Search Results (49 total)

We study in this Letter the neutrinoless double beta decay nuclear matrix elements (NME’s) in the framework of the interacting shell model. We analyze them in terms of the total angular momentum of the decaying neutron pair and as a function of the seniority truncations in the nuclear wave functions. This point of view turns out to be very adequate to gauge the accuracy of the NME’s predicted by different nuclear models. In addition, it gives back the protagonist role in this process to the pairing interaction, the one which is responsible for the very existence of double beta decay emitters. We show that low seniority approximations, comparable to those implicit in the quasiparticle RPA in a spherical basis, tend to overestimate the NME’s in several decays.

The γ decay of excited states in the waiting-point nucleus ¹³⁰Cd₈₂ has been observed for the first time. An 8⁺ two-quasiparticle isomer has been populated both in the fragmentation of a ¹³⁶Xe beam as well as in projectile fission of ²³⁸U, making ¹³⁰Cd the most neutron-rich N=82 isotone for which information about excited states is available. The results, interpreted using state-of-the-art nuclear shell-model calculations, show no evidence of an N=82 shell quenching at Z=48. They allow us to follow nuclear isomerism throughout a full major neutron shell from ⁹⁸Cd₅₀ to ¹³⁰Cd₈₂ and reveal, in comparison with ⁷⁶Ni₄₈ one major proton shell below, an apparently abnormal scaling of nuclear two-body interactions.

The energies of the excited states in very neutron-rich ⁴²Si and ^41,43P have been measured using in-beam γ-ray spectroscopy from the fragmentation of secondary beams of ^42,44S at 39A  MeV. The low 2⁺ energy of ⁴²Si, 770(19) keV, together with the level schemes of ^41,43P, provides evidence for the disappearance of the Z=14 and N=28 spherical shell closures, which is ascribed mainly to the action of proton-neutron tensor forces. New shell model calculations indicate that ⁴²Si is best described as a well-deformed oblate rotor.

Large-scale shell-model calculations, with dimensions reaching 10⁹, are carried out to describe the recently observed deformed (ND) and superdeformed (SD) bands based on the first and second excited 0⁺ states of ⁴⁰Ca at 3.35 and 5.21 MeV, respectively. A valence space comprising two major oscillator shells, sd and pf, can accommodate most of the relevant degrees of freedom of this problem. The ND band is dominated by configurations with four particles promoted to the pf shell (4p-4h in short). The SD band by 8p-8h configurations. The ground state of ⁴⁰Ca is strongly correlated, but the closed shell still amounts to 65%. The energies of the bands are very well reproduced by the calculations. The out-band transitions connecting the SD band with other states are very small and depend on the details of the mixing among the different np-nh configurations; in spite of that, the calculation describes them reasonably. For the in-band transition probabilities along the SD band, we predict a fairly constant transition quadrupole moment Q₀(t)~170  e fm² up to J=10 that decreases toward the higher spins. We submit also that the J=8 states of the deformed and superdeformed bands are maximally mixed.

In an effort to understand the magical status of N=32 and N=34 at the very neutron rich edge, experiments have been carried out in the titanium isotopes up to A=56. The measured staggering of the B(E2)'s is not reproduced by the shell model calculations using the best effective interactions. We argue that this may be related to the choice of the isovector effective charge and to the value of the N=34 neutron gap.

The β decay of ³¹Mg has been investigated at GANIL. The ions were implanted in a silicon detector array surrounded by germanium γ detectors. β particles and subsequent γ rays have been observed. The ³¹Mg half-life has been measured (T_1/2=237±25 ms) in good agreement with previously reported values. The decay scheme of ³¹Mg has been enriched with two new low-spin levels and seven new γ transitions in ³¹Al. Most importantly, very weak feeding to the ³¹Al ground and lowest excited states has been established. These results are compared to new shell-model calculations in the sd-fp valence space, using an interaction that was modified in order to reproduce the recently established low-energy level scheme of ³¹Mg. A good agreement for all observables is found, supporting the idea that ³¹Al is outside and ³¹Mg is inside the so-called “island of inversion.”

Large scale shell model calculations in the valence space spanned by two major oscillator shells (sd and pf) describe simultaneously the superdeformed excited band of ³⁶Ar and its spherical ground state. We explain the appearance of this superdeformed band at low excitation energy as a consequence of the very large quadrupole correlation energy of the configurations with many particles and many holes (np-nh) relative to the normal filling of the spherical mean field orbits (0p-0h). We study the mechanism of mixing between the different configurations to understand why the superdeformed band survives and how it finally decays into the low-lying spherical states via the indirect mixing of the 0p-0h and 4p-4h configurations.

The last decade has witnessed both quantitative and qualitative progress in shell-model studies, which have resulted in remarkable gains in our understanding of the structure of the nucleus. Indeed, it is now possible to diagonalize matrices in determinantal spaces of dimensionality up to 10⁹ using the Lanczos tridiagonal construction, whose formal and numerical aspects are analyzed in this review. In addition, many new approximation methods have been developed in order to overcome the dimensionality limitations. New effective nucleon-nucleon interactions have been constructed that contain both two- and three-body contributions. The former are derived from realistic potentials (i.e., potentials consistent with two-nucleon data). The latter incorporate the pure monopole terms necessary to correct the bad saturation and shell-formation properties of the realistic two-body forces. This combination appears to solve a number of hitherto puzzling problems. The present review concentrates on those results which illustrate the global features of the approach: the universality of the effective interaction and the capacity of the shell model to describe simultaneously all the manifestations of the nuclear dynamics, either single-particle or collective in nature. The review also treats in some detail the problems associated with rotational motion, the origin of quenching of the Gamow-Teller transitions, double-β decays, the effect of isospin nonconserving nuclear forces, and the specificities of neutron-rich nuclei. Many other calculations—which appear to have “merely” spectroscopic interest—are touched upon briefly, although the authors are fully aware that much of the credibility of the shell model rests on them.

Due to the immiscibility of ³He into ⁴He at very low temperatures, mixed helium droplets consist of a core of ⁴He atoms coated by a ³He layer whose thickness depends on the number of atoms of each isotope. When these numbers are such that the centrifugal kinetic energy of the ³He atoms is small and can be considered as a perturbation to the mean-field energy, a novel shell structure arises, with magic numbers different from these of pure ³He droplets. If the outermost shell is not completely filled, the valence atoms align their spins up to the maximum value allowed by the Pauli principle.

γ decays from high-spin states in the N=Z-1 nucleus ₂₇⁵³Co₂₆ have been identified for the first time. Level energies and Coulomb energy differences between these states and their analogs in its mirror nucleus ⁵³Fe have been compared with large-scale pf shell-model calculations, which offer excellent agreement. New information has been obtained on two-proton Coulomb matrix elements needed in the interpretation. These have been extracted from the data via a number of methods and are shown to exhibit an anomalous behavior for the J=2 coupling.

High-spin states in ⁴⁶Ti have been investigated with the reaction ²⁸Si(²⁴Mg,α2p) at 100 MeV, using the GASP γ-ray array and the charged-particle detector ISIS. The positive parity yrast sequence has been observed up to the 14⁺ terminating state and several high-energy transitions have been established above the termination state. Both the signatures of a negative parity band have been observed up to the 16^- and 17^- terminating states, plus one more transition from an 18^- state at 19.086 MeV excitation. The observed states are interpreted within the frame of large scale shell model calculations in the full pf space (positive parity) and as particle-hole excitations from the d_3/2 shell (negative parity).

The isovector and isotensor energy differences between yrast states of isobaric multiplets in the lower half of the pf region are quantitatively reproduced in a shell model context. The isospin nonconserving nuclear interactions are found to be at least as important as the Coulomb potential. Their isovector and isotensor channels are dominated by J=2 and J=0 pairing terms, respectively. The results are sensitive to the radii of the states, whose evolution along the yrast band can be accurately followed.

Experimental evidence for the coexistence of states with different K^π value was found in ⁵⁰Cr. The bandcrossing of the K=0⁺ ground state band with a K=10⁺ one is confirmed. Large scale shell model calculations could explain all of the observed experimental features and in particular the known experimental Coulomb energy differences in the mirror pair ⁵⁰Fe-⁵⁰Cr.

An extensive spectroscopic study of ⁴⁸V has been performed using fusion-evaporation reactions to deduce the decay scheme and to measure transition probabilities through the Doppler shift attenuation method. Experimental levels were grouped into five bands, labeled with Nilsson configurations. Bands become nearly spherical approaching band termination. Large-scale shell model calculations were performed, producing results in very good agreement with the experimental data.

The neutron-rich ^66,68Ni have been produced at GANIL via interactions of a 65.9A MeV ⁷⁰Zn beam with a ⁵⁸Ni target. Their reduced transition probability B(E2;0₁⁺→2⁺) has been measured for the first time by Coulomb excitation in a ²⁰⁸Pb target at intermediate energy. The B(E2) value for ⁶⁸Ni₄₀ is unexpectedly small. An analysis in terms of large scale shell model calculations stresses the importance of proton core excitations to reproduce the B(E2) values and indicates the erosion of the N  =  40 harmonic-oscillator subshell by neutron-pair scattering.

Excited states in ³⁸Ar have been investigated by means of the heavy-ion fusion-evaporation reaction ²⁸Si+¹⁶O. The level scheme reveals a subtle interplay between spherical, deformed, and superdeformed shapes. Large-scale shell-model calculations in the sd-fp space are invoked to study the microscopic aspects of deformation and shape changes.

Gamma rays from the N  =  Z-2 nucleus ⁵⁰Fe have been observed, establishing the rotational ground state band up to the state J^π  =  11⁺ at 6.994 MeV excitation energy. The experimental Coulomb energy differences, obtained by comparison with the isobaric analog states in its mirror ⁵⁰Cr, confirm the qualitative interpretation of the backbending patterns in terms of successive alignments of proton and neutron pairs. A quantitative agreement with experiment has been achieved by exact shell model calculations, incorporating the differences in radii along the yrast bands, and properly renormalizing the Coulomb matrix elements in the pf model space.

Lifetimes have been measured in a superdeformed rotational band recently identified in the N=Z nucleus ³⁶Ar. A large low-spin quadrupole deformation (β₂=0.46±0.03) is confirmed and a decrease in the collectivity is observed as the high-spin band termination at I^π=16⁺ is approached. Detailed comparisons of the experimental B(E2) values with the results of cranked Nilsson-Strutinsky and large-scale (s_1/2d_3/2)-pf spherical shell model calculations indicate the need for a more refined treatment of transition matrix elements close to termination in the former, and the inclusion of the complete sd-pf model space in the latter description of this highly-collective rotational band.

Gamma decays from excited states in the T_z=-1 / 2 nucleus ⁵¹Fe have been observed for the first time. The differences in excitation energies as compared with those of the mirror partner, ⁵¹Mn, have been interpreted in terms of Coulomb effects and the resulting Coulomb energy differences (CED) can be understood intuitively in terms of particle-alignment effects. A new CED effect has been observed, in which different CED trends have been measured for each signature of the rotational structures that characterize these mid-f_7/2 shell nuclei. Large-scale fp-shell model calculations have been used to compute the trends of the CED as a function of spin. The result of comparing these calculations with the data demonstrates an ability to reproduce the fine details of the Coulomb effects with a precision far greater than has been previously achieved.

A superdeformed rotational band has been identified in ³⁶Ar, linked to known low-spin states, and observed to its high-spin termination at I^π  =  16⁺. Cranked Nilsson-Strutinsky and spherical shell model calculations assign the band to a configuration in which four pf-shell orbitals are occupied, leading to a low-spin deformation β₂≈0.45. Two major shells are active for both protons and neutrons, yet the valence space remains small enough to be confronted with the shell model. This band thus provides an ideal case to study the microscopic structure of collective rotational motion.

The nucleus ⁴⁹Cr has been studied using the reactions ²⁸Si(²⁸Si,α2pn) at 115 MeV and ⁴⁶Ti(α,n) at 12 MeV, respectively. A high-K band with K^π=13/2⁺ has been identified, whose bandhead acts as an yrast trap for the low-lying levels with positive parity. This peculiar phenomenon is well reproduced by large-scale shell model calculations in the pf configuration space plus a proton hole in the 1d_3/2 orbital. As an essential part of the work, lifetimes of some nonyrast states, as well as of a state in the K^π=5⁺ band in ⁵⁰Mn, have been determined using the Doppler shift attenuation method.

High spin states in the odd-odd N=Z nucleus ⁴⁶V have been identified. At low spin, the T=1 isobaric analog states of ⁴⁶Ti are established up to I^π=6⁺. Other high spin states, including the band terminating state, are tentatively assigned to the same T=1 band. The T=0 band built on the low-lying 3⁺ isomer is observed up to the 1f_7/2-shell termination at I^π=15⁺. Both signatures of a negative parity T=0 band are observed up to the terminating states at I^π=16^- and I^π=17^-, respectively. The structure of this band is interpreted as a particle-hole excitation from the 1d_3/2 shell. Spherical shell model calculations are found to be in excellent agreement with the experimental results.

Structures of the medium- to high-spin states in the doubly magic nucleus ⁵⁶Ni have been investigated using the reaction ²⁸Si(³⁶Ar,2α) and the γ-ray spectrometer Gammasphere in conjunction with the 4π charged-particle detector array Microball. Two well-deformed rotational bands have been identified. There is evidence that one of the bands, which is identical to a sequence in the odd-odd neighbor ⁵⁸Cu, partially decays via proton emission into the ground state of ⁵⁵Co. Predictions of extensive large-scale shell-model and cranked Hartree-Fock and Hartree-Fock-Bogolyubov calculations are compared with the experimental data.