Fotonové silové funkce v jádře 177Lu z měření dvoukrokových gamma kaskád

V předložené práci studujeme fotonové silové funkce, jejichž studium probíhá již více než padesát let. V průběhu této doby byla vytvořena celá řada modelů bud na základě čistě teoretických, či fenomenologických přístupu, které se pokoušejí popsat experimentální výsledky. Korektnost těchto modelů je stále diskutabilní a její ověření je v současnosti předmětem intenzivní experimentální i teoretické činnosti. Výsledky analýzy měření dvoukrokových kaskád doprovázejících záchyt tepelného neutronu na jádru 176Lu jsou porovnány s výstupy Monte Carlo simulace založené na předpokladu platnosti tzv. Extrémního Statistického Modelu. Porovnání výsledků zpracování experimentálních dat s výsledky získanými pomocí počítačových simulací se stává základním nástrojem studia korektnosti teoretických modelů. Námi analyzovaný experiment poskytuje informace zejména o E1 a M1 fotonové silové funkci, především pak o tzv. nůžkové rezonanci.The subject of the study made in this work are photon strength functions. Many models, both based on theoretical as well as phenomenological approaches, were proposed for these quantities, describing the gamma decay of the nucleus, during last 50 year. However, the correctness of these models is still questionable and its verification is the subject of intensive experimental and theoretical activity at the present time. The results of the analysis of measurements of the two-step cascades following thermal neutron capture on the 176Lu nucleus are compared with the outputs of the Monte Carlo simulations based on the validity of the so-called Extreme Statistical Model. Comparison of experimental data with outcomes of simulations thus becomes the basic tool for studying correctness of theoretical models. The experiment analyzed in the present work provides information mainly about E1 and M1 photon strength functions, especially about the so-called scissors resonance.Institute of Particle and Nuclear PhysicsÚstav částicové a jaderné fyzikyFaculty of Mathematics and PhysicsMatematicko-fyzikální fakult

Luminescence of free-standing versus matrix-embedded oxide-passivated silicon nanocrystals: The role of matrix-induced strain:

We collect a large number of experimental data from various sources to demonstrate that free-standing (FS) oxide-passivated silicon nanocrystals (SiNCs) exhibit considerably blueshifted emission, by 200 meV on average, compared to those prepared as matrix-embedded (ME) ones of the same size. This is suggested to arise from compressive strain, exerted on the nanocrystals by their matrix, which plays an important role in the light-emission process; this strain has been neglected up to now as opposed to the impact of quantum confinement or surface passivation. Our conclusion is also supported by the comparison of low-temperature behavior of photoluminescence of matrix-embedded and free-standing silicon nanocrystals

First Results of the 140^{140}Ce(n,γ)141^{141}Ce Cross-Section Measurement at n_TOF

An accurate measurement of the $^{140}$Ce(n,γ) energy-dependent cross-section was performed at the n_TOF facility at CERN. This cross-section is of great importance because it represents a bottleneck for the s-process nucleosynthesis and determines to a large extent the cerium abundance in stars. The measurement was motivated by the significant difference between the cerium abundance measured in globular clusters and the value predicted by theoretical stellar models. This discrepancy can be ascribed to an overestimation of the $^{140}$Ce capture cross-section due to a lack of accurate nuclear data. For this measurement, we used a sample of cerium oxide enriched in $^{140}$Ce to 99.4%. The experimental apparatus consisted of four deuterated benzene liquid scintillator detectors, which allowed us to overcome the difficulties present in the previous measurements, thanks to their very low neutron sensitivity. The accurate analysis of the p-wave resonances and the calculation of their average parameters are fundamental to improve the evaluation of the $^{140}$Ce Maxwellian-averaged cross-section

First Results of the 140^{140}Ce(n,γ)141^{141}Ce Cross-Section Measurement at n_TOF

An accurate measurement of the $^{140}$Ce(n,γ) energy-dependent cross-section was performed at the n_TOF facility at CERN. This cross-section is of great importance because it represents a bottleneck for the s-process nucleosynthesis and determines to a large extent the cerium abundance in stars. The measurement was motivated by the significant difference between the cerium abundance measured in globular clusters and the value predicted by theoretical stellar models. This discrepancy can be ascribed to an overestimation of the $^{140}$Ce capture cross-section due to a lack of accurate nuclear data. For this measurement, we used a sample of cerium oxide enriched in $^{140}$Ce to 99.4%. The experimental apparatus consisted of four deuterated benzene liquid scintillator detectors, which allowed us to overcome the difficulties present in the previous measurements, thanks to their very low neutron sensitivity. The accurate analysis of the p-wave resonances and the calculation of their average parameters are fundamental to improve the evaluation of the $^{140}$Ce Maxwellian-averaged cross-section

Nuclear Fussion and its Utilization in Energetics

Ústav částicové a jaderné fyzikyInstitute of Particle and Nuclear PhysicsFaculty of Mathematics and PhysicsMatematicko-fyzikální fakult

Studium statistických vlastností rozpadu jader

γ rozpad vysoce vzbuzených jaderných stavů lze popisovat v rámci statistického modelu jádra pomocí hustoty hladin a sady fotonových si- lových funkcí. Dobrá znalost těchto veličin umožuje přesnější výpočty účinných průřezů mnohých jaderných reakcí, které jsou důležité zejména v jaderné ast- rofyzice a při dalším vývoji jaderných reaktorů. Přestože jsou fotonové silové funkce studovány již řadu desetiletí, stále nejsou vyřešeny některé rozpory experimentálních výsledků o nízkoenergetickém chování dipólové síly. Jedním z takových případů je i otázka tvaru elektrické dipólové fotonové silové funkce a síly nůžkového módu v dobře deformovaných jádrech vzácných zemin. V této práci jsou prezentovány analýzy γ spekter měřených dvěma rozdílnými experimentálními technikami. Dvoukrokové γ kaskády byly naměřeny pro lichá gadoliniová terčíková jádra na výzkumném reaktoru LVR-15 v Centru výzkumu Řež. Vícekrokové γ kaskády z rezonančního záchytu neutronů na terčíkových jádrech 161−163 Dy byly naměřeny pomocí vysoce segmentovaného γ kalorimetru DANCE v Los Alamos Neutron Science Center, Los Alamos National Laboratory. Experimentální spektra byla porovnána se svými simu- lovanými...The γ decay of highly excited nuclear levels can be described within the statistical model of nucleus in terms of the level density and a set of photon strength functions. The knowledge of these quantities enables more accurate calculations of reaction rates in many different reactions which are important especially in nuclear astrophysics and in the development of advanced nuclear reactors. Despite the fact that the photon strength functi- ons have been studied for decades, there are still contradicting experimental results regarding the low energy behavior of dipole strength. One of these ca- ses is the shape of electric dipole photon strength function and the strength of the scissors mode in well-deformed rare-earth nuclei. In this thesis the ana- lyses of γ-ray spectra measured by two different experimental setups are pre- sented. The two-step γ cascades measurements with odd gadolinium targets were performed at the research reactor LVR-15 at the Research Centre Řež. In the multi-step γ cascades experiments the γ rays following resonance ne- utron capture on 161−163 Dy targets were measured with the highly-segmented γ-ray calorimeter Detector for Advanced Neutron Capture Experiments in the Los Alamos Neutron Science Center at Los Alamos National Labora- tory. Experimental spectra were compared...Ústav částicové a jaderné fyzikyInstitute of Particle and Nuclear PhysicsMatematicko-fyzikální fakultaFaculty of Mathematics and Physic

Photon strength functuins in 177Lu from two-step gamma cascade measurement

The subject of the study made in this work are photon strength functions. Many models, both based on theoretical as well as phenomenological approaches, were proposed for these quantities, describing the gamma decay of the nucleus, during last 50 year. However, the correctness of these models is still questionable and its verification is the subject of intensive experimental and theoretical activity at the present time. The results of the analysis of measurements of the two-step cascades following thermal neutron capture on the 176Lu nucleus are compared with the outputs of the Monte Carlo simulations based on the validity of the so-called Extreme Statistical Model. Comparison of experimental data with outcomes of simulations thus becomes the basic tool for studying correctness of theoretical models. The experiment analyzed in the present work provides information mainly about E1 and M1 photon strength functions, especially about the so-called scissors resonance

Nuclear Fussion and its Utilization in Energetics

Ústav částicové a jaderné fyzikyInstitute of Particle and Nuclear PhysicsFaculty of Mathematics and PhysicsMatematicko-fyzikální fakult

Study of statistical decay in well deformed rare-earth nuclei

The γ decay of highly excited nuclear levels can be described within the statistical model of nucleus in terms of the level density and a set of photon strength functions. The knowledge of these quantities enables more accurate calculations of reaction rates in many different reactions which are important especially in nuclear astrophysics and in the development of advanced nuclear reactors. Despite the fact that the photon strength functi- ons have been studied for decades, there are still contradicting experimental results regarding the low energy behavior of dipole strength. One of these ca- ses is the shape of electric dipole photon strength function and the strength of the scissors mode in well-deformed rare-earth nuclei. In this thesis the ana- lyses of γ-ray spectra measured by two different experimental setups are pre- sented. The two-step γ cascades measurements with odd gadolinium targets were performed at the research reactor LVR-15 at the Research Centre Řež. In the multi-step γ cascades experiments the γ rays following resonance ne- utron capture on 161−163 Dy targets were measured with the highly-segmented γ-ray calorimeter Detector for Advanced Neutron Capture Experiments in the Los Alamos Neutron Science Center at Los Alamos National Labora- tory. Experimental spectra were compared..