels in the neutron deficient ¹⁴²Gd and ^142,144Dy nuclei have been identified using multiple particle-gamma coincidence techniques. The isotopes were produced in the ^54,56Fe + ⁹²Mo and ⁵²Cr + ⁹⁶Ru reactions at 20–30 MeV above the Coulomb barrier. The new data are discussed in the context of the systematic trend toward deformation in light rare-earth nuclei and the disappearance of the Z = 64 subshell gap.     

Nuclear structure of the heavyN=82 isotones:144Sm and146Gd

For single-closed shell nuclei with a large number of extra-protons, outside theZ=50,N=82 double-closed shell nucleus, a standard shell-model calculation yields dimensions of the Hamiltonian matrices beyond the scope of present computer technology. A quasi-particle (QP) calculation however, is able to describe the most important proton-excitations in a much restricted configuration space. Extended BCS+TDA calculations are performed on the single-closed shell¹⁴⁴Sm and¹⁴⁶Gd nuclei with a Gaussian force as effective two-body interaction. The influence of the nonconservation of the exact number of extra-protons in the BCS-approach is studied by performing a projection onto the states with an exact number of extra-protons (BCS+P). The first excited 2⁺, 4⁺ and 6⁺ states in¹⁴⁶Gd are described mainly by the (1g7/2) ₀ ⁸ (2d5/2) ₀ ⁴ \((1h11/2)_{J^\pi }^2 \) configuration.