L. Calabretta

MACISTE: the forward spectrometer for heavy ions at LNS-Catania

The compact heavy ion spectrometer MACISTE is going to be realized at the Laboratorio Nazionale del Sud-Catania (LNS). It should allow one to resolve the single mass unit up to A/spl ap/70 and the single charge up to Z/spl ap/45 when operating on ions around 50 MeV/amu. Coupling this device with other multidetectors already installed at LNS will give access to a large series of experiments concerning the projectile-like and the quasi-fusion physics at intermediate energy.

Experimental investigation on the possible complexity in the potential energy surface of232Th

The reaction232Th(γ,f) is studied using bremsstrahlungγ-beams in the (5.45–6.85) MeV end point energy range. Fragments angular distributions are measured and yield components with opposite parities are separated. The unfolded cross sections have resonant behaviour which indicates the existence of low damped vibrational states. Furthermore, a near degeneracy forKπ=0+ andKπ= 0− resonances at ∼5.5 MeV and ∼6 MeV is observed. Data are analysed in terms of a double humped barrier configuration within the doorway state model for fission. For each component maxima with almost equal heights and a shallow intermediate minimum are determined. Moreover almost equal potential barriers withK=0 and opposite parities are found. This is a signature for mass-asymmetric shapes and seems a clear indication for a threehumped barrier configuration with a low inner maximum and a third mass asymmetric shallow minimum due to the fragmentation of the normal mass-asymmetric second maximum.

Measurements of subthreshold photofission angular distributions of238U and fission channel analysis

SummaryMeasurements of photofission fragment angular distributions of238U between 5.1 and 6.0 MeV are presented. As γ-source the intense bremsstrahlung beam from the Catania University Microtron is used. The results are analysed within the framework of the double-humped barrier model with a potential constructed with three smoothly joined parabolae and a damping term in the second well. The values of the fission barrier parameters for the involved 2+0, 1−0 and 1−11 channels are extracted and a comparison with the theoretical predictions and experimental results in the literature is made. Delayed-fission contributions for the involved channels are taken into account in the 1−1 channel parameter determination.RiassuntoSono presentate misure di distribuzioni angolari dei frammenti della fotofissione del238U tra 5.1 e 6.0 MeV. Come sorgente γ è stato usato l’intenso fascio di bremsstrahlung ottenuto dal microtrone dell’Università di Catania. I risultati sono analizzati nell’ambito di un modello a doppia barriera con un potenziale costruito con tre parabole tangenti ed un termine di smorzamento in seconda buca. Si estraggono i valori dei parametri di barriera per i canali coinvolti 2+ 0, 1−0, 1−1 e si fa un confronto con previsioni teoriche e risultati sperimentali esistenti in letteratura. Nella determinazione dei parametri per il canale 1−1 si tiene conto dei contributi da fissione ritardata per i canali coinvolti.

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.

Multicounter system for high-multiplicy γ-ray cascades: Performances and first experiment at the Laboratorio Nazionale del Sud

SummaryA multicounter system for high-multiplicity γ-ray cascade measurements has been designed. The detection system, consisting of 12 NaI(Tl) scintillation counters, is described. Tests concerning highmultiplicity cascade detection probability and cross-talk are discussed. Performances of the multiplicity filter in investigations on nuclei at very high spin produced in heavy-ion fusion are presented.RiassuntoÈ stato realizzato un sistema a molti rivelatori per misure di cascate γ di elevata molteplicità. Si descrive il sistema di rivelazione costituito da 12 contatori a scintillazione NaI(Tl). Sono presentate le prove riguardanti il cross-talk e la probabilità di rivelare cascate di elevata molteplicità. Sono descritte le prestazioni del filtro nello studio di nuclei ad alto spin prodotti in reazioni di fusione con ioni pesanti.

Coulomb Suppression Effects in the Proton-Proton Elastic Scattering Extracted from the 2 H(p,pp)n Reaction

The behaviour of the quasi-free p-p cross section is investigated in the framework of the Trojan Horse Method at relative enegies where the free p-p cross section is sensitive to the Coulomb interaction. For this reason the 2H(p,pp)n reaction was studied by using a proton beam energy of 6 MeV at Laboratori Nazionali del Sud, INFN, Catania. The experimental data have been compared with a simulation based on a Plane Wave Impulse Approximation approach.

Three-humped fission barrier transmission including vibrational damping

SummaryThe total penetrability through a three-humped fission barrier including vibrational damping is calculated by using an optical model for fission. The Bondorf’s stationary probability current theory is used for transitions among class-I, class-II and class-III phases. A method to calculate the partial-transmission coefficients is developed.RiassuntoLa penetrabilità totale attraverso una barriera di fissione a tre picchi, includente smorzamento vibrazionale, è calcolata usando un modello ottico per la fissione. Le transizioni tra le fasi di classe I, classe II e classe III sono trattate usando la teoria di Bondorf delle correnti stazionarie di probabilità. Si sviluppa un metodo per calcolare i coefficienti di trasmissione parziale.РеэюмеИспольэуя оптическую модель для деления, вычисляется полная проницаемость трех-горбого барьера деления, с учетом вибрационного эатухания. Для переходов между фаэами класса I, класса II и класса III испольэуется теория стационарного потока вероятности Бондорфа. Раэвивается метод вычисления парцальных козффициентов прохождения.

NUMEN project @ LNS: Status and perspectives

The aim of the NUMEN project is to access the Nuclear Matrix Elements (NME), involved in the half life of the neutrinoless double beta decay (0νββ), by measuring the cross sections of Heavy Ions (HI) induced Double Charge Exchange (DCE) reactions with high accuracy. First evidence of the possibility to get quantitative information about NME from experiments is shown in the reaction 40Ca(18O,18Ne)40Ar at 270 MeV, performed with MAGNEX spectrometer using Superconducting Cyclotron (CS) beams at INFN - Laboratory Nazionali del Sud (LNS) in Catania. Preliminary tests on 116Sn and 116Cd target are already performed. High beam intensity is the new frontiers for these studies.

NURE: An ERC project to study nuclear reactions for neutrinoless double beta decay

Neutrinoless double beta decay (0νββ) is considered the best potential resource to determine the absolute neutrino mass scale. Moreover, if observed, it will signal that the total lepton number is not conserved and neutrinos are their own anti-particles. Presently, this physics case is one of the most important research “beyond Standard Model” and might guide the way towards a Grand Unified Theory of fundamental interactions. Since the ββ decay process involves nuclei, its analysis necessarily implies nuclear structure issues. The 0νββ decay rate can be expressed as a product of independent factors: the phase-space factors, the nuclear matrix elements (NME) and a function of the masses of the neutrino species. Thus the knowledge of the NME can give information on the neutrino mass scale, if the 0νββ decay rate is measured. In the NURE project, supported by a Starting Grant of the European Research Council, nuclear reactions of double charge-exchange (DCE) will be used as a tool to extract information on the ββ NME. In DCE reactions and ββ decay, the initial and final nuclear states are the same and the transition operators have similar structure. Thus the measurement of the DCE absolute cross-sections can give crucial information on ββ matrix elements.

The NUMEN project @ LNS: Status and perspectives

The NUMEN project aims at accessing experimentally driven information on Nuclear Matrix Elements (NME) involved in the half-life of the neutrinoless double beta decay (0νββ), by high-accuracy measurements of the cross sections of Heavy Ion (HI) induced Double Charge Exchange (DCE) reactions. Particular attention is given to the (18O,18Ne) and (20Ne,20O) reactions as tools for β+β+ and β-β-decays, respectively. First evidence about the possibility to get quantitative information about NME from experiments is found for both kind of reactions. In the experiments, performed at INFN - Laboratory Nazionali del Sud (LNS) in Catania, the beams are accelerated by the Superconducting Cyclotron (CS) and the reaction products are detected the MAGNEX magnetic spectrometer. The measured cross sections are challengingly low, limiting the present exploration to few selected isotopes of interest in the context of typically low-yield experimental runs. A major upgrade of the LNS facility is foreseen in order to increase the experimental yield of at least two orders of magnitude, thus making feasible a systematic study of all the cases of interest. Frontiers technologies are going to be developed, to this purpose, for the accelerator and the detection systems. In parallel, advanced theoretical models will be developed in order to extract the nuclear structure information from the measured cross sections.