Search Results (49 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.

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.

We report on the g factor measurement of an isomer in the neutron-rich ₂₆⁶¹Fe (E^*=861  keV and T_1/2=239(5)  ns). The isomer was produced and spin aligned via a projectile-fragmentation reaction at intermediate energy, the time dependent perturbed angular distribution method being used for the measurement of the g factor. For the first time, due to significant improvements of the experimental technique, an appreciable residual alignment of the nuclear spin ensemble has been observed, allowing a precise determination of its g factor, including the sign: g=-0.229(2). In this way we open the possibility to study moments of very neutron-rich short-lived isomers, not accessible via other production and spin-orientation methods.

A study of high-energy (43–68  MeV∕nucleon) one-neutron removal reactions on a range of neutron-rich psd-shell nuclei (Z=5–9,  A=12–25) has been undertaken. The inclusive longitudinal and transverse momentum distributions for the core fragments together with the cross sections have been measured for breakup on a carbon target. Momentum distributions for reactions on tantalum were also measured for a subset of nuclei. An extended version of the Glauber model incorporating second-order noneikonal corrections to the Jeukenne, Lejeune, and Mahaux parametrization of the optical potential has been used to describe the nuclear breakup, while the Coulomb dissociation is treated within first-order perturbation theory. The projectile structure has been taken into account via shell-model calculations employing the psd interaction of Warburton and Brown. Both the longitudinal and transverse momentum distributions together with the integrated cross sections were well reproduced by these calculations and spin-parity assignments are thus proposed for ¹⁵B,¹⁷C,^19–21N,^21,23O,^23–25F. In addition to the large spectroscopic amplitudes for the ν2s_1∕2 intruder configuration in the N=9 isotones, ¹⁴B and ¹⁵C, significant ν2s_1∕2² admixtures appear to occur in the ground state of the neighboring N=10 nuclei ¹⁵B and ¹⁶C. Similarly, crossing the N=14 subshell, the occupation of the ν2s_1∕2 orbital is observed for ²³O, ^24,25F. Recent claims of a modified shell structure for ²³O are investigated and the original suggestion of a ground state J^π=1∕2⁺ is confirmed. Analysis of the longitudinal and transverse momentum distributions reveals that both carry spectroscopic information, often of a complementary nature. The general utility of high-energy nucleon removal reactions as a spectroscopic tool is also examined.

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.

A new isomeric 0⁺ state was identified as the first excited state in the self-conjugate (N=Z) nucleus ⁷²Kr. By combining for the first time conversion-electron and gamma-ray spectroscopy with the production of metastable states in high-energy fragmentation, the electric-monopole decay of the new isomer to the ground state was established. The new 0⁺ state is understood as the band head of the known prolate rotational structure, which strongly supports the interpretation that ⁷²Kr is one of the rare nuclei having an oblate-deformed ground state. This observation gives in fact the first evidence for a shape isomer in a N=Z nucleus.

Spin-polarized ²⁷Na and ³¹Al secondary beams have been produced at GANIL by the fragmentation of a 77.5 MeV/nucleon ³⁶S¹⁶⁺ primary beam onto a ⁹Be target. The primary beam was deflected at an angle of -2(1)° with respect to the entrance of the LISE3 spectrometer, where the target was placed. For linear momenta higher than that of the ³⁶S projectiles, a deduced spin polarization P=-6.2(9)% and P=-1.5(4)% was obtained for ²⁷Na and ³¹Al projectile fragments. These results demonstrate for the first time that fragments far from the projectile mass can be substantially polarized.

The β⁺ decay half-lives of 22 neutron-deficient nuclei in the cobalt-to-krypton region have been measured following the fragmentation of a primary ⁷⁸Kr beam at an energy of 73 MeV per nucleon. The half-lives of the T_z=-1 nuclei ⁶²Ge, ⁶⁴As, and ⁶⁶Se are determined for the first time with values of (129±35) ms, 18_-7⁺⁴³ ms, and (33±12) ms, respectively. The impact of these results on the nucleosynthesis and time scale of the astrophysical rapid proton-capture process are discussed.

In an experiment at the SISSI-LISE3 facility of GANIL, the decay of the proton drip line nucleus ⁴⁵Fe has been studied. Fragment-implantation events have been correlated with radioactive decay events in a 16×16 pixel silicon-strip detector. The decay-energy spectrum of ⁴⁵Fe implants shows a distinct peak at (1.14±0.04)  MeV with a half-life of T_1/2=(4.7_-1.4^+3.4)  ms. None of the events in this peak is in coincidence with β particles. For a longer correlation interval, daughter decays of the two-proton daughter ⁴³Cr can be observed after ⁴⁵Fe implantation. The decay energy for ⁴⁵Fe agrees nicely with several theoretical predictions for two-proton radioactivity.

A systematic study of the population probabilities of microsecond isomers produced following the fragmentation of ²⁰⁸Pb projectiles at 1 GeV/nucleon has been undertaken at the SIS/FRS facility at GSI Darmstadt. Gamma decays from approximately 20 isomeric states, mainly in the rare-earth and transitional nuclei with A∼180, were identified and the corresponding isomeric ratios deduced. The results are compared with a model based on the statistical abrasion-ablation description of relativistic fragmentation and simple assumptions concerning gamma cascades in the final nucleus (sharp cutoff). The model is found to represent an upper limit for the population of isomeric states in relativistic projectile fragmentation.

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.

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.

Isomeric ratios and momentum distributions of nuclei produced in the fragmentation of a 60A MeV ⁹²Mo beam on a thin ²⁷Al target have been studied in detail. A strong dependence of the isomeric ratio on the structure of the isomer and on the reaction mechanism has been observed for the first time at intermediate energies. The results are quantitatively reproduced in a framework of kinematical and statistical models of nuclear reactions.

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.

A new transition is reported in the β-delayed neutron decay of ¹⁶C. The energy of the associated neutrons is 3.29±0.03 MeV, leading to the feeding, with a branching ratio of ∼1%, of a probable 1⁺ level in ¹⁶N at 6.00±0.03 MeV. Such an observation is in good accordance with shell model calculations carried out within the 0p1s0d model space.

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 masses of 31 neutron-rich nuclei in the range A  =  29–47 have been measured. The precision of 19 masses has been significantly improved and 12 masses were measured for the first time. The neutron-rich Cl, S, and P isotopes are seen to exhibit a change in shell structure around N  =  28. Comparison with shell model and relativistic mean field calculations demonstrate that the observed effects arise from deformed prolate ground state configurations associated with shape coexistence. Evidence for shape coexistence is provided by the observation of an isomer in ⁴³S.

γ-ray decays depopulating isomeric states have been observed in a number of very neutron deficient nuclei around A∼80 following the projectile fragmentation of a ⁹²Mo primary beam. Previously unobserved decays have been identified in the N=Z+2 nuclei ₃₉⁸⁰Y and ₄₁⁸⁴Nb and the N=Z nucleus, ₄₃⁸⁶Tc, making the latter the heaviest N=Z nucleus to date in which a discrete γ-ray transition has been assigned. The lifetime of the previously reported I^π=9 / 2⁺ isomeric state in ⁷³Kr has also been measured and a clearer picture of its decay properties has been deduced. Isomeric ratios have been measured and have been interpreted in terms of the yrast or nonyrast nature of the isomeric state.

In an experiment at the SISSI/LISE3 facility of GANIL, we used the projectile fragmentation of a primary ⁵⁸Ni²⁶⁺ beam at 74.5 MeV/nucleon with an average current of 3 μA on a natural nickel target to produce very neutron-deficient isotopes. In a 10-day experiment, 287 ⁴²Cr isotopes, 53 ⁴⁵Fe isotopes, 106 ⁴⁹Ni isotopes, and 4 ⁴⁸Ni isotopes were unambiguously identified. The doubly magic nucleus ⁴⁸Ni, observed for the first time, is the most proton-rich isotope ever identified with an isospin projection T_z  =  -4. It is probably the last doubly magic nucleus with “classical” shell closures accessible for present-day facilities. Its observation allows us to deduce a lower limit for the half-life of ⁴⁸Ni of 0.5 μs.