Dissertations / Theses on the topic 'HPGe spectrometry'

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Dissertacao (Mestrado)
IPEN/D
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Dissertation in Environmental Physics to be publicly examined in Häggsalen (Ångström Laboratory), Uppsala University, on Friday, November 8, 2002 at 10:00 am for the degree of doctor of philosophy in Physics. The examination will be conducted in English.

Gamma spectrometry is one of the tools commonly used for the measurement of various environmental radionuclides. Simultaneous determination of the absolute activity of gamma emitting radiotracers in a wide range of environmental matrices and fractions necessitates proper and accurate evaluation of the sample-to-detector efficiency. Several radiotracers require, in addition, the use of sub-routines for self-absorption corrections.

Gamma spectrometry is an important and elegant tool for assessing environmental changes. Optimisation of ultra low-level gamma spectrometry for reliable assessment of such changes requires harmonisation of laboratory needs with sampling and site conditions.

Different aspects of the calculation of sample-to-detector efficiencies using empirical and Monte Carlo approaches are discussed here, including the uncertainties related to the simulation of the performance of different HPGe detectors and the effects of the incomplete collection of charges in Ge-crystals. Various simulation codes for the computation of peak efficiencies in planar and well Ge-detectors have been developed from scratch. The results of the simulations have been tested against experimental data and compared to other simulation results obtained with the Monte Carlo N-Particle code (MCNP). The construction of calibration sources with improved absorption and collimation characteristics have been, also, described in this work. These sources have been especially designed for the efficiency calibration of Ge-detectors at energies below 100 keV.

Flexible, fully tested and prototype approaches for the evaluation of self-absorption corrections, based on Monte Carlo simulations, are described. Special consideration is given to the problems related to the sample's variability in size, density and composition. Several examples of the absolute and simultaneous determination of environmental multitracers which benefited from self-absorption corrections and the optimised efficiency calibration algorithms are, also, presented and discussed. These examples include, among other things, a comprehensive analysis of the gamma spectrometry of ²³⁴Th in a wide range of matrices and the speciation of several radionuclides in sediments from a hard-water lake.

A quantificação da atividade de radionuclídeos emissores de raios gama em amostras medidas por espectrometria gama com detectores HPGe depende da análise dos fotopicos presentes no espectro, especialmente da determinação exata das suas áreas líquidas. Tal análise é geralmente realizada com o auxílio de ferramentas de software proprietário. Este trabalho apresenta uma metodologia, descrição de algoritmos e um aplicativo de código aberto, denominado OpenGamma, para a busca e análise de fotopicos a fim de se obter seus parâmetros relevantes e as atividades dos radionuclídeos na amostra. A implementação computacional é distribuída sob licença aberta para o código principal e com o uso de pacotes de software aberto para o projeto da interface e para bibliotecas matemáticas. O procedimento para a busca de picos é realizado em três etapas. Primeiramente, executa-se uma pesquisa preliminar com o método da segunda diferença, que consiste na geração de um espectro derivado para a busca de picos candidatos. Na segunda etapa, calculam-se as larguras experimentais dos picos, dos quais escolhem-se aqueles bem formados e isolados para a obtenção da função de ajuste da largura vs. canal, por meio do método de Levenberg-Marquardt para ajustes não-lineares. Por fim, regiões do espectro com picos agrupados são delimitadas e novo ajuste não-linear é aplicado a cada região para a obtenção dos termos da linha de base e dos fotopicos. A partir destes termos, são calculadas as áreas líquidas dos picos. Para a determinação da atividade, curvas de calibração de eficiência previamente obtidas, bem como dados de transições gama dos radionuclídeos, são incorporadas ao código e aplicadas. O código foi escrito em linguagem C++ e a interface foi desenvolvida com a ferramenta Qt. Para as funções matemáticas e procedimentos de ajuste, foi usada a biblioteca científica GNU (GSL). A validação de código foi feita por meio da análise de: 1) espectros sintéticos de teste da AIEA, especialmente desenhados para a avaliação de desempenho de software; 2) espectros obtidos com amostras de exercícios de intercomparação e 3) espectros da rotina de trabalho do Laboratório de Radiometria Ambiental (LRA) do IPEN e analisados com o aplicativo comercial InterWinner. Os resultados obtidos são consistentes com os valores de referência e com aqueles obtidos pelo aplicativo citado, sugerindo que o código OpenGamma pode ser utilizado com segurança na espectrometria de raios gama de uso geral.
Radioactivity quantification of gamma-ray emitter radionuclides in samples measured by HPGe gamma-ray spectrometry relies on the analysis of the photopeaks present in the spectra, especially on the accurate determination of their net areas. Such a task is usually performed with the aid of proprietary software tools. This work presents a methodology, algorithm descriptions and an open source application, called OpenGamma, for the peak search and analysis in order to obtain the relevant peaks parameters and radionuclides activities. The computational implementation is released entirely in open-source license for the main code and with the use of open software packages for interface design and mathematical libraries. The procedure for the peak search is performed on a three step approach. Firstly a preliminary search is done by using the second-difference method, consisting in the generation of a derived spectrum in order to find candidate peaks. In the second step, the experimental peaks widths are assessed and well formed and isolated ones are chosen to obtain a FWHM vs. channel relationship, by application of the Levenberg-Marquardt minimization method for non-linear fitting. Lastly, regions of the spectrum with grouped peaks are marked and a non-linear fit is again applied to each region to obtain baseline and photopeaks terms; from these terms, peaks net areas are then assessed. For the activity determination, previously obtained efficiency calibration curves, as well as nuclides gamma-ray data, are incorporated in the code and applied. The code was developed in C++ language and the interface was developed with Qt GUI software toolkit. GNU scientific library, GSL, was employed to perform fitting procedures as needed. Validation of code was done by analyzing: 1) synthetic test spectra from IAEA, especially designed for software performance evaluation; 2) spectra obtained from samples of intercomparison exercises and 3) spectra from routine activities of the Environmental Radiometric Laboratory (LRA) at IPEN and analyzed with the commercial software package InterWinner. Results obtained are consistent with the reference values and with those obtained by the aforementioned package. The results suggest that the OpenGamma code could be safely used in general-purpose gamma-ray spectrometry.

The work covered by this thesis was carried out at the Brasimone ENEA Research Centre, the National Agency for New Technologies, Energy and Sustainable Economic Development, with the aim of optimising a gamma spectrometry system in anticoincidence for the detection of noble gases, in particular the radioactive isotopes of xenon 131mXe, 133Xe, 133mXe and 135Xe. The laboratory of the ENEA Research Center of Brasimone, where the experimental apparatus is found to carry out the measurements of 131mXe, 133Xe, 133mXe and 135Xe, collaborates constantly with the monitoring network and is able to provide, if necessary, data and analysis on noble gases. The signals produced by the interaction of cosmic rays that manage to pass the screen have been recognized as the main cause of the increase of the detector background because they give rise to the Compton continuum and, as a result, they increase the value of detectable MDA. For this reason, a system in anticoincidence has been developed through the use of two plastic scintillators, placed over the shielding of the Hpge detector, which sends pulses recording within a gate located in the germanium multichannel analyzer: at the time the signal arrives from the scintillator, the gate blocks data acquisition to avoid recording pulses generated by cosmic radiation. For both configurations of the system, therefore, both with the anti-coincidence apparatus inactive and in operation, energy, FWHM and efficiency calibrations had to be carried out using a certified multi-peak source. The solution proposed, in conclusion of the thesis, to eliminate any electronic interference with the efficiency of the detector provides for the replacement of the current electronic apparatus with a new fully digitized, made by the company CAEN, Nuclear Electronic Construction Equipment.

The environmental radioactivity monitoring programs start in the late 1950s of the 20th century following the global fallout from testing of nuclear weapons in the atmosphere, becoming a cause of concern regarding health effects. Later, the necessity of world industrialization for new energy sources led to develop national plans on electricity production from nuclear technology, initializing in this context world wide exploration for fuel minerals: uranium exploration gained a particular attention in late 1940's in USA, Canada and former USSR and in 1951 in Australia with respective national plans. Nowadays there are about 440 nuclear power plants for electricity generation with about 70 more NPP under construction giving rise to the nuclear emergency preparedness of a large number of states (like Radioactivity Environmental Monitoring (REM) data bank and EUropean Radiological Data Exchange Platform (EURDEP). Furthermore, a lot of applications in the field of geosciences are related to the environmental radioactivity measurements going from geological mapping, mineral exploration, geochemical database construction to heat -flow studies. Gamma-ray spectroscopy technique is widely used when dealing with environmental radioactivity monitoring programs. The purpose of this work is to investigate the potentialities that such a technique offers in monitoring radioactivity concentration through three different interventions in laboratory, in-situ and airborne measurements. An advanced handling of gamma-ray spectrometry method is realized by improving the performances of instruments and realizing and testing dedicated equipments able to deal with practical problems of radioactivity monitoring. For each of these gamma-ray spectrometry methods are faced also the problems of calibration, designing of monitoring plans and data analyzing and processing. In the first chapter I give a general description for the common radionuclides present in the environment having a particular interest for monitoring programs. Three categories of environmental radionuclides classified according to their origin as cosmogenic, primordial and man-made are discussed. The cosmic rays continuously produce radionulides and also direct radiation, principally high energetic muons. Cosmogenic radionuclides are originated from the interaction of cosmic rays with stable nuclides present in the Earth’s atmosphere. Primordial radionuclides are associated with the phenomenon of nucleosynthesis of the stars and are present in the Earth’s crust. Man-made radionuclides commonly present in natural environments are principally derived from radioactive fallout from atmospheric nuclear weapons testing and peaceful applications of nuclear technology like nuclear power plants for electricity generation and the associated nuclear fuel cycle facilities. A relevant contribution, generally with local implication comes from the so called non-nuclear industries which are responsible for technologically enhancement of natural radioelements producing huge amounts of naturally occurring radioactive materials (NORM/TENORM). In the second chapter is described a homemade approach to the solution of the problem rising in monitoring situations in which a high number of samples is to be measured through gamma-ray spectrometry with HPGe detectors. Indeed, in such cases the costs sustaining the manpower involved in such programs becomes relevant to the laboratory budget and sometimes becomes a limitation of their capacities. Manufacturers like ORTEC® and CANBERRA produce gamma-ray spectrometers supported by special automatic sample changers which can process some tens of samples without any human attendance. However, more improvements can be done to such systems in shielding design and detection efficiency. We developed a fully automated gamma-ray spectrometer system using two coupled HPGe detectors, which is a well known method used to increase the detection efficiency. An alternative approach on shielding design and sample changer automation was realized. The utilization of two coupled HPGe detectors permits to achieve good statistical accuracies in shorter time, which contributes in drastically reducing costs and man power involved. A detailed description of the characterization of absolute full-energy peak efficiency of such instrument is reported here. Finally, the gamma-ray spectrometry system, called MCA_Rad, was used to characterize the natural radioactivity concentration of bed-rocks in Tuscany Region, Italy. More than 800 samples are measured and reported here together with the potential radioactivity concentration map of bed rocks in Tuscany Region. In the third chapter is described the application of portable scintillation gamma -ray spectrometers for in-situ monitoring programs focusing on the problems of calibration and spectrum analysis method. In-situ γ-ray spectrometry with sodium iodide scintillators is a well developed and consolidated method for radioactive survey. Conventionally, a series of self-constructed calibration pads prevalently enriched with one of the radioelements is used to calibrate this portable instrument. This method was further developed by introducing the stripping (or window analysis) described in International Atomic Energy Agency (IAEA) guidelines as a standard methods for natural radioelement exploration and mapping. We realized a portable instrument using scintillation gamma-ray spectrometers with sodium iodide detector. An alternative calibration method using instead well-characterized natural sites, which show a prevalent concentration of one of the radioelements, is developed. This procedure supported by further development of the full spectrum analysis (FSA) method implemented in the non-negative least square (NNLS) constrain was applied for the first time in the calibration and in the spectrum analysis. This new approach permits to avoid artifacts and non physical results in the FSA analysis related with the χ2 minimization process. It also reduces the statistical uncertainty, by minimizing time and costs, and allows to easily analyze more radioisotopes other than the natural ones. Indeed, as an example of the potentialities of such a method 137Cs isotopes has been implemented in the analysis. Finally, this method has been tested by acquiring gamma Ombrone -ray spectra using a 10.16 cm×10.16 cm sodium iodide detector in 80 different sites in the basin, in Tuscany. The results from the FSA method with NNLS constrain have been compared with the laboratory measurements by using HPGe detectors on soil samples collected. In the forth chapter is discussed the self-construction of an airborne gamma-ray spectrometer, AGRS_16.0L. Airborne gamma-ray spectrometry (AGRS) method is widely considered as an important tool for mapping environmental radioactivity both for geosciences studies and for purposes of radiological emergency response in potentially contaminated sites. Indeed, they have been used in several countries since the second half of the twentieth century, like USA and Canada, Australia, Russia, Checz Republic, and Switzerland. We applied the calibration method described in the previous chapter using well -characterized natural sites and implemented for the first time in radiometric data analysis FSA analysis method with NNLS constrain. This method permits to decrease the statistical uncertainty and consequently reduce the minimum acquisition time (which depend also on AGRS system and on the flight parameters), by increasing in this way the spatial resolution. Finally, the AGRS_16.0L was used for radioelement mapping survey over Elba Island. It is well known that the natural radioactivity is strictly connected to the geological structure of the bedrocks and this information has been taken into account for the analysis and maps construction. A multivariate analysis approach was considered in the geostatistical interpolation of radiometric data, by putting them in relation with the geology though the Collocated Cokriging (CCoK) interpolator. Finally, the potential radioelement maps of potassium, uranium and thorium are constructed for Elba Island.

L'esperimento JUNO è stato proposto con l'obiettivo principale di risolvere il problema relativo all'ordinamento di massa dei neutrini attraverso misure accurate del flusso di antineutrini prodotto da reattori nucleari. A causa della sezione estremamente ridotta dei neutrini, il numero di eventi segnale previsti è molto piccolo, circa 60 eventi IBD al giorno, ed è quindi essenziale tenere sotto controllo il tasso di eventi di fondo. Ciò può essere ottenuto minimizzando tutte le sorgenti che contribuiscono alla produzione di eventi spuri e in primo luogo quelle generate dal fondo radioattivo. Per ogni capostipite delle catene naturali (U238 e Th232), per il 40K, e per alcuni nuclidi chiave, come 60Co e 210Pb, è necessario imporre forti limiti alla concentrazione che può essere presente all'interno dei materiali del rivelatore. Data la struttura del rivelatore JUNO, il materiale più critico è il liquido scintillante per il quale sono richieste concentrazioni di uranio e torio inferiori a 1E-15 g/g e potassio inferiori a 1E-16 g/g. In questa tesi, presento il lavoro che ho svolto in questo contesto con due scopi principali. Il primo è la validazione del software Monte Carlo dell'esperimento JUNO applicato alle simulazioni di fondo con l'obiettivo di verificare i limiti di radiopurezza imposti per i materiali e determinare il background-budget dell'esperimento. La seconda è l'implementazione di una tecnica di misura che permette di raggiungere le sensibilità richieste per la misura del contenuto di uranio, torio e potassio nel liquido scintillante. La validazione del software Monte Carlo dell'esperimento JUNO (SNiPER) è stata effettuata confrontandone i risultati con quelli di altri due codici di simulazione, in particolare con il software Arby, sviluppato presso l'Università degli studi di Milano-Bicocca. Ho potuto studiare diversi aspetti e molte criticità della simulazione del fondo e dei risultati riportati dallo strumento ufficiale, come l'applicazione del quenching factor e la forma degli spettri beta. Gli spettri dell'energia depositata prodotta dalle contaminazioni nei componenti principali del rivelatore JUNO sono stati calcolati con i codici Monte Carlo. Il tasso di eventi indotti nel rivelatore è stato valutato in base ai limiti di radiopurezza imposti, ottenendo il rate totale di eventi di fondo previsto. Il valore ottenuto è inferiore al limite imposto per garantire la sensibilità finale dell'esperimento. Ciò ha consentito di correggere e validare la risposta del software ufficiale dell'esperimento JUNO e di verificare l'attualità dei limiti di radiopurezza inizialmente definiti per i componenti del rivelatore. Durante il mio dottorato di ricerca ho completato lo sviluppo del nuovo sistema di misura, chiamato GeSparK che sfrutta la coincidenza tra uno scintillatore liquido e un rivelatore HPGe per ridurre il fondo del rivelatore HPGe singolo. È stata anche sviluppata una nuova tecnica di coincidenza ritardata che sfrutta la struttura nucleare del 239Np, il prodotto di attivazione del 238U, al fine di ottenere un marcatore estremamente forte di questo particolare decadimento e aumentare significativamente la sensibilità di misura rispetto all'approccio tradizionale. La sensibilità ottenuta era ancora insufficiente rispetto alle richieste di JUNO e per questo si decise di attuare una serie di trattamenti radiochimici. Diversi trattamenti sono stati proposti, testati e implementati con i due obiettivi di aumentare la massa del campione misurabile e ridurre la concentrazione di nuclidi interferenti. La tecnica sviluppata per uranio e torio prevede una fase di estrazione liquido-liquido e di estrazione cromatografia con resine UTEVA e TEVA rispettivamente prima e dopo l’irraggiamento. Due misurazioni condotte su campioni "bianchi" con la procedura finale hanno permesso di ottenere una sensibilità compatibile con i limiti imposti da JUNO per lo scintillatore liquido a livello del ppq.
The JUNO experiment was proposed with the main aim of solving the problem related to the neutrino mass ordering through accurate measurements of the antineutrinos flow produced by nuclear reactors. Due to the extremely small cross-section of neutrinos, the number of expected signal events is very small, about 60 IBD events per day, and it is therefore essential to keep under control the rate of background events. This can be achieved by minimizing all the sources that contribute to the generation of spurious events and in the first place those generated by the radioactive background. For each progenitor of the natural chains (U238 and Th232), for the 40K, and for some key nuclides, such as 60Co and 210Pb, it is necessary to impose strong limits on the concentration that may be present within the materials of the detector. Given the structure of the JUNO detector, the most critical material is the liquid scintillator for which uranium and thorium concentrations below 1E-15 g/g and potassium below 1E-16 g/g are required. In this thesis, I present the work I did in this context with two main purposes. The first one is the validation of the Monte Carlo software of the JUNO experiment applied to the background simulations with the aim of verifying the radiopurity limits imposed for the materials and determining the background budget of the experiment. The second one is the implementation of a measurement technique that allows reaching the sensitivities required for the measurement of the content of uranium, thorium, and potassium in the liquid scintillator. The validation of the Monte Carlo software of the JUNO experiment (SNiPER) was performed by comparing its results with those of two other simulation codes, in particular with the software Arby, developed at the University of Milano-Bicocca. I was able to study different aspects and many critical issues of the simulation of the background and the results reported by the official tool, such as the application of the quenching factor and the shape of the radioactive β-decay spectra. The spectra of the deposited energy produced by the contaminations in the main components of the JUNO detector were computed with the Monte Carlo codes. The rate of events induced in the detector was assessed based on the imposed radiopurity limits, obtaining the expected total background event rate. The value obtained is lower than the limit set to ensure the final sensitivity of the experiment. This allowed correcting and validating the answer of the official software of the JUNO experiment and verifying the actuality of the radiopurity limits initially defined for the components of the detector. During my Ph.D. I completed the development of the new measurement system, called GeSparK that exploits the coincidence between a liquid scintillator and an HPGe detector to reduce the background of the single HPGe detector. I also worked on the development of a new delayed coincidence technique that exploits the nuclear structure of 239Np, the activation product of 238U, in order to obtain an extremely strong marker of this particular decay and significantly increase the measurement sensitivity compared to the traditional approach. The sensitivity obtained was still insufficient compared to the requests of JUNO and for this reason, it was decided to implement a series of radiochemical treatments. Different treatments have been proposed, tested, and implemented with the two aims of increasing the mass of the measurable sample and reducing the concentration of interfering nuclides. The technique developed for uranium and thorium involves a liquid-liquid extraction phase and the extraction chromatography with UTEVA and TEVA resins respectively before and after irradiation. Two measurements conducted on "blank samples” with the final procedure allowed us to achieve a sensitivity that is compatible with the limits imposed by JUNO for the liquid scintillator at the ppq level.

Since 1896, when Henri Becquerel discovered that penetrating radiation was given off in the radioactive decay of uranium, the studies on radioactivity have been an interest of scientific world. With the spread of nuclear technologies applied to energy, health and industrial production, the theme of environmental radioactivity monitoring increasingly is becoming important to the policies of the health public protection both national and European level. Italy is required to comply with the recommendation of the European Commission of 8 June 2000 on the application of Article 36 of the Euratom Treaty concerning the monitoring of levels of radioactivity in the environment for the purpose of assessing the exposure of the population as a whole. In addition, the World Health Organization has identified the first group of carcinogens gas 222Rn, which is considered the second leading cause, after smoking, of lung tumors. In our environment there are various sources of radioactivity that can be natural or artificial origin. Gamma-ray spectrometry is a widely used and powerful method that can be employed both to identify and quantify radionuclides. The purpose of this work is calibration and performances of in situ a portable gamma ray spectrometer. In the first chapter I have given the necessary concepts for understanding the phenomenon of radioactivity. Qualitatively has been described the process of radioactive decay and its three types which can occur in nature. Three categories of environmental radionuclides, cosmogenic, primordial and man-made are discussed. We are exposed to environmental radiation from different sources. The origin of radioactivity in the environment can be divided into two main sources: (a) natural and (b) man-made sources. Mostly the naturally occurring radiation arises from terrestrial radioactive nuclides that are widely distributed in the earth’s crust and extra-terrestrial sources arising from cosmic ray. Also from human activities arise some other sources concerned with the use of radiation and radioactive materials from which releases of radionuclides into the environment may occur. In the second chapter is described the gamma radiation interacts with matter via three main processes: the photoelectric effect, Compton scattering and pair production The operation of a detector is based on the interaction of photons constituting the incident radiation with the material that constitutes the detector itself.. Thanks to these processes, all or part of the energy possessed by the radiation is transferred to the mass of the detector and then converted into an electrical signal. The basic notions related to the interaction of electromagnetic radiation with matter that we will provide in this chapter will therefore be useful to understand the mechanisms that are at the basis of the generation of a gamma spectrum. In addition, this chapter will briefly describe the two main types of gamma radiation detectors, i.e. the semiconductor detector and the scintillation, in particular the high-pure germanium detector (HPGe) and a sodium iodide detector activated by thallium NaI(Tl). In the third chapter is described the study area in which are performed the measurements of natural radioactivity. The area under consideration is the Ombrone basin located in southern Tuscany and Commune of Schio located in Region of Veneto. During the campaign were acquired in situ 338 spectra, including 80 with the ZaNaI_1.0L placed on the ground (Ombrone), 80 with the ZaNaI_1.0L placed on a tripod at 1m height (Ombrone), 89 with the ZaNaI_1.0L placed on the ground (Schio) and 89 spectra are acquired with a backpack placed on the shoulders of an operator (Schio). In each of the 80 sites which have been realized the measurements of radioactivity with the ZaNaI_1.0L instrument, also have been taken 5 different soil samples, for a total of 400 samples. The abundances of 40K, 238U and 232Th were obtained from the analysis of 338 spectra taken with the ZaNaI_1.0L and 400 spectra measured on soil samples in the laboratory with a high-pure germanium detector (MCA_Rad). Also it is described the procedures of ZaNaI_1.0L portable scintillation gamma-ray spectrometers for in-situ measurements. In the fourth chapter is described the procedure for the preparation of soil samples to be analyzed with the MCA_Rad system. The gamma-ray spectrometry system, called MCA_Rad introduces an innovative configuration of a laboratory high-resolution gamma-ray spectrometer featured with a complete automation measurement process, which can conduct measurements on each type of material (solid, liquid or gaseous) in less than 1 hour. The utilization of two coupled HPGe detectors permits to achieve good statistical accuracies in shorter time, which contributes in drastically reducing costs and man power involved. It is made a description of the characterization of absolute full-energy peak efficiency of such instrument reported here. In the fifth chapter are discussed the correlations between the abundances of 40K, 238U and 232Th measured with the ZaNaI_1.0L and those obtained from laboratory analysis on soil samples. The analysis was focused in particular on the study of four different types of correlation: correlation between in-situ acquisition on ground and laboratory measurements, correlation between in-situ acquisition on tripod and laboratory measurements, correlation between in-situ acquisition on ground and on tripod and correlation between in-situ acquisition on ground and on operator shoulder and the influence of vegetative cover during measurements in-situ.

У докторској дисертацији  приказани су резултати развијених и примењених коинцидентних метода за временску анализу нуклеарних процеса. Експериментални коинцидентни системи засновани су на  HPGe  и пластичним сцинтилационим детекторима.  Извршена су три експеримента, која су показала примењивост развијених коинцидентних система. Први део експерименталног дела обухвата анализе временских варијација интензитета космичког зрачења посредством нискоенергијских фотона. Показано је да се праћењем интензитета нискоенергијских фотона у току времена у различитим енергијским регионима остварује бољи увид у анализу фонских догађаја, стварајући предуслове за извођење експеримената који трагају за ретким нуклеарним процесима. Поред праћења временских варијација интензитета, метода се може применити и  на детекцију периодичних и апериодичних догађаја повезаних са активношћу Сунца. Други део дисертације обухватао је истраживања везана  за  детекцију потенцијалних  флуктуација константе распада 22 Na. Развијен је коинцидентни систем  и  у току времена је  праћен интензитет анихилационе линије, која је резултат анихилације позитрона емитованог распадом 22Na.  За време трајања  аквизиције података, нису пронађена значајна одступања испитиване константе распада од стандардног експоненцијалног закона радиоактивног распада. У трећем делу експерименталног рада, приказана је коинцидентна метода за временско раздвајање догађаја индукованих мионима и неутронима у околини детекторског система. Добијена је временска крива у експерименту и извршена је детаљна анализа различитих временских региона. Извршене су Монте Карло симулације, на основу којих је добијена временска крива. Анализом различитих региона симулиране временске криве, показано је да се ови догађаји могу раздвојити у две велике групе–брзе и споре догађаје. Међу спорим догађајима, показано је да се може направити разлика између догађаја индукованих мионима, међу којима доминира анихилација, и догађаја индукованих неутронима, који спадају у закаснеле догађаје у групи спорих догађаја. Добијено је да неутрони највише доприносе нискоенергијском региону, првенствено у региону до  ≈50  keV,  што их чини  нежељеном кариком фонских догађаја у експериментима који трагају за ретким нуклеарним догађајима. На основу резултата симулација, анализирани су електромагнетни и хадронски процеси индуковани мионима и неутронима, као и удели мионске и неутронске компоненте у коинцидентном спектру HPGe  детектора и директном спектру пластичног сцинтилационог детектора. Закључено је да нискоенергијском спектру,  који је превасходно значајан за ретке нуклеарне процесе,  доминантно доприносе неутрони.
U doktorskoj disertaciji  prikazani su rezultati razvijenih i primenjenih koincidentnih metoda za vremensku analizu nuklearnih procesa. Eksperimentalni koincidentni sistemi zasnovani su na  HPGe  i plastičnim scintilacionim detektorima.  Izvršena su tri eksperimenta, koja su pokazala primenjivost razvijenih koincidentnih sistema. Prvi deo eksperimentalnog dela obuhvata analize vremenskih varijacija intenziteta kosmičkog zračenja posredstvom niskoenergijskih fotona. Pokazano je da se praćenjem intenziteta niskoenergijskih fotona u toku vremena u različitim energijskim regionima ostvaruje bolji uvid u analizu fonskih događaja, stvarajući preduslove za izvođenje eksperimenata koji tragaju za retkim nuklearnim procesima. Pored praćenja vremenskih varijacija intenziteta, metoda se može primeniti i  na detekciju periodičnih i aperiodičnih događaja povezanih sa aktivnošću Sunca. Drugi deo disertacije obuhvatao je istraživanja vezana  za  detekciju potencijalnih  fluktuacija konstante raspada 22 Na. Razvijen je koincidentni sistem  i  u toku vremena je  praćen intenzitet anihilacione linije, koja je rezultat anihilacije pozitrona emitovanog raspadom 22Na.  Za vreme trajanja  akvizicije podataka, nisu pronađena značajna odstupanja ispitivane konstante raspada od standardnog eksponencijalnog zakona radioaktivnog raspada. U trećem delu eksperimentalnog rada, prikazana je koincidentna metoda za vremensko razdvajanje događaja indukovanih mionima i neutronima u okolini detektorskog sistema. Dobijena je vremenska kriva u eksperimentu i izvršena je detaljna analiza različitih vremenskih regiona. Izvršene su Monte Karlo simulacije, na osnovu kojih je dobijena vremenska kriva. Analizom različitih regiona simulirane vremenske krive, pokazano je da se ovi događaji mogu razdvojiti u dve velike grupe–brze i spore događaje. Među sporim događajima, pokazano je da se može napraviti razlika između događaja indukovanih mionima, među kojima dominira anihilacija, i događaja indukovanih neutronima, koji spadaju u zakasnele događaje u grupi sporih događaja. Dobijeno je da neutroni najviše doprinose niskoenergijskom regionu, prvenstveno u regionu do  ≈50  keV,  što ih čini  neželjenom karikom fonskih događaja u eksperimentima koji tragaju za retkim nuklearnim događajima. Na osnovu rezultata simulacija, analizirani su elektromagnetni i hadronski procesi indukovani mionima i neutronima, kao i udeli mionske i neutronske komponente u koincidentnom spektru HPGe  detektora i direktnom spektru plastičnog scintilacionog detektora. Zaključeno je da niskoenergijskom spektru,  koji je prevashodno značajan za retke nuklearne procese,  dominantno doprinose neutroni.
In  this  doctoral  thesis,  the  results  of  the  developed and  applied  coincidence   methods  on  the  timeanalysis  of  nuclear  processes  are  presented. Coincidence systems, used in presented experiments, are  based  on  the  HPGe  and  plastic  scintillation detectors.  Three  experiments  were  performed,showing  a  wide  application  of  the  developed coincidence  systems.  The  first  part  of  the experimental work involves the analysis of the time  variations of cosmic rays via low-energy photons. It was  shown  that  by  analyzing  the  intensity  of  lowenergy photons better insight into the behavior of thebackground  events  is  provided,  which  is   especially important  as  a  precondition  for  rare  nuclear  events experiments.  Furthermore,  it  was  shown  that  this method can be applied in order to search for periodic or aperiodic events resulting from the Sun activity. The  second  part  contains  the  research  of  the  22Na decay constant fluctuations. The coincidence system was  developed  and  the  intensity  of  the  annihilation line,  resulting  from  annihilation  of  the  positrons emitted from 22Na, was followed with time. During  he  acquisition  time,  no  significant  deviations  from the standard exponential radioactive decay law were found. In  the  third  part  of  the  experimental  work,  the coincidence  method  for  time  separation  of  the events,  induced  by  cosmic  muons  and  neutrons  in the  vicinity  of  the  detectors  system,  was  presented. The time curve was obtained in the experiment and the  detailed  analysis  of  the  different  time  regions was  performed.  The  Monte  Carlo  simulations  were conducted  and  the  time  curve  from  the  simulation results was obtained. Analyzing the different regions of the simulated time curve, it was noticed that these events can be separated into two groups–prompt and delayed.  Between  delayed  events,  it  was  concluded that  events  induced  by  muons,  dominantly annihilation  line,  and  events  induced  by  neutrons, which  may  be  classified  as  more  delayed  events  in the  group  of  the  delayed  events,  can  be distinguished.  It  was  concluded  the  neutrons dominantly  contribute  to  the  low-energy  region, mostly  in  the  region  to  ≈50  keV,  which  makes neutrons  an  important  background  in  the experiments searching for rare nuclear events. Based on  the  simulation  results,  electromagnetic  and hadronic processes induced by muons and neutrons, as  well  as  portions  of  muon’s  and  neutron’s component  in  the  coincidence  spectrum  of  HPGe detector  and  direct  spectrum  of  the  plastic scintillation  detector  were  analyzed.  It  was concluded  that  in  the  low-energy  part  of  the spectrum,  primarily important for the search for rare nuclear  events,  dominant  influence  is  originated from cosmic neutrons.

La spectrométrie gamma à haute résolution offre un outil d'analyse performant pour effectuer des mesurages environnementaux. Dans le cadre de la caractérisation radiologique d'un site (naturelle ou artificielle) ainsi que pour le démantèlement d'installations nucléaires, la cartographie des radionucléides est un atout important. Le principe consiste à déplacer un spectromètre HPGe sur le site à étudier et, à partir des données nucléaires et de positionnements, d'identifier, de localiser et de quantifier les radionucléides présents dans le sol. Le développement de cet outil fait suite à une intercomparaison où un exercice orienté intervention a montré les limites des outils actuels. Une partie de ce travail s'est portée sur la représentation cartographique des données nucléaires. La connaissance des paramètres d'un spectre in situ a permis la création d'un simulateur modélisant la réponse d'un spectromètre se déplaçant au-dessus d'un sol contaminé. Ce simulateur a lui-même permis de développer les algorithmes de cartographie et de les tester dans des situations extrêmes et non réalisables. Ainsi, ce travail ouvre sur la réalisation d'un prototype viable donnant en temps réel les informations nécessaires sur l'identité et la position possible des radionucléides. La recherche réalisée sur la déconvolution des données permet de rendre en post traitement une carte de l'activité du sol par radionucléide mais également une indication sur la profondeur de la source. Le prototype nommé OSCAR (Outil Spectrométrique de Cartographie de Radionucléides) a ainsi été testé sur des sites contaminés (Suisse et Japon) et les résultats obtenus sont en accord avec des mesures de référence.

Ce travail s’insère dans le cadre de l’expérience NEMO cherchant à mettre en évidence une radioactivité extrêmement rare : la double désintégration bêta sans émission de neutrino et par suite d’obtenir des informations sur la nature Dirac-Majorana et sur la masse du neutrino. La particularité de cette expérience actuellement en fin de prise de données au Laboratoire Souterrain de Modane est l’extrême rareté du signal recherché (T1/2 > 1024 ans, quelques événements par an). Il s’ensuit donc des conditions très contraignantes sur toutes les composantes du bruit de fond, et parmi celles-ci sur les niveaux de radon et de thoron. Après quelques rappels sur les propriétés du radon et du thoron et leur influence sur les données de l’expérience NEMO, ce travail de thèse a porté sur la détection de ces gaz radioactifs par la technique de collection électrostatique des descendants sur la surface d’une diode silicium et la détection des alpha émis. Nous avons montré que les rendements de détection sont fortement influencés par les conditions expérimentales et que le niveau de radon pouvait être contrôlé actuellement au niveau du mBq/m3. Une série des mesures de spectrométrie gamma nous a permis de comprendre l’origine du bruit de fond du détecteur de radon, et ainsi d’envisager dans l’avenir un gain en sensibilité en augmentant le volume de détection et en effectuant une sélection très poussée des matériaux non radioactifs. Par contre, pour le thoron, qui n’avait jamais été étudié jusqu’à présent, les efficacités de détection sont très faibles à cause des périodes courtes mises en jeu. Un monitorage continu du thoron dans le gaz de NEMO est donc exclu, ce qui souligne l’importance pour l’expérience NEMO que le dispositif expérimental puisse contrôler son propre bruit de fond
This work is part of the NEMO (Neutrino Ettore Majorana Observatory) experiment that is looking for an extremely rare radioactivity: the double beta decay without neutrino emission. Such process could prove the Majorana nature of the neutrino and could give an estimation of its absolute mass. The particularity of this experiment, currently running in the Modane underground laboratory (LSM), is the extreme weak signal (for T1/2 ~ 1024 years, a few events per year). It requires therefore very stringent conditions on all components of the background among which the level of radon and thoron activity. After a reminding of the general properties of radon and thoron as well as their influence on the NEMO data, this thesis focuses on the detection of these radioactive gases using the technique of electrostatic collection. The radon and/or the thoron daughters are collected by an electrostatic field on the surface of a silicon diode, where their characteristic alphas are detected. We have shown that the detection efficiencies are strongly influenced by the experimental conditions and that sensitivity around 1 mBq/m3 can be achieved for the radon in a gas. A series of measures through low background gamma spectrometer allowed us to understand the origin of the radon background, and thus showing that better sensitivity could be obtained by increasing the detection volume and by carrying out a strict selection of non-radioactive materials. For the thoron gas, which had never been studied before, the detection efficiencies have been found very low as a consequence of the short periods involved. Therefore, a continuous monitoring of the level of thoron in the NEMO gas is excluded, which underlines the importance for the NEMO experimental device to be able to control its own background
Nghiên cứu này nằm trong khuôn khổ thí nghiệm NEMO, hiện đang trong những bước lấy số liệu cuối cùng tại phòng thí nghiệm ngầm Modane, nhằm ghi nhận phân rã bêta kép không kèm theo phát xạ neutrino, để từ đó chứng minh bản chất Majorana của hạt neutrino và cho phép ước lượng khối lượng tuyệt đối của hạt này. Phân rã này, nếu thật sự tồn tại, là loại phân rã cực hiếm (T1/2 > 1024 năm, tương đương với vài tín hiệu ghi nhận được mỗi năm). Vì thế, nó đòi hỏi những điều kiện rất gắt gao đối với mọi thành phần cấu thành nên phông nền bức xạ, trong đó bao gồm hoạt độ của radon và thoron. Sau khi nhắc lại những tính chất cơ bản của radon và thoron, cũng như ảnh hưởng của chúng tới số liệu của NEMO, bản luận văn tập trung vào việc đo hoạt độ của hai khí phóng xạ này bằng cách thu nhận các hạt nhân con của chúng thông qua trường tĩnh điện, tiếp đó ghi nhận các hạt alpha phát ra tại bề mặt một đi-ốt silic. Nghiên cứu này đã chỉ ra rằng hiệu suất đo phụ thuộc vào nhiều điều kiện thực nghiệm, và thiết bị này cho phép chúng tôi kiểm soát khí radon ở mức mBq/m3. Một lọat các phép đo với phổ kế gamma đã giúp chúng tôi hiểu được căn nguyên phông nền bức xạ của thiết bị, và từ đó dự kiến có thể cải thiện độ nhạy của thiết bị bằng cách tăng thể tích đo và chọn lọc các loại vật liệu có độ phóng xạ thấp. Tuy nhiên, với khí thoron, vốn vẫn chưa được nghiên cứu kỹ cho tới hiện nay, hiệu suất đo được là rất nhỏ do chu kỳ bán rã quá ngắn của các hạt nhân liên quan. Vì vậy, việc theo dõi thoron trong khí ga của NEMO đã bị loại trừ, đồng nghĩa với việc khẳng định tầm quan trọng của việc kiểm soát phông nền bức xạ thoron của chính thiết bị ghi nhận bêta kép trong thí nghiệm NEMO

Integral, projet de telescope spatial de l'agence spatiale europeenne, est destine a l'observation des emissions x et gamma d'origine astrophysique. Il associe les performances d'un imageur et d'un spectrometre. Le spectrometre spi s'appuie sur la haute resolution fournie par 19 detecteurs en germanium ultra pur de type n. Nous presentons les essais effectues sur 12 detecteurs des series prototype et modele d'etude de spi en vue de la qualification des modeles de vol et de la preparation des calibrations. Nous synthetisons les resultats obtenus en termes de resolution et d'efficacite sur toute la gamme d'energie couverte par spi (20 kev 8 mev), et analysons l'influence de la tension de polarisation et de la temperature de fonctionnement. Des simulations de type monte-carlo, basees sur les modules geant du cern, realisees en parallele avec les essais, sont confrontees systematiquement aux mesures et font l'objet d'une etude detaillee. Un ensemble de tests : vibrations, cycles thermiques, irradiations par des particules energetiques, a permis de qualifier les detecteurs et d'etudier leur comportement dans les conditions de vol. Notamment, 9 detecteurs ont ete irradies au cours de 6 campagnes avec des faisceaux de neutrons d'energies differentes, de 5 a 70 mev. Ces campagnes ont permis d'evaluer la degradation des performances des detecteurs en fonction de la dose recue, de l'energie des neutrons incidents et de la temperature de fonctionnement. Finalement, nous exploitons le code de simulation developpe pour modeliser les detecteurs. Les caracteristiques d'un ensemble de 19 detecteurs associes selon la configuration du plan de detection de spi sont analysees. Nous nous attachons plus particulierement a l'etude des evenements multiples et a leur influence sur la reponse spectrale et spatiale de la camera.

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A espectrometria gama é uma técnica que proporciona informações diversas em uma única análise, além de ser rápida e não destrutiva. Por meio de um detector de radiação acoplado a um sistema eletrônico de aquisição de dados, a técnica identifica os radionuclídeos emissores gama em uma amostra com radioatividade natural ou induzida, bem como fornece informações para calcular as suas atividades, a partir do registro e análise do espectro gama. A eficiência é um dos principais parâmetros a ser considerado quando se trabalha com detectores de radiação gama. Seu conhecimento possibilita uma melhor exatidão na quantidade de quantuns de radiação que o detector pode registrar dentre o total que emerge da fonte ou da amostra em estudo. A eficiência pode ser determinada experimentalmente, por métodos teóricos e semiempíricos. Usualmente, a determinação por método experimental é a mais exata, porém exige a aquisição de mais dados, o que torna o procedimento mais trabalhoso. Por sua vez, os métodos teóricos e os semiempíricos são muito menos laboriosos, apesar dos riscos de apresentarem maiores incertezas. Neste trabalho, foi realizado um estudo comparativo para verificar se as equações de eficiência determinadas por métodos semiempíricos e experimental teriam desempenhos similares na determinação das concentrações elementares da amostra de referência IAEA/Soil7, em geometria puntual e não puntual, irradiadas no reator de pesquisa TRIGA MARK I IPR-R1. Por métodos semiempíricos, foram aplicados os programas KayZero for Windows, V. 2.42 específico para cálculo de concentração elementar no método k0 de Ativação Neutrônica e o ANGLE V3.0 desenvolvido para cálculo de eficiência de detectores semicondutores para diversas geometrias. Os resultados indicaram, por meio de avaliação estatística, que as eficiências determinadas experimentalmente e por métodos semiempíricos são similares e igualmente eficazes tanto para a amostra puntual quanto para a não puntual. As diferenças observadas nos resultados das concentrações elementares foram relacionadas às correções aplicadas pelo programa e não consideradas quando se usa a planilha eletrônica e não devido às eficiências determinadas. Este estudo mostrou também que o programa KayZero for Windows analisa e considera como puntual amostras com massa cinco vezes maior que a massa das amostras usualmente analisadas, o que expande o campo de aplicação do programa.
Gamma spectrometry is a technique that provides a piece of information on a sample in one measurement, in a fast and non destructive assay. Such technique identifies gamma emitter radionuclides in natural and induced radioactivity, using a radiation detector linked to an electronic system in order to acquire the data and gamma spectra. To determine the full energy peak efficiency of High-Purity Germanium detector is important for gamma-ray spectrometry experiments. The efficiency of a detector is proportionality constant, which relates the activity of the source being counted and the number of counts observed. This efficiency can be determined applying experimental methods, using theoretical and semi empirical methods. Usually, the determination via an experimental procedure is more accurate, however, it is necessary more data acquisition that makes the procedure more tiring. On the other hand, the theoretical and semi empirical methods are less laborious procedures, despite the risks of higher uncertainties. This comparative study was carried out in order to verify whether the full energy peak efficiency curves determined by experimental and semi empirical methods, would present a similar performance on elemental concentration of reference material IAEA/Soil7, prepared in punctual and non-punctual geometries, and irradiated in the TRIGA MARK I IPR-R1 research reactor. It was applied were applied the KayZero for Windows, V. 2.42 a specific software for elemental concentration determination in the k0-stardadization neutron activation analysis - and ANGLE V3.0 specific method to determine semi conductor detectors gamma efficiencies for several sample geometries. Based on statistical tests u-score and Relative Tendency - the results pointed out for two types of sample geometries, the efficiencies determined experimentally and by semi empirical methods are similar, which were fitted well and worked properly. The deviations observed in the results were related to corrections made by the KayZero for Windows software and didnt apply when the values were calculated using the spread sheet and not related to the efficiency curves themselves. Additionally, it was evidenced that KayZero for Windows software is able to analyze a non-punctual sample, with mass 5 times higher that the usual size, as it were punctual. It points out the versatility of the software and expands the application field.

The nuclear design of fusion devices such as ITER (International Thermonuclear Experimental Reactor), which is an experimental fusion reactor based on the "tokamak" concept, rely on the results of neutron physical calculations. These depend on the knowledge of the neutron and photon flux spectra which is particularly important because it permits to anticipate the possible answers of the whole structure to phenomena such as nuclear heating, tritium breeding, atomic displacements, radiation shielding, power generation and material activation. The flux spectra can be calculated with transport codes, but validating measurements are also required. An important constituent of structural materials and divertor areas of fusion reactors is tungsten. This thesis deals with the measurement of the neutron fluence and neutron energy spectrum in a tungsten assembly by means of multiple foil neutron activation technique. In order to check and qualify the experimental tools and the codes to be used in the tungsten benchmark experiment, test measurements in the D-T and D-D neutron fields of the neutron generator at Technische Universität Dresden were performed. The characteristics of the D-D and D-T reactions, used to produce monoenergetic neutrons, together with the selection of activation reactions suitable for fusion applications and details of the activation measurements are presented. Corrections related to the neutron irradiation process and those to the sample counting process are discussed, too. The neutron fluence and its energy distribution in a tungsten benchmark, irradiated at the Frascati Neutron Generator with 14 MeV neutrons produced by the T(d, n)4He reaction, are then derived from the measurements of the neutron induced γ-ray activity in the foils using the STAYNL unfolding code, based on the linear least-square-errors method, together with the IRDF-90.2 (International Reactor Dosimetry File) cross section library. The differences between the neutron flux spectra measured by means of neutron foil activation and the neutron flux spectra obtained in the same assembly, making use of an NE213 liquid-scintillation spectrometer were studied. The comparison of measured neutron spectra with the spectra calculated with the MCNP-4B (Monte Carlo neutron and photon transport) code, which allows a crucial test of the evaluated nuclear data used in fusion reactor design, is discussed, too. In conclusion, this thesis shows the applicability of the neutron foil activation technique for the measurement of neutron flux spectra inside a thick tungsten assembly irradiated with 14 MeV from a D-T generator
Die Konstruktion von Fusionsreaktoren wie ITER (International Thermonuclear Experimental Reactor), der ein experimenteller Fusionsreaktor ist und auf dem "Tokamak"-Konzept beruht, basiert unter neutronenphysikalischen Gesichtspunkten auf den Ergebnissen von umfangreichen Simulationsrechnungen. Diese setzen die Kenntnis der Spektren des Neutronen- und Photonenflusses voraus die besonders wichtig ist, weil sie, die möglichen Antworten der ganzen Struktur auf physikalische Prozesse vorauszuberechnen erlaubt wie z.B.: Heizen durch nukleare Prozesse, Tritium-Brüten, Atomverschiebung, Abschirmung von Strahlung, Leistungserzeugung und Materialaktivierung. Die Flußspektren können mittels Transportcodes berechnet werden, aber es werden auch Messungen zu ihrer Bestätigung benötigt. Ein wichtiger Bestandteil des Strukturmaterials und der Divertor-Flächen der Fusionsreaktoren ist Wolfram. Diese Dissertation behandelt die Messungen der Neutronspektren und ?fluenz in einer Wolfram-Anordnung mittels der Multifolien-Neutronenaktivierungstechnik. Um die anzuwendenden experimentellen Geräte und die Codes, die im Wolfram-Benchmark-Experiment eingesetzt werden, zu überprüfen und zu bestimmen, wurden Testmessungen in den D-T und D-D Neutronenfeldern des Neutronengenerator der Technischen Universität Dresden durchgeführt. Die Eigenschaften der D-T und D-D Reaktionen, die für die Erzeugung von monoenergetischen Neutronen verwendet werden, sowie die Auswahl der Aktivierungsreaktionen, die für Fusionsanwendungen geeignet sind und die Aktivierungsmessung werden detailliert vorgestellt. Korrekturen, die sich auf den Neutronen-Bestrahlungsprozess und auf den Probenzählungsprozess beziehen, werden ebenfalls besprochen. Die Neutronenfluenz und ihre Energieverteilung in einem Wolfram-Benchmark, bestrahlt am Frascati Neutronen Generator mit 14 MeV-Neutronen aus der T(d, n)4He Reaktion, werden aus den Messungen der γ-Strahlenaktivität, die von Neutronen in den Folien induziert ist, durch den STAYNL Entfaltungscode, der auf der Methode der kleinsten Fehlerquadrate basiert, zusammen mit der IRDF-90.2 Wirkungsquerschnitt-Bibliothek abgeleitet. Die Unterschiede zwischen den Neutronenflußspektren, die mit Hilfe der Multifolien-Neutronenaktivierung ermittelt wurden, und den Neutronenflußspektren, gemessen im selben Aufbau mit einem NE-213 Flüssigszintillator, wurden untersucht. Die gemessenen Neutronenspektren werden den aus MCNP-4B Rechnungen (Monte Carlo neutron and photon transport) ermittelten Spektren gegenüber gestellt. Der Vergleich stellt einen wichtigen Test der evaluierten Kerndaten für Fusionsreaktorkonzepte dar. Zusammenfassend zeigt diese Arbeit die Anwendbarkeit der Multifolien-Neutronenaktivierungstechnik bei Messungen der Neutronenflussspektren innerhalb eines massiven Wolframblocks bei Bestrahlung mit schnellen Neutronen aus D-T Generatoren

Ce travail s'insère dans le cadre de l'expérience NEMO cherchant à mettre en évidence une radioactivité extrêmement rare : la double désintégration bêta sans émission de neutrino et par suite d'obtenir des informations sur la nature Dirac-Majorana et sur la masse du neutrino. La particularité de cette expérience actuellement en fin de prise de données au Laboratoire Souterrain de Modane est l'extrême rareté du signal recherché (T1/2 > 1024 ans, quelques événements par an). Il s'ensuit donc des conditions très contraignantes sur toutes les composantes du bruit de fond, et parmi celles-ci sur les niveaux de radon et de thoron. Après quelques rappels sur les propriétés du radon et du thoron et leur influence sur les données de l'expérience NEMO, ce travail de thèse a porté sur la détection de ces gaz radioactifs par la technique de collection électrostatique des descendants sur la surface d'une diode silicium et la détection des alpha émis. Nous avons montré que les rendements de détection sont fortement influencés par les conditions expérimentales et que le niveau de radon pouvait être contrôlé actuellement au niveau du mBq/m3. Une série des mesures de spectrométrie gamma nous a permis de comprendre l'origine du bruit de fond du détecteur de radon, et ainsi d'envisager dans l'avenir un gain en sensibilité en augmentant le volume de détection et en effectuant une sélection très poussée des matériaux non radioactifs. Par contre, pour le thoron, qui n'avait jamais été étudié jusqu'à présent, les efficacités de détection sont très faibles à cause des périodes courtes mises en jeu. Un monitorage continu du thoron dans le gaz de NEMO est donc exclu, ce qui souligne l'importance pour l'expérience NEMO que le dispositif expérimental puisse contrôler son propre bruit de fond.

tesis (doctor en física)--universidad nacional de córdoba. facultad de matemática, astronomía, física y computación, 2019.
en esta tesis, se desarrolla un método basado en simulaciones monte carlo que permite el cálculo y el análisis de espectros gamma complejos a partir de los espectros medidos, sin el uso de muestras calibradas. el método, es adecuado para calcular eficiencias de pico y eficiencias totales, detectar la presencia de líneas entremezcladas u ocultas, y determinar el área de picos de líneas situadas sobre el fondo real de otras de mayor energía. el método, es eficaz para determinar y estudiar efectos de coincidencia y también para obtener y estudiar efectos de auto-absorción en muestras de formas y tamaños variados.
in this thesis, a method based on monte carlo simulations is developed that allows the calculation and analysis of complex gamma spectra starting from the measured spectra, without the use of calibrated samples. the method is suitable for calculating peak efficiencies and total efficiencies, detecting the presence of intermixed or hidden lines, and determining the area of peaks of lines located on the real background of others of higher energy. the method is effective for determining and studying coincidence effects and also for obtaining and studying self-absorption effects in samples of varied shapes and sizes.
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fil: gonzalez, enrique. universidad nacional de córdoba. facultad de matemática, astronomía, física y computación; argentina.