Search Results (31 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.

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 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.

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 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.

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 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.

We studied β-delayed proton and γ emission from ⁴⁰Ti decay. We found t_1/2=52.7±1.5 ms and observed 28 proton groups that we organized into a ⁴⁰Ti decay scheme with 21 branches. The reduced transition strengths of these decay branches were then used to compute the neutrino detection efficiency of the ICARUS liquid argon time-projection chamber. We found ⁴⁰Ar(ν,e) cross sections (for an electron energy threshold W=5 MeV) of (14.0±0.3)×10^-43 cm², (75.1±1.8)×10^-43 cm², and (3.2±0.1)×10^-41 cm² for ⁸B neutrinos, hep neutrinos, and supernova neutrinos characterized by a temperature of 4.5 MeV.

The short-lived isomeric states in the T_Z=1 nuclei, ^94mPd [T_1/2=0.6(1) μs] and ^96mAg [T_1/2=0.7(2) μs], were identified among the fragmentation products of the ¹¹²Sn (63A MeV) + ^natNi (93.5 mg/cm²) reaction at GANIL. The separation and identification of the reaction products was done by means of the Alpha and LISE3 magnetic spectrometers combined with time-of-flight, energy-loss, and total kinetic energy measurements. Evidence for isomeric states in ⁸⁰Y, ⁹⁸Cd, and ¹⁰²Sn was also obtained.

The neutron-rich isotopes ^54-57Ti and ^58-60Cr are produced by fragmentation of a 64.5 MeV/nucleon ⁶⁵Cu²⁶⁺ beam in a 90 mg/cm^{2 9}Be target. Following particle identification by energy loss and time of flight, the radioactive decay was observed by β singles and βγ-coincidence measurements. The results obtained for ^58-60Cr are compared to previous results, whereas the decay of the ^54-57Ti isotopes is studied here. γ-ray intensities and energies are estimated. The new experimental results are compared to quasi-random-phase-approximation predictions. © 1996 The American Physical Society.

The reaction cross section for ⁸B and ⁷Be and the breakup cross section for ⁸B on silicon have been measured for incident energies between 10 and 40 MeV/nucleon with a stack of silicon detectors. Secondary ⁸B and ⁷Be beams were obtained by use of the LISE spectrometer at GANIL. The stack of silicon detectors had the multiple purpose of slowing down the incident secondary beams, identifying the reaction products, and measuring their energy. The separate contributions of diffraction dissociation and absorption to the breakup have been determined. The parallel momentum distribution of ⁷Be resulting from the breakup of ⁸B has also been determined. The data are compared to theoretical calculations. © 1996 The American Physical Society.

The new neutron-deficient nuclei ¹⁰³Sb, ¹⁰⁴Sb, ⁹⁸In, ⁹¹Pd, ⁸⁹Rh, and ⁸⁷Ru have been identified among the quasifragmentation products of a ¹¹²Sn beam (63 MeV/nucleon). The fragment identification based on energy-loss, total kinetic energy, and time-of-flight measurements has been independently confirmed via observation of γ radiation following the decay of known short-lived isomers. The region of known isotopes is extended to the predicted proton drip line for indium and silver, and beyond it for antimony and rhodium.

Beta-decay half-lives and β-delayed neutron-emission probabilities of the very neutron-rich nuclei ⁴⁴S and ^45–47Cl have been measured. These isotopes, which lie at or close to the N=28 magic shell, were produced in interactions of a 60 MeV/u ⁴⁸Ca beam from GANIL (Grand Accélérateur National d’Ions Lourds) with a ⁶⁴Ni target, and were separated by the doubly achromatic spectrometer LISE (Ligne d’Ions Super Epluchés). Their decay was studied by a β-n time correlation measurement. The results are compared to recent model predictions and indicate a rapid weakening of the N=28 shell effect below ₂₀⁴⁸Ca₂₈. The nuclear structure effects reflected in the decay properties of the exotic S and Cl isotopes may be the clue for the astrophysical understanding of the unusual ⁴⁸Ca/⁴⁶Ca abundance ratio measured in the solar system as well as the Ca-Ti-Cr anomalies observed in E. King inclusions of the Allende meteorite.

The different β-delayed particle-decay modes of ³¹Ar are investigated by means of a new silicon-detector telescope. Identification of β-delayed two-proton decay, and β-delayed three-proton decay observed for the first time, is made possible by Monte Carlo simulations of the detector response. Other β-delayed single-proton branches are also investigated, and none of the energetically allowed β-α and β-pα decay modes are found. Finally, the results of this experiment are compared to shell-model calculations.

An attempt has been made to synthesize the extremely neutron-rich isotope ²⁶O in the nuclear reaction 44 MeV/nucleon ⁴⁸Ca+n/rTa. Use was made of magnetic separation and identification methods including time-of-flight and ΔE,E measurements. The ²⁶O nucleus appears to be unstable against particle emission since no events attributable to the ²⁶O nucleus were observed at a level one order of magnitude lower than that predicted from the extrapolated yields. The previously unobserved isotope ³²Ne was found to be particle stable and the isotope ³¹Ne particle unstable. Neutron-separation energies calculated with different models are tabulated.

The spectroscopy of neutron-rich isotopes of ⁶⁷Ni and ⁶⁸Ni is studied using the quasi-elastic transfer reactions (¹⁴C,¹⁶O) and (¹⁴C,¹⁷O) on a ⁷⁰Zn mass separated target. The structure of these exotic nuclei is investigated in the framework of a microscopic collective model based on the Hartree-Fock-Bogoliubov theory. Gogny’s two-body effective interaction is used. Collective excited states of ⁶⁸Ni are obtained by solving the Bohr Hamiltonian in which inertia parameters are calculated in the cranking approximation. Spin and parity assignments to observed excited levels are suggested on the basis of information deduced from this analysis. This assignment is further checked by comparing measured angular distributions to predictions. Predictions of the level structure of ⁷⁰Ni and ⁷⁸Ni isotopes are given. A more precise test of the 0⁺ wave functions is provided by the calculation of monopole operator of the 0₁⁺→0₂⁺ transition in ⁶⁸Ni. An impressive agreement is obtained between the measured and calculated half-life.

First evidence of the nucleus ²²Si (T_z=-3) is observed by fragmentation of 85-MeV/nucleon ³⁶Ar projectiles from the Grand Accélérateur National d’Ions Lourdes.

Reaction products from the bombardment of ²⁰Ne and ¹⁶O gas targets with ²⁰Ne beams have been measured at five laboratory energies between 70 and 160 MeV. Fusion, deep inelastic, and quasielastic processes were studied. Large deep inelastic yields (σ_Di≳600 mb) with average Q values of -50 MeV are present at the highest energy. Analysis of the fusion cross sections for ²⁰Ne+¹⁶O suggests that the maximum angular momentum in ³⁶Ar populated via this entrance channel may be as low as 29±2ℏ.

NUCLEAR REACTIONS ²⁰Ne+²⁰Ne, ²⁰Ne+¹⁶O; E_lab=80 to 160 MeV. Measured elastic scattering, direct processes, deep inelastic processes, and fusion. All products with Z>4 identified. Measured angular distributions. Monte Carlo simulation of deep inelastic and fusion.

Fusion cross sections were measured for the ¹²C + ¹⁵N system at two ¹²C bombarding energies, 98 and 117 MeV. The extracted fusion cross sections are 946 ± 74 mb for E(¹²C)=98 MeV and 889 ± 89 mb for E(¹²C)=117 MeV. The critical angular momenta for fusion deduced from the cross section measurements equal, within 1ℏ, the values deduced from ¹⁴N + ¹³C and ¹⁰B + ¹⁷O at the same excitation energies, thus indicating a compound nucleus limitation for fusion.

NUCLEAR REACTIONS ¹²C + ¹⁵N, E(¹²C)=98 and 117 MeV; gas cell target; measured d²σ / dΩdE for reaction products with Z=7 to 12. Extracted σ_fus.

The ground-state transition is observed in the double-charge-exchange reaction ⁴⁸Ca(¹⁸O, ¹⁸C) ⁴⁸Ti at 100 MeV. A mass excess of 24.82 ± 0.30 MeV is measured for ¹⁸C.

NUCLEAR REACTIONS ⁴⁸Ca(¹⁸O, ¹⁸C), E=100 MeV; measured ¹⁸C mass; enriched target.

The (¹⁴C, ¹⁶O) reaction on even Ni and Zn isotopes has been investigated at 72 MeV bombarding energy. The mass excesses of ⁶²Fe and ⁶⁸Ni are determined to be -58.84±0.04 and -63.55±0.04 MeV, respectively. Previously unknown excited states are found. Spectroscopic factors for transitions to ground states and to the first excited states are shown to increase with the neutron number.

[NUCLEAR REACTIONS ^{58,  60,  62,  64}Ni(¹⁴C, ¹⁶O) and ^{64,  66,  68,  70}Zn(¹⁴C, ¹⁶O), E=72 MeV; measured σ(E,θ); deduced binding energies of ⁶²Fe and ⁶⁸Ni and excited states energies. DWBA analysis, deduced spectroscopic factors.]

Cross section angular distributions of ²⁴Mg(¹⁸O, ¹⁶O)²⁶Mg reaction at 50 MeV incident energy leading to the first four excited states in ²⁶Mg and the elastic and inelastic scattering for ¹⁸O + ²⁴Mg at 50 MeV and ¹⁶O + ²⁶Mg at 56 MeV have been measured in order to investigate the reaction mechanism of the two-nucleon transfer reaction including the analysis of incident and exit channels. The measured angular distributions were analyzed in the framework of the exact finite-range distorted-wave Born approximation. Two neutron transfer amplitudes were determined using the 2s-1d shell model amplitudes calculated by Chung and Wildenthal. The large experimental cross section in contrast to the smallness of the direct transfer amplitude and the peculiar shape of the measured angular distribution of the lowest 2₁⁺ state suggests that this level is mainly excited by two-step processes. It is shown that a coupled-channel Born approximation calculation describes successfully the angular distributions of the ground state and the 2₁⁺ state in ²⁶Mg.

NUCLEAR REACTIONS ²⁴Mg(¹⁸O, ¹⁶O)²⁶Mg, E_¹⁸O=50 MeV, measured σ(θ, E_x); ²⁴Mg(¹⁸O, ¹⁸O)²⁴Mg, E_¹⁸O=50 MeV and ²⁶Mg(¹⁶O, ¹⁶O)²⁶Mg, E_¹⁶O=56 MeV, elastic and inelastic scattering, measured σ(θ); enriched targets; optical model parameters deduced from scattering: microscopic EFR-DWBA and CCBA analysis with shell-model wave functions.

The reaction ²⁶Mg(¹⁸O, ¹⁶O)²⁸Mg has been studied at a bombarding energy of 50 MeV between laboratory angles of 4° and 17°. The differential cross sections of the lowest four states of ²⁸Mg have been compared to exact finite-range calculations in the distorted-wave Born approximation and coupled-channels Born approximation formulations. The reaction-mechanism calculations employed wave functions for the initial and final nuclear states which were generated in shell-model calculations carried out in the full d_{5 / 2}-s_{1 / 2}-d_{3 / 2} basis space. The relative importance of one-step and two-step processes in the population of the different final states is evaluated and the effectiveness of current reaction theories in accounting for phenomena such as are exemplified by the present data is discussed.

NUCLEAR REACTIONS ²⁶Mg(¹⁸O, ¹⁶O)²⁸Mg; enriched target, E=50 MeV, measured σ(θ), microscopic DWBA and CCBA analysis with shell-model wave functions; deduced levels of ²⁸Mg.