${}^{18}\mathrm{O}$-induced single-nucleon transfer reactions on ${}^{40}\mathrm{Ca}$ at $15.3A$ MeV within a multichannel analysis

Background: Nucleon transfer reactions are selective tools for nuclear physics investigations. The theoretical and computational limits affecting in the past their data analysis could be nowadays surmounted thanks to the advent of methods with refined approximations and constraints, even when heavy-ion collisions are considered.

Purpose: Modern microscopic calculations of heavy-ion-induced transfer reactions combined with precise experimental data offer the chance for accurately testing different reaction models as well as the nuclear structure description of the involved nuclear states.

Method: Single proton and neutron transfer reactions were measured with the MAGNEX magnetic spectrometer for the ${}^{18}\mathrm{O}+{}^{40}\mathrm{Ca}$ system at $15.3A$ MeV. Excitation energy spectra and angular differential cross section distributions were extracted. The experimental results are compared with theoretical calculations performed in distorted wave and coupled channel Born approximation. The use of a coupled channel equivalent polarization potential to effectively describe the coupling effects affecting the initial state interaction is also considered. Spectroscopic amplitudes derived from a large-scale shell model with appropriate interactions adapted for the involved nuclei are employed.

Results: Our theoretical calculations are in good agreement with experimental data, without the need for any scaling factor, validating the adopted reaction and nuclear structure parameters. Moreover, under the present experimental conditions, a weak dependence of the obtained results on the choice of the reaction models was observed.

Conclusions: The good agreement between experimental and theoretical results validates the reliability of the parameter sets entering the calculations. They are extracted from or tested in complementary analyses of other reaction channels under the same experimental conditions. Such a multichannel approach represents the best option to pursue a solid, comprehensive, and model-independent description of the single-nucleon transfer reactions. The successful description of the present one-nucleon transfer data is also propaedeutic to the accurate assessment, under the same theoretical description, of higher-order transfer processes, like the sequential nucleon transfer mechanisms which are in competition with the direct charge exchange reactions.

Figure 1

Excitation energy spectrum for the (${}^{18}\mathrm{O}$, ${}^{17}\mathrm{O}$) one-neutron stripping reaction in the $5.0^{\circ }<{\theta }_{\mathrm{lab}}<5.5^{\circ }$ angular range. The visible structures up to $\approx 4$ MeV are fitted by a global curve (solid red line) given by the sum of five Gaussian functions; the first two (the diagonally shaded magenta and the horizontally shaded blue) correspond to single transitions while the third one (the vertically shaded cyan) corresponds to the excitation of two different ${}^{41}\mathrm{Ca}$ excited states (see Table 1). The auxiliary Gaussian functions that peak beyond 2 MeV (dashed green and dotted orange) are introduced to better reproduce the shape in such a region. The peak labeled with an asterisk is polluted by target contamination contributions.

Figure 2

Excitation energy spectrum for the (${}^{18}\mathrm{O}$, ${}^{19}\mathrm{F}$) one-proton pickup reaction in the $5.0^{\circ }<{\theta }_{\mathrm{lab}}<5.5^{\circ }$ angular range. The visible structures up to $\approx 5$ MeV are fitted by a global curve (solid red line) given by the sum of four Gaussian functions, each of them accounting for several different transitions (see Table 2 for the details). The peaks labeled with asterisks are polluted by the target contamination contributions.

Figure 3

Comparison between theoretical and experimental one-neutron transfer angular distributions. The different panels refer to the different peaks shown in the excitation energy spectrum of Fig. 1. (a, b) The $\mathrm{OM}\left(\mathrm{SPP}\right)+\mathrm{DWBA}$, $\mathrm{OM}\left(\mathrm{CCEP}\right)+\mathrm{DWBA}$ and CCBA(SPP) single-transition calculations are indicated by the dash-dotted blue, dashed green, and solid red lines, respectively. (c) The same symbol code is adopted for the total summed curves whereas the different single transitions, calculated in the CCBA(SPP) scheme, are reported as dotted lines.

Figure 4

Comparison between theoretical and experimental one-proton transfer angular distributions. The different panels refer to the different peaks shown in the excitation energy spectrum of Fig. 2. The sum of the OM(SPP)$+\mathrm{DWBA}$, $\mathrm{OM}\left(\mathrm{CCEP}\right)+\mathrm{DWBA}$, and CCBA(SPP) calculations are indicated by the dash-dotted blue, dashed green, and solid red lines, respectively. Single transitions as well as partial sums, calculated in the CCBA(SPP) scheme, are reported as dotted lines.

Figure 5

Coupling schemes for the projectile-ejectile and target-residual systems for the described (a) one-proton and (b) one-neutron transfer reactions. The solid blue arrows refer to the couplings considered only in the $\mathrm{OM}+\mathrm{DWBA}$ calculations. In the CCBA framework, instead, both the solid blue and the dashed red arrows are included.