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| Author(s): |
Marcelo Gimenez Del Santo
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Instituto de Física (IF/SBI) |
| Defense date: | 2009-02-10 |
| Examining board members: |
Marcelo Gameiro Munhoz;
Roberto Meigikos dos Anjos;
Valdir Guimaraes;
Takeshi Kodama;
Roberto Vicencotto Ribas
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| Advisor: | Marcelo Gameiro Munhoz |
| Abstract | |
Nuclear Astrophysics is the key to explain, among other things, the production of energy in stars, stellar evolution and the synthesis of chemical elements and isotopes in the Universe. In such cases, nuclear reactions are the main structure, where cross sections and reaction rates must be determined with reasonable accuracy in the laboratory. Because the extreme conditions found in stars, the understanding of nuclear processes that occur in their interiors have become a big challenge for theoretical and experimental nuclear physicists. In the last 40 years, physicists are getting the rates of these reactions but the uncertainty in these values are high due to difficulties found in the experimental cross section measurements at very low energies (Gamow peak region). Thus, only in some cases it is possible to measure directly the cross section and the behavior at low energies is usually extrapolated from the region of higher energy. To avoid the procedure of extrapolation, some indirect methods are being used successfully in recent years. In particular, the Trojan Horse Method gives the Astrophysics 7 S(E) factor of nuclear reactions involving charged particles at low energies without extrapolation and without electron screening effects. The reactions 10B(p,a)7Be and 11B(p,a)8Be are the main responsible for the burning process of boron inside F and G main sequence stars. The cross sections of these reactions have been obtained in previous direct experiments, but the data did not reach the Gamow peak and the behavior of the S(E) factor is then extrapolated from higher energies. In this work, we extract the S(E) factor for the reactions 10B(p,a)7Be and 11B(p,a)8Be through the indirect Trojan Horse Method (THM) applied to the three body reactions 2H(10B,a7Be)n e 2H(11B,a8Be)n without extrapolation. The astrophysical S(E)-factor for the 10B(p,a)7Be reaction was extracted by means of the THM and it is a factor 2 less in the Gamow peak if compared with previous direct studies. For the 11B(p,a)8Be reaction both a0 e a1 channels were studied by means of the THM and the astrophysical S(E)-factor extracted is in good agreement with direct previous studies. (AU) | |