Study of neutron transmission on 154,155,157Gd at neutron time-of-flight facility GELINA

The measurements of transmission on 154,155,157Gd for energies below 100 eV are discussed in this thesis. This study aimed at reducing the uncertainty to provide very accurate cross section data relevant for nuclear technology, nuclear medicine and detector development. In particular, I have analysed the transmission measurements carried out at the neutron time-of-flight facility GELINA at the EC-JRC-Geel (Belgium). The measurements were performed under different neutron irradiation conditions, using 9 different samples in order to minimize systematic uncertainties related to the samples, which was the main limitation in previous experiments. The samples analysed in this work are five isotopically enriched 154,155,157Gd metallic disks used in a previous capture measurement at the n_TOF facility at CERN and four samples consisting in an alloy of magnesium with a 0.25%wt of natGd. Transmission experiments were carried at a measurement station located at 10 m from the neutron source. I have performed the data reduction of these transmission measurements using dedicated software programs. The resulting cross sections data for 154,155,157Gd will be delivered to the publicly available experimental nuclear reaction database EXFOR

Misura della sezione d'urto delle reazioni nucleari 155,157Gd(n,g) e loro impatto sulle tecnologie nucleari

In questo lavoro di tesi sono presentati i primi risultati delle misure delle sezioni d'urto di cattura neutronica degli isotopi dispari del gadolinio 155Gd e 157Gd e le loro incertezze ottenute presso la facility n_ToF del CERN. Queste misurazioni sono state effettuate nel range di energie compreso tra l'energia termica e il MeV. Nella prima parte di questo elaborato verranno esposte le motivazioni scientifiche e tecnologiche alla base di tale esperimento, le cui applicazioni spaziano dall'ambito energetico a quello medico-sanitario. Verranno successivamente descritte la struttura e il funzionamento della facility di ricerca e le tecniche sperimentali ivi utilizzate, con particolare interesse sui nuovi rivelatori che utilizzano come liquido scintillatore il benzene deuterato. Verranno infine esposti i dati sperimentali ottenuti, a partire dalla calibrazione energetica di tali rivelatori fino alla stima del segnale di fondo, dato di fondamentale importanza al fine di poter diminuire l'incertezza sulla misura della sezione d'urto

NEAR: A New Station to Study Neutron-Induced Reactions of Astrophysical Interest at CERN-n_TOF

We present NEAR, a new experimental area at the CERN-n_TOF facility and a possible setup for cross section measurements of interest to nuclear astrophysics. This was recently realized with the aim of performing spectral-averaged neutron-capture cross section measurements by means of the activation technique. The recently commissioned NEAR station at n_TOF is now ready for the physics program, which includes a preliminary benchmark of the proposed idea. Based on the results obtained by dedicated Monte Carlo simulations and calculation, a suitable filtering of the neutron beam is expected to enable measurements of Maxwellian Averaged Cross Section (MACS) at different temperatures. To validate the feasibility of these studies we plan to start the measurement campaign by irradiating several isotopes whose MACS at different temperatures have recently been or are planned to be determined with high accuracy at n_TOF, as a function of energy in the two time-of-flight measurement stations. For instance, the physical cases of 88Sr(n,γ ), 89Y(n,γ ), 94Zr(n,γ ) and 64Ni(n,γ ) are discussed. As the neutron capture on 89Y produces a pure β -decay emitter, we plan to test the possibility to perform activation measurements on such class of isotopes as well. The expected results of these measurements would open the way to challenging measurements of MACS by the activation technique at n_TOF, for rare and/or exotic isotopes of interest for nuclear astrophysic

Presolar Grain Isotopic Ratios as Constraints to Nuclear and Stellar Parameters of Asymptotic Giant Branch Star Nucleosynthesis

Recent models for evolved low-mass stars (with M ≲ 3 M⊙), undergoing the asymptotic giant branch (AGB) phase assume that magnetic flux-tube buoyancy drives the formation of 13C reservoirs in He-rich layers. We illustrate their crucial properties, showing how the low abundance of 13C generated below the convective envelope hampers the formation of primary 14N and the ensuing synthesis of intermediate-mass nuclei, like 19F and 22Ne. In the mentioned models, their production is therefore of a purely secondary nature. Shortage of primary 22Ne has also important effects in reducing the neutron density. Another property concerns AGB winds, which are likely to preserve C-rich subcomponents, isolated by magnetic tension, even when the envelope composition is O-rich. Conditions for the formation of C-rich compounds are therefore found in stages earlier than previously envisaged. These issues, together with the uncertainties related to several nuclear physics quantities, are discussed in the light of the isotopic admixtures of s-process elements in presolar SiC grains of stellar origin, which provide important and precise constraints to the otherwise uncertain parameters. By comparing nucleosynthesis results with measured SiC data, it is argued that such a detailed series of constraints indicates the need for new measurements of weak-interaction rates in ionized plasmas, as well as of neutron-capture cross sections, especially near the N = 50 and N = 82 neutron magic numbers. Nonetheless, the peculiarity of our models allows us to achieve fits to the presolar grain data of a quality so far never obtained in previously published attempts