Search

The Na23(α,p)Mg26 and Na23(α,n)Al26 reactions are important for our understanding of the Al26 abundance in massive stars. The aim of this work is to report on a direct and simultaneous measurement of these astrophysically important reactions using an active target system. The reactions were investigated in inverse kinematics using He4 as the active target gas in the detector. We measured the excitation functions in the energy range of about 2 to 6 MeV in the center of mass. We have found that the cross sections of the Na23(α,p)Mg26 and the Na23(α,n)Al26 reactions are in good agreement with previous experiments and with statistical-model calculations. The astrophysical reaction rate of the Na23(α,n)Al26 reaction has been reevaluated and it was found to be larger than the recommended rate.

Free Access to the whole issue; International audience

Measurements of the excitation function for the fusion of 24Mg + 30Si (Q = 17.89 MeV) have been extended toward lower energies with respect to previous experimental data. The S-factor maximum observed in this large, positive-Q-value system is the most pronounced among such systems studied thus far. The significance and the systematics of an S-factor maximum in systems with positive fusion Q values are discussed. This result would strongly impact the extrapolated cross sections and reaction rates in the carbon and oxygen burnings and thus, the study of the history of stellar evolution.

Previous explanations for the resonance behavior of C12+C12 fusion at low energies were based on a nonresonant compound-nucleus background and an additional contribution from a series of resonances. This separation into "resonance" and "background" contributions of the cross section is artificial. We propose to explain this phenomenon through the impact on the cross section of the relatively large spacings and the narrow widths of Mg24 compound levels in the corresponding excitation-energy region.

The fusion excitation function of 32S+48Ca has been measured in a wide energy range, from above the Coulomb barrier down to cross sections in the sub-μb region. The excitation function has a smooth behavior below the barrier with a rather flat slope, and no maximum of astrophysical factor S vs. energy has been observed. However, other interesting features of the dynamics of this system can be noted. In particular, the fusion barrier distribution has an unusual shape with two peaks of similar height, lower and higher than the Akyu¨z-Winther barrier. Preliminary coupledchannels calculations and a comparison with nearby systems yield information on the possible influence of nucleon transfer channels with positive Q-value.

Fusion evaporation cross sections for the Al27+Sc45 (Q=9.63 MeV) system are measured down to about 300 nb. Deviations from standard coupled-channels calculations were observed in this system at the lowest energies. The steep fall-off of the fusion cross sections can be reproduced by calculations using a shallow potential model, which was originally developed to explain the hindrance behavior of heavy-ion fusion in medium-mass systems with negative Q values. Comparisons of the hindrance behavior between the present experiment and other systems, for example, Si28+Si30 (Q=14.3 MeV) and S36+Ca48 (Q=7.55 MeV) are presented. © 2010 The American Physical Society. Fil: Jiang, C. L.. Argonne National Laboratory; Estados Unidos Fil: Rehm, K. E.. Argonne National Laboratory; Estados Unidos Fil: Esbensen, H.. Argonne National Laboratory; Estados Unidos Fil: Back, B. B.. Argonne National Laboratory; Estados Unidos Fil: Janssens, R. V. F.. Argonne National Laboratory; Estados Unidos Fil: Collon, P.. University of Notre Dame; Estados Unidos Fil: Deibel, C. M.. Michigan State University; Estados Unidos. Argonne National Laboratory; Estados Unidos Fil: Digiovine, B.. Argonne National Laboratory; Estados Unidos Fil: Figueira, Juan Manuel. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Argonne National Laboratory; Estados Unidos. Laboratorio Tandar; Argentina Fil: Greene, J. P.. Argonne National Laboratory; Estados Unidos Fil: Henderson, D. J.. Argonne National Laboratory; Estados Unidos Fil: Lee, H. Y.. Los Alamos National Laboratory; Estados Unidos. Argonne National Laboratory; Estados Unidos Fil: Notani, M.. University of Notre Dame; Estados Unidos Fil: Marley, S. T.. Argonne National Laboratory; Estados Unidos. Western Michigan University; Estados Unidos Fil: Pardo, R. C.. Argonne National Laboratory; Estados Unidos Fil: Patel, N.. Argonne National Laboratory; Estados Unidos. Colorado School Of Mines; Estados Unidos Fil: Seweryniak, D.. Argonne National Laboratory; Estados Unidos Fil: Tang, X. D.. University of Notre Dame; Estados Unidos Fil: Ugalde, C.. Argonne National Laboratory; Estados Unidos Fil: Zhu, S.. Argonne National Laboratory; Estados Unidos

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.

Classical novae are a site of explosive nucleosynthesis where hydrogen rich material from a companion giant star accretes onto the surface of a white warf. Critical to our understanding of nova explosions are proton-capture reaction rates involved in the nucleosynthesis. While, ideally, all of the relevant (p,γ) reactions would be measured directly, in practice, such measurements are very challenging and are only possible in a few cases. This provides considerable scope for indirect measurements including transfer reactions, mass measurements, beta-decay and gamma- ray spectroscopy. The latter technique, until recently largely neglected as an input in nuclear astrophysics analyses, has clear advantages in locating resonances with high energy precision and assisting in determining the spin and parity of resonances. Such information is very valuable in a complementary approach to indirect determinations of key reaction rates.

An excitation function for the fusion reaction Si28 + Si30 (Q=14.3MeV) has been measured down to 40μb. Deviations from the behavior predicted by the optical model and standard coupled-channels calculations have been observed in this system. The fusion cross sections can be reproduced by a shallow potential model well, which was originally developed to explain the hindrance of heavy-ion fusion for systems with negative Q-values.

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.

The extrapolations of cross sections for fusion reactions involving C12 and O16 nuclei down to energies relevant for explosive stellar burning have been reexamined. Based on a systematic study of fusion in heavier systems, it is expected that a suppression of the fusion process will also be present in these light heavy-ion systems at extreme sub-barrier energies due to the saturation properties of nuclear matter. Previous phenomenological extrapolations of the S factor for light heavy-ion fusion based on optical model calculations may therefore have overestimated the corresponding reaction rates. A new "recipe" is proposed to extrapolate S factors for light heavy-ion reactions to low energies taking the hindrance behavior into account. It is based on a fit to the logarithmic derivative of the experimental cross section which is much less sensitive to overall normalization discrepancies between different data sets than other approaches. This method, therefore, represents a significant improvement over other extrapolations. The impact on the astrophysical reaction rates is discussed.

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.

A maximum of the S(E) factor is evidence for an onset of sub-barrier fusion hindrance and it can be well described by a radius-of-curvature expression near the maximum. The systematics of this radius of curvature has been studied over a wide range of projectile-target combinations. It follows a tentative general trend as a function of the parameter ζ=Z1Z2μ, and is strongly affected by effects associated with the nuclear structure of the nuclei in the entrance channel. It also explains the reason why the S factor maximum is not easily recognized visually for lighter, astrophysically interesting fusion systems.

The excitation function for the fusion-evaporation reaction 28Si + 64Ni has been measured down to a cross section of 25 nb. This is the first observation of fusion hindrance at extreme sub-barrier energies for a system with a small, negative Q-value (- 1.78 MeV). This result is further proof that heavy-ion fusion hindrance, reported earlier only for systems with large, negative Q-values, is a general phenomenon. The measured behavior can be reproduced by coupled-channels calculations with a modified ion-ion potential incorporating the effects of nuclear incompressibility.

The Ti44(t1/2=59yr) nuclide, an important signature of supernova nucleosynthesis, has recently been observed as live radioactivity by γ-ray astronomy from the Cas A remnant. We investigate in the laboratory the major Ti44 production reaction Ca40(α,γ)Ti44 (Ec.m.∼0.6-1.2MeV/u) by direct off-line counting of Ti44 nuclei. The yield, significantly higher than inferred from previous experiments, is analyzed in terms of a statistical model using microscopic nuclear inputs. The associated stellar rate has important astrophysical consequences, increasing the calculated supernova Ti44 yield by a factor ∼2 over previous estimates and bringing it closer to Cas A observations.

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 fusion excitation function for 64Ni + 64Ni down to a cross section level of 10 nb and have observed a strong fusion hindrance at extreme sub-barrier energies. The onset of the sub-barrier fusion hindrance in 64Ni + 64Ni was found to occur 8.4 MeV lower in center-of-mass energy, whereas a 2 MeV higher energy was expected on the basis of systematics. The effect was found to appear to be associated with the nuclear structure of the interacting nuclei. A strong dependence of the energy where the hindrance was also found to occur on the stiffness of the interacting nuclei.

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 astrophysically important reaction 14O(α,p)17F has been studied through a measurement of the time-inverse p(17F,α)14O reaction using a radioactive 17F beam. Resonance parameters for several states above an excitation energy of 7 MeV in 18Ne have been obtained. Through a measurement of the partial widths for elastic and inelastic proton scattering, it was determined that for these resonances the contribution of the 14O(α,p)17F* branch populating the first excited state in 17F is small. The results indicate that the contribution of resonances above Ex=7 MeV to the astrophysical (α,p) reaction rate is smaller than was previously assumed.

We have studied the [Formula Presented] reaction and have identified new states in [Formula Presented] at [Formula Presented] and [Formula Presented] Based on these measurements and other recent evidence, we suggest spin-parity assignments of [Formula Presented] for the 5.678 MeV state and [Formula Presented] for the 5.945 MeV state, which would account for all the “missing” unnatural parity states in [Formula Presented] in the excitation energy region important to hydrogen burning in novae. New reaction rates are presented for the [Formula Presented] reaction based on this possible assignment of states.

Cross sections for the 44Ti(α, p)47V reaction which significantly affects the yield of 44Ti in supernovae were measured in the energy range 5.7MeV ≤ Ec.m. ≤ 9 MeV, using a beam of radioactive 44Ti. The cross sections and the deduced astrophysical reaction rates are larger than the results from theoretical calculations by about a factor of 2. The implications of this increase in the reaction rate for the search of supernovae using space-based gamma detectors are discussed.

A method has been developed that can provide beams of many short-lived nuclei of interest in nucleosynthesis along the rp process path. With a 17F beam (T1/2=64s) the excitation function of the 17F(p, α)14O reaction was measured to determine properties of excited states in 18Ne. These states influence the rate of the 14O(α, p)17F reaction which is important for understanding energy generation and nucleosynthesis in x-ray bursts. The present direct measurements yield a pattern of resonances and cross sections which differ substantially from previous estimates.

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.