Theory of Single Charge Exchange Heavy Ion Reactions

The theory of heavy ion single charge exchange reactions is reformulated. In momentum space the reaction amplitude factorizes into a product of projectile and target transition form factors, folded with the nucleon-nucleon isovector interaction and a distortion coefficient which accounts for initial and final state ion-ion elastic interactions. The multipole structure of the transition form factors is studied in detail for Fermi-type non-spin flip and Gamow-Teller-type spin flip transitions, also serving to establish the connection to nuclear beta decay. The reaction kernel is evaluated for central and rank-2 tensor interactions. Initial and final state elastic ion-ion interaction are shown to be dominated by the imaginary part of the optical potential allowing to evaluate the reaction coefficients in the strong absorption limit, realized by the black disk approximation. In that limit the distortion coefficient is evaluated in closed form, revealing the relation to the total reaction cross section and the geometry of the transition form factors. It is shown that at small momentum transfer distortion effects reduce to a simple scaling factor, allowing to define reduced forward-angle cross section which is given by nuclear matrix elements of beta decay-type. The response function formalism is used to describe nuclear charge changing transitions. Spectral distributions obtained by a self-consistent HFB and QRPA approach are discussed for $\tau_\pm$ excitations of $^{18}O$ and $^{40}Ca$, respectively, and compared to spectroscopic data. The interplay of nuclear structure and reaction dynamics is illustrated for the single charge exchange reaction $^{18}O+^{40}Ca \to ^{18}F+^{40}K$ at $T_{lab}=270$ MeV

Clustering effects in the 6^6Li(p,3^3He)4^4He reaction at astrophysical energies

Background: The understanding of nuclear reactions between light nuclei at energies below the Coulomb barrier is important for several astrophysical processes, but their study poses experimental and theoretical challenges. At sufficiently low energies, the electrons surrounding the interacting ions affect the scattering process. Moreover, the clustered structure of some of these nuclei may play a relevant role on the reaction observables. Purpose: In this article, we focus on a theoretical investigation of the role of clustered configurations of $^6$Li in reactions of astrophysical interest. Methods: The $^6$Li(p,$^3$He)$^4$He reaction cross section is described considering both the direct transfer of a deuteron as a single point-like particle in Distorted Wave Born Approximation (DWBA), and the transfer of a neutron and a proton in second-order DWBA. A number of two- and three-cluster structure models for $^6$Li are compared. Results: Within the two-cluster structure model, we explore the impact of the deformed components in the $^6$Li wave-function on the reaction of interest. Within the three-cluster structure model, we gauge the degree of $\alpha$-d clustering and explicitly probe its role on specific features of the reaction cross section. We compare the energy trend of the astrophysical $S$ factor deduced in each case. Conclusions: Clustered $^6$Li configurations lead in general to a significant enhancement of the astrophysical factor in the energy region under study. This effect only originates from clustering, whereas static deformations of the ground-state configuration play a negligible role at very low energies

Theory of single-charge exchange heavy-ion reactions

The theory of heavy-ion single-charge exchange reactions is reformulated. In momentum space, the reaction amplitude factorizes into a product of projectile and target transition form factors, folded with the nucleon-nucleon isovector interaction. The multipole structure of the transition form factors is studied in detail for Fermi-type non-spin-flip and Gamow-Teller-type spin-flip transitions, also serving to establish the connection to nuclear β decay. The reaction kernel is evaluated for central and rank-2 tensor interactions. Initial- and final-state ion-ion elastic interactions are accounted for by a distortion coefficient. Since the ion-ion interactions are dominated by the imaginary part of the optical potentials, the distortion coefficients can be evaluated in the strong absorption limit. For a Gaussian potential form factor, the distortion coefficient is evaluated in closed form, revealing the relation to the total reaction cross section. It is shown that at small momentum transfer distortion effects reduce to a simple scaling factor, allowing us to define a reduced forward-angle cross section which is given by nuclear matrix elements of β decay type. Thus we introduce new unit cross sections, as those traditionally used with light projectiles for spectroscopic purposes, for heavy-ion charge-exchange reactions. Results are discussed for τ ± excitations of 18 O and 40 Ca , respectively. Spectral distributions of nuclear-charge-changing transitions are obtained by self-consistent Hartree-Fock-Bogolubov (HFB) and quasiparticle random phase approximation (QRPA) theory and compared to spectroscopic data. The interplay of nuclear structure and reaction dynamics is illustrated for the single-charge exchange (SCE) reaction 18 O + 40 Ca → 18 F + 40 K at T lab = 270 MeV, by performing full-scale numerical calculations of the SCE cross section. We also show that the latter compare rather well with the results obtained within the strong absorption limit, thus confirming the possibility to factorize the forward-angle cross section into intrinsic nuclear transition dynamics and reaction dynamics.Programa Horizonte 2020 de la Unión Europea.654002Ministerio de Ciencia, Innovación y Universidades de España y Fondos FEDER. FIS2017- 88410-

Integral measurement of the 12C(n, p)12B reaction up to 10 GeV

The integral measurement of the 12C(n, p)12B reaction was performed at the neutron time-offlight facility n TOF at CERN. The total number of 12B nuclei produced per neutron pulse of the n TOF beam was determined using the activation technique in combination with a time-of-flight technique. The cross section is integrated over the n TOF neutron energy spectrum from reaction threshold at 13.6 MeV to 10 GeV. Having been measured up to 1 GeV on basis of the 235U(n, f) reaction, the neutron energy spectrum above 200 MeV has been re-evaluated due to the recent extension of the cross section reference for this particular reaction, which is otherwise considered a standard up to 200 MeV. The results from the dedicated GEANT4 simulations have been used to evaluate the neutron flux from 1 GeV up to 10 GeV. The experimental results related to the 12C(n, p)12B reaction are compared with the evaluated cross sections from major libraries and with the predictions of different GEANT4 models, which mostly underestimate the 12B production. On the contrary, a good reproduction of the integral cross section derived from measurements is obtained with TALYS-1.6 calculations, with optimized parameters.European Atomic Energy Communitys (Euratom) Seventh Framework Programme FP7/2007-2011-CHANDA (No. 605203)Narodowe Centrum Nauki (NCN)-UMO-2012/04/M/ST2/00700Croatian Science Foundation-No. 168

A new computational technique for re-entry flow calculations based upon a shock-fitting technique for unstructured grids

An in-house developed, 2D/3D unstructured CFD solver has been extended to deal with a mixture of thermally perfect gases in chemical non-equilibrium. The Euler equations have been coupled with a state-to-state kinetic model for argon plasma. The spatial discretization uses compact stencil Residual Distribution Schemes and shock waves can be modelled using either shock-capturing or shock-fitting. Promising results have been obtained using the shock-fitting approach for a 2D hypersonic flow past the fore-body of a circular cylinder