Reduced probabilities of gamma transitions in even-even deformed nuclei

The reduced probabilities of the E2 intraband and interband transitions in even-even erbium isotopes (A=158–170) andN=94 isotones (A=158–164) are analysed in the frame of the perturbation theory assuming the mixing of the wave functions of ground, beta- and gammavibrational states as well as different values of the intrinsic quadrupole moments of these states.We would like to thank J. Dobe and J. Zvolský for useful discussions.     

Real and complex boson expansions in even-even deformed nuclei

Analysis of real and complex boson expansions of the Kishimoto-Tamura type is performed in a deformed basis in order to allow a further study of the anharmonicities of vibrations in deformed nuclei. It is shown that complex solutions cannot be found in the cases where no real one exits.     

On the description of collective negative parity bands in even-even deformed nuclei

The collective bands of negative parity are studied via a microscopic approach where Coriolis forces and octupole correlations are treated on an equal footing. The apparent moment of inertia of these bands are nicely reproduced.     

Strongly deformed nuclei near the magic proton shell 82?

The sudden change of the isotope phase shift in the Hg isotopes is explained by studying the deformation energy with the Strutinsky shell correction method. It is shown that going from the spherical isotope ²⁰⁴Hg to the isotope ¹⁸⁴Hg a small negative deformation (up to β₂ = −0.19) is built up. Between the mass numbers 186 and 182 the deformation is shifted through the γ instability valley to larger positive deformations (β₂ ≈ 0.3). The experimentally observed anomaly can be explained with the theoretical isotope shift constant, if this change of ground state deformations is taken into account.     

An extended phonon projection model for even-even nuclei

An extension of the projection model of Lipas et al. is presented. This extended phonon projection model (EPM) includes diagonalization in a projected-state basis and the extension of the laboratory hamiltonian to include anharmonic terms. The oriented system is used in generating the model space. A comparison of the EPM predictions with those of the six-parameter 1BA-1 model is made for ^148–152Sm.     

Spin-flipβ − decay of even-even deformed nuclei110Ru and112Ru

Neutron-rich nuclides¹¹⁰Ru and¹¹²Ru produced in symmetric fission of²³⁸U by 20 MeV protons have been studied at the IGISOL facility by means ofβ-ray,γ-ray and conversion electron spectroscopy. A total of 12 and 6γ-transitions were observed in the decays of¹¹⁰Ru and¹¹²Ru, respectively. Multipolarities were determined for a few transitions. The beta decay half-life was determined to be 11.6±0.6 s for¹¹⁰Ru and 1.75±0.07 s for¹¹²Ru. As a side product, a new value of 2.1±0.3 s for theβ half-life of the¹¹²Rh 1⁺ state was obtained. The decay energy measured with the plastic scintillator was 2.81 ±0.05 MeV for¹¹⁰Ru and 4.52 ±0.08 MeV for¹¹²Ru. The beta decay schemes of¹¹⁰Ru and¹¹²Ru isotopes indicate that the main fraction of their beta strength resides in two 0⁺→1⁺ spin-flip transitions with their logf t values between 4.4 and 4.7. The decay energies and the energies of the 1⁺ GT states are compared with the macroscopic-microscopic model calculations. The observed GT-strengths are discussed in the framework of the deformed single-particle model.     

IBM-2 calculations of some even-even selenium nuclei

In this study, we determined the most appropriate Hamiltonian that is needed for present calculations of nuclei in the A≅ 80 region by the view of interacting boson model (IBM-2). Using the best-fitted values of parameters in the Hamiltonian, we have calculated energy levels and B(E2) values for a number of transitions in some doubly even Se nuclei. The results were compared with the previous experimental and theoretical data and it is observed that they are in good agreement. The calculations have been extended to Se isotopes with A < 76 for which some B(E2) values are still not known.     

Experimental evidence for anomalous L/M conversion ratio of the 2+→0+ transitions in some deformed even-even nuclei

TheL-subshell andM conversion coefficients ratios of the 2⁺→0⁺ transitions in some deformed even-even nuclei have been determined using a high resolution iron free double focusing beta-ray spectrometer. All possible uncertainties in these measurements have been minimized and properly evaluated. TheL ₁+L ₁₁/L ₁₁₁ ratios for the 2⁺→0⁺ E2 transitions in Dy¹⁶⁰, Er¹⁶⁶, Yb¹⁷⁰, Hd¹⁷⁶ and W¹⁸² are found to be in agreement with the theoretical ones within an experimental error of about 4%. However, large deviations from the theory of about two order of magnitude are observed forL/M conversion ratios. The deviations indicate the possibility of considerable approximation in the theoretical calculation of theM-internal conversion coefficients.     

Deformed even-even nuclei in a quark-meson coupling model with tensor coupling

Neutron total cross sections of ¹⁹⁷Au and ^natTa have been measured at the nELBE photoneutron source in the energy range 0.1–10MeV with a statistical uncertainty of up to 2% and a total systematic uncertainty of 1%. This facility is optimized for the fast neutron energy range and combines an excellent time structure of the neutron pulses (electron bunch width 5ps) with a short flight path of 7m. Because of the low instantaneous neutron flux transmission measurements of neutron total cross sections are possible, that exhibit very different beam and background conditions than found at other neutron sources.     

Subthreshold photofission of even-even nuclei

Within quantum-mechanical fission theory, the angular distributions of fragments originating from the subthreshold photofission of the even-even nuclei ²³²Th, ²³⁴U, ²³⁶U, ²³⁸U, ²³⁸Pu, ²⁴⁰Pu, and ²⁴²Pu are analyzed for photon energies below 7 MeV. Special features of various fission channels are assessed under the assumption that the fission barrier has a two-humped shape. It is shown that the maximum value of the relative orbital angular momentum L _m of fission fragments can be found upon taking into account deviations from the predictions of A. Bohr’s formula for the angular distributions of fission fragments. The result is L _m ≈ 30. The existence of an “isomeric shelf” for the angular distributions of fragments from ²³⁶U and ²³⁸U photofission in the low-energy region is confirmed.     

Spin dependence of the shape of even-even nuclei within the Davydov-Chaban model

Within a model of a nonaxial even-even nucleus soft in β vibrations, the shape parameters of the ¹⁵⁴Gd, ^156,158,160Dy, ^164,168Er, ¹⁶⁸Yb, ¹⁷⁶Hf, and ¹⁸⁰W nuclei are calculated in the quadrupole approximation as functions of the excited-state spin.     

Classification of the even-even nuclei in symplectic multiplets

A classification of the even-even nuclei withZ20,A-Z-20, in terms of the boson representation of thesp(4,R) algebra is proposed. All even-even nuclei whose valence nucleons occupy the same major nuclear shell are united in two symplectic multiplets and thus treated in a unified way. A qualitative analysis of the spectrum of the 2⁺ energy levels of the ground (quasiground) bands is carried out. This analysis shows the expediency of the classification scheme proposed—a periodic structure of the same type is observed in the different shells. This periodic structure is especially stable in the case of the heavy and superheavy nuclei.     

Shell model calculations for heavy deformed nuclei

The pseudo SU(3) model is shown to be a powerful scheme for describing the excitation spectra as well asB(E2) andB(M1) transition strengths in heavy deformed nuclei. It is also useful for describing double beta decay amplitudes for transitions from the ground state of an even-even nucleus to the ground and excited states of the daughter nucleus, both for the two and zero neutrino emitting modes. The existence of selection rules which strongly restricts the decays is discussed. Anti-correlations between the quadrupole deformation and the Gamow-Teller (GT⁺) strength are found in an extension of the pseudo SU(3) model which explicitly includes pairing, which is also able to describe the fragmentation of the scissors mode. The projected shell model is introduced and proposed as an alternate means for studying single and double beta decay processes. Presented by J.G. Hirsch at the Workshop on calculation of double-beta-decay matrix elements (MEDEX’97), Prague, May 27–31, 1997. This work was supported in part by CONACyT (México), CONICET (Argentina), NSF and DOE (U.S.A.).     

Heavy deformed nuclei in the shell model Monte Carlo method

We extend the shell model Monte Carlo approach to heavy deformed nuclei using a new proton-neutron formalism. The low excitation energies of such nuclei necessitate low-temperature calculations, for which a stabilization method is implemented in the canonical ensemble. We apply the method to study a well-deformed rare-earth nucleus, 162Dy. The single-particle model space includes the 50-82 shell plus 1f_{7/2} orbital for protons and the 82-126 shell plus 0h_{11/2}, 1g_{9/2} orbitals for neutrons. We show that the spherical shell model reproduces well the rotational character of 162Dy within this model space. We also calculate the level density of 162Dy and find it to be in excellent agreement with the experimental level density, which we extract from several experiments.