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A methodology for the production of PbSe targets for 79Se neutron capture cross section studies is presented. PbSe material was synthesized by direct reaction of its constituents at high temperature, and characterized by X-ray diffraction. Thin PbSe targets, produced for cross section experiments with the surrogate reaction method, were obtained by applying a physical vapor deposition technique, and their morphology and composition were analyzed by X-ray fluorescence, Scanning Electron Microscopy, and Energy dispersive X-ray spectroscopy. Pb79Se targets produced for cross section measurements with the Time of Flight method were characterized by γ-ray spectroscopy. Finally, a procedure for the recovery of Se from PbSe is suggested. The purity of the retrieved Se was determined with Inductively Coupled Plasma Optical Emission Spectroscopy., Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 1029, ISSN:0168-9002, ISSN:1872-9576

The 26Al(n,p)26Mg reaction is the key reaction impacting on the abundances of the cosmic γ-ray emitter 26Al produced in massive stars and impacts on the potential pollution of the early solar system with 26Al by asymptotic giant branch stars. We performed a measurement of the 26Al(n,p)26Mg cross section at the high-flux beam line EAR-2 at the n_TOF facility (CERN). We report resonance strengths for eleven resonances, nine being measured for the first time, while there is only one previous measurement for the other two. Our resonance strengths are significantly lower than the only previous values available. Our cross-section data range to 150 keV neutron energy, which is sufficient for a reliable determination of astrophysical reactivities up to 0.5 GK stellar temperature.

Que comprende un colimador mecánico (1) que recibe partículas de una fuente emisora de radiación gamma (61) y/o de neutrones (62), un material absorbente de neutrones y prácticamente transparente a la radiación gamma; un primer detector (2) que comprende un cristal centelleador con capacidad de discriminación neutrón-gamma y un fotosensor sensible a las posiciones, acoplado al colimador mecánico (1); uno o más bloques detectores con cristales centelleadores de radiación gamma o neutrón-gamma (3) y fotosensores sensibles a las posiciones, a continuación del primer detector (2), en una cara opuesta al colimador mecánico (1); y un módulo de adquisición y procesado (4), conectado a los detectores (2, 3), siendo el dispositivo apto para utilizar en inspecciones de seguridad nuclear, seguridad portuaria, amenazas y accidentes nucleares y tratamientos de hadrón-terapia y medicina nuclear. [ES], The invention comprises a mechanical collimator (1) which receives particles from a gamma radiation source (61) and/or a neutron source (62), a neutron-absorbent material that is virtually transparent to gamma radiation; a first detector (2) that comprises a scintillation crystal with neutron-gamma discrimination capability and a position-sensitive photosensor, coupled to the mechanical collimator (1); one or more detector blocks with gamma or neutron-gamma radiation scintillation crystals (3) and position-sensitive photosensors, after the first detector (2), on a face opposite the mechanical collimator (1); and an acquisition and processing module (4) connected to the detectors (2, 3), the device being suitable for use in inspections relating to nuclear safety, port safety, nuclear threats and accidents, and hadron therapy treatments and nuclear medicine. [EN], Consejo Superior de Investigaciones Científicas (CSIC), Universitat de València, A1 Solicitud de patente con informe sobre el estado de la técnica

The first crucial ingredient of reactor and fuel cycle analysis is nuclear data. When designing or assessing the safety of a reactor system, nuclear data for a wide range of reactions and materials has to be known. Designers and physicists must address many variants of nuclear plants and undertake extensive calculations to estimate the performance of a critical nuclear system. For accurate and reliable estimates, these studies should incorporate the most accurate and reliable nuclear data and neutron cross-sections, compiled in evaluated libraries such as ENDF/B-VII.1, JENDL-4.0a, or JEFF-3.2 . These evaluated libraries are tested against relevant benchmark experimental data, thus validating the current knowledge of nuclear cross-sections and nuclear data. These major evaluated libraries predict the measured criticality of nuclear systems extremely well (for many assemblies, although not for all). However, such good performance in integral testing creates a false sense of optimism due to compensating errors, calibration of some critical parameters, and discrepancies between libraries. Over the years, great efforts have been made to obtain reliable neutron-induced cross-sections of the $^{235}$U, which are the most important physical constants in nuclear energy applications. In particular, for the neutron capture cross-section, there are several measurements. The neutron-induced cross-sections for this isotope are very important, not only for major nuclear thermal reactors but for Fast Breeder Reactors (FBRs) because many critical experiments for FBRs have been performed at critical assemblies where UO2 fuels were used as driver fuels. The experimental data obtained at such critical assemblies has a great impact on design work for FBRs. Recent studies show that calculated sodium void reactivity values for BFS experiments underestimate the experimental results by 30-50%. These significant discrepancies not only exceed the target accuracy of 20% for an FBR design but also undermine the design accuracy estimated with the cross-section adjustment and bias factor techniques. To tackle the discrepancies in the neutron cross-section data of the major nuclides, the IAEA CIELO pilot project is re-evaluating the major nuclides important for the nuclear applications: $^{1}$H, $^{16}$U, $^{235}$U, $^{238}$U, and $^{239}$Pu. The main goal of this project is the production of improved and validated evaluated nuclear data files. The work presented in this manuscript focuses on the framework for improving the neutron capture cross-section at low neutron energy and improving current knowledge of the resonance parameters for the $^{235}$U isotope.

Las actuales y futuras demandas de energía requieren el desarrollo de nuevas fuentes de producción así como la mejora de las ya existentes. El primer y crucial ingrediente para el diseño de los reactores nucleares y el ciclo de combustible son los datos nucleares. A la hora de diseñar y evaluar la seguridad de los reactores nucleares es necesario una gran cantidad de datos nucleares para distintos materiales. Los ingenieros y físicos deben afrontar diferentes variantes de los reactores nucleares y realizar grandes cálculos para estimar el rendimiento de los sistemas nucleares críticos. Para tener estimaciones fiables, estos estudios deben incoporar las secciones eficaces más precisas que se conocen, habitualmente incluidas en las librerias evualadas tales como ENDF/B-VII.1, JENDL-4.0a o JEFF-3.2. Estas librer¿¿as evaluadas son probadas con experimentos integrales y de esta manera se valida los actuales datos y secciones eficaces nucleares. Estas librer¿¿as predicen de manera muy precisa para la mayoria de sistemas nucleares cr¿¿ticos su comportamiento. Sin embargo, la buena estimación en estos experimentos integrales crea la falsa sensación de optimismo debido a la compensación de errores, calibraciones de los datos y discrepancias entre librer¿¿as. El objetivo de este manuscrito es presentar el análisis y resultados de la medida de la sección eficaz de captura del 235 U llevado a cabo en la instalación n TOF del CERN en el rango de energ¿¿as del neutrón desde 0.2 a 200 eV. Estos datos han sido utilizados para el desarrollo de una nueva libreria evaluada de secciones eficaces de neutrones que será usada en el diseño y desarrollo de nuevos reactores nucleares.

El proceso (-s) de captura lenta de neutrones es responsable de la formación de la mitad de los elementos más pesados ​​que el hierro en el universo. A pesar de la larga escala de tiempo de este proceso, la larga vida media de algunos isótopos inestables a lo largo del flujo de reacción del proceso-s crea puntos de ramificación que conducen a una división de la ruta de la nucleosíntesis. 79Se (t1/2 = 3.27 x 10^5 y) representa uno de los núcleos ramificados-s más relevantes y debatidos por dos razones principales. Por un lado, la existencia de estados excitados de baja energía en el 79Se, cuya población puede variar con la temperatura del medio estelar, hace que el patrón de abundancia local alrededor de esta ramificación sea especialmente sensible a las condiciones térmicas. Por otro lado, las abundancias observadas de los isótopos de criptón solo-s (80,82Kr) son muy conocidas a partir de datos meteóricos. Por lo tanto, al comparar estas abundancias con las predichas por los modelos estelares, se puede obtener información sobre las condiciones térmicas del medio estelar en el que ocurre el proceso-s. Para este objetivo, los modelos estelares hidrodinámicos de última generación necesitan datos de entrada experimentales sobre la sección transversal de captura de neutrones de los isótopos implicados en la ramificación en un amplio rango de energía térmica. La última afirmación es cierta para el núcleo inestable 79Se y sus núcleos vecinos más cercanos, 78,80Se. Sin embargo, las mediciones de captura de neutrones en núcleos radiactivos son muy desafiantes y, de hecho, hasta el momento, no hay datos experimentales sobre la reacción del 79Se(n,gamma). Además, los datos experimentales previos sobre el 80Se eran bastante limitados en términos de resolución e integridad. En este contexto, el presente trabajo ha contribuido en dos frentes diferentes con el objetivo de arrojar luz sobre la ramificación 79Se del proceso-s. La primera parte de este trabajo describe la medición de la sección transversal de captura de neutrones de 80Se en el CERN n_TOF, con una resolución de energía muy alta y que cubre por primera vez todo el rango de energía estelar de interés. La medición anterior en 80Se(n,gamma) adolece de una resolución de energía muy limitada y un rango corto de energía de neutrones. Estos inconvenientes se han mejorado notablemente en este trabajo mediante una medición del tiempo de vuelo (ToF) de alta resolución empleando una muestra de 80Se de alta pureza de 3.8 g de masa. El uso de detectores de energía total C6D6D en combinación con la técnica de ponderación de altura de pulso (PHWT), nos ha permitido obtener un rendimiento de captura con alta precisión y cubriendo todo el rango de energía de interés astrofísico entre 1 eV y 100 keV. Se han analizado ciento trece resonancias mediante el formalismo de matriz-R, noventa y ocho de ellas por primera vez. El impacto es considerable, siendo el MACS a kT = 8 keV un 36 % más pequeño que el valor recomendado en KADoNiS. La incertidumbre estadística que afecta a este nuevo MACS se ha reducido del 10% al 1%. La precisión sistemática lograda entre el 3,2 % y el 5,7 % es comparable a las incertidumbres de las abundancias isotópicas de los isótopos de Kr solo s, que es el requisito de los modelos hidrodinámicos estelares para ofrecer resultados precisos. La segunda contribución principal de este trabajo al estudio de la ramificación del 79Se consistió en los primeros desarrollos hacia un nuevo sistema de detección, denominado i-TED, para medir secciones transversales (n,gamma) con una relación señal/fondo mejorada. Este nuevo sistema de detección se aplicará por primera vez en la medición de la sección transversal de 79Se(n,gamma) en el CERN n_TOF en 2022. El detector de energía total con capacidad de generación de imágenes i-TED aprovecha la técnica de generación de imágenes de Compton para seleccionar principalmente la gamma -rayos generados en la muestra por los neutrones capturados en la misma, al tiempo que rechaza los rayos gamma contaminantes procedentes de los neutrones perdidos capturados en el entorno. Para implementar técnicamente este concepto, i-TED consta de dos planos de detección que operan en coincidencia temporal, en los que se registran la posición, la energía y el tiempo de las interacciones de los rayos gamma. En este trabajo de tesis se ha desarrollado y caracterizado un primer demostrador denominado i-TED5.3, con tres detectores sensibles a la posición (PSDs), y se ha realizado la primera prueba de concepto experimental. En i-TED5.3, un PSD se coloca en el plano de dispersión mientras que los dos restantes se disponen en una configuración vertical dentro de la capa absorbente. Cada PSD consta de un cristal de centelleo monolítico de LaCl3(Ce) acoplado ópticamente a un fotomultiplicador de silicio, que está conectado a un sistema de lectura basado en ASIC fabricado por PETsys Electronics. Una caracterización completa de este prototipo arrojó resoluciones de posición que oscilan entre 1 mm y 2 mm FWHM, y resoluciones de energía de 6% y 7% FWHM a 661 keV para los espectros de energía simple y depositada por coincidencia, respectivamente. Finalmente, un primer experimento experimental de prueba de concepto realizado en el CERN n_TOF con i-TED5.3 nos permitió validar técnicamente el sistema para experimentos ToF y demostrar las capacidades de rechazo en segundo plano. Se logró una reducción de fondo de hasta un factor de 3.8 después de comparar los espectros de energía de neutrones de 56Fe(n,gamma) medidos con el demostrador i-TED5.3 y los detectores C6D6 de última generación. Otras mejoras realizadas fuera del alcance de este trabajo de tesis comprenden el ensamblaje y la caracterización de una matriz de 4 detectores i-TED, cada uno con 5 PSD, y el uso de técnicas de inteligencia artificial y aprendizaje automático para mejorar aún más la capacidad de rechazo de fondo. y el rendimiento general del sistema. The slow neutron capture (s-) process is responsible for the formation of half of the elements heavier than iron in the universe. Despite the long time scale of this process, the long half-life of some unstable isotopes throughout the s-process reaction flow creates branching points that lead to a split of the nucleosynthesis path. 79Se (t1/2 = 3.27 x 10^5 y) represents one of the most relevant and debated s-branching nuclei for two main reasons. On the one hand, the existence of low-energy excited states in 79Se, whose population can vary with the temperature of the stellar medium, makes the local abundance pattern around this branching especially sensitive to the thermal conditions. On the other hand, the observed abundances of the s-only krypton isotopes (80,82Kr) are very well-known from meteoric data. Thus, by comparing these abundances with those predicted by stellar models, information about the thermal conditions of the stellar media in which the s-process occurs can be obtained. To this aim, state-of-the-art hydrodynamic stellar models need experimental input data on the neutron capture cross section of the isotopes involved in the branching over a broad thermal energy range. The latter statement is certainly true for the unstable 79Se and its closest neighboring nuclei, 78,80Se. However, neutron capture measurements on radioactive nuclei are very challenging and indeed, thus far, there is no experimental data on the 79Se(n,gamma) reaction. Also, previous experimental data on 80Se was rather limited in terms of resolution and completeness. In this context, the present work has contributed in two different fronts with the aim of shedding light on to the 79Se s-process branching. The first part of this work describes the neutron capture cross section measurement of 80Se at CERN n_TOF, with very high energy resolution and covering the full stellar energy range of interest for the first time. The previous measurement on 80Se(n,gamma) suffers from a very limited energy resolution and a short neutron-energy range. These drawbacks have been remarkably improved in this work by means of a high-resolution time of flight (ToF) measurement employing a high purity 80Se sample of 3.8 g of mass. The use of C6D6D total energy detectors in combination with the Pulse-Height Weighting Technique (PHWT), have allowed us to obtain a capture yield with high accuracy and covering the entire energy range of astrophysical interest between 1 eV and 100 keV. One hundred and thirteen resonances have been analyzed by means of the R-matrix formalism, ninety-eight of them for the first time. The impact is sizable, being the MACS at kT = 8 keV 36% smaller than the value recommended in KADoNiS. The statistical uncertainty affecting this new MACS has been reduced from 10% down to 1%. The achieved systematic accuracy between 3.2% and 5.7% is comparable to the uncertainties of the isotopic abundances of the s-only Kr-isotopes, which is the requirement of hydrodynamic stellar models to deliver accurate results. The second main contribution of this work to the study of the 79Se branching consisted of the first developments towards a novel detection system, called i-TED, for measuring (n,gamma) cross sections with enhanced signal-to-background ratio. This new detection system will be applied for the first time in the measurement of the 79Se(n,gamma) cross-section at CERN n_TOF in 2022. The i-TED imaging capable Total Energy Detector exploits the Compton imaging technique to select mainly the gamma-rays generated in the sample by neutrons captured therein, while rejecting contaminant gamma-rays coming from stray neutrons captured in the surroundings. In order to technically implement this concept, i-TED consists of two detection planes operating in time coincidence, in which the position, energy and time of the gamma-ray interactions are registered. A first demonstrator called i-TED5.3, with three position sensitive detectors (PSDs), has been developed and characterized in this thesis work and the first experimental proof of concept has been carried out. In i-TED5.3, one PSD is placed in the scatter plane while the remaining two are arranged in a vertical configuration within the absorber layer. Each PSD consists of a monolithic LaCl3(Ce) scintillation crystal optically coupled to a silicon photomultiplier, which is connected to an ASIC-based readout system manufactured by PETsys Electronics. A complete characterization of this prototype yielded position resolutions ranging between 1 mm and 2 mm FWHM, and energy resolutions of 6% and 7% FWHM at 661 keV for the singles and coincidence deposited energy spectra, respectively. Finally, a first experimental proof of concept experiment carried out at CERN n_TOF with i-TED5.3 allowed us to technically validate the system for ToF experiments, and demonstrate the background rejection capabilities. A background reduction by up to a factor of 3.8 was achieved after comparing the 56Fe(n,gamma) neutron energy spectra measured with the i-TED5.3 demonstrator and state-of-the-art C6D6D detectors. Further improvements undertaken outside of the scope of this thesis work comprise the assembly and characterization of an array of 4 i-TED detectors, each one comprising 5 PSDs, and the use of artificial intelligence and machine-learning techniques for enhancing further the background rejection capability and overall system performance.

The NeNa and the MgAl cycles play a fundamental role in the nucleosynthesis of asymptotic giant branch stars undergoing hot bottom burning. The Na 23 ( p , γ ) 24 Mg reaction links these two cycles and a precise determination of its rate is required to correctly estimate the contribution of these stars to the chemical evolution of various isotopes of Na, Mg and Al. At temperatures of 50 ≲ T ≲ 110 MK , narrow resonances at E p = 140 and 251 keV are the main contributors to the reaction rate, in addition to the direct capture that dominates in the lower part of the temperature range. We present new measurements of the strengths of these resonances at the Laboratory for Underground Nuclear Astrophysics (LUNA). We have used two complementary detection approaches: high efficiency with a 4π BGO detector for the 140 keV resonance, and high resolution with a HPGe detector for the 251 keV resonance. Thanks to the reduced cosmic ray background of LUNA, we were able to determine the resonance strength of the 251 keV resonance as ω γ = 482 ( 82 ) μ eV and observed new gamma ray transitions for the decay of the corresponding state in Mg 24 at E x = 11931 keV . With the highly efficient BGO detector, we observed a signal for the 140 keV resonance for the first time in a direct measurement, resulting in a strength of ω γ 140 = 1.46 − 0.53 + 0.58 neV (68% CL). Our measurement reduces the uncertainty of the Na 23 ( p , γ ) 24 Mg reaction rate in the temperature range from 0.05 to 0.1 GK to at most − 35 % + 50 % at 0.07 GK. Accordingly, our results imply a significant reduction of the uncertainties in the nucleosynthesis calculations.

The suppression of the internal detector background in 3He counters through pulse shape discrimination methods is becoming an area of interest for a growing number of experiments that want to detect low event rate signals. We discuss two discrimination methods for signals coming from high-pressure (thus high efficiency) 3He counters read out through charge sensitive preamplifiers. We present a digital filter to convert the integrated signal to a current pulse. The application of a discrimination method based on the peak shape suppresses the internal alpha-induced background by > 98.5%. In addition a semi-empirical model of the signal creation in 3He counters is presented and compared to experimental data.