National Repository of Grey Literature 11 records found  1 - 10next  jump to record: Search took 0.01 seconds. 
Study of reaction cross-sections important for advanced nuclear systems
Chudoba, Petr ; Wagner, Vladimír (advisor) ; Krása, Antonín (referee) ; Krtička, Milan (referee)
The doctoral thesis is focused on measurements of neutron cross-sections mainly cross-sections of (n,xn) threshold reactions. With the dawn of advanced nuclear systems such as fast reactors of GenIV, accelerator driven system or even fusion reactors, monitoring of fast neutron fields will gain on importance. The activation detectors are one of the viable options how to monitor these neutron fields and (n,xn) reactions would be one of the best options how to do such monitoring. There is one condition though. We need to have good cross- section data in high neutron energy region. Unfortunately, the current situation is rather bad. There is only a small amount of reliable data for neutron energies above 20 MeV and with increasing energy, the situation gets only worse. For this reason, we measure the cross-sections of (n,xn) reactions on perspective materials such as yttrium, gold and tantalum with neutron energies from 17.4 up to 94 MeV. In this thesis, the measurements together with motivation and methodology are described. Obtained results are in agreement with existing data. Severe discrepancies were observed between the predictions and experimental data for high order (n,xn) reactions. This problem is discussed and solved. The results in this region are in form of cross-section values and also...
Measurements and usage of cross sections of various (n,chi n) threshold reactions
Chudoba, Petr ; Vrzalová, Jitka ; Svoboda, Ondřej ; Krása, Antonín ; Kugler, Andrej ; Majerle, Mitja ; Suchopár, Martin ; Wagner, Vladimír
Current trend in nuclear reactor physics is a transition from technologies using thermal neutrons to technologies utilizing fast neutrons. Unfortunately focus was put mainly on the thermal neutrons for a long time and lead to very good knowledge about this low energy region, but very scarce coverage of the high energy region. This means that there is a gap in the knowledge of excitation functions for higher energies. This gap spreads from 20 MeV up to 1 GeV and higher. This is exactly the energy region needed for description of advanced nuclear systems such as accelerator driven systems (ADS). Our group from Nuclear Physics Institute (NPI) of the CAS is a member of an international collaboration Energy & Transmutation of Radioactive Waste (E&T RAW). This collaboration focuses on ADS for many years. In order to measure neutron field within ADS models it is necessary to know excitation functions of reactions used to monitor the neutron field. In many cases there are almost no experimental data for suitable reactions. Worse and quite common case is that there are no data at all. Therefore we are also focusing on measurements of these data in order to fill the databases as well as to allow further improvements of codes for nuclear data calculations.
Two- neutrino double beta decay of 150Nd in the NEMO-3 experiment and scintillator characterization for the SUPER NEMO
Žukauskas, Aivaras ; Vorobel, Vít (advisor) ; Lokajíček, Miloš (referee) ; Wagner, Vladimír (referee)
The half-life of two-neutrino double beta decay of 150 Nd has been measured with data taken by the NEMO-3 experiment at the Modane Underground Laboratory. Using 1918.5 days of data recorded with 36.55 g of 150 Nd the half-life of this process is measured to be T2ν 1/2 = (8.99 ± 0.18 (stat.) ± 0.71 (syst.)) × 1018 y. SuperNEMO is the successor to NEMO-3 and will be one of the next generation of 0νββ experiments. It aims to measure 82 Se with an half-life sensitivity of 1026 yr corresponding to ⟨mββ⟩ < 50 - 100 meV. In order to achieve this target, a testing apparatus for the scintillator characterization has been developed and 140 scintillator blocks were characterized for the calorimeter of the SuperNEMO. It was determined that none of the blocks violate defined criteria of uniformity (± 3% for the energy resolution at 1 MeV) nor the criteria of energy resolution (< 16% at 1 MeV). The average uniformity of the blocks was found to be 0.7% and the average energy resolution of these blocks is 12% at 1 MeV. However, it was discovered during the visual tests that 23 out of 140 blocks contain more than 20 visible dust specks, thus alarming the possible contamination of these blocks with the natural radioactive isotopes.
Study of Interactions of Cosmic Rays at Ultra-high Energies
Nečesal, Petr ; Řídký, Jan (advisor) ; Vorobel, Vít (referee) ; Wagner, Vladimír (referee)
This thesis is dedicated to study of interactions of utra-high energy cosmic rays using measured data from the Pierre Auger Observatory, automatic alarm system called Shift Guard and analysis of correlation of temperature and detector response. The Pierre Auger Observatory is the largest experiment to study ultra-high energy cosmic rays. The assumed relation between temperature and fluorescence detector response is studied together with descriptive temperature analysis, which shows that the air-conditioning system is not able to stabilize temperature enough. The temperature influence on camera response is investigated in order to measure the calibration correction factor. Shift Guard - the alarm system dedicated to protect fluorescence detectors and data taking is introduced and its functionality is described. The alarm system informs shifters about some events and situations that can endanger detector or deteriorate data quality. The basic parameters and alarm system logic are described together with light and sound signals. The discrepancy between measured and predicted number of muons at ground level calculated by EAS generators is showed and possible sources of this disagree- ment are studied. The result of massive dark photons decay in EAS as one of exotic muon source is presented. The analysis...
Use of activation detectors for neutron field measurement in models of ADTS
Chudoba, Petr ; Wagner, Vladimír (advisor) ; Krtička, Milan (referee)
Accelerator driven transmutation systems could be a solution to the problem with long-lived nuclear waste and opening the way to thorium fuel cycle. Due to intensive neutron source based on spallation reaction are these systems very litle dependent on the arrangement of the core and fuel quality. These systems can transmute the spent fuel, eventually 232Th or 238U without affecting maintenance of fission reaction. Additionally subcritical blanket ensures high safety. For these systems it is necessary to know the cross sections of reactions of fast neutrons produced in the spallation reaction with different materials. This data is necessary not only for the selection of appropriate construction materials, but also for creation of programs simulating accelerator driven transmutation systems. This thesis is focused on the experimental determination of cross sections of reactions 89Y(n,2n)88Y and 89Y(n,3n)87Y with neutron energies from 17,6 to 33,6 MeV. Yttrium is analyzed for its (n,xn) threshold reactions, which makes it appropriet activation detector for study of the neutron fields in model configurations of accelerator driven transmutation systems. The obtained cross sections are unique, in that so far there are no experimental data for used neutron energies.
Production of neutrons in reaction of relativistic protons on thick or how to burn the nuclear waste by accelerator
Krása, V. ; Wagner, Vladimír ; Belina, L. ; Henzlová, D. ; Henzl, V. ; Kugler, Andrej ; Adam, Jindřich ; Čaloun, Pavel ; Krivopustov, M. I. ; Tumendelger, Th.
We look for the way how to burn the nuclear waste by the transmutation of radioactive nuclei by the fast neutrons produced by proton beam.
Heavy-ion collisions: The study of elliptic flow of neutral mesons in Au+Au reaction at 800 A MeV
Pleskač, Radek ; Kugler, Andrej ; Tlustý, Pavel ; Wagner, Vladimír ; Hlaváč, S.
The combination of the photon spectrometer TAPS - an electromagnetic calorimeter with good energy, time and spatial resolution - and the charged particle-detecting KAOS Forward Wall (FW) offers the possibility to study azimuthal distribution of neutral mesons relative to the reaction plane. The experimental data from the Au+Au reaction at 800 MeV were analyzed. The reaction plane was determined on an event by event by event basis and angular distribution of ă.sup.o./sup. and .eta. mesons was reconstructed. The observed elliptic flow signal is negative for .eta. mesons, indicating a preferred emission perpendicular to the reaction plane. The elliptic flow for ă.sup.o./sup. mesons is significantly observed in peripheral Au+Au collisions only.
Electron-positron pair production in nuclear matter - first data from HADES
Tlustý, Pavel ; Kugler, Andrej ; Pleskač, Radek ; Wagner, Vladimír ; Suk, M. ; Hlaváč, S. ; Žovinec, D.
Study of hadron properties inside nuclear matter at normal and high temperatures and baryon densities represents one of important topics of the present nuclear research. Several models predict change of effective masses and widths of hadrons due to the chiral symmetry restoration at extreme conditions of the nuclear matter. Existing experimental data do not allow unambiguous conclusions with respect to medium modifications due to the limited invariant mass resolution and separation. To improve and extend the existing experimental evidence the new High Acceptance DiElectron Spectrometer HADES was build at GSI, Darmstadt.

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2 Wagner, Vojtěch
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