M. Cavallaro

Heavy-ion double charge exchange reactions: A tool toward

Abstract.The knowledge of the nuclear matrix elements for the neutrinoless double beta decay is fundamental for neutrino physics. In this paper, an innovative technique to extract information on the nuclear matrix elements by measuring the cross section of a double charge exchange nuclear reaction is proposed. The basic point is that the initial- and final-state wave functions in the two processes are the same and the transition operators are similar. The double charge exchange cross sections can be factorized in a nuclear structure term containing the matrix elements and a nuclear reaction factor. First pioneering experimental results for the 40Ca(18O,18Ne)40Ar reaction at 270 MeV incident energy show that such cross section factorization reasonably holds for the crucial 0+

0 \nu\beta\beta

\rightarrow

nuclear matrix elements

0+ transition to 40Args, at least at very forward angles.

Study of the rainbow-like pattern in the elastic scattering of 16O on 27Al at Elab. = 100 MeV

Recently, a rainbow pattern in the elastic scattering of 16O + 27Al at Elab. = 100 MeV was reported. In the present paper, we show that the predicted change of slope of the cross section, as a function of angle, is mostly due to the far-side component of the scattering, which is affected by the inelastic couplings. The experimental data is consistent with the calculations up to the inflection point, where the effect of the couplings is significant. New experimental data, in very good agreement with the theoretical expectations around the Coulomb rainbow angle, are also presented.

The MAGNEX spectrometer: results and perspectives

Abstract.This review discusses the main achievements and future perspectives of the MAGNEX spectrometer at the INFN-LNS laboratory in Catania (Italy). MAGNEX is a large-acceptance magnetic spectrometer for the detection of the ions emitted in nuclear collisions below Fermi energy. In the first part of the paper an overview of the MAGNEX features is presented. The successful application to the precise reconstruction of the momentum vector, to the identification of the ion masses and to the determination of the transport efficiency is demonstrated by in-beam tests. In the second part, an overview of the most relevant scientific achievements is given. Results from nuclear elastic and inelastic scattering as well as from transfer and charge-exchange reactions in a wide range of masses of the colliding systems and incident energies are shown. The role of MAGNEX in solving old and new puzzles in nuclear structure and direct reaction mechanisms is emphasized. One example is the recently observed signature of the long searched Giant Pairing Vibration. Finally, the new challenging opportunities to use MAGNEX for future experiments are briefly reported. In particular, the use of double charge-exchange reactions toward the determination of the nuclear matrix elements entering in the expression of the half-life of neutrinoless double beta decay is discussed. The new NUMEN project of INFN, aiming at these investigations, is introduced. The challenges connected to the major technical upgrade required by the project in order to investigate rare processes under high fluxes of detected heavy ions are outlined.

Enhancement of the two neutron transfer channel in 18O induced reactions at 84 MeV

A study of the yields for different reaction channels has been performed at the Catania INFN-LNS laboratory using a 18O beam on 13C and 12C targets. The ejectiles have been momentum analyzed by the MAGNEX magnetic spectrometer. The achieved mass resolution (about 1/160) has allowed to identify the reaction products corresponding to different reaction channels. The measured yields show an enhancement of two neutrons transfer channel compared to one. This result demonstrates that the (18O, 16O) reaction proceeds mainly by the direct transfer of the neutron pair, with small contributions from second order processes.

(18O,18Ne) double charge-exchange with MAGNEX

An experimental study concerning Double Gamow-Teller (DGT) modes in (18O,18Ne) Double Charge-Exchange reactions has been very recently performed at INFN-LNS laboratory in Catania. The experiment was performed using a 40Ca solid target and a 18O Cyclotron beam at 270 MeV incident energy. Charged ejectiles produced in the reaction were momentum analyzed and identified by MAGNEX spectrometer at very forward angles. Preliminary results are presented in the present paper.

The NUMEN project: NUclear Matrix Elements for Neutrinoless double beta decay

Abstract.The article describes the main achievements of the NUMEN project together with an updated and detailed overview of the related R&D activities and theoretical developments. NUMEN proposes an innovative technique to access the nuclear matrix elements entering the expression of the lifetime of the double beta decay by cross section measurements of heavy-ion induced Double Charge Exchange (DCE) reactions. Despite the fact that the two processes, namely neutrinoless double beta decay and DCE reactions, are triggered by the weak and strong interaction respectively, important analogies are suggested. The basic point is the coincidence of the initial and final state many-body wave functions in the two types of processes and the formal similarity of the transition operators. First experimental results obtained at the INFN-LNS laboratory for the 40Ca(18O,18Ne)40Ar reaction at 270MeV give an encouraging indication on the capability of the proposed technique to access relevant quantitative information. The main experimental tools for this project are the K800 Superconducting Cyclotron and MAGNEX spectrometer. The former is used for the acceleration of the required high resolution and low emittance heavy-ion beams and the latter is the large acceptance magnetic spectrometer for the detection of the ejectiles. The use of the high-order trajectory reconstruction technique, implemented in MAGNEX, allows to reach the experimental resolution and sensitivity required for the accurate measurement of the DCE cross sections at forward angles. However, the tiny values of such cross sections and the resolution requirements demand beam intensities much larger than those manageable with the present facility. The on-going upgrade of the INFN-LNS facilities in this perspective is part of the NUMEN project and will be discussed in the article.

Preliminary Study of Two‐Neutron States via the (18O,16O)Reaction at 84 MeV

A study of light neutron‐rich nuclei has been performed at the INFN‐LNS laboratory in Catania (Italy) by the (18O, 16O) reaction at 84 MeV incident energy on 9Be, 11B and 13C targets. The 16O ejectiles have been momentum analyzed and detected at forward angles by the MAGNEX magnetic spectrometer. The energy spectra of 15C show several known low lying states up to about 7 MeV excitation energy and unknown resonant structures at higher excitation energy. The strong excitation of these latter together with the measured width of about 2 MeV FWHM could indicate the presence of collective modes of excitation connected to the transfer of a correlated neutron pair. Also considerations regarding the reaction mechanism and the nuclei involved in the transfer seem to confirm such a conclusion. Similar features characterize the energy spectra of the 11Be and 13B nuclei. The achieved mass resolution (about 1/160) guarantying the identification of the reaction products corresponding to different reaction channels has a...

A NEW COOLING TECHNIQUE FOR TARGETS OPERATING UNDER VERY INTENSE BEAMS

The NUMEN experiment aims to study Double Charge Exchange nuclear reactions at low energy as complementary information of the Neutrino-less Double Beta Decay half-life, for the evaluation of the Majorana effective neutrino mass. Special targets of Sn, Cd, Te, Ge and Se are needed, and their thickness must be in the range between 200 and 500nm, in order to satisfy the energy resolution requirements of the produced ions. The very low cross-section requires high intensity of the beams; thus, the energy loss becomes a constant heat source in the target region illuminated by the beam spot. To avoid the increase of the target temperature up to the melting point, some way must be found to dissipate the heat outside the target. The solution adopted in previous experiments at much lower intensity (thin films of isotopes supported by metal frames) is not suitable in this case. Solving the heat equation in steady state condition and cylindrical coordinates gives temperatures higher than the materials melting point, due to the low internal conductivity of the targets. Therefore, we designed a new cooling technique, which takes advantage of the electron stripper located beyond the target. We changed the stripper material (usually Mylar) in pyrolytic graphite of equivalent thickness (≈10µm), which becomes the substrate of the isotope deposition. Due to the presence of discontinuities in the material and heat source geometry, the temperature equation has been numerically solved. Thanks to the high internal conductivity of the graphite, the results are promising. Concerning the deposition, the goal is twofold: to obtain an isotope film with uniform density, and to create a good degree of adhesion to the graphite substrate. After the first trials with Sn, heating up the substrate during the deposition seems to provide reliable and satisfactory results.