Search Results (36 total)

The structure of ^19-22N nuclei was investigated by means of in-beam γ-ray spectroscopic technique using fragmentation reactions of both stable and radioactive beams. Based on particle-γ and particle-γγ coincidence data, level schemes are constructed for the neutron-rich nitrogen nuclei. The experimental results are compared with shell model calculations. The strength of the N=14 and Z=8 shell closures and the weakening of the shell model interaction WBT are discussed.

The β decay of the neutron-rich ₃₀⁸¹Zn₅₁ has been investigated at the PARRNe mass separator at the IPN Orsay. The sources of ⁸¹Zn were produced using the ISOL (Isotopic Separation On Line) technique by the fission of ^natU exposed to the neutron flux produced by the 26-MeV deuteron beam delivered by the MP-Tandem. With γ and γ-γ coincidence measurements, excited levels were attributed to ₃₁⁸¹Ga₅₀ for the first time. A partial decay scheme for ⁸¹Zn is proposed. The proposed level scheme is well reproduced by shell model calculations using the most recent effective empirical interaction. We show that the structure of this nucleus is consistent with that of the heavier odd-proton N=50 isotones within the assumption of strong proton Z=28 and neutron N=50 effective shell effects. The observed states can be associated to rather simple and clean configurations of three protons placed in the 1f_5/2 and 2p_3/2 orbits.

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.

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.

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.

Nuclear structure of the neutron rich ^25-29Ne nuclei has been investigated through the in-beam γ-ray spectroscopy technique using fragmentation reactions of both stable and radioactive beams. Level schemes have been deduced for these Ne isotopes. In order to examine the importance of intruder fp configurations, they are compared to shell model calculations performed either in the restricted sd or in the larger sdpf valence space. The ^25,26Ne and ²⁷Ne nuclei were found to be in agreement with the sd shell model calculations, whereas ²⁸Ne exhibits signatures of the intruder fp shell contribution.

A pure neutron-rich ²⁶Ne beam was obtained at the ISOLDE facility using isobaric selectivity. This was achieved by a combination of a plasma ion source with a cooled transfer line and subsequent mass separation. The high quality of the beam and good statistics allowed us to obtain new experimental information on the ²⁶Ne β-decay properties and resolve a contradiction between earlier experimental data and prediction of shell-model calculations.

Information on β-decay properties of neutron-rich ⁴⁷Ar was obtained at the ISOLDE facility at CERN using isobaric selectivity. This was achieved by a combination of a plasma-ion source with a cooled transfer line and subsequent mass separation. A doubly charged beam was used in order to improve the signal-to-background ratio associated with multi-charged noble gas fission products. The identification of the ⁴⁷Ar γ-ray transitions was performed by comparing the spectra obtained from direct proton bombardment of the target and of the neutron converter. New excited levels in the daughter ⁴⁷K nucleus corresponding to the negative-parity states were observed. The obtained data are compared to the result of large-scale shell model calculations and quasiparticle random-phase approximation predictions.

In-beam γ-ray spectroscopy using fragmentation reactions of both stable and radioactive beams has been performed in order to study the structure of excited states in neutron-rich oxygen isotopes with masses ranging from A=20 to 24. For the produced fragments, γ-ray energies, intensities, and γ-γ coincidences have been measured. Based on this information new level schemes are proposed for ^21,22O up to the neutron separation energy. The nonobservation of any γ-decay branch from ²³O and ²⁴O suggests that their excited states lie above the neutron decay thresholds. From this, as well as from the level schemes proposed for ²¹O and ²²O, the size of the N=14 and 16 shell gaps in oxygen isotopes is discussed in the light of shell-model calculations.

The yrast J^π=8⁺ states in neutron-rich ^70,72,74,76Ni nuclei are predicted to be isomeric. The present paper describes two GANIL experiments. In the first of them a search was made for the 8⁺ isomeric states in ^72,74Ni nuclei via fragmentation of ⁷⁶Ge using the ion γ-decay correlation technique. Although these states were not observed, limits for their lifetimes were determined. In the second experiment the decay spectroscopy of ^70,72Co nuclei was performed using fragmentation of a ⁸⁶Kr³⁶⁺ beam and the new LISE2000 spectrometer. The β delayed γ rays from the decay of ^70,72Co to ^70,72Ni were observed using the EXOGAM germanium detectors. The half life of ⁷²Co was measured to be 62(3) ms and the level sequence of the lowest excited states in ⁷²Ni was suggested, with the 2⁺ state at 1096  keV. An attempt to reproduce the level scheme in terms of shell-model calculations was undertaken. The reasons for the disappearance of the 8⁺ isomer in ⁷²Ni are discussed.

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.

A new approach to the production and detection of bound neutron clusters is presented. The technique is based on the breakup of beams of very neutron-rich nuclei and the subsequent detection of the recoiling proton in a liquid scintillator. The method has been tested in the breakup of intermediate energy (30–50 MeV/nucleon) ¹¹Li, ¹⁴Be, and ¹⁵B beams. Some six events were observed that exhibit the characteristics of a multineutron cluster liberated in the breakup of ¹⁴Be, most probably in the channel ¹⁰Be+⁴n. The various backgrounds that may mimic such a signal are discussed in detail.

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.

Three-body correlations in the dissociation of two-neutron halo nuclei are explored using a technique based on intensity interferometry and Dalitz plots. This approach provides for the combined treatment of both the n-n and core-n interactions in the exit channel. As an example, the breakup of ¹⁴Be into ¹²Be+n+n by Pb and C targets has been analyzed and the halo n-n separation extracted. Evidence for a finite delay between the emission of the neutrons in the reaction on the C target was observed and is attributed to ¹³Be resonances populated in sequential breakup.

The β⁺-decay half-lives of the neutron-deficient odd-odd N=Z nuclei ⁷⁴Rb, ⁷⁸Y, ⁸²Nb, and ⁸⁶Tc have been measured following the fragmentation of a primary ⁹²Mo beam at an energy of 60 MeV per nucleon at the GANIL laboratory, France. This was achieved by correlating β⁺ decays with the implantation of unambiguously identified fragments at the final focus of the LISE3 separator. The deduced log₁₀ ft_1/2 values are consistent with 0⁺→0⁺, Fermi superallowed transitions, which together with the measured β⁺-detection efficiencies, suggest T=1, I^π=0⁺ ground states for these odd-odd N=Z nuclei. These data represent the heaviest N=Z systems for which Fermi superallowed decays have been established.

The breakup of ^10,12Be into He clusters has been studied using the p,¹²C(¹²Be,⁶He,⁶He) and ¹²C(¹²Be,⁴He,⁶He) inelastic scattering and two neutron transfer reactions with a 378 MeV ¹²Be beam incident on ¹²C and (CH₂)_n targets. Evidence has been found for three new states in ¹⁰Be at excitation energies of 13.2, 14.8, and 16.1 MeV, which may be associated with a ⁴He+⁶He cluster structure. The evidence for He cluster states in ¹²Be in the excitation energy range 12 to 25 MeV is also discussed.

The two-neutron halo nucleus ¹⁴Be has been investigated in a kinematically complete measurement of the fragments ( ¹²Be and neutrons) produced in dissociation at 35 MeV/nucleon on C and Pb targets. Two-neutron removal cross sections, neutron angular distributions, and invariant mass spectra were measured, and the contributions from electromagnetic dissociation (EMD) were deduced. Comparison with three-body model calculations suggests that the halo wave function contains a large ν(2s_1/2)² admixture. The EMD invariant mass spectrum exhibited enhanced strength near threshold consistent with a nonresonant soft-dipole excitation.

The level structure of the unbound nucleus ¹¹N has been studied by ¹⁰C+p elastic resonance scattering in inverse geometry with the LISE3 spectrometer at GANIL, using a ¹⁰C beam with an energy of 9.0 MeV/nucleon. An additional measurement was done at the A1200 spectrometer at MSU. The excitation function above the ¹⁰C+p threshold has been determined up to 5 MeV. A potential-model analysis revealed three resonance states at energies 1.27_-0.05^+0.18 MeV (Γ=1.44±0.2 MeV), 2.01_-0.05^+0.15 MeV (Γ=0.84±0.2 MeV), and 3.75±0.05 MeV (Γ=0.60±0.05 MeV) with the spin-parity assignments I^π=1 / 2⁺,1 / 2^-,5 / 2⁺, respectively. Hence, ¹¹N is shown to have a ground state parity inversion completely analogous to its mirror partner ¹¹Be. A narrow resonance in the excitation function at 4.33±0.05 MeV was also observed and assigned spin parity 3 / 2^-.

The γ radiation and neutrons emitted following the β decays of ²⁴O, ^25–27F, and ^28-30Ne have been measured. The nuclides were produced in the quasifragmentation of a 2.8 GeV ³⁶S beam, separated in-flight and identified through time-of-flight and energy-loss measurements. The ions were stopped in a silicon detector telescope, which was used to detect the β particles emitted in their subsequent radioactive decay. The coincident γ rays were measured using four large volume germanium detectors mounted close to the implantation point and the neutrons were detected using 42 ³He proportional counters. The measured γ-ray energy spectra are compared with shell model calculations and, where available, the level energies deduced from multinucleon transfer reactions.

The ¹⁴Be two-neutron halo nucleus is described within a two-neutron pairing model. In order to reproduce the measured two-neutron separation energy in ¹⁴Be and the d_5/2 resonance in ¹³Be at the measured energy of 2 MeV, one has to assume in ¹³Be the inversion of 1p_1/2 and 2s_1/2 shells as in ¹¹Be and ¹⁰Li. We thus predict the ground state of ¹³Be to be a 1/2^- state unbound by about 0.3 MeV, instead of a 1/2⁺ state as usually accepted.

The halo neutron breakup cross section for ¹¹Be on Si has been obtained in a wide energy range by applying an integral method and separately determining the contributions of stripping and dissociation mechanisms. A new breakup mechanism, for which the core energy is strongly dumped, has also been observed. Parallel momentum distributions of ¹⁰Be resulting from breakup have been deduced for both stripping and dissociation and angular and energy distributions of the neutrons coincident with different reaction products have been measured. Charge changing cross sections for ^10,11Be complemented the measurements. An extended Glauber model has been elaborated in order to provide a unitary interpretation for all the data. It takes into account both the specific structure of ¹¹Be and the reaction mechanism, practically without free parameters. The effects of reaction mechanisms on the widths of observed momentum distributions are particularly important.

Measured angular and energetic distributions of neutrons obtained by bombarding Be, C, and U thick targets with ²H at 17, 20, and 28 MeV incident energies are reported. The data were obtained using the time-of-flight method. The energetic distributions of neutrons were determined at 0°, 5°, 10°, and 20°. The data are compared with a modelization based on stripping formalism extended for thick targets.

The breakup of ¹²Be into ⁶He+⁶He and ⁴He+⁸He has been studied using a 378 MeV ¹²Be beam inelastically excited by ¹²C and (CH₂)_n targets. The measurements indicate that breakup occurs from rotational states in the 10 to 25 MeV excitation energy interval, with spins in the range of 4ħ to 8ħ. The inferred moment of inertia is consistent with the cluster decay of an exotic molecular structure in ¹²Be, which may be associated with an α-4n-α cluster configuration.