Search Results (12 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 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 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 masses of neutron-deficient nuclei close to the proton drip line are an important input for the rapid proton-capture process modeling above ⁵⁶Ni. The measurement of the masses of proton-rich nuclei with 32<~Z<~35 has been made using a direct time-of-flight technique. The masses of the nuclides ⁶⁶As,⁶⁸Se, and ⁷¹Br are reported for the first time, with mass excesses of -51 500(680), -53 620(1000), and -57 060(570) keV being found. The masses agree well in most cases with the Audi-Wapstra systematics.

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 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.

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.

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.

In an experiment at the SISSI/LISE facility of GANIL, we used the projectile fragmentation of a ⁷⁸Kr primary beam at 73 MeV/nucleon to produce new isotopes of astrophysical interest. We obtained clear evidence for the existence of the five new isotopes ⁶⁰Ga, ⁶⁴As, ^69,70Kr, and ⁷⁴Sr. However, we did not find any evidence for ⁶⁹Br, whereas comparable nuclei were observed with more than 1000 counts. The isotope ⁶⁹Br is thus deduced to be a proton-unbound nucleus with a half-life shorter than about 100 ns. The influence of these results on our understanding of the astrophysical RP process is discussed.