Beta Spectrum Shapes Study for the Prediction of Reactors’ Antineutrino Spectra

Gustavo Adolfo Alcalá Escalona

NUEVAS MEDIDAS DE LA EMISIÓN DE NEUTRONES EN LA DESINTEGRACIÓN β DE 85 Ge, 85,86 As, 91 Br, 135 Sb, 137,138 Te y 138,139,140 I

Jorge Agramunt Ros

High resolution 80 Se(n,γ) cross section measurement at CERN n_TOF and development of the novel i-TED detection system

Victor Babiano Suarez

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. 79 Se (t 1/2 = 3.27 × 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 79 Se, 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 ( 80,82 Kr) 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 79 Se and its closest neighbouring nuclei, 78,80 Se. However, neutron capture measurements on radioactive nuclei are very challenging and indeed, thus far, there is no experimental data on the 79 Se(n, γ) reaction. Also, previous experimental data on 80 Se 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 79 Se s-process branching. The first part of this work describes the neutron capture cross section measurement of 80 Se 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 80 Se(n,γ) 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 80 Se sample of 3.8 g of mass. The use of C 6 D 6 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   xxv
Chapter 0. Abstract 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 79 Se branching consisted of the first developments towards a novel detection system, called i-TED, for measuring (n,γ) cross sections with enhanced signal-to-background ratio. This new detection system will be applied for the first time in the measurement of the 79 Se(n, γ) 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 γ-rays generated in the sample by neutrons captured therein, while rejecting contaminant γ-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 γ-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 LaCl 3 (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 56 Fe(n,γ) neutron energy spectra measured with the i-TED5.3 demonstrator and state-of-the-art C 6 D 6 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.

Beta-decay of 8B into highly excited states of 8Be: Isospin mixing and proton-halo contributions

Silvia Vinyals i Onsés

Neutron capture cross section measurement of the heaviest s-process branching 204Tl and of 203Tl at CERN n_TOF

Adrià Casanovas Hoste

Study of beta-delayed neutron emitters in the region of 78Ni and its impact on r-process nucleosynthesis

Álvaro Tolosa Delgado

Uso de detectores de la radiación para el desarrollo de técnicas de reconstrucción espacial en residuos radiactivos

José Luis Leganés Nieto

Tesis industrial

Collectivity along N=50: Nuclear Structure stu- dies on the neutron-magic nuclei 92 Mo and 94 Ru with AGATA and VA- MOS++

Rosa María Pérez Vidal

The present Thesis focusses on the experimental study of the evolution of collectivity in the vicinity of the doubly-magic nucleus 100 Sn. In particular, the conservation of seniority along the g 9/2 shell has been investigated. This shell is the first one in which the seniority might not be conserved. Excellent cases for studying the properties of this shell are the the N = 50 isotones in the vicinity of 100 Sn and the Z = 28 isotopes towards 78 Ni, which have the same valence nucleons for protons and neutrons, respectively. In the former case, shell model calculations predict the conservation of seniority towards the complete occupation of the g 9/2 orbital due to an increasing pairing-strength along the N = 50 isotones, which differs from what the calculations suggest for the latter case. To test this phenomenon, the reduced transition probabilities for the 4 + → 2 + yrast transitions, in 92 Mo and 94 Ru nuclei, have been determined experimentally for the first time via lifetime measurements at the GANIL laboratory. The multi-nucleon transfer reaction mechanism was unconventionally used to populate the proton-rich nuclei of interest: a energy of 716.9 MeV impinged on the stretched 92 92 Mo beam with an Mo target of the IKP Cologne plunger, while a 24 Mg foil was used to degrade the energy of the reaction products to measure the lifetime with the RDDS technique. The reaction products of interest have been identified with the magnetic spectrometer VAMOS++, while the gamma-rays have been measured using AGATA. From this experiment the reduced transition probabilities for the 4 + → 2 + and 2 + → 0 + yrast transitions in 92 Mo and 94 Ru and for the 4 + → 2 + and 6 + → 4 + yrast transitions in 90 Zr have been determined. This made possible a complete study of the seniority scheme in the πg 9/2 . The results on the obtained reduced transition probabilities have been interpreted on the basis of realistic shell model predictions in the f 5/2 , p 3/2 , p 1/2 , g 9/2 proton valence space, allowing also for the comparison of the nuclear structure trends between the valence mirror symmetry partners. Additionally, part of this Thesis is devoted to the performance of state-of-the art high-resolution gamma-ray spectroscopy detector used in the experiment. The characterization of AGATA is provided based on the analysis of source and in-beam data taken with 23 segmented crystals.

Isospin mixing and in-beam study of non-yrast states in $^{56}$Co

Ana Montaner-Pizá

In the present work we have studied the fusion-evaporation 56Fe(p,n gamma)56Co reaction with an incident 10 MeV proton-beam at the Maier-Leibnitz-Laboratory (MLL) of the Technische Universität München (TUM, Germany). The gamma-rays emitted in the de-excitation of the odd-odd 56Co nucleus were measured in-beam with four high-resolution MINIBALL-triple germanium (Ge) detectors. The experiment provided excellent data in gamma-gamma coincidences. A total of 223 gamma-transitions have been observed and placed in the level scheme, 169 of which were previously unobserved. A total of 77 excited states have been observed: 37 of them were previously known states for which no gamma-de-excitation had been observed and 14 of them have been observed in this work for the first time. In 42 cases the precision of the excitation energy has been improved. The spin and parity assignments to the excited states were made based on their gamma-decay pattern and the angular distributions of the gamma-ray de-excitations measured. The angular distributions of 53 gamma-transitions were measured, obtaining the corresponding angular coefficients A2. A total of 36 new J-pi assignments have been made, 10 improvements of previous J-pi assignments have been suggested and in 4 cases J-pi assignments ambiguities have been resolved. For the remainder of states, previously assigned J-pi values have been confirmed. Large-scale shell-model calculations in a truncated fp-shell valence space were performed, using the NuShellX@MSU and ANTOINE codes with the KB3G, GXPF1a and KB3GR effective interactions, to compare with the experimental results.

TAGS measurements for neutrino physics and applications

Víctor Guadilla

En la actualidad un gran número de temas de interés en Física Nuclear continúan planteando interrogantes fundamentales. Una de las fuentes de información más esclarecedoras es el proceso de la desintegración beta. Gracias al estudio experimental de este fenómeno en núcleos alejados de la estabilidad, es posible extraer valiosa información sobre estructura nuclear. En esa línea, esta tesis se centra en la determinación de las intensidades beta que pueblan los estados excitados del núcleo hijo en la desintegración beta de núcleos exóticos de gran interés. Para ello se ha utilizado la técnica de Espectroscopía gamma de Absorción Total (TAGS, por sus siglas en inglés), basada en el uso de grandes cristales centelleadores para detectar con gran eficiencia la cascada gamma completa que se emite tras la desintegración beta. Esta técnica requiere un proceso de deconvolución con la función de respuesta del detector para extraer las intensidades beta. Los resultados obtenidos con esta técnica permiten evitar un error sistemático conocido como Pandemonio, que afecta a las medidas con detectores de germanio que convencionalmente se llevan a cabo para obtener las intensidades beta. Las medidas de este trabajo han sido realizadas en la instalación de haces radioactivos IGISOL-IV (Jyväskylä, Finlandia). El montaje experimental consistió en el nuevo espectrómetro de absorción total DTAS, un nuevo detector plástico para partículas beta y un detector de germanio. Uno de los objetivos de este trabajo fue el montaje y caracterización de los detectores usados en estos experimentos, especialmente de DTAS, un instrumento diseñado por el grupo de Valencia para el proyecto FAIR. La detallada caracterización de los detectores se llevó a cabo comparando medidas de fuentes radioactivas de calibración con simulaciones Monte Carlo realizadas con el código de simulación GEANT4. Se obtuvo un excelente nivel de reproducción de las medidas experimentales, validando así las simulaciones que han permitido calcular las funciones de respuesta de los isótopos de interés. En esta tesis se han medido y analizado diez casos de interés. Nueve de ellos son desintegraciones de alta relevancia por tratarse de fragmentos de fisión que contribuyen de forma destacada en el calor residual generado en un reactor nuclear y en el espectro de antineutrinos emitido por los reactores. Por este motivo, la Agencia Internacional de la Energía Atómica (IAEA por sus siglas en inglés) considera la medida de estas desintegraciones con la técnica TAGS como altamente prioritaria para mejorar las bases de datos nucleares que se utilizan en los cálculos que tratan de reproducir tanto el calor residual como el espectro de antineutrinos de los reactores nucleares. En los nueve casos estudiados se ha identificado intensidad beta que no había sido detectada hasta ahora, y en dos de los casos las distribuciones de intensidades beta han sido obtenidas por vez primera. El impacto de estos nuevos datos, libres del efecto Pandemonio, en los cálculos mencionados resulta reseñable. Además, estos análisis han permitido extraer valiosa información sobre estructura nuclear, en relación con el proceso de emisión retardada de neutrones y con la forma de los núcleos. Cabe señalar también la importancia y complejidad de varias de estas medidas por tratarse de estados isoméricos muy próximos en energía. Finalmente, el caso restante incluido en este trabajo resulta de interés por ser una desintegración beta de importancia para mejorar los cálculos teóricos del proceso de desintegración doble beta. El análisis TAGS de este caso confirma las intensidades beta previamente conocidas, garantizando datos libres del efecto Pandemonio.

FIRST MEASUREMENT OF BETA-DECAY HALF-LIVES AND NEUTRON EMISSION PROBABILITIES IN SEVERAL ISOTOPES BEYOND N=126

Roger Caballero Folch

In this work beta-decay half-lives and neutron-emission probability values have been experimentally determined for first time in several nuclei beyond the neutron-shell closure at N=126. To this aim the accelerator complex at the GSI center for heavy ion research (Germany), in combination with the FRS fragment separator, were employed. The beta-decay detection system consisted of a charged particle detector named SIMBA, which served to detect both ion and beta-particles, surrounded by an array of He-3 based neutron counters named BELEN. The half-life values of eighteen isotopes of Au, Hg, Tl, Pb and Bi were determined, as well as the neutron-branching ratios (or upper limits) for seven nuclei. A comparison of the present results with previous half-life measurements in this mass-region shows a generally good agreement, thus reflecting the systematic validity of the decay data available in this region. Compared to theoretical models, our results show a reasonable agreement for half of the analyzed isotopes, and large discrepancies of up to a factor of 10 for the other half. The measured neutron-branching ratios represent the first set of experimental data available in this mass-region for several isotopes. Owing to the absence of previous experimental results, the values reported here can be only compared with theoretical predictions. In summary, a fair compatibility is found between FRDM+QRPA calculations, which however are underestimating by a factor of 5 the neutron-branching ratio of Tl-215, which is the most exotic nucleus measured in this work.

Decay Heat Measurements of Fission Fragments 86Br, 91Rb and 94Sr Using Total Absorption Gamma-ray Spectroscopy

Simon Rice

During a nuclear reactor’s normal operation, approximately 7-8% of the total heat produced is due to delayed β-decay of the initial fission products, and is known as decay heat. Once a reactor is shut down this decay heat contributes 100% of the heat produced by the fuel. Total absorption γ-ray spectroscopy utilises a near 4π geometry with a high efficiency to collect γ rays produced from a source placed in its centre. This total absorption can be used to determine the β-feeding levels of fission fragments that have large Qβ values. Conventional methods using HPGe detectors to determine β-feeding can be affected by the “Pandemonium effect”. This occurs due to the low detection efficiency of high-energy γ rays in HPGe detectors. Current decay heat calculations predict lower values than calorimetry measurements and this needs to be addressed. An experiment was carried out in Jyvaskyla to measure the β-feeding levels of key nuclei (86Br,91Rb and 94Sr) for decay heat calculations using a BaF2 Total Absorption Spectrometer (TAS). This thesis describes the experimental method, the calibration of the TAS and, the analysis procedure to obtain the average mean γ ray and β particle energy for each isotope as well as the β decay strength function. The final results from this work have provided new mean energy values for the β decays of 86Br (Eγ=3822(6)(54) keV Eβ=1670(4)(28) keV) and 91Rb (Eγ=2788(5)(29) keV Eβ=1330(3)(22) keV) showing that the “Pandemonium effect” was present in the previously recorded data and a reduced uncertainty was obtained for the decay of 94Sr (Eγ=1472(9)(15) keV Eβ=826(5)(6) keV). The results have given increased validity to previous TAS measurements by Greenwood et al. and subsequently questions work by Rudstam et al. on the measurements of β-particle and γ-ray spectra of many fission fragments.

Neutron capture measurement of 54Fe and 57Fe at CERN n_TOF

Giuseppe Giubrone

TAS measurements for neutrino physics and nuclear structure study of the beta decays of 150 Er, 152,156 Yb and 188,190,192Pb

Maria Esther Estevez Aguado

Ground-State Shape Determination of N~Z nuclei from beta-decay studies

Ana Belén Pérez Cerdán

Beta decay of the Tz=-1 nuclei and Comparison with CE reaction Experiments

Francisco Gabriel Molina Palacios

Beta -decay Total Absorption Spectroscopy measurements for reactor decay heat calculations

María Dolores Jordán Martín

Double Octupole States in 146Gd

Luis Caballero Ontanaya

New radiative neutron capture measurement of 207Pb and 209Bi

Cesar Domingo Pardo

Beta decay studies in the N≈Z and the rare-earth regions using Total Absorption Spectroscopy techniques

Enrique Nácher González

An RFQ Buncher for Accumulation and Cooling of Heavy Radionuclides at SHIPTRAP and High Precision Mass Measurements on Unstable Kr Isotopes at ISOLTRAP

Daniel Rodriguez Rubiales

High spin studies in bromine nuclei with N-Z

Trino Martínez

Estudio de la Resonancia Gamow-Teller en la Región del 146Gd mediante Espectroscopía Gamma de Absorción Total

Daniel Cano Ott

Estudio de la estructura del nucleo ⁷⁵ Br producido por la desintegración ß+ del ⁷⁵Kr

José Bea Gilabert

Espectroscopía Total en 145Sm: Estudio de estado de tres cuasipartículas y acoplamiento a las vibraciones del "core"

Lluís Miquel García Raffi

Estudio experimental de la desintegración Gamow-Teller de núcleos impar-impar en la zona de los lantánidos

Andrés Gadea Raga

EN ESTA TESIS SE HA ESTUDIADO EXPERIMENTALMENTE LA DESINTEGRACION GAMOW-TELLER DE LOS ESTADOS FUNDAMENTALES E ISOMERICOS DE LOS NUCLEOS IMPAR-IMPAR, EN LA REGION DEL 146GD, CON Z 64 Y N 82. PARA ESTOS NUCLEOS EXISTE UNA UNICA TRANSICION GAMOW-TELLER PERMITIDA: H11/2 H9/2. EN PARTICULAR SE HAN ESTUDIADO CON TECNICAS DE ESPECTROSCOPIA DE ALTA RESOLUCION LA DESINTEGRACION DE LOS NUCLEOS 150, 152 Y 154 HO, 152, 154 Y 156 TM Y 154, 156 Y 158 LU. POR MEDIO DE TECNICAS DE ABSORCION TOTAL SE HA PODIDO CALCULAR LA B(GT) (PROBABILIDAD DE DESINTEGRACION GAMOW-TELLER) TOTAL DE LA DESINTEGRACION DE NUCLEOS 156 TM Y 158 LU. EN ADICION SE HA MEDIDO EN NUCLEO 156 TM EN HAZ HASTA 10 MEV DE ENERGIA DE EXCITACION.

Determinación experimental de la interacción residual de dos huecos de protón en el núcleo 144Sm

Francisco Javier Rico