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The excitation energy of deformed intruder states (specifically the 2p2h bandhead) as a function of proton number Z along N=20 is of interest both in terms of better understanding the evolution of nuclear structure between spherical Ca40 and the Island of Inversion nuclei, and for benchmarking theoretical descriptions in this region. At the center of the N=20 Island of Inversion, the npnh (where n=2,4,6) neutron excitations across a diminished N=20 gap result in deformed and collective ground states, as observed in Mg32. In heavier isotones, npnh excitations do not dominate in the ground states but are present in the relatively low-lying level schemes. With the aim of identifying the expected 2p2h - s1/2+ state in P35, the only N=20 isotone for which the neutron 2p2h excitation bandhead has not yet been identified, the S36(d,He3)P35 reaction has been revisited in inverse kinematics with the HELical Orbit Spectrometer (HELIOS) at the Argonne Tandem Linac Accelerator System (ATLAS). While a candidate state has not been located, an upper limit for the transfer reaction cross section to populate such a configuration within a 2.5 to 3.6 MeV energy range provides a stringent constraint on the wave function compositions in both S36 and P35.

International audience; The sequential two-proton decay of the second excited state in Ne17, produced by inelastic excitation at intermediate energy, is studied. This state is found to be highly spin aligned, providing another example of a recently discovered alignment mechanism. The fortuitous condition that the second decay step is slightly more energetic than the first, permits the lifetime of the one-proton daughter, the ground state of F16, to be determined from the magnitude of the final-state interactions between the protons. This new method gave a result [Γ=20.6(57) keV] consistent with that obtained by directly measuring the width of the state [Γ=21.3(51) keV]. This width allows one to determine the continuum coupling constant in this mass region. Real-energy continuum-shell-model studies yield a satisfactory description of both spectra and widths of low-energy resonances in F16 and suggest an unusual large ratio of proton-proton to proton-neutron continuum couplings in the vicinity of the proton drip line.

Background: Obtaining reaction rates for nuclear astrophysics applications is often limited by the availability of radioactive beams. Indirect techniques to establish reaction rates often rely heavily on the properties of excited states inferred from mirror symmetry arguments. Mirror energy differences can depend sensitively on nuclear structure effects. Purpose: The present work sets out to establish a detailed comparison of mirror symmetry in the A=23, T=1/2 mirror nuclei 23Na and 23Mg both to high spin, and high excitation energy, including beyond the proton threshold. These data can be used to benchmark state-of-the-art shell-model calculations of these nuclei. Methods: Excited states in 23Na and 23Mg were populated using the 12C(12C,p) and 12C(12C,n) reactions at beam energies of 16 and 22 MeV, and their resulting γ decay was measured with Gammasphere. Results: Level schemes for 23Na and 23Mg have been considerably extended; highly excited structures have been found in 23Na, as well as their counterparts in 23Mg for previously known rotational structures in 23Na. Mirror symmetry has been investigated up to an excitation energy of 8 MeV and spin-parity of 13/2+. Excited states in the region above the proton threshold have been studied in both nuclei. Conclusions: A detailed exploration of mirror symmetry has been performed which heavily constrains expectations as to how mirror energy differences should evolve for different structures. Agreement with shell-model calculations provides confidence in using such estimations where real data are absent.

We describe the investigation of high-spin states in 24Mg populated with the 12C(16O,α)24Mg reaction at 62- and 68-MeV beam energy. The excited states were established through the coincident detection of up to four α and γ particles in complete decay cascades toward a final state of angular momentum zero, the ground state of either 20Ne or 16O. We describe a new analysis method for the angular correlations in such events and apply it with the goal to assign spin and parity to α-unbound states in 24Mg. We establish a number of high-spin, natural-parity states between spins 6 and 12, including the lowest known 10+ and 12 + levels. The energy systematics of positive- and negative-parity high-spin states are compared to the predictions of modern shell-model calculations.

The decay branchings of a resonance in the 12C(12C,γ)24Mg reaction at Ec.m. = 8.0 MeV have been studied with high resolution using the Gammasphere array. Radiative capture residues were discriminated from scattered beam and the dominant evaporation channels using the fragment mass analyzer coupled to a multistage Parallel Grid Avalanche Counter (PGAC)/ion chamber system. The clean selection of residues has allowed the population of excited states up to 10 MeV in 24Mg to be examined in detail. Strong feeding of an excited Kπ=0- band is observed. A Jπ = 4+ assignment to the resonance is strongly favored.

A measurement of the β-delayed α decay of N16 using a set of twin ionization chambers is described. Sources were made by implantation, using a N16 beam produced via the In-Flight Technique. The energies and emission angles of the C12 and α particles were measured in coincidence and very clean α spectra, down to energies of 450 keV, were obtained. The structure of the spectra from this experiment is in good agreement with results from previous measurements. An analysis of our data with the same input parameters as used in earlier studies gives SE1(300)=86±22 keVb for the E1 component of the S-factor. This value is in excellent agreement with results obtained from various direct and indirect measurements. In addition, the influence of new measurements including the phase shift data from Tischhauser on the value of SE1(300) is discussed.

We report on a study of the structure of the unbound nucleus He7 utilizing the proton-removal reaction 2H(8Li,3He)7He. Combining the present results with those of our prior measurements of the neutron-adding reaction 2H(6He,p)7He, a consistent picture emerges for the low-lying excitations in He7. Specifically, the negative-parity sequence of resonances, in order of excitation energies, is consistent with 3/2-,1/2-, and 5/2-. The stable-beam reactions 2H(7Li,t)6Li and 2H(7Li,3He)6He were also measured. The results are compared with the predictions of nuclear structure models, including those of ab initio quantum Monte Carlo calculations.

A new measurement of the β-delayed α decay of N16 has been performed using a set of high efficiency ionization chambers. Sources were made by implantation of a N16 beam, yielding very clean α spectra down to energies as low as 400 keV. Our data are in good agreement with earlier results. For the S factor SE1, we obtain a value of 74±21keVb. In spite of improvements in the measurement, the error in SE1 remains relatively large because of the correlations among the fit parameters and the uncertainties inherent to the extrapolation.

The heavy-ion radiative capture reaction, 12C( 12C,γ), has been investigated at beam energies around 16 MeV. Two different experiments were performed. Capture cross sections were obtained by measuring fused 24Mg residues and were found to significantly exceed values reported earlier. Subsequently, the decay pathways associated with radiative capture were delineated using the Gammasphere array. A substantial fraction of the decay was found to proceed through a few high-lying doorway states near 10 MeV in excitation.

We have studied the 2H(8Li, p)9Li reaction to obtain information on the spins, parities, and single-neutron spectroscopic factors for states in 9Li, using a radioactive 8Li beam. The deduced properties of the lowest three states are compared to the predictions of a number of calculations for the structure of 9Li. The results of ab initio quantum Monte Carlo calculations are in good agreement with the observed properties.

We have studied the properties of low-lying levels in He7 using the H2(He6,p)He7 reaction at 11.5 MeV/u. This reaction probes the He6g.s.+n character of states in He7. The ground state was populated with a spectroscopic factor comparable to that obtained from ab initio calculations, supporting the tentative spin-parity assignment of 3/2- in the literature. In addition to the ground state, a broad structure is observed between EX=2-3 MeV, the excitation-energy range expected for the 1/2- state in He7. No evidence was found for a lower-lying, first-excited state reported recently.

The heavy-ion radiative capture reaction, $^{12}C(^{12}C,\gamma), has been investigated at beam energies around 16 MeV. Two different experiments were performed. Capture cross sections were obtained by measuring fused $^{24}$Mg residues and were found to significantly exceed values reported earlier. Subsequently, the decay pathways associated with radiative capture were delineated using the Gammasphere array. A substantial fraction of the decay was found to proceed through a few high-lying doorway states near 10 MeV in excitation.

The use of detailed spectroscopy to understand the processes which create and destroy 22Na was discussed. A reduction in the yield of 22Na was inferred from the hydrodynamical calculations incorporating the upper and lower limits. The accuracy of the excitation energy of the previous states was improved and the new states were identified in the relevant excitation region. A reduction in the maximum detectability distance for 22Na gamma rays from the novae was also shown.

Large angle elastic scattering of alpha particles from 44Ti, which is the lightest unstable alpha-particle nucleus above A=40, was measured at backward angles. The angular distributions of elastic scattering of alpha particles from 40 and 44Ti were compared. Results indicated that for 40Ca, at angles close to 180° elastic cross sections higher than Rutherford values were confirmed, while corresponding yields for 44Ti were about a factor 20 smaller. An optical model analysis of α+44Ti data produced parameters which were in good agreement with the results.

The power of quadruple angular correlations combined with a new analysis method was used to identify the first 10+ state at an energy of 19.2(1) MeV in 24Mg. Evidence for the γ decay of this state was also obtained. The results indicate that the rotational band is yrast in 24Mg at spin 10ℏ and is associated with a rotational frequency of ℏω≈2.9 MeV.

The fusion-fission cross section for the system 17F + 208Pb involving the drip line nucleus 17F has been measured at energies in the vicinity of the Coulomb barrier. No enhancement of the fusion-fission yields due to breakup or to a large interaction radius was observed.

The single-particle character of states outside the doubly magic (radioactive) nucleus 56Ni has been determined through a measurement of the (d, p) neutron transfer reaction using inverse kinematics. From the spectroscopic factors of the low-lying states in 57Ni, the astrophysically interesting yield for the 56Ni(p, γ) reaction to the mirror nucleus 57Cu has been calculated, utilizing charge symmetry. The rate for this reaction in the temperature range typical of novae, supernovae, and x-ray bursts is found to be more than 10 times higher than previously assumed.

International audience

The fully energy-damped yields for the 36Ar + 12C and 20Ne + 28Si reactions at Ec.m. = 47.0 MeV and 45.5 MeV, respectively, are explored using particle-particle-γ coincidence data. These reactions reach a similar excitation energy of E*CN= 59.5 MeV in the 48Cr compound nucleus as was obtained in an earlier particle-particle coincidence study of the 24Mg + 24Mg reaction. The overall mass and total kinetic energy distributions of the fission fragments are found to be well reproduced by statistical-model calculations. These calculations are also found to reproduce structure seen in the excitation-energy spectra for the 20Ne + 28Si and 24Mg + 24Mg exit channels for all three reactions. In previous excitation-function measurements, strong heavy-ion resonance behavior has been observed in elastic and inelastic cross sections for the 24Mg - 24Mg system. There has been speculation that peaks observed in the corresponding excitation-energy spectra at more negative Q values may also be a consequence of this resonance phenomenon. The observation of very similar behavior with the asymmetric-mass entrance channels makes it less likely, though, that the peaks arise from any special configuration of the compound system. Instead, an analysis of the γ-ray data and the results of statistical-model calculations support the conclusion that most of the observed high-lying structure can be accounted for in terms of statistical fission from a fully energy- and shape-equilibrated compound nucleus. For the 24Mg + 24Mg entrance channel, however, comparisons with the statistical model indicate a reduction of high-angular-momentum partial cross sections, leading to the 24Mg + 24Mg fission channel. For the first time, we are able to deduce the nature of the competition between the resonance and statistical-fission mechanisms in this mass region.