Summary: The growing demand for energy has increased interest in the development of nuclear fission reactors. Whether intended to improve the performance of reactors currently in operation (U Cycles of Th), as if they were looking for a procedure for the transmutation of radioactive waste long half-life, there is a need for more precise data on sections effective the interaction of neutrons on minor actinides and fission products long half-life. Currently available data in databases nuclear (ENDF, JENDL, ...) show large discrepancies and both OCDE-NEA as the IAEA has developed a series of recommendations on the most relevant data to be measured within this context, including the U-234, which is the main objective of this thesis work. The work presented in this thesis deals with the measures undertaken within the partnership n_TOF of CERN, for the purpose of obtaining accurate data on sections effective fission of these isotopes of uranium, with a beam of neutrons very high resolution in energy, in the range extended from 1 eV to 1 GeV. 1. Basics of fission induced by the fission neutrons is an extraordinarily complex process in which the collective behavior of the components of a núleo heavily deformed produce nuclear breaking into two lighter nuclei of comparable masses, which we call fission fragments (FF) . These FF occur with mass distributions and kinetic energies that are dependent on energy neutron incident. Also depending on the neutron energy of the incident, there was a certain angular distribution of the FF. In Chapter 2 of this report discusses these concepts, only deepened those points which are relevant to the interpretation of the data obtained in this work. 2. The experimental device. The work has led to this doctoral thesis has been done at the facilities of CERN (Geneva, Switzerland) called n_TOF. It has been used a pulsed beam proton accelerator with a PS energy 20 GeV / c, one of 4-7E +12 proton intensity and duration of the pulse of only 7 ns rms The proton beam impact on a target in the lead, by spallation, there is a pulse of neutrons. The main feature of n_TOF was to produce a pulsed beam of high-intensity neutron with a white spectrum of energies in the range 1 eV to 1 GeV. His alt'i sima intensity makes it unique in the world. The beam, due colimado, addresses the pilot area over a vacuum tube, approximately 185 m in length, so that the higher energy neutrons arrive earlier than those with less. So, by measuring the time of flight, it can have a precise determination of the energy of the neutrons that make an impact on the target, which has placed a sample of nuclei whose sections are effective want to analyze. The detectors used have been specially designed and built for this experiment. They are kind of gaseous detectors PPAC (Parallel Plate Avalanche Counter), whose main features are: high-speed signal and greatly reduced downtime, which enables the accurate detection of fragments fissión in coincidence with high rates of accounting and strong rejection of noise-answer almost proportional to the energy deposited, making it easy to discriminate the pulses produced by the stimulated emission of alpha particles - high resistance to ionizing radiation; possibility of covering large areas of detection at low cost; section little effective interaction with gamma radiation, allowing measurements at times very close to the initial pulse, is 8 say, has 120f ta maximum energy of the neutrons incident-the little mass interposed in the flow of neutrons can measure up to nine targets simultaneously. The sophisticated data acquisition system, based converters rapid analgico to digital (FADC) has been specifically developed for collaboration n_TOF, taking advantage of the large bandwidth and data storage, as well as the computing power available at CERN . 3. The data reduction. An important part of the work consists of a description of the procedure specially developed to transform the information recorded by the detectors that were used in n_TOF to obtain the values of the rates of reaction, defined as the fraction of neutrons affecting samples and produced fission reactions. To define an event fission is needed in coincidence detection of the two fragments. To that end, both digitizers as individual electrodes of the PPACs used have been properly calibrated to get the close temporal resolutions required. 4. Procedures for obtaining sections effective. The determination of absolute values of sections effective is a very complicated task. In our case we get measures, taking advantage of the fact that we can put several targets simultaneously under the same neutron flux. Most of the discussion on how to obtain effective sections focuses on knowing the efficiency of detection with the experimental device based on the PPACs, which is mainly determined by its aceptancia angle, the loss of events due to the absorption in the device and by signals cathodes in some small extent not registered with the data acquisition system and change in the angular distribution of fission fragments with energía.Esta latter has been studied in detail obtaining a measure of the anisotropy in the issuance of fragments for each blanco.5. Results Sections effective neutron-induced fission of U-234 that have been obtained in this work are shown in Capí Title 6, for the entire range of energí ace n_TOF. The measures are derived measures, a standardized assessment of ENDF for energies between 1 and 4 MeV. The results have been studied in detail by dividing the cross section into three regions: Region resonances resolved, the region of the fission threshold to 20 MeV (maximum energy to which data are evaluated for this kernel) and the region above the 20 MeV, compared with parametrizations extracted data sections effective induced protones.En the range of low energy resolution at the extraordinary measure that will allow the resonances soon conduct a new analysis of the parameters of the resonances improving measured by James et al. In year
1977. Conclusions The objective of this study was to measure the cross section of induced fission neutrons in the range energí as from 1 eV to hundreds of MeV. It has developed a new method suitable for the treatment of data from the detectors PPAC and the data acquisition system for the installation n_TOF including a detailed study of the behavior of the detectors on the particular operating conditions of the facility. In addition to the extent of the section effective, it has also been able to measure the angular anisotropy in the distribution of fission fragments emitted in the reaction fission. These actions have proved the ability of the detectors to measure sections PPAC effective until radioactive nuclei energies as high as hundreds of MeV, depending on which is planning further steps in extending the n_TOF beginning in the year 2006.
