Heavy-ion double charge exchange reactions as tools for 0bb decays. The 40Ca(18O,18Ne)40Ar reaction at 270 MeV by using MAGNEX

This study is inserted in a research line that aims to access the Nuclear Matrix Element (NME) involved in the half-life of the 0 bb decay, by measuring the cross sections of heavy-ion induced Double Charge Exchange (DCE) reactions with high accuracy. The basic point is that the initial and nal state of both 0 bb decay and DCE processes are the same. In addition, both processes pass through the same intermediate state and the transition operators have a similar mathematical structure. This work shows for the rst time experimental data on heavy-ion DCE reaction in a wide range of transferred momenta, with an acceptable statistical signi cance and good angular and energy resolution. In particular (18O,18Ne) reaction at 270 MeV incident energy on 40Ca target was investigated. In order to estimate the contribution of the concurrent channels the 40Ca(18O,18F)40K single charge exchange intermediate channel and the competing processes 40Ca(18O,20Ne)38Ar two-proton transfer and 40Ca(18O,16O)42Ca two-neutron transfer were also studied. The experiment was performed at Laboratori Nazionali del Sud (LNSINFN) in Catania using a 270 MeV energy 18O Cyclotron beam impinging on a 279 g/cm2 thick 40Ca target. The ejectiles were momentum analysed by the MAGNEX large acceptance magnetic spectrometer and detected by its focal plane detector. The energy spectra and angular distributions have been extracted. The data analysis of experimental results have established that the transition to 40Args: is dominated by the direct processes. Finally, an innovative technique to infer on the nuclear matrix elements by measuring the cross section of a double charge exchange nuclear reaction was proposed. The main assumption are that the DCE reaction is a twostep charge exchange and a surface localized process. The model adopted to describe the cross section of the DCE reaction consists in a generalization of the well-established factorization of the single charge-exchange cross section, valid under certain hypothesis, discussed in the thesis. Therefore, the cross section could be factorized in a nuclear structure term, containing the matrix elements, and a nuclear reaction one (unit cross section). Despite the used approximations, the extracted strength and nuclear matrix elements are reasonable within +-50%, signalling that the main physics content has been kept.