Analysis of one-proton transfer reaction in ${}_{18}O+{}_{76}\mathrm{Se}$ collisions at 275 MeV

Background: A systematic exploration of one-nucleon transfer reactions induced by the (${}_{18}O,{}_{19}F$) and (${}_{18}O,{}_{17}O$) reactions on different targets (${}_{12}C, {}_{16}O, {}_{27}\mathrm{Al}, {}_{40}\mathrm{Ca}, {}_{48}\mathrm{Ti}, {}_{76}\mathrm{Se}, {}_{116}\mathrm{Sn}$) is being performed at the Istituto Nazionale di Fisica Nucleare–Laboratori Nazionali del Sud (INFN-LNS) at beam energies higher than Coulomb barrier. A featured aspect is the adoption of a multichannel reaction approach, where several quasielastic processes are studied consistently from both the experiment and theory sides. Resembling the case of light-ion induced direct reactions, for which a large amount of data exists, the multichannel heavy-ion direct reaction is a powerful tool to characterize nuclear mean field as well as few-nucleon correlations in low-lying nuclear states. In this view, the study of different reaction mechanisms and nuclear structure models helps to characterize the nuclear wave functions and accurately scrutinize the parameters that control the uncertainties in the calculations of nuclear matrix elements (NMEs). In this context, special attention is recently paid to NMEs involved in second-order isotensor processes such as double charge exchange (DCE) and neutrinoless double beta ($0\nu \beta \beta$) decay.

Purpose: We perform the experiment and the data analysis based on theoretical models of one-nucleon transfer reactions induced by the ${}_{18}O+{}_{76}\mathrm{Se}$ collision at energies above the Coulomb barrier in a multichannel approach. The ${}_{76}\mathrm{Se}$ nucleus attracts nowadays much interest since it is the daughter in the ${}_{76}\mathrm{Ge} \beta \beta$ decay, and the nuclear matrix elements involved in the ${}_{76}\mathrm{Se}_{g.s.}\to {}_{76}\mathrm{Ge}_{g.s.}$ and ${}_{76}\mathrm{Ge}_{g.s.}\to {}_{76}\mathrm{Se}_{g.s.}$ transitions are the same for time reversal symmetry. In particular, we intend to analyze transitions to low-lying excited states of the residual and ejectile nuclei in the ${}_{76}\mathrm{Se}({}_{18}O,{}_{19}F){}_{75}\mathrm{As}$ one-proton pickup reaction at 275 MeV incident energy by measuring the cross section. An additional goal is to determine the role of the coupling channels in the measured cross sections, testing different model descriptions of the involved nuclear states.

Methods: Nuclear reactions induced by the ${}_{18}O+{}_{76}\mathrm{Se}$ collision were measured at INFN-LNS using the MAGNEX large acceptance magnetic spectrometer for the detection of the ejectiles. The missing mass technique was used for the reconstruction of the reaction kinematics. The excitation energy spectrum and the differential cross section angular distributions were the key extracted observables. The experimental data were compared with theoretical calculations based on the distorted wave Born approximation, the coupled-channels Born approximation, and coupled reaction channels. The adopted spectroscopic amplitudes for the projectile and target overlaps were derived by large-scale shell-model and interacting boson-fermion model calculations. In the calculations the initial state interaction and the nuclear structure model inputs were the same as those adopted in the study of elastic and inelastic scattering and (${}_{18}O,{}_{17}O$) one-neutron stripping reaction, published elsewhere.

Results: Peaks in the cross section energy spectra corresponding to groups of transitions to ${}_{75}\mathrm{As}$ and ${}_{19}F$ were identified and the experimental angular distributions were compared with theoretical calculations. A fair agreement between theory and experiment both in cross section values and diffraction pattern is obtained, without the need for any scaling factor, validating the adopted reaction and nuclear structure approaches.

Conclusions: Resembling the case of the (${}_{18}O,{}_{17}O$) one-neutron stripping reaction, the couplings to the inelastic channels of projectile and target are significant for the one-proton pickup reaction and are likely to also play a role in the single and double charge exchange reactions. The fair description of the data is remarkable since no free parameter was used for this analysis, highlighting that the multichannel approach guarantees an accurate investigation of all the interesting reactions induced by the ${}_{18}O+{}_{76}\mathrm{Se}$ collision.