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Experimental nuclear data and simulations are crucial for new radiotherapies of cancer. In Boron Neutron Capture Therapy (BNCT), an accurate knowledge of neutron interaction with the elements present in body tissues is key for dosimetry. At low neutron energies, the reaction between neutrons and nitrogen, 14N(n,p), becomes a main component of the dose in most human tissues. There are several discrepancies in previous measurements of this reaction, which increase the uncertainty in dose estimations needed for treatment planning. Furthermore, Neutron Capture Therapy needs intense and clean neutron sources, suitable for therapy and with the possibility of being installed in hospital environments, getting over previous facilities at nuclear reactors. This thesis aims to contribute to these two topics. The nuclear data from a new measurement of the 14N(n,p) reaction, carried out at the n TOF Facility at CERN will be shown. The new high-accuracy data span from 8 meV to 800 keV, covering the range of interest for BNCT, including the thermal point (25.3 meV), whose cross-section value was found to be at 1.809 0.045 b. The implications of this new nuclear data to BNCT dosimetry will be explored via Monte Carlo simulations. A new Beam Shaping Assembly (BSA) will be proposed for a proton acceleratorbased neutron source, using 30 mA of 2.1 MeV protons onto a lithium target. This BSA produces a high-intensity and well-collimated neutron eld that also provides low contamination from gamma radiation and both thermal and fast neutrons. The spectrum of the beam coming out of the BSA aperture will be shown to be adequate for BNCT treatments, full lling all recommendations from the IAEA and also performing well in in-phantom dose simulations. The new nuclear data and the proposed neutron beam will be put together as input in order to develop a Treatment Planning System (TPS). The TPS processes medical images (DICOM) and runs Monte Carlo simulations of neutron transport through the model of the patient, allowing to compute dose estimations in the tumor and surrounding tissues, including organs at risk. The TPS will be used to perform a pilot test simulation with a realistic case of a Glioblastoma patient., El uso de datos nucleares experimentales y simulaciones es crucial para nuevas formas de radioterapia contra el cáncer. En la Terapia mediante Captura de Neutrones por Boro (BNCT, por sus siglas en ingles), un conocimiento preciso de la interacción de los neutrones con los elementos presentes en los tejidos corporales es clave para la dosimetría. La reacción de los neutrones con el nitrógeno, 14N(n,p), es una componente muy importante de la dosis en la mayor a de tejidos humanos para neutrones de baja energía. Existen discrepancias entre varias medidas anteriores de esta reacción, lo que aumenta la incertidumbre en las estimaciones de dosis necesarias para plantificar un tratamiento. Por otra parte, la Terapia por Captura de Neutrones necesita fuentes de neutrones intensas y limpias, adecuadas para terapia y que tengan la posibilidad de instalarse en el entorno hospitalario, a diferencia de las instalaciones anteriores que se han construido en reactores nucleares. Esta tesis pretende contribuir a resolver estos dos problemas. Se mostrarán los datos nucleares obtenidos en una nueva medida de la reacción 14N(n,p) realizada en las instalaciones de neutrones por tiempo de vuelo (n TOF) del CERN. Los nuevos datos de alta precisión abarcan el rango entre 8 meV y 800 keV, cubriendo por completo el rango de interés para BNCT, lo que incluye el punto térmico (25.3 meV), para el cual se ha encontrado que la sección e caz es 1.809 0.045 b. Las repercusiones de estos nuevos datos nucleares para la dosimetría en BNCT serán exploradas usando simulación Monte Carlo. Se propone un nuevo dispositivo conformador de haces (Beam Shaping Assembly, BSA), diseñado para una fuente de neutrones basada en acelerador, con 30 mA de protones a 2.1 MeV sobre un blanco de litio. Esta BSA produce un campo de neutrones colimado de alta intensidad que además presenta una contaminación baja por radiación gamma y neutrones rápidos o térmicos. Se mostrar a que el espectro del haz de neutrones a la salida de la apertura de la BSA es adecuado para tratamientos de BNCT, cumpliendo todas las recomendaciones del Organismo Internacional para la Energía Atómica (OIEA) y además mostrando buenos resultados con simulaciones de dosis en maniquíes. Tanto los nuevos datos nucleares como el haz de neutrones propuesto se combinarán como inputs en el desarrollo de un Sistema de Planificación de Tratamientos (TPS, por sus siglas en inglés). El TPS procesa imágenes médicas DICOM y ejecuta simulaciones Monte Carlo de transporte de neutrones en un modelo del paciente, lo que permite calcular la dosis en el tumor y los tejidos circundantes, incluyendo los órganos de riesgo. El TPS ser a usado para realizar una simulación de prueba piloto con un caso realista de paciente con Glioblastoma., Tesis Univ. Granada.

This manuscript describes the design work and the R&D activities carried out for the third-generation neutron production target in the neutron Time-Of-Flight (n_TOF) facility at the European Laboratory for Particle Physics (CERN). The facility is composed of a neutron source coupled to two flight paths delivering a white-spectrum, pulsed neutron beam to two experimental areas. The neutrons are produced by the spallation reaction induced in lead-based target by a pulsed high-intensity proton beam with a momentum of 20 GeV/c extracted by the Proton Synchrotron, one of the circular particle accelerators of the CERN complex. The impact of high-energy proton beam pulses induces severe thermo-mechanical loads on the target, posing challenges for the target design. A description of the design process is provided, which started with the examination of multiple potential target concepts by means of numerical codes aimed to find a design offering an optimal combination of physics performance and thermo-mechanical response. The process converges towards a final design based on a series of pure-lead blocks supported by an anticreep structure in aluminum alloy and cooled by nitrogen gas. Novel approaches to simulate material response are presented and degraded operational scenarios are analyzed. The execution of two beam irradiation tests on target prototypes in a dedicated CERN facility is described, followed by post-irradiation examinations including neutron tomography inspections. The manuscript ends presenting the results of cyclic mechanical characterization tests on pure lead at different temperatures and strain-rates, and the proposal of a new constitutive model to better simulate the evolution of the material behaviour when subjected to repeated impacts of proton beam pulses.

The cerium oxide material for this measurement was provided by T. Katabuchi of the Tokyo Institute of Technology., An accurate measurement of the 140Ce(n,g) energy-dependent cross-section was performed at the n_TOF facility at CERN. This cross-section is of great importance because it represents a bottleneck for the s-process nucleosynthesis and determines to a large extent the cerium abundance in stars. The measurement was motivated by the significant difference between the cerium abundance measured in globular clusters and the value predicted by theoretical stellar models. This discrepancy can be ascribed to an overestimation of the 140Ce capture cross-section due to a lack of accurate nuclear data. For this measurement, we used a sample of cerium oxide enriched in 140Ce to 99.4%. The experimental apparatus consisted of four deuterated benzene liquid scintillator detectors, which allowed us to overcome the difficulties present in the previous measurements, thanks to their very low neutron sensitivity. The accurate analysis of the p-wave resonances and the calculation of their average parameters are fundamental to improve the evaluation of the 140Ce Maxwellian-averaged cross-section.

An accurate measurement of the140Ce(n,ү) energy-dependent cross-section was performed at the n_TOF facility at CERN. This cross-section is of great importance because it represents a bottleneck for the s-process nucleosynthesis and determines to a large extent the cerium abundance in stars. The measurement was motivated by the significant difference between the cerium abundance measured in globular clusters and the value predicted by theoretical stellar models. This discrepancy can be ascribed to an overestimation of the140Ce capture cross-section due to a lack of accurate nuclear data. For this measurement, we used a sample of cerium oxide enriched in140Ce to 99.4%. The experimental apparatus consisted of four deuterated benzene liquid scintillator detectors, which allowed us to overcome the difficulties present in the previous measurements, thanks to their very low neutron sensitivity. The accurate analysis of the p-wave resonances and the calculation of their average parameters are fundamental to improve the evaluation of the140Ce Maxwellian-averaged cross-section. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

This work was supported by the Austrian Science Fund FWF (J3503), the Adolf Messer Foundation (Germany), the UK Science and Facilities Council (ST/M006085/1), and the European Research Council ERC-2015-StG No. 677497. We also acknowledge the support of the National Science Centre, Poland, under the grant UMO-2016/22/M/ST2/00183, the MSMT of the Czech Republic and the Croatian Science Foundation under the project IP-2018-01-8570., This manuscript summarizes the results of radiative neutron capture cross-section measurements on two stable germanium isotopes, Ge-70 and Ge-73. Experiments were performed at the n_TOF facility at CERN via the time-of-flight technique, over a wide neutron energy range, for all stable germanium isotopes (70,72,73,74, and 76). Results for Ge-70 [Phys. Rev. C 100, 045804 (2019)] and Ge-73 [Phys. Lett. B 790, 458 (2019)] are already published. In the field of nuclear structure, such measurements allow to study excited levels close to the neutron binding energy and to obtain information on nuclear properties. In stellar nucleosynthesis research, neutron induced reactions on germanium are of importance for nucleosynthesis in the weak component of the slow neutron capture processes., Austrian Science Fund (FWF) J3503, Adolf Messer Foundation (Germany), UK Science and Facilities Council ST/M006085/1, European Research Council (ERC), European Commission 677497, National Science Centre, Poland UMO-2016/22/M/ST2/00183, Ministry of Education, Youth & Sports - Czech Republic, Croatian Science Foundation IP-2018-01-8570

We measured the neutron capture-to-fission cross-section ratio (α ratio) and the capture cross section of $^{235}$U between 0.2 and 200 eV at the n_TOF facility at CERN. The simultaneous measurement of neutron-induced capture and fission rates was performed by means of the n_TOF BaF$_{2}$ Total Absorption Calorimeter (TAC), used for detection of γ rays, in combination with a set of micromegas detectors used as fission tagging detectors. The energy dependence of the capture cross section was obtained with help of the $^{6}$Li(n,t) standard reaction determining the n_TOF neutron fluence; the well-known integral of the $^{235}$U(n,f) cross section between 7.8 and 11 eV was then used for its absolute normalization. The α ratio, obtained with slightly higher statistical fluctuations, was determined directly, without need for any reference cross section. To perform the analysis of this measurement we developed a new methodology to correct the experimentally observed effect that the probabilities of detecting a fission reaction in the TAC and the micromegas detectors are not independent. The results of this work have been used in a new evaluation of $^{235}$U performed within the scope of the Collaborative International Evaluated Library Organisation (CIELO) Project, and are consistent with the ENDF/B-VIII.0 and JEFF-3.3 capture cross sections below 4 eV and above 100 eV. However, the measured capture cross section is on average 10% larger between 4 and 100 eV.

In the measurement of neutron capture cross-sections of fissile isotopes, the fission channel is a source of background which can be removed efficiently using the so-called fission-tagging or fission-veto technique. For this purpose a new compact and fast fission chamber has been developed. The design criteria and technical description of the chamber are given within the context of a measurement of the 233U(n, ������) cross-section at the n_TOF facility at CERN, where it was coupled to the n_TOF Total Absorption Calorimeter. For this measurement the fission detector was optimized for time resolution, minimization of material in the neutron beam and for alpha-fission discrimination. The performance of the fission chamber and its application as a fission tagging detector are discussed., This work was partially supported by the French NEEDS/NACRE Project and by the European Commission within HORIZON2020 via the EURATOM Project EUFRAT.

In the measurement of neutron capture cross-sections of fissile isotopes, the fission channel is a source of background which can be removed efficiently using the so-called fission-tagging or fission-veto technique. For this purpose a new compact and fast fission chamber has been developed. The design criteria and technical description of the chamber are given within the context of a measurement of the 233U(n, ) cross-section at the n_TOF facility at CERN, where it was coupled to the n_TOF Total Absorption Calorimeter. For this measurement the fission detector was optimized for time resolution, minimization of material in the neutron beam and for alpha-fission discrimination. The performance of the fission chamber and its application as a fission tagging detector are discussed. French NEEDS/NACRE Project European Commission within HORIZON2020 via the EURATOM Project EUFRAT

The 16O(n,alpha)13C reaction was proposed to be measured at the neutron time-of-flight (n_TOF) facility of CERN. To this purpose, a Double Frisch Grid Ionization Chamber (DFGIC) containing the oxygen atoms as a component in the counting gas coupled with a switch device in order to prevent the charge collection from the so-called gamma-flash has been developed at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), in Germany. The first 16O(n, alpha)13C measurement without seeing the charge of the γ-flash at n_TOF has been performed in November 2018. After the electronics did not suffer from the gamma-flash any more, another huge charge collection was discovered. Due to the high instantaneous flux at the n_TOF facility the amount of that induced charge from neutron induced background reactions was piling up so much that the recognition of 16O(n, alpha)13C reactions from that background was very difficult. For that reason another 16O(n, alpha)13C measurement at the time-of-flight facility nELBE at HZDR which has a low instantaneous flux, has been performed in April 2019. Both measurements from n_TOF and nELBE will be presented here.

The [Formula: see text]U(n,f) cross section plays a key role for nuclear physics due to its widespread use as a standard reference for neutron cross section measurements and for neutron flux measurements. Recent experimental data of the fission cross section have suggested the presence of discrepancies around 6–8% with respect to the most used libraries, precisely in the range between 10 keV and 30 keV. In order to shed light on this disagreement, an accurate measurement of the [Formula: see text]U(n,f) fission cross section has been performed at n_TOF facility @CERN, using the standard reactions 6Li(n,t) and [Formula: see text]B(n,[Formula: see text] as reference. A custom experimental setup based on a stack of silicon detectors sandwiched between pairs of [Formula: see text]U, 6Li and [Formula: see text]B targets, has been installed along the neutron beam line to intercept the same neutron flux, allowing the detection of the fission fragments and the products of the reference reactions at the same time. Such a technique allows calculation of the cross section via the “ratio method”, by normalizing the [Formula: see text]U(n,f) reaction yields with respect to the reference reactions and to the recommended data in the IAEA libraries; in particular, the integral between 7.8 and 11 eV has been chosen. Accurate Monte Carlo simulations have allowed evaluation of the neutron absorption in the different layers, as well as the detection efficiency of each detector. The data are in excellent agreement with the standard values and highlight the overestimation of the [Formula: see text]U(n,f) cross section between 9 and 18 keV in the most recent libraries.

The measurement of neutron induced reactions on unstable isotopes is of interest in many fields, from nuclear energy to astrophysics or applications; in particular transuranic isotopes are essential for the development of innovative nuclear reactors and for the management of the radioactive waste. In such measurements, the quality of the associated radioactive target is crucial for the success of the experiment, but in many cases the geometry, amount of mass and encapsulation of the target are not optimal, leading to limited results. In this work we propose to produce high quality radioactive targets for capture as a stack of thin targets using the techniques usually employed for fission measurements. In particular, we have succeeded in making a 242Pu target of nearly 100 mg by combining seven thin (1 mg/cm2) fission-like targets with 45 mm in diameter achieving a total backing thickness of only of aluminum. The target has been shown to perform successfully in experiments at both a neutron time-of-flight facility (n_TOF at CERN) and a thermal neutron beam (BRR at KFKI), providing the most accurate data from thermal up to 500 keV to date., This measurement has received funding from the EC FP7 Programme under the projects NEUTANDALUS (Grant No. 334315) and CHANDA (Grant No. 605203), the Spanish Ministry of Economy and Competitiveness projects FPA2014-53290-C2-2-P and FPA2016-77689-C2-1-R and the V Plan Propio de Investigación from the University of Seville .

This paper presents the results of a sensitivity and uncertainty (S&U) analysis on k eff performed on a BWR fuel assembly (FA) poisoned with gadolinia, as well as of a criticality and sensitivity study of the ZED-2 research reactor, in whose moderator has been dissolved. The evidence collected from the ZED-2 analysis, the BWR FA S&U analysis, and from other criticality benchmarks indicates that improvements should be introduced in the 157 Gd (n, γ) cross section evaluation, especially in the thermal and near-thermal ranges, and in its uncertainty. In 2016, to cope with this need, the n_TOF Collaboration conducted an experimental campaign to obtain new 157 Gd and 155 Gd cross section measurements. The preliminary information available from the analysis of the experimental data suggests that these new data have the potential to correct some of the discrepancies found between some experimental benchmarks and their simulation.

The 235U(n,f) cross section was measured in a wide energy range at n_TOF relative to 6Li(n,t) and 10B(n,alpha), with high resolution and in a wide energy range, with a setup based on a stack of six samples and six silicon detectors placed in the neutron beam. This allowed us to make a direct comparison of the reaction yields under the same experimental conditions, and taking into account the forward/backward emission asymmetry. A hint of an anomaly in the 10÷30 keV neutron energy range had been previously observed in other experiments, indicating a cross section systematically lower by several percent relative to major evaluations. The present results indicate that the evaluated cross section in the 9÷18 keV neutron energy range is indeed overestimated, both in the recent updates of ENDF/B-VIII.0 and of the IAEA reference data. Furthermore, these new high-resolution data confirm the existence of resonance-like structures in the keV neutron energy region. The new, high accuracy results here reported may lead to a reduction of the uncertainty in the 1÷100 keV neutron energy region. Finally, the present data provide additional confidence on the recently re-evaluated cross section integral between 7.8 and 11 eV.

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

Neutron capture data on intermediate mass nuclei are of key importance to nucleosynthesis in the weak component of the slow neutron capture processes, which occurs in massive stars. The (n, γ) cross section on 70Ge, which is mainly produced in the s process, was measured at the neutron time-of-flight facility n_TOF at CERN. Resonance capture kernels were determined up to 40 keV neutron energy and average cross sections up to 300 keV. Stellar cross sections were calculated from kT = 5 keV to kT = 100 keV and are in very good agreement with a previous measurement by Walter and Beer (1985) and recent evaluations. Average cross sections are in agreement with Walter and Beer (1985) over most of the neutron energy range covered, while they are systematically smaller for neutron energies above 150 keV. We have calculated isotopic abundances produced in s-process environments in a 25 solar mass star for two initial metallicities (below solar and close to solar). While the low metallicity model reproduces best the solar system germanium isotopic abundances, the close to solar model shows a good global match to solar system abundances in the range of mass numbers A = 60–80.

73Ge(n,γ) cross sections were measured at the neutron time-of-flight facility n_TOF at CERN up to neutron energies of 300 keV, providing for the first time experimental data above 8 keV. Results indicate that the stellar cross section at kT=30 keV is 1.5 to 1.7 times higher than most theoretical predictions. The new cross sections result in a substantial decrease of 73Ge produced in stars, which would explain the low isotopic abundance of 73Ge in the solar system.

The accurate calculations of neutron-induced reaction cross sections are relevant for many nuclear applications. The photon strength functions and nuclear level densities are essential inputs for such calculations. These quantities for 235U are studied using the measurement of the gamma de-excitation cascades in radiative capture on 234U with the Total Absorption Calorimeter at n_TOF at CERN. This segmented 4π gamma calorimeter is designed to detect gamma rays emitted from the nucleus with high efficiency. This experiment provides information on gamma multiplicity and gamma spectra that can be compared with numerical simulations. The code diceboxc is used to simulate the gamma cascades while geant4 is used for the simulation of the interaction of these gammas with the TAC materials. Available models and their parameters are being tested using the present data. Some preliminary results of this ongoing study are presented and discussed.

233U is of key importance among the fissile nuclei in the Th-U fuel cycle. A particularity of 233U is its small neutron capture cross-section, which is on average about one order of magnitude lower than the fission cross-section. The accuracy in the measurement of the 233U capture cross-section depends crucially on an efficient capture-fission discrimination, thus a combined set-up of fission and γ-detectors is needed. A measurement of the 233U capture cross-section and capture-to-fission ratio was performed at the CERN n_TOF facility. The Total Absorption Calorimeter (TAC) of n_TOF was employed as γ-detector coupled with a novel compact ionization chamber as fission detector. A brief description of the experimental set-up will be given, and essential parts of the analysis procedure as well as the preliminary response of the set-up to capture are presented and discussed.

International audience; A novel MicroMegas detector based on microbulk technology with an embedded XY strip structure was developed, obtained by segmenting both the mesh and the anode in perpendicular directions. This results in a very low-mass device with good energy and spatial resolution capabilities. Such a detector is practically “transparent” to neutrons, being ideal for in-beam neutron measurements and can be used as a quasi-online neutron beam profiler at neutron time-of-flight facilities. A dedicated front end electronics and acquisition system has been developed and used. The first studies of this new detection system are presented and discussed.

International audience; This manuscript describes the production of three targets obtained by implantation of different activities of $^7$Be into thin aluminium disks. Two of the produced targets were used to measure the $^7$Be(n, p)$^7$Li cross section in the energy range of interest for the Big-Bang Nucleosynthesis. A third target was used to measure the cross sections of $^7$Be(n, p)$^7$Li and $^7$Be(n, $\alpha$)$^7$Li nuclear reactions with cold and thermal neutrons, respectively. This paper describes also the characterization of the first two targets, performed after the neutron irradiation, in terms of implanted $^7$Be activities and spatial distributions.

The spent fuel of current nuclear reactors contains fissile plutonium isotopes that can be combined with uranium to make mixed oxide (MOX) fuel. In this way the Pu from spent fuel is used in a new reactor cycle, contributing to the long-term sustainability of nuclear energy. However, an extensive use of MOX fuels, in particular in fast reactors, requires more accurate capture and fission cross sections for some Pu isotopes. In the case of 242Pu there are sizable discrepancies among the existing capture cross-section measurements included in the evaluations (all from the 1970s) resulting in an uncertainty as high as 35% in the fast energy region. Moreover, postirradiation experiments evaluated with JEFF-3.1 indicate an overestimation of 14% in the capture cross section in the fast neutron energy region. In this context, the Nuclear Energy Agency (NEA) requested an accuracy of 8% in this cross section in the energy region between 500 meV and 500 keV. This paper presents a new time-of-flight capture measurement on 242Pu carried out at n_TOF-EAR1 (CERN), focusing on the analysis and statistical properties of the resonance region, below 4 keV. The 242Pu(n, γ) reaction on a sample containing 95(4) mg enriched to 99.959% was measured with an array of four C6D6 detectors and applying the total energy detection technique. The high neutron energy resolution of n_TOF-EAR1 and the good statistics accumulated have allowed us to extend the resonance analysis up to 4 keV, obtaining new individual and average resonance parameters from a capture cross section featuring a systematic uncertainty of 5%, fulfilling the request of the NEA.

The 33S(n, α)30Si cross section has been measured at the neutron time-of-flight (n_TOF) facility at CERN in the neutron energy range from 10 to 300 keV relative to the 10B(n, α)7Li cross-section standard. Both reactions were measured simultaneously with a set of micromegas detectors. The flight path of 185 m has allowed us to obtain the cross section with high-energy resolution. An accurate description of the resonances has been performed by means of the multilevel multichannel R-matrix code SAMMY. The results show a significantly higher area of the biggest resonance (13.45 keV) than the unique high-resolution (n, α) measurement. The new parametrization of the 13.45-keV resonance is similar to that of the unique transmission measurement. This resonance is a matter of research in neutron-capture therapy. The 33S(n, α)30Si cross section has been studied in previous works because of its role in the production of 36S in stars, which is currently overproduced in stellar models compared to observations.

We report on the measurement of the 7Be(n, p)7Li cross section from thermal to approximately 325 keV neutron energy, performed in the high-flux experimental area (EAR2) of the n_TOF facility at CERN. This reaction plays a key role in the lithium yield of the big bang nucleosynthesis (BBN) for standard cosmology. The only two previous time-of-flight measurements performed on this reaction did not cover the energy window of interest for BBN, and they showed a large discrepancy between each other. The measurement was performed with a Si telescope and a high-purity sample produced by implantation of a 7Be ion beam at the ISOLDE facility at CERN. While a significantly higher cross section is found at low energy, relative to current evaluations, in the region of BBN interest, the present results are consistent with the values inferred from the time-reversal 7Li(p, n)7Be reaction, thus yielding only a relatively minor improvement on the so-called cosmological lithium problem. The relevance of these results on the near-threshold neutron production in the p+7Li reaction is also discussed.

The 241Am(n, γ) cross section has been measured at the n_TOF facility at CERN with the n_TOF BaF2 Total Absorption Calorimeter in the energy range between 0.2 eV and 10 keV. Our results are analyzed as resolved resonances up to 700 eV, allowing a more detailed description of the cross section than in the current evaluations, which contain resolved resonances only up to 150–160 eV. The cross section in the unresolved resonance region is perfectly consistent with the predictions based on the average resonance parameters deduced from the resolved resonances, thus obtaining a consistent description of the cross section in the full neutron energy range under study. Below 20 eV, our results are in reasonable agreement with JEFF-3.2 as well as with the most recent direct measurements of the resonance integral, and differ up to 20–30% with other experimental data. Between 20 eV and 1 keV, the disagreement with other experimental data and evaluations gradually decreases, in general, with the neutron energy. Above 1 keV, we find compatible results with previously existing values.

The slow neutron capture process (s-process) is responsible for producing about half of the elemental abundances heavier than iron in the universe. Neutron capture cross sections on stable isotopes are a key nuclear physics input for s-process studies. The 72Ge(n, γ) cross section has an important influence on production of isotopes between Ge and Zr during s-process in massive stars and therefore experimental data are urgently required. 72Ge(n, γ) was measured at the neutron time-of-flight facility n_TOF (CERN) for the first time at stellar energies. The measurement was performed using an enriched 72GeO2 sample at a flight path of 185m with a set of liquid scintillation detectors (C6D6). The motivation, experiment and current status of the data analysis are reported.

The neutron time-of-flight (n_TOF)facility at CERN is a unique worldwide pulsedneutron source to measure the nuclear data over thewidest energy range with two beam lines currentlyexploited. One is horizontal with a ~185 metersflight path, sending neutrons to experimental area-1(EAR-1). The second one is a new line sendingneutrons vertically to experimental area-2 (EAR-2)with a ~20 meters flight path.The first part of this PhD work is dedicated to thecharacterization of the beam (flux, geometricalprofile, energy spectrum) of the new EAR-2 neutronbeam, of the utmost importance for the experimentalproposals and analyses. An experiment was carriedout at EAR-2, based on PPAC detectors looking atfission of 235U, and the beam profile and neutronflux have been obtained for the entire availableenergy range (from thermal to 200 MeV).The second part of the thesis aims to study theneutron-induced fission of 237Np. 237Np isabundantly produced in present nuclear reactor andis one of the major long-lived components ofnuclear waste which can be considered as apotential target of incineration in fast neutronreactors. Consequently its neutron-induced fissioncross section has been measured at differentfacilities. However, significant discrepancies existbetween different experiments. Especially, therecent one performed at n_TOF in 2010 is about 6%higher by comparison to the evaluation data basedon previous experiments. Therefore an experimenthas been performed at n_TOF EAR-1 to measure itsfission cross section, in a configuration allowing anaccurate control of the detection efficiency, aimingto give a definite answer to the puzzle. In this work,we found that the efficiencies for different targetelements are different, even though they havesimilar thickness, reflecting the conditions ofelectrodeposition. After application of thisefficiency correction in the energy range from 1 to 5MeV, the newly extracted fission cross section is 2-3% lower compared with our previous measurementin 2010, however they are 3-4% higher than the; L’installation n_TOF au CERN est unesource pulsée de neutrons, unique au monde pour lamesure de données nucléaires sur le spectre enénergie le plus étendu, avec deux lignesactuellement exploitées. Une ligne horizontalealimente l’aire expérimentale 1 (EAR-1) avec unebase de vol de ~185 mètres. La seconde ligne estverticale et alimente l’aire 2 (EAR-2) à ~20 mètresde la cible de productionLa première partie de ce travail de thèse concernela caractérisation du faisceau de neutrons (flux,profil géométrique, spectre en énergie) de lanouvelle ligne EAR-2, particulièrement importantepour la définition des expériences et leur analyse.Une mesure a été réalisée à EAR-2, basée sur desdétecteurs PPAC enregistrant la fission de 235U, àpartir de laquelle nous avons obtenu le profil et leflux sur la gamme en énergie accessible (thermiqueà 200 MeV).La seconde partie de la thèse a pour but l’étudede la fission de 237Np. Cet isotope est abondammentproduit dans les réacteurs nucléaires actuels et estun des constituants des déchets à vie longue. A cetitre on peut le considérer comme une ciblepotentielle pour l’incinération en réacteur rapide.Ceci a motivé des mesures récentes de sa sectionefficace de fission. Cependant des déviationsimportantes sont apparues, en particulier la mesureeffectuée à n_TOF en 2010 est 6% supérieure auxévaluations basées sur les mesures antérieures. Cecia motivé une nouvelle mesure à n_TOF avec uneconfiguration permettant une mesure précise del’efficacité de détection, pour apporter une réponseau problème. Ce travail a permis de mettre enévidence une dépendance de l’efficacité dedétection avec l’élément, résultant des conditions del’électrodéposition. Après application de cettecorrection d’efficacité dans la région 1 à 5 MeV lasection efficace de fission ainsi extraite est 2 à 3%plus petite par rapport à la mesure de 2010,cependant elle reste 3 à 4% plus forte que lesévaluations

The aim of this work is to provide a precise and accurate measurement of the 238U(n, γ) reaction cross section in the energy region from 1 eV to 700 keV. This reaction is of fundamental importance for the design calculations of nuclear reactors, governing the behavior of the reactor core. In particular, fast reactors, which are experiencing a growing interest for their ability to burn radioactive waste, operate in the high energy region of the neutron spectrum. In this energy region most recent evaluations disagree due to inconsistencies in the existing measurements of up to 15%. In addition, the assessment of nuclear data uncertainty performed for innovative reactor systems shows that the uncertainty in the radiative capture cross section of 238U should be further reduced to 1–3% in the energy region from 20 eV to 25 keV. To this purpose, addressed by the Nuclear Energy Agency as a priority nuclear data need, complementary experiments, one at the GELINA and two at the n_TOF facility, were proposed and carried out within the 7th Framework Project ANDES of the European Commission. The results of one of these 238U(n, γ) measurements performed at the n_TOF CERN facility are presented in this work. The γ-ray cascade following the radiative neutron capture has been detected exploiting a setup of two C6D6 liquid scintillators. Resonance parameters obtained from this work are on average in excellent agreement with the ones reported in evaluated libraries. In the unresolved resonance region, this work yields a cross section in agreement with evaluated libraries up to 80 keV, while for higher energies our results are significantly higher.

The radiative capture cross section of a highly pure (99.999%), 6.125(2) grams and 9.56(5)×10^−4 atoms/barn areal density 238U sample has been measured with the Total Absorption Calorimeter (TAC) in the 185 m flight path at the CERN neutron time-of-flight facility n_TOF. This measurement is in response to the NEA High Priority Request list, which demands an accuracy in this cross section of less than 3% below 25 keV. These data have undergone careful background subtraction, with special care being given to the background originating from neutrons scattered by the 238U sample. Pileup and dead-time effects have been corrected for. The measured cross section covers an energy range between 0.2 eV and 80 keV, with an accuracy that varies with neutron energy, being better than 4% below 25 keV and reaching at most 6% at higher energies.

The accurate knowledge of the neutron-induced fission cross-sections of actinides and other isotopes involved in the nuclear fuel cycle is essential for the design of advanced nuclear systems, such as Generation-IV nuclear reactors. Such experimental data can also provide the necessary feedback for the adjustment of nuclear model parameters used in the evaluation process, resulting in the further development of nuclear fission models. In the present work, the 240Pu(n, f) cross-section was measured at CERN’s n_TOF facility relative to the well-known 235U(n, f) cross section, over a wide range of neutron energies, from meV to almost MeV, using the time-of-flight technique and a set-up based on Micromegas detectors. This measurement was the first experiment to be performed at n_TOF’s new experimental area (EAR-2), which offers a significantly higher neutron flux compared to the already existing experimental area (EAR-1). Preliminary results as well as the experimental procedure, including a description of the facility and the data handling and analysis, are presented.

The neutron time of flight (n_TOF) facility at CERN is a spallation source characterized by a white neutron spectrum. The innovative features of the facility, in the two experimental areas, (20 m and 185 m), allow for an accurate determination of the neutron cross section for radioactive samples or for isotopes with small neutron capture cross section, of interest for Nuclear Astrophysics. The recent results obtained at n_TOF facility are presented.

Significant work has been performed on the development of STEFF (SpecTrometer for Exotic Fission Fragments), a 2E2V (2-Energy 2-Velocity) spectrometer built by the University of Manchester Fission Group. The majority of this work was in the development of the time-of-flight systems, in particular the stop detector; with the main goals of improving the timing resolution and the detection efficiency of the ssion fragments. Further development of the STEFF spectrometer was done to enable 2E2V measurements of the $^{235}$U(n,f) reaction with coincident measurements using a white neutron spectra of energies ranging from 10 meV to 200 MeV provided by the n_TOF (neutron Time Of Flight) facility, CERN. The STEFF spectrometer was successfully operated twice on the Experimental Area-2 high flux pulsed neutron beam line resulting in 2E2V measurements for ssion events with neutron energies ranging from 20 meV to 10 MeV. The first experiment received 1.36 X 10$^{18}$ POT (Protons On Target) with stable conditions and the second received 1.53 X 10$^{18}$ POT with stable conditions. The development of the stop detector resulted in a replacement MWPC (Multi- Wire Proportional Counter) detector for the second of the two experiments. This allowed direct comparison for the timing coincidence resolution, $\sigma_{c}$, between the start and stop detectors and gave $\sigma_{c}$ = 0.81 $\pm$0:08 ns for the prior PPAC (Parallel Plate Avalanche Counter) detector and $\sigma_{c}$ = 0.40 $\pm$ 0:04 ns for the MWPC. The MWPC gave improved the detection efficiency per fission fragment of $\epsilon$= 0:67 compared to $\epsilon$ = 0:43 for the PPAC. The methods and research described in this work also provided alternate stop detector designs with greater performance. This work produced two large data sets from the two successful deployments of the STEFF spectrometer on the n TOF beam line that will be the future work of many nuclear structure scientists to come.

International audience; Nuclear data in general, and neutron-induced reaction cross sections in particular, are important for a wide variety of research fields. They play a key role in the safety and criticality assessment of nuclear technology, not only for existing power reactors but also for radiation dosimetry, medical applications, the transmutation of nuclear waste, accelerator-driven systems, fuel cycle investigations and future reactor systems as in Generation IV. Applications of nuclear data are also related to research fields as the study of nuclear level densities and stellar nucleosynthesis. Simulations and calculations of nuclear technology applications largely rely on evaluated nuclear data libraries. The evaluations in these libraries are based both on experimental data and theoretical models. Experimental nuclear reaction data are compiled on a worldwide basis by the international network of Nuclear Reaction Data Centres (NRDC) in the EXFOR database. The EXFOR database forms an important link between nuclear data measurements and the evaluated data libraries. CERN's neutron time-of-flight facility n TOF has produced a considerable amount of experimental data since it has become fully operational with the start of the scientific measurement programme in 2001. While for a long period a single measurement station (EAR1) located at 185 m from the neutron production target was available, the construction of a second beam line at 20 m (EAR2) in 2014 has substantially increased the measurement capabilities of the facility. An outline of the experimental nuclear data activities at CERN's neutron time-of-flight facility n TOF will be presented.