Advances and new ideas for neutron-capture astrophysics experiments at CERN n_TOF

The version of record of this article, first published in [The Europen Physics Journal A], is available online at Publisher’s website: http://dx.doi.org/10.1140/epja/s10050-022-00876-7This article presents a few selected developments and future ideas related to the measurement of (n,¿) data of astrophysical interest at CERN n_TOF. The MC-aided analysis methodology for the use of low-efficiency radiation detectors in time-of-flight neutron-capture measurements is discussed, with particular emphasis on the systematic accuracy. Several recent instrumental advances are also presented, such as the development of total-energy detectors with ¿ - ray imaging capability for background suppression, and the development of an array of small-volume organic scintillators aimed at exploiting the high instantaneous neutron-flux of EAR2. Finally, astrophysics prospects related to the intermediate i neutron-capture process of nucleosynthesis are discussed in the context of the new NEAR activation areaPeer ReviewedArticle escrit per 139 auors/autores C. Domingo-Pardo, V. Babiano-Suarez, J. Balibrea-Correa, L. Caballero, I. Ladarescu, J. Lerendegui-Marco, J. L. Tain, A. Tarifeño-Saldivia, O. Aberle, V. Alcayne, S. Altieri, S. Amaducci, J. Andrzejewski, M. Bacak, C. Beltrami, S. Bennett, A. P. Bernardes, E. Berthoumieux, M. Boromiza, D. Bosnar, M. Caamaño, F. Calviño, M. Calviani, D. Cano-Ott, A. Casanovas, F. Cerutti, G. Cescutti, S. Chasapoglou, E. Chiaveri, N. M. Chiera, P. Colombetti, N. Colonna, P. Console Camprini, G. Cortés, M. A. Cortés-Giraldo, L. Cosentino, S. Cristallo, S. Dellmann, M. Di Castro, S. Di Maria, M. Diakaki, M. Dietz, R. Dressler, E. Dupont, I. Durán, Z. Eleme, S. Fargier, B. Fernández, B. Fernández-Domínguez, P. Finocchiaro, S. Fiore, F. García-Infantes, A. Gawlik-Ramięga , G. Gervino, S. Gilardoni, E. González-Romero, C. Guerrero, F. Gunsing, C. Gustavino, J. Heyse, W. Hillman, D. G. Jenkins, E. Jericha, A. Junghans, Y. Kadi, K. Kaperoni, F. Käppeler, G. Kaur, A. Kimura, I. Knapová, U. Köster, M. Kokkoris, M. Krtička, N. Kyritsis, C. Lederer-Woods, G. Lerner, A. Manna, T. Martínez, A. Masi, C. Massimi, P. Mastinu, M. Mastromarco, E. A. Maugeri, A. Mazzone, E. Mendoza, A. Mengoni, P. M. Milazzo, I. Mönch, R. Mucciola, F. Murtas, E. Musacchio-Gonzalez, A. Musumarra, A. Negret, A. Pérez de Rada, P. Pérez-Maroto, N. Patronis, J. A. Pavón-Rodríguez, M. G. Pellegriti, J. Perkowski, C. Petrone, E. Pirovano, J. Plaza, S. Pomp, I. Porras, J. Praena, J. M. Quesada, R. Reifarth, D. Rochman, Y. Romanets, C. Rubbia, A. Sánchez, M. Sabaté-Gilarte, P. Schillebeeckx, D. Schumann, A. Sekhar, A. G. Smith, N. V. Sosnin, M. Stamati, A. Sturniolo, G. Tagliente, D. Tarrío, P. Torres-Sánchez, J. Turko, S. Urlass, E. Vagena, S. Valenta, V. Variale, P. Vaz, G. Vecchio, D. Vescovi, V. Vlachoudis, R. Vlastou, T. Wallner, P. J. Woods, T. Wright, R. Zarrella, P. ŽugecPostprint (published version

Constraints on the dipole photon strength for the odd uranium isotopes

Nuclear level densities (NLDs) and photon strength functions (PSFs), also called ¿ -ray or radiation strength functions, represent average properties of the nucleus in the regime of excitation where individual levels and transition probabilities by ¿ decay are not readily accessible by experimental or theoretical means. They are key ingredients for statistical calculations of the reaction cross sections involving ¿ rays via the Hauser-Feshbach approach [1], like inelastic scattering or neutron capture reactions.Peer ReviewedAquest article té 124 autors/autores J. Moreno-Soto, S. Valenta, E. Berthoumieux, A. Chebboubi, M. Diakaki, W. Dridi, E. Dupont, F. Gunsing, M. Krticka, O. Litaize, O. Serot, O. Aberle, V. Alcayne, S. Amaducci, J. Andrzejewski, L. Audouin, V. Bécares, V. Babiano-Suarez, M. Bacak, M. Barbagallo, Th. Benedikt, S. Bennett, J. Billowes, D. Bosnar, A. Brown, M. Busso, M. Caamaño, L. Caballero-Ontanaya, F. Calviño, M. Calviani, D. Cano-Ott, A. Casanovas, F. Cerutti, E. Chiaveri, N. Colonna, G. Cortés, M. A. Cortés-Giraldo, L. Cosentino, Cristallo, L. A. Damone, P. J. Davies, M. Dietz, C. Domingo-Pardo, R. Dressler, Q. Ducasse, I. Durán, Z. Eleme, B. Fernández-Domínguez, A. Ferrari, P. Finocchiaro, V. Furman, K. Göbel, A. Gawlik-Rami, S. Gilardoni, I. F. Gonçalves, E. González-RomeroC. Guerrero, S. Heinitz, J. Heyse, D. G. Jenkins, A. Junghans, F. Käppeler, Y. Kadi, A. Kimura, I. Knapová, M. Kokkoris, Y. Kopatch, D. Kurtulgil, I. Ladarescu, C. Lampoudis, C. Lederer-Woods, S. J. Lonsdale, D. Macina, A. Manna, T. Martínez, A. Masi, C. Massimi, P. Mastinu, M. Mastromarco, E. A. Maugeri, A. Mazzone, E. Mendoza, A. Mengoni, V. Michalopoulou, P. M. Milazzo, F. MingroneA. Musumarra, A. Negret, R. Nolte, F. Ogállar, A. Oprea, N. Patronis, A. Pavlik, J. Perkowski, L. Piersanti, C. Petrone, E. Pirovano, I. Porras, J. Praena, J. M. Quesada, D. Ramos-Doval, T. Rauscher, R. Reifarth, D. Rochman, M. Sabaté-Gilarte, A. Saxena, P. Schillebeeckx, D. Schumann, A. Sekhar, A. G. Smith, N. V. Sosnin, P. Sprung, A. Stamatopoulos, G. Tagliente, J. L. Tain, A. Tarifeño-Saldivia, L. Tassan-Got, P. Torres-Sánchez, A. Tsinganis, J. Ulrich, S. Urlass, G. Vannini, V. Variale, P. Vaz, A. Ventura, D. Vescovi, V. Vlachoudis, R. Vlastou, A. Wallner, P. J. Woods, T. Wright, P. ŽugecPostprint (published version

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

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

Simultaneous Gamma-Neutron Vision device: a portable and versatile tool for nuclear inspections

Abstract This work presents GN-Vision, a novel dual γ-ray and neutron imaging system, which aims at simultaneously obtaining information about the spatial origin of γ-ray and neutron sources. The proposed device is based on two position sensitive detection planes and exploits the Compton imaging technique for the imaging of γ-rays. In addition, spatial distributions of slow- and thermal-neutron sources (<100 eV) are reconstructed by using a passive neutron pin-hole collimator attached to the first detection plane. The proposed gamma-neutron imaging device could be of prime interest for nuclear safety and security applications. The two main advantages of this imaging system are its high efficiency and portability, making it well suited for nuclear applications were compactness and real-time imaging is important. This work presents the working principle and conceptual design of the GN-Vision system and explores, on the basis of Monte Carlo simulations, its simultaneous γ-ray and neutron detection and imaging capabilities for a realistic scenario where a 252Cf source is hidden in a neutron moderating container

A first prototype of C6D6 total-energy detector with SiPM readout for neutron capture time-of-flight experiments

Low efficiency total-energy detectors (TEDs) are one of the main tools for neutron capture cross section measurements utilizing the time-of-flight (TOF) technique. State-of-the-art TEDs are based on a CD liquid-scintillation cell optically coupled to a fast photomultiplier tube. The large photomultiplier tube represents yet a significant contribution to the so-called neutron sensitivity background, which is one of the most conspicuous sources of uncertainty in this type of experiments. Here we report on the development of a first prototype of a TED based on a silicon-photomultiplier (SiPM) readout, thus resulting in a lightweight and much more compact detector. Apart from the envisaged improvement in neutron sensitivity, the new system uses low voltage (+28 V) and low current supply (50 mA), which is more practical than the kV supply required by conventional photomultipliers. One important difficulty hindering the earlier implementation of SiPM readout for this type of detector was the large capacitance for the output signal when all pixels of a SiPM array are summed together. The latter leads to long pulse rise and decay times, which are not suitable for time-of-flight experiments. In this work we demonstrate the feasibility of a Schottky-diode multiplexing readout approach, that allows one to preserve the excellent timing properties of SiPMs, hereby paving the way for their implementation in future neutron TOF experiments.Postprint (published version