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Register a new userRole of {alpha}-cluster transfer in formation 20Ne+16O cross sections at backward angles
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The experimental angular distributions for 20Ne+16O elastic transfer are reanalyzed using different forms of potential both phenomenological and semi-microscopic. The significant increase in cross sections at backward hemisphere due to the contribution of alphacluster transfer is investigated using the distorted wave Born approximation (DWBA) method. The spectroscopic amplitude (SA) for the configuration 20Ne as consisting of 16O (core) and an alpha-particle orbiting this core at the different concerned energies is extracted. The agreement between the experimental data and theoretical calculations using the two considered approaches is reasonably good.
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Inelastic $alpha$ scattering on 16O is studied at 400 MeV by using an ice target. Near the 4-alpha breakup threshold of 14.4 MeV, a broad peak is observed at an excitation energy of 13.6+/-0.2 MeV with a width of 0.6+/-0.2 MeV. The spin-parity is estimated to be 0+ from the momentum-transfer dependence. The observed width is significantly larger than those of the neighboring 0+ states indicating a state with a well-developed alpha cluster structure. The magnitude of the cross section is sensitive to the density distribution of the constituent alpha clusters. The observed cross section is consistent with the theoretical prediction for the alpha cluster condensed state characterized by its dilute density distribution with a large root-mean-square radius of about 4.3 fm.
Background $alpha$-nucleus potentials play an essential role for the calculation of $alpha$-induced reaction cross sections at low energies in the statistical model. Uncertainties of these calculations are related to ambiguities in the adjustment of the potential parameters to experimental elastic scattering angular distributions (typically at higher energies) and to the energy dependence of the effective $alpha$-nucleus potentials. Purpose The present work studies cross sections of $alpha$-induced reactions for $^{64}$Zn at low energies and their dependence on the chosen input parameters of the statistical model calculations. The new experimental data from the recent Atomki experiments allow for a $chi^2$-based estimate of the uncertainties of calculated cross sections at very low energies. Method The recent data for the ($alpha$,$gamma$), ($alpha$,$n$), and ($alpha$,$p$) reactions on $^{64}$Zn are compared to calculations in the statistical model. A survey of the parameter space of the widely used computer code TALYS is given, and the properties of the obtained $chi^2$ landscape are discussed. Results The best fit to the experimental data at low energies shows $chi^2/F approx 7.7$ per data point which corresponds to an average deviation of about 30% between the best fit and the experimental data. Several combinations of the various ingredients of the statistical model are able to reach a reasonably small $chi^2/F$, not exceeding the best-fit result by more than a factor of 2. Conclusions The present experimental data for $^{64}$Zn in combination with the statistical model calculations allow to constrain the astrophysical reaction rate within about a factor of 2. However, the significant excess of $chi^2/F$ of the best-fit from unity asks for further improvement of the statistical model calculations and in particular the $alpha$-nucleus potential.
The $mathrm{^{16}O}(alpha, alpha^{prime})$ reaction was studied at $theta_{lab} = 0^circ$ at an incident energy of $textrm{E}_{lab}$ = 200 MeV using the K600 magnetic spectrometer at iThemba LABS. Proton and $alpha$-decay from the natural parity states were observed in a large-acceptance silicon-strip detector array at backward angles. The coincident charged particle measurements were used to characterize the decay channels of the $0_{6}^{+}$ state in $mathrm{^{16}O}$ located at $E_{x} = 15.097(5)$ MeV. This state is identified by several theoretical cluster calculations to be a good candidate for the 4-$alpha$ cluster state. The results of this work suggest the presence of a previously unidentified resonance at $E_{x}approx15$ MeV that does not exhibit a $0^{+}$ character. This unresolved resonance may have contaminated previous observations of the $0_{6}^{+}$ state.
We study the cluster structure of 20Ne and show that the available experimental data can be well described by a bi-pyramidal structure with D(3h) symmetry. Strong evidence for the occurrence of this symmetry comes from the observation of all nine expected vibrational modes (3 singly degenerate and 3 doubly degenerate) and of six (singly degenerate) double vibrational modes. 20Ne appears to be another example of the simplicity in complexity program, in which simple spectroscopic features arise out of a complex many-body system.
Background: alpha-nucleus potentials play an essential role for the calculation of alpha-induced reaction cross sections at low energies in the statistical model... Purpose: The present work studies the total reaction cross section sigma_reac of alpha-induced reactions at low energies which can be determined from the elastic scattering angular distribution or from the sum over the cross sections of all open non-elastic channels. Method: Elastic and inelastic 64Zn(a,a)64Zn angular distributions were measured at two energies around the Coulomb barrier at 12.1 MeV and 16.1 MeV. Reaction cross sections of the (a,g), (a,n), and (a,p) reactions were measured at the same energies using the activation technique. The contributions of missing non-elastic channels were estimated from statistical model calculations. Results: The total reaction cross sections from elastic scattering and from the sum of the cross sections over all open non-elastic channels agree well within the uncertainties. This finding confirms the consistency of the experimental data. At the higher energy of 16.1 MeV, the predicted significant contribution of compound-inelastic scattering to the total reaction cross section is confirmed experimentally. As a by-product it is found that most recent global alpha-nucleus potentials are able to describe the reaction cross sections for 64Zn around the Coulomb barrier. Conclusions: Total reaction cross sections of alpha-induced reactions can be well determined from elastic scattering angular distributions. The present study proves experimentally that the total cross section from elastic scattering is identical to the sum of non-elastic reaction cross sections. Thus, the statistical model can reliably be used to distribute the total reaction cross section among the different open channels.
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