Dy Nuclei Research Articles

GDR dissipation and nuclear shape in hot fast-rotating Dy nuclei

The statistical γ-ray decay of the GDR built on excited states in Dy nuclei has been investigated for selected domains of angular momentum up to about 70ħ and temperatures in the range 1–2 MeV. The GDR strength distribution extracted from the data indicate large average nuclear deformations ( β ∼ 0.35) at high angular momentum and average temperatures T ⩾ 1.5 MeV. The experimental observation is supported by results from calculations in which thermal shape fluctuations are taken into account around an oblate equilibrium deformation β eq. Although this equilibrium deformation increases with angular momentum, the calculations show rather large and constant average deformations 〈 β ∼0.35.

Giant dipole resonances built on superdeformed states in Dy nuclei

The properties of the Superdeformed giant dipole resonance (SDGDR) in rotating 146-152Dy even nuclei are studied based on the linear response theory with the self-consistently determined superdeformed mean field. The obtained features may serve as the criteria of observing the SDGDR.

Shape transition at high spin manifested in theγ-ray decay of the giant dipole resonance in hot Dy nuclei

Shape transition at high spin manifested in the<i>γ</i>-ray decay of the giant dipole resonance in hot Dy nuclei

Structure of even-even Dy nuclei in the interacting boson model with two-quasiparticle states.

The energy levels of g.s. bands, ..beta.. bands, and ..gamma.. bands of /sup 154-162/Dy isotopes are studied in the model of the traditional interacting boson approximation, allowing one boson to break and form a quasiparticle pair. The two quasiparticles are allowed to excite to the i/sub 13/2/ and h/sub 11/2/ orbitals. It was found that the energy levels of the g.s., ..beta.., and ..gamma.. bands of even-even Dy nuclei can be reproduced very well. The backbends of the moment of inertia can also be reasonably described. The yrast B(E2) values are also calculated and compared with the experimental data.

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.

High-spin states and the boson cutoff in rotational nuclei: Application to even-even Dy nuclei

We present a model in which core excitations within the framework of the interacting boson model are studied. First, a detailed study of high-spin states and backbending in the even-even /sup 154en-dash164/Dy nuclei is carried out. We also address the problem of the cutoff in the yrast B(E2) values in rotational nuclei. Application, again for the even-even /sup 154en-dash164/Dy nuclei, is performed in detail. We also discuss the intrinsic quadrupole moments and compare them to the existing data. We relate the present description, using the classical limit expressions for a given sd-boson Hamiltonian, to the geometrical study of the even-even Dy nuclei, which also is carried out in detail.

Rotation-induced shape transitions in Dy nuclei

Lifetimes of states with spins up to 30ħ have been measured in the nuclei 156Dy, 157Dy, and ll58Dy using the recoil-distance technique together with inverse reactions of the type Mg( 136 Xe, x n) . The applied method, which benefited from the high velocities of the fusion residues as well as from improvements of the recoil-distance technique, allowed us to determine lifetimes and feeding times down to 0.1 ps. Below the first backbending the resultant B(E2) values in the ground-state band of 156, 158Dy increase faster with increasing rotational frequency than expected for rigid rotors, reaching values similar to those observed for the well-deformed neutron-rich Dy isotopes. In contrast to this, the E2-transition probabilities between high-spin states are clearly retarded. The retardation gradually evolves from the rotation alignment of nucléons and indicates deformation changes most likely towards a triaxial shape. From the analysis of the side-feeding times of the high-spin yrast states it could be furthermore deduced that the E2 component of the preyrast γ-decay stems from transitions along highly collective bands.

The 161Dy and 57Fe Mössbauer studies of the Dy2Fe17−yAly compounds

The hyperfine interaction parameters at the 161 Dy and 57 Fe nuclei in Dy 2 Fe 17− y Al y compounds (0 ⩽ y ⩽ 8) have been determined from the Mössbauer effect measurements at 4.2 K. The excess of the hyperfine magnetic field at the 161 Dy nuclei is discussed according to the 5d electron transfer model.

Level structure of 153Dy and the competition between collective and few-particle excitation modes in Dy Nuclei

The 153Dy level structure has been studied through the 124Sn( 34S, 5n) reaction. The low-spin collective character of 153Dy gives way to single particle yrast configurations at I > 41 2 ; an I= 47 2 isomer at 5591 ke V shows the single particle character (and overall oblate shape) to be well established at that spin. Nuclear shape charges in Dy nuclei are discussed.

High spin states and the boson cut-off in rotational nuclei

We present a schematic model in order to include core-excited states in the interacting boson nuclei. It is shown that thereby the cut-off on the B(E2) values in the yrast band becomes less pronounced. Application to the even-even Dy nuclei is carried out.

Level structure of 153Dy and the competition between collective and few-particle excitation modes in Dy Nuclei

Level structure of 153Dy and the competition between collective and few-particle excitation modes in Dy Nuclei

Measurement of the linear polarization of continuum gamma rays from ( 32S, xn) reactions to prolate and oblate rare-earth nuclei

We describe the techniques and report the results of a measurement of the linear polarization, P γ of continuum γ-rays from Er and Dy nuclei produced in ( 32S, xn) reactions on 130Te and 124Sn targets. For the prolate 156–158Er isotopes we find P γ ≈ 0.35 in the γ-ray energy range if0.3 MeV ≦ E γ ≦ 1.5 MeV, thus confirming the E2 nature of the unresolved yrast cascades in rotational nuclei. In contrast, the rather small polarization P γ ≈ 0.05 for E γ ≦ 1.0 MeV measured for the oblate 151,152Dy isotopes, and the shape of the spectrum of continuum γ-rays suggest a rather direct feeding of the yrast states by statistical cascades. For 1.0 MeV ≦ E γ ≦ 1.5 MeV we find P γ ≈ 0.3 indicating collective rotations of the Dy nuclei at spin I ≧ 40 h z.shtsls; . Measurements of γ-ray angular distributions with a collimated NaI detector support these results. From the reduction of the P γ , and A 2 values of the γ-ray continuum in 156–158Er, compared to those measured for discrete E2 yrast transitions, we obtain estimates for the intensities of the M1 and statistical components in the continuum of these nuclei.

Study of quasi-continuum gamma-ray deexcitation of 152–161Dy nuclei populated in α- and 12C-induced reactions

The influence of the reaction mechanism and of the nuclear structure at high angular momentum on the γ-ray deexcitation process is studied for Dy nuclei produced in a (40–80 MeV) and 12C (80–110 MeV) induced reactions on 154, 155, 158, 160Gd and 146, 150Nd targets, respectively. The quasi-continuum (q.c.) γ-ray spectra for selected exit channels are decomposed into collective and statistical parts for which the γ-ray energies, the multiplicities and partly the multi-polarities are determined. Large differences in the constitution of the q.c. γ-ray cascades and in the population strengths of high-spin yrast states in 152Dy and 156Dy are attributed to nuclear structure effects at high angular momentum. This constitution and the centroids of the γ-ray entry regions in the energy versus angular-momentum plane are also evaluated as functions of beam energy and projectile. The γ-ray multiplicities and both coordinates of the entry region centroids increase initially with bombarding energy and then level off or even decrease at the highest energies. The observed increase is reproduced by a statistical-decay model, but at the higher bombarding energies strong deviations occur which are discussed in view of the reaction mechanisms, including incomplete fusion and non-equilibrium neutron emission, particularly absorptive breakup.

Crystal field of Dy implanted in Al single crystals, obtained by Mossbauer spectroscopy

The field dependence of the hyperfine interaction of 161Dy in Al has been studied in external magnetic fields up to 3.21 T by means of Mossbauer spectroscopy. 161Dy was introduced into a single crystal of Al by means of low-temperature ion implantation. From the measurements the authors determine the parameters of the cubic crystalline electric field acting on the Dy nuclei: A4(r4)=-32+or-32K and A6(r6)=14.1+or-1.4K. These parameters correspond to a Gamma 7 ground doublet separated from a Gamma 8 excited quartet by Delta =26+or-6K and an overall multiplet splitting of 143+or-25K.

Crystal field of Dy implanted in Au single crystals, obtained by Mossbauer spectroscopy

The field dependence of the hyperfine interaction of 161Dy in Al has been studied in external magnetic fields up to 3.21 T by means of Mossbauer spectroscopy. 161Dy was introduced into a single crystal of Al by means of low-temperature ion implantation. From the measurements the authors determine the parameters of the cubic crystalline electric field acting on the Dy nuclei: A4(r4)=-32+or-32K and A6(r6)=14.1+or-1.4K. These parameters correspond to a Gamma 7 ground doublet separated from a Gamma 8 excited quartet by Delta =26+or-6K and an overall multiplet splitting of 143+or-25K.

Crystal field of Dy implanted in Ag obtained by Mössbauer spectroscopy

The magnetic field dependence of the hyperfine interaction of $^{161}\mathrm{Dy}$ impurities in Ag has been studied in external fields up to 3.2 T by means of M\"ossbauer spectroscopy. $^{161}\mathrm{Dy}$ was introduced into a single crystal of Ag by means of low-temperature implantation. From the measurements we can determine the parameters of the cubic crystalline electric field (CEF) acting on the Dy nuclei: ${A}_{4}〈{r}^{4}〉=\ensuremath{-}13.5\ifmmode\pm\else\textpm\fi{}4.5$ K and ${A}_{6}〈{r}^{6}〉=+5.7\ifmmode\pm\else\textpm\fi{}0.3$ K. These results seem to be at variance with those of other methods. However, we will show that the above CEF parameters can account for the other experimental results as well. It may be concluded that in this case M\"ossbauer spectroscopy is probably the only method to determine the CEF parameters ${A}_{4}〈{r}^{4}〉$ and ${A}_{6}〈{r}^{6}〉$ with reasonable accuracy.

Crystal field of Dy implanted in Cu single crystals obtained by M�ssbauer spectroscopy

The field dependence of the hyperfine interaction of161Dy impurities in Cu has been studied in external magnetic fields up to 3.21 T by means of Mossbauer spectroscopy. 161Dy was introduced into a single crystal of Cu by means of low temperature ion implantation. From the measurements we determine the parameters of the cubic crystalline electric field acting on the Dy nuclei:A 4 =−28±58 K and $$A_6 \langle r^6 \rangle = 20.6_{ - 0.2}^{ + 3.1} K$$ . The ground state is a $$\Gamma _7 $$ doublet, separated well from a $$\Gamma _8 $$ excited quartet by $$\Delta = 44.8_{ - 13.3 }^{ + 1.8} K$$ .

Intensity and Multipolarity of Low-Energy Components in the Quasicontinuumγ-Ray Spectrum Followingα- andC12-Induced Reactions

Measurements of $K$-shell ionization yields for ($\ensuremath{\alpha}$,$\mathrm{xn}$) and ($^{12}\mathrm{C}$,$\mathrm{xn}$) evaporation residues exclude the existence of a large $M1$ component in the quasicontinuum spectrum of well-deformed Dy nuclei below 500 keV. Upper intensity limits are deduced for $M1$, $E1$, and $E2$ components.

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

Measurements of hyperfine interactions at 155Gd nuclei in metallic compounds between Gd and 3 d 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 4 f moments is directly related to that caused by 3 d moments.

NMR of Dy nuclei in ferromagnetic DyAl 2 and Dy metal

The NMR spin echo of Dy nuclei in ferromagnetic DyAl 2 yields for the Dy 161 hyperfine field constant, a=−845.2±0.3 MHz and quadrupolar splitting, 2 P=420.0±0.3 MHz compared to a=−830.0±0.5 MHz and 2 P= 387.8±0.5 MHz in ferromagnetic Dy metal. The different contributions to these parameters are discussed. A line, observed at 1273 MHz Dy metal, is attributed to Δ m I = 2 transition.