Search Results (48 total)

The (d,²He) charge-exchange reaction on the double-β decay (ββ) nucleus ⁶⁴Zn has been studied at an incident energy of 183 MeV. The two protons in the ¹S₀ state (indicated as ²He) were both momentum analyzed and detected simultaneously by the BBS magnetic spectrometer and its position-sensitive detector. ²He spectra with a resolution of about 115 keV (FWHM) have been obtained allowing identification of many levels in the residual nucleus ⁶⁴Cu with high precision. ⁶⁴Zn is one of the rare cases undergoing a ββ decay in β⁺ direction. In the experiment presented here, Gamow-Teller (GT⁺) transition strengths have been extracted. Together with the GT^- transition strengths from ⁶⁴Ni(³He,t) data to the same intermediate nucleus ⁶⁴Cu, the nuclear matrix elements of the ββ decay of ⁶⁴Zn have been evaluated. Finally, the GT^± distributions are compared with shell-model calculations and a critical assessment is given of the various residual interactions presently employed for the pf shell.

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 neutron-rich Fe isotopes from A=61 to 66 were studied through multinucleon transfer reactions by bombarding a ²³⁸U target with a 400 MeV ⁶⁴Ni beam. Unambiguous identification of prompt γ rays belonging to each nucleus was achieved using coincidence relationships with the ions detected in a high-acceptance magnetic spectrometer. The new data extend our knowledge of the level structure of Fe isotopes, which is discussed in terms of the systematics of the region and compared with large-scale shell-model calculations.

A Reply to the Comment by Angelo Signoracci and B. Alex Brown.

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.

The excited states of the neutron-rich nucleus ₁₅³⁷P₂₂ have been populated in grazing reactions, using a beam of ³⁶S ions (at 215 MeV) delivered onto a thin ²⁰⁸Pb target. Emitted γ rays from excited projectile-like nuclei were detected using the CLARA array of 25 escape-suppressed Ge clover detectors in coincidence with reaction products detected and identified with the PRISMA magnetic spectrometer. A level scheme is presented for ³⁷P together with proposed spin assignments. The level structure of ³⁷P is discussed within the context of shell-model calculations by using an improved sdpf effective interaction.

The N=28 shell closure has been investigated via the ⁴⁶Ar(d,p)⁴⁷Ar transfer reaction in inverse kinematics. Energies and spectroscopic factors of the neutron p_3/2, p_1/2, and f_5/2 states in ⁴⁷Ar were determined and compared to those of the ⁴⁹Ca isotone. We deduced a reduction of the N=28 gap by 330(90) keV and spin-orbit weakenings of ≃10(2) and 45(10)% for the f and p states, respectively. Such large variations for the f and p spin-orbit splittings could be accounted for by the proton-neutron tensor force and by the density dependence of the spin-orbit interaction, respectively. This contrasts with the picture of the spin-orbit interaction as a surface term only.

Excited states of N=22 ³⁶Si, populated in deep-inelastic processes produced by the interaction of a 215 MeV beam of ³⁶S ions with a ²⁰⁸Pb target, were studied in the present work. γ rays from the binary fragments detected using CLARA, an array of 25 Ge Clover detectors, were measured in coincidence with projectile-like fragments detected by PRISMA, a large solid angle magnetic spectrometer. Two new γ-ray photopeaks at energies of 1442 and 842 keV were observed and tentatively assigned to the 4⁺→2⁺ and 6⁺→4⁺ transitions, respectively. The systematics of the level structures of N=22 isotones are presented, and a comparison is made of the behavior of Si, Mg, and S isotopes. The level structure of ³⁶Si is also compared with the results of sdpf shell model calculations.

The reduced transition probabilities B(E2;0⁺→2₁⁺) of the neutron-rich ⁷⁴Zn and ⁷⁰Ni nuclei have been measured by Coulomb excitation in a ²⁰⁸Pb target at intermediate energy. These nuclei have been produced at Grand Accélérateur National d’Ions Lourds via interactions of a 60A  MeV ⁷⁶Ge beam with a Be target. The B(E2) value for ⁷⁰Ni₄₂ is unexpectedly large, which indicates that neutrons added above N=40 strongly polarize the Z=28 proton core. In the Zn isotopic chain, the steep rise of B(E2) values beyond N=40 continues up to ⁷⁴Zn₄₄. The enhanced proton core polarization in ⁷⁰Ni is attributed to the monopole interaction between the neutron in the g_9/2 and protons in the f_7/2 and f_5/2 spin-orbit partner orbitals. This interaction could result in a weakening of magicity in ⁷⁸Ni₅₀.

The unstable neutron-deficient ¹⁰⁸Sn isotope has been studied in inverse kinematics by intermediate-energy Coulomb excitation using the RISING/FRS experimental setup at GSI. This is the highest Z nucleus studied so far with this method. Its reduced transition probability B (E2;0_g.s.⁺→2₁⁺) has been measured for the first time. The extracted B(E2) value of 0.230(57)e²  b² has been determined relative to the known value in the stable ¹¹²Sn isotope. The result is discussed in the framework of recent large-scale shell model calculations performed with realistic effective interactions. The roles of particle-hole excitations of the ¹⁰⁰Sn core and of the Z=50 shell gap for the E2 polarization are investigated.

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.”

The NEMO 3 detector, which has been operating in the Fréjus underground laboratory since February 2003, is devoted to the search for neutrinoless double-beta decay (ββ0ν). The half-lives of the two neutrino double-beta decay (ββ2ν) have been measured for ¹⁰⁰Mo and ⁸²Se. After 389 effective days of data collection from February 2003 until September 2004 (phase I), no evidence for neutrinoless double-beta decay was found from ∼7  kg of ¹⁰⁰Mo and ∼1  kg of ⁸²Se. The corresponding limits are T_1/2(ββ0ν)>4.6×10²³   yr for ¹⁰⁰Mo and T_1/2(ββ0ν)>1.0×10²³   yr for ⁸²Se (90% C.L.). Depending on the nuclear matrix element calculation, the limits for the effective Majorana neutrino mass are ⟨m_ν⟩<0.7–2.8  eV for ¹⁰⁰Mo and ⟨m_ν⟩<1.7–4.9  eV for ⁸²Se.

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.

The nucleus ³⁴S has been studied up to relatively high spin (I=10ℏ) and up to an excitation energy of 16.64 MeV. Data are compared with different shell model calculations where effective interactions involving two main shells, the sd and the fp, are used. For the first time in an N≠Z nucleus, it is observed that the promotion of an isoscalar (T=0) proton-neutron pair to the fp shell becomes energetically favored at high spin.

A core-excited I^π=(12⁺) spin-gap isomer was identified in ⁹⁸Cd in an experiment at EUROBALL IV. It was found to feed the known I^π=(8⁺) seniority isomer by an E4 transition. Half-lives of T_1∕2=0.23(+4 / −3)  μs and 0.17(+6 / −4)  μs were measured for the two states at E_x=6635  keV and 2428  keV, respectively. From the excitation energy of the core-excited isomer a ¹⁰⁰Sn shell gap of 6.46(15)  MeV is inferred. The measured E4 and E2 strengths, ¹⁰⁰Sn core excitations and the origin of empirical polarization charges are discussed in the framework of large-scale shell model calculations. An E2 polarization charge for protons of δe_π<0.4  e is found, which corresponds to the empirical value δe_π=0.45(+20 / −25)  e in the pure proton hole valence space.

Low-lying excited states in the odd-odd N=Z nucleus ⁶²Ga have been investigated following the heavy-ion fusion-evaporation reaction ⁴⁰Ca(²⁴Mg,pn)⁶²Ga near the Coulomb barrier. Special emphasis is devoted to the search for nonyrast states. The extended decay scheme of ⁶²Ga is compared to spherical shell-model calculations employing the pf_5∕2g_9∕2 valence space.

We calculate the low-lying B(E2,0_g.s.⁺→2_f⁺) distribution of strength in ⁶⁸Ni and other nickel isotopes using several theoretical approaches. We find that in ⁶⁸Ni the calculated B(E2) transition to the first 2⁺ state exhausts only a fraction of the low-lying B(E2) strength, while the remainder of the low-lying strength is mainly collected in the group of states lying above 4 MeV. This fragmentation is sensitive to the size of the N=40 gap. We argue that the small experimental B(E2) value to the first 2⁺ state is not a strong evidence for the double-magic character of ⁶⁸Ni.

The structure of neutron-rich ^40,42,44S nuclei has been investigated through in-beam γ-ray spectroscopy using the fragmentation reaction of a 60 MeV A ⁴⁸Ca beam on a thin Be target. E_γ, I_γ, γγ-coincidence, and γ-ray angular distributions were measured for each produced fragment. The level schemes previously containing only a single γ transition were extended, and spin values were proposed for the new states. The experimental results were interpreted by use of microscopic collective-model and large-scale shell-model calculations. The results of the model calculations are consistent with each other, and give a reasonable description of the experimental results. Both models predict an erosion of the N=28 shell closure at Z=16 and suggest a deformed ground state for ^40,42S and a spherical-deformed mixed configuration for ⁴⁴S.

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.