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Data: 29-mar-2013
Autori: Spartà, Roberta
Titolo: Indirect 2H(d,p)3H and 2H(d,n)3He fusion reactions measurement at energies relevant for Big Bang Nucleosynthesis
Abstract: The work presented in this thesis is focused on the experimental study of the two deuterium burning channels 2H(d,p)3H and 2H(d,n)3He in the Big Bang Nucleosynthesis (BBN) astrophysical scenario. In the beginning is presented this phase of the early universe and why the BBN model is still fine tuned with new results coming from observations and laboratory measurements. This model (if BBN is considered standard) can be adjusted such to have only a free parameter, the baryon to photon ratio of the universe eta at a certain time. Also the WMAP satellite results have been of great help to fix eta (and then all the others BBN numbers), but still persist some discrepancies, as for the lithium primordial abundances, between what is theoretically predicted and what is observed. The deuterium (and the reactions in which is involved) plays a key role in the evaluation of eta, thus for all BBN, so that is called the best baryometer. Then is explained that the need of new cross sections (and reaction rates) measurements for astrophysics can not be satisfied. This is because of the problems given by their measurements in the laboratory, as the presence of Coulomb barrier (that makes cross sections exponentially decrease in the energy range of interest) and the electron screening effect. For all these reasons the present measurements have been performed through the Trojan Horse Method (THM), an indirect method that allow to have a bare-nucleus cross section of the two-body reaction of astrophysical interest that is free of the Coulomb suppression. This is accomplished via the selection of the quasi-free mechanism in an appropriate three-body reaction, whose center-of-mass energy is greater than the Coulomb barrier. Two experimental runs have been carried out at the Nuclear Physics Institute of the Academy of Science of Czech Republic, in Rez (Prague). In the first one, with a 17 MeV 3He beam (in which only 2H(d,p)3H has been measured) the presence of quasi-free mechanism events has been ascertained. The result obtained is fair but not good enough for the error reduction needed for astrophysics. Instead, to optimize the result in the region relecant for astrophysics, a the second run (where the 3He beam energy was 18 MeV) has been performed. In particular for the first time the technique of measuring one of the two-body reactions participant ejectile and the spectator particle, in this case a proton, instead of both the ejectiles. This has also allowed the measure of the 2H(d,n)3He reaction without the complexity of the neutron detection, so with a very good precision. All the off-line analysis done until the S-factor extraction is detailed explained, including the MPWBA analysis by Dr. S. Typel. Also the screening potential has been evaluated, obtaining a value of 13.2±1.8 eV for 2H(d,p)3H and 11.7±1.6 eV for 2H(d,n)3He, very close to the adiabatic limit, as expected. A pole invariance test has been provided comparing present results with previous TH data, where the 6Li was used as TH nucleus. Reaction rates from present TH data for the two d+d channels, and from TH cross section of 3He(d,p)4He and 7Li(p,a)4He have been calculated. The new rates have been also compared with previous direct data compilations and with a new updated one that exclude questionable data sets. Using these new TH rates as input for the BBN code developed by prof. Bertulani, with eta fixed at the WMAP value, the primordial abundances have been obtained. These results are coherent with the whole model and will be soon compared with the observational results: a further analysis will provide stronger constraints on the values and a reduction of the involved uncertainties. This result reasserts that THM is a powerful tool for nuclear astrophysics and gives further validation to the BBN model.
InArea 02 - Scienze fisiche

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