Vliv detekčního prahu na odezvu C6D6 detektorů

Scintilačné C6D6 detektory sa používajú na meranie účinného prierezu radiačného neutrónového záchytu v experimentálnych zariadeniach, ktoré fungujú na princípe metódy určovania doby letu (n_TOF v CERN, GELINA v IRMM Geel). Pri stanovení účinných prierezov pomocou metódy váhovej funkcie sa obvykle zanedbáva vplyv nedetekovateľnej energie, pochádzajúcej z dvoch javov - vnútornej elektrónovej konverzie a detekčného prahu pre nízke energie fotónov. Tento vplyv však v niektorých prípadoch nie je úplne zanedbateľný a doposiaľ kvôli malej znalosti gama rozpadov jadier nebol dostatočne skúmaný. Účelom tejto práce je zo simulácií spektier fotónov z radiačného záchytu neutrónov pozorovať vplyv týchto dvoch javov, pričom na testovanie boli použité jadrá 197 Au a 238 U.Scintillation C6D6 detectors are used for radiative capture cross-section measurements in various neutron time-of-flight facilities (n_TOF in CERN, GELINA in IRMM Geel). To determinate cross-section with the help of Pulse Height Weighting Technique we usually neglect influence of undetectable energy, derived from two phenomena - internal conversion and detection threshold for low photon energies. However, in some cases their impact cannot be completely omitted and due to lack of knowledge it has not been the subject of research yet. The purpose of this thesis is to study the influence of these two effects from simulations of neutron radiative capture induced photon spectra. Two nuclei: 197 Au and 238 U were used for testing.Institute of Particle and Nuclear PhysicsÚstav částicové a jaderné fyzikyMatematicko-fyzikální fakultaFaculty of Mathematics and Physic

Fotonové silové funkce v jádře 168Er z měření gamma kaskád detektorem DANCE

Subject of the thesis is study of photon strength functions describing the gamma decay of the nucleus. During last 50 years a number of theoretical models for these quantities were proposed, however, their accuracy is still de- batable and its verification has recently been a subject of intensive experimental and theoretical research. In this work measurement of multi-step gamma cas- cades following the radiative neutron capture on target nucleus 167 Er is used to analyse photon strength functions. The experiment was performed with DANCE calorimeter located in Los Alamos Neutron Scattering Center. The experimen- tal spectra are compared with Monte Carlo simulations of electromagnetic decay using the DICEBOX algorithm, based on assumptions of Extreme Statistical Model. Comparison of experimental and simulated spectra is a fundamental tool for studying correctness of theoretical models of photon strength functions. This study provides information mainly about E1 and M1 photon strength functions, especially properties of the so-called scissor mode are analysed.Predmetom tejto práce je štúdium fotónových silových funkcií popisujúcich gama rozpad jadier. V uplynulých päťdesiatich rokoch bolo navrhnutých mnoho teoretických modelov týchto veličín, avšak ich presnosť je otázna a jej overovanie je objektom intenzívneho experimentálneho a teoretického výskumu. V tejto práci k analýze fotónových silových funkcií slúži meranie viackrokových gama kaskád, ktoré sprevádzajú neutrónový záchyt na terčíkovom jadre 167 Er. Experiment bol uskutočnený pomocou kalorimetru DANCE, ktorý sa nachádza v Los Alamos Neutron Scattering Center. Experimentálne spektrá sú porovnané s Monte Carlo simuláciami elektromagnetického rozpadu pomocou DICEBOX algoritmu a sú založené na predpokladoch Extrémneho štatistického modelu. Porovnanie experimentálnych a simulovaných spektier je dôležitým nástrojom pre štúdium správnosti teoretických modelov fotónových silových funkcií. Táto práca poskytuje informácie hlavne o E1 a M1 fotónových silových funkciách, skúmané sú predovšetkým vlastnosti takzvaného nožnicového módu.Institute of Particle and Nuclear PhysicsÚstav částicové a jaderné fyzikyFaculty of Mathematics and PhysicsMatematicko-fyzikální fakult

Studium gamma rozpadu jádra 168Er z neutronového záchytu

The γ-ray emission process in highly-excited nuclei is typically described within the statistical model of nucleus using the two main ingredients - the level density (LD) and the photon strength functions (PSFs). The knowledge of LD and PSFs is essential in the modeling of nuclear reactions with applications in astrophysics or advanced nuclear reac- tors. In this work, we studied the multi-step γ-ray cascade spectra following the radiative neutron capture 167 Er(n, γ)168 Er measured with the Detector for Advanced Neutron Cap- ture Experiments in Los Alamos National Laboratory. The experimental γ-ray spectra were compared with the simulated spectra exploiting the dicebox Monte Carlo code for simulation of radiative decay to test different models of LD and PSFs. Furthermore, the measured spectra allowed us to determine the population of the isomeric state in 168 Er with 109 ns half-life and compare it with the simulated isomeric population, which provides an important test of the applicability of the statistical model. The simulated isomeric population was found to be significantly lower than the experi- mental one if no structure effects are assumed above the excitation energy of the isomer. If we adopt the decay scheme up to excitation energy well above 2 MeV, we obtain the simulated isomeric ratio...Proces emisie γ žiarenia vo vysoko excitovaných jadrách je typicky popisovaný pomo- cou štatistického modelu jadra, využívajúc dve hlavné veličiny - hustota hladín (LD) a fotónové silové funkcie (PSFs). Znalosť LD a PSFs je dôležitá pre simulovanie jadrových reakcií a pre aplikácie v astrofyzike a oblasti pokročilých jadrových reaktorov. V tejto práci sme študovali spektrá fotónov nasledujúcich po radiačnom neutrónovom záchyte 167 Er(n, γ)168 Er, ktorá bola meraná pomocou Detektoru pre pokročilé experimenty s neu- trónovým záchytom. Experimentálne γ spektrá boli porovnané so simulovanými spek- trami, ktoré boli modelované pomocou Monte Carlo kódu dicebox, určenému na simulo- vanie γ rozpadu, aby sme mohli otestovať rôzne modely LD a PSFs. Namerané spektrá nám taktiež dovolili určiť populáciu izomérneho stavu, ktorý má polčas rozpadu 109 ns a porovnať ju so simulovanou izomérnou populáciou, čo nám posky- tuje dôležitý test pre použiteľnosť štatistického modelu. Získaná simulovaná izomérna populácia je podstatne menšia než experimentálna, ak neuvažujeme žiadne štruktúrne efekty nad excitačnou energiou izoméru. Ak použijeme rozpadovú schému hladín do en- ergií viac než 2 MeV, dostaneme simulovanú populáciu porovnateľnú s experimentálnou. Tento výsledok naznačuje, že štruktúrne efekty môžu v 168 Er stále hrať...Institute of Particle and Nuclear PhysicsÚstav částicové a jaderné fyzikyMatematicko-fyzikální fakultaFaculty of Mathematics and Physic

Study of gamma decay in 168Er from neutron capture

The γ-ray emission process in highly-excited nuclei is typically described within the statistical model of nucleus using the two main ingredients - the level density (LD) and the photon strength functions (PSFs). The knowledge of LD and PSFs is essential in the modeling of nuclear reactions with applications in astrophysics or advanced nuclear reac- tors. In this work, we studied the multi-step γ-ray cascade spectra following the radiative neutron capture 167 Er(n, γ)168 Er measured with the Detector for Advanced Neutron Cap- ture Experiments in Los Alamos National Laboratory. The experimental γ-ray spectra were compared with the simulated spectra exploiting the dicebox Monte Carlo code for simulation of radiative decay to test different models of LD and PSFs. Furthermore, the measured spectra allowed us to determine the population of the isomeric state in 168 Er with 109 ns half-life and compare it with the simulated isomeric population, which provides an important test of the applicability of the statistical model. The simulated isomeric population was found to be significantly lower than the experi- mental one if no structure effects are assumed above the excitation energy of the isomer. If we adopt the decay scheme up to excitation energy well above 2 MeV, we obtain the simulated isomeric ratio..

Influence of the Detection Threshold on the Response of the C6D6 Detectors

Scintillation C6D6 detectors are used for radiative capture cross-section measurements in various neutron time-of-flight facilities (n_TOF in CERN, GELINA in IRMM Geel). To determinate cross-section with the help of Pulse Height Weighting Technique we usually neglect influence of undetectable energy, derived from two phenomena - internal conversion and detection threshold for low photon energies. However, in some cases their impact cannot be completely omitted and due to lack of knowledge it has not been the subject of research yet. The purpose of this thesis is to study the influence of these two effects from simulations of neutron radiative capture induced photon spectra. Two nuclei: 197 Au and 238 U were used for testing

Photon strength functions in 168Er from multi-step gamma cascade measurement at DANCE

Subject of the thesis is study of photon strength functions describing the gamma decay of the nucleus. During last 50 years a number of theoretical models for these quantities were proposed, however, their accuracy is still de- batable and its verification has recently been a subject of intensive experimental and theoretical research. In this work measurement of multi-step gamma cas- cades following the radiative neutron capture on target nucleus 167 Er is used to analyse photon strength functions. The experiment was performed with DANCE calorimeter located in Los Alamos Neutron Scattering Center. The experimen- tal spectra are compared with Monte Carlo simulations of electromagnetic decay using the DICEBOX algorithm, based on assumptions of Extreme Statistical Model. Comparison of experimental and simulated spectra is a fundamental tool for studying correctness of theoretical models of photon strength functions. This study provides information mainly about E1 and M1 photon strength functions, especially properties of the so-called scissor mode are analysed

Accurate measurement of the standard 235U(n,f) cross section from thermal to 170 keV neutron energy

An accurate measurement of the 235U(n,f) cross section from thermal to 170 keV of neutron energy has recently been performed at n_TOF facility at CERN using 6Li(n,t)4He and 10B(n,α)7Li as references. This measurement has been carried out in order to investigate a possible overestimation of the 235U fission cross section evaluation provided by most recent libraries between 10 and 30 keV. A custom experimental apparatus based on in-beam silicon detectors has been used, and a Monte Carlo simulation in GEANT4 has been employed to characterize the setup and calculate detectors efficiency. The results evidenced the presence of an overestimation in the interval between 9 and 18 keV and the new data may be used to decrease the uncertainty of 235U(n,f) cross section in the keV region

Fission program at n_TOF

Since its start in 2001 the n_TOF collaboration developed a measurement program on fission, in view of advanced fuels in new generation reactors. A special effort was made on measurement of cross sections of actinides, exploiting the peculiarity of the n_TOF neutron beam which spans a huge energy domain, from the thermal region up to GeV. Moreover fission fragment angular distributions have also been measured. An overview of the cross section results achieved with different detectors is presented, including a discussion of the 237Np case where discrepancies showed up between different detector systems. The results on the anisotropy of the fission fragments and its implication on the mechanism of neutron absorption, and in applications, are also shown

First results of the 241Am(n,f) cross section measurement at the Experimental Area 2 of the n_TOF facility at CERN

Feasibility, design and sensitivity studies on innovative nuclear reactors that could address the issue of nuclear waste transmutation using fuels enriched in minor actinides, require high accuracy cross section data for a variety of neutron-induced reactions from thermal energies to several tens of MeV. The isotope 241Am (T1/2= 433 years) is present in high-level nuclear waste (HLW), representing about 1.8 % of the actinide mass in spent PWR UOx fuel. Its importance increases with cooling time due to additional production from the β-decay of 241Pu with a half-life of 14.3 years. The production rate of 241 Am in conventional reactors, including its further accumulation through the decay of 241Pu and its destruction through transmutation/incineration are very important parameters for the design of any recycling solution. In the present work, the 241 Am(n,f) reaction cross-section was measured using Micromegas detectors at the Experimental Area 2 of the n_TOF facility at CERN. For the measurement, the 235U(n,f) and 238U(n,f) reference reactions were used for the determination of the neutron flux. In the present work an overview of the experimental setup and the adopted data analysis techniques is given along with preliminary results

Monte Carlo simulations and n-p differential scattering data measured with Proton Recoil Telescopes

The neutron-induced fission cross section of 235U, a standard at thermal energy and between 0.15 MeV and 200 MeV, plays a crucial role in nuclear technology applications. The long-standing need of improving cross section data above 20 MeV and the lack of experimental data above 200 MeV motivated a new experimental campaign at the n_TOF facility at CERN. The measurement has been performed in 2018 at the experimental area 1 (EAR1), located at 185 m from the neutron-producing target (the experiment is presented by A. Manna et al. in a contribution to this conference). The 235U(n,f) cross section from 20 MeV up to about 1 GeV has been measured relative to the 1H(n,n)1H reaction, which is considered the primary reference in this energy region. The neutron flux impinging on the 235U sample (a key quantity for determining the fission events) has been obtained by detecting recoil protons originating from n-p scattering in a C2H4 sample. Two Proton Recoil Telescopes (PRT), consisting of several layers of solid-state detectors and fast plastic scintillators, have been located at proton scattering angles of 25.07° and 20.32°, out of the neutron beam. The PRTs exploit the ΔE-E technique for particle identification, a basic requirement for the rejection of charged particles from neutron-induced reactions in carbon. Extensive Monte Carlo simulations were performed to characterize proton transport through the different slabs of silicon and scintillation detectors, to optimize the experimental set-up and to deduce the efficiency of the whole PRT detector. In this work we compare measured data collected with the PRTs with a full Monte Carlo simulation based on the Geant-4 toolkit