Neutron radiative capture on 242Pu: addressing the target accuracies for innovative nuclear systems

A low-carbon energy oulook to mitigate the impact of the climate change requires the progressive replacement of fossil fuel technologies by sources with low CO2 emissions. In this context, nuclear energy is expected to play a relevant role. Ensuring the long-term sustainability of nuclear energy points to the use of innovative nuclear systems, such as Accelerator Driven Systems and Generation-IV reactors and new fuel compositions, such MOX fuels aimed at the reduction of the nuclear waste. The design and operation of these nuclear innovative systems requires a better knowledge of the capture and ssion cross sections of the Pu isotopes. For the case of 242Pu, a reduction of the uncertainty in the fast region (2-500 keV) from the current 35% down to 8-12% is required. Moreover, aiming at improving the evaluation of the fast energy range in terms of average parameters, the OECD NEA High Priority Request List, requests high-resolution capture measurements with improved accuracy below 2 keV. The uncertainties also afect the thermal point, where previous experimental results deviate from each other by 20%. This thesis presents the new measurement of the 242Pu(n,) cross section from thermal to 500 keV combining diferent neutron beams and techniques. In collaboration with JGU Mainz and HZ Dresden-Rossendorf, we produced a sample consisting of a stack of sevenssion-like targets making a total of 95(4) mg of 242Pu electrodeposited on thin (11.5 um) aluminium backings. The thermal point was determined at the Budapest Research Reactor by means of Neutron Activation Analysis and Prompt Gamma Analysis, and the Resolved (1 eV - 4 keV) and Unresolved Resonance Regions (1 - 500 keV) were measured using a set of four Total Energy detectors at n TOF-EAR1. This manuscript deals with the description of the facilities and experimental techniques, the detailed data reduction for both experiments, and the discussion of the nal results and achieved accuracies for the capture cross section in each energy region

Geant4 simulations of the neutron production and transport in the n TOF spallation target

The neutron production and transport in the spallation target of the n TOF facility at CERN has been simulated with Geant4. The results obtained with the different hadronic Physics Lists provided by Geant4 have been compared with the experimental neutron flux in n TOF-EAR1. The best overall Agreement in both the absolute value and the energy dependence of the flux from thermal to 1GeV, is obtained with the INCL++ model coupled with the Fritiof Model(FTFP). This Physics List has been thus used to simulate and study the main features of the new n TOF-EAR2 beam line, currently in its commissioning phase

Measurement of the Pu-242(n, γ) cross section from thermal to 500 keV at the Budapest research reactor and CERN n_TOF-EAR1 facilities

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 FPA2013-45083-P, FPA2014-53290-C2-2-P and FPA2016-77689-C2-1-R and the V Plan Propio de Investigacion Programme from the University of Sevilla. Support from the German Federal Ministry for Education and Research (BMBF), contract number 03NUK13A, is gratefully acknowledged.The design and operation of innovative nuclear systems requires a better knowledge of the capture and fission cross sections of the Pu isotopes. For the case of capture on Pu-242. a reduction of the uncertainty in the fast region down to 8-12% is required. Moreover, aiming at improving the evaluation of the fast energy range in terms of average parameters, the OECD NEA High Priority Request List (HPRL) requests high-resolution capture measurements with improved accuracy below 2 keV. The current uncertainties also affect the thermal point, where previous experiments deviate from each other by 20%. A fruitful collaboration betwen JGU Mainz and HZ Dresden-Rossendorf within the EC CHANDA project resulted in a Pu-242 sample consisting of a stack of seven fission like targets making a total of 95(4) mg of Pu-242 electrodeposited on thin (11.5 mu m) aluminum backings. This contribution presents the results of a set of measurements of the Pu-242(n,gamma) cross section from thermal to 500 keV combining different neutron beams and techniques. The thermal point was determined at the Budapest Research Reactor by means of Neutron Activation Analysis and Prompt Gamma Analysis, and the resolved (1 eV - 4 keV) and unresolved (1 - 500 keV) resonance regions were measured using a set of four Total Energy detectors at the CERN n_TOF-EAR1.EC FP7 Programme under the project NEUTANDALUS 334315EC FP7 Programme under the project CHANDA 605203Spanish Ministry of Economy and Competitiveness FPA2013-45083-P FPA2014-53290-C2-2-P FPA2016-77689-C2-1-RV Plan Propio de Investigacion Programme from the University of SevillaFederal Ministry of Education & Research (BMBF) 03NUK13

On the role of secondary pions in spallation targets

We use particle-transport simulations to show that secondary pions play a crucial role for the development of the hadronic cascade and therefore for the production of neutrons and photons from thick spallation targets. In particular, for the n_TOF lead spallation target, irradiated with 20 GeV/c protons, neutral pions are involved in the production of ~90% of the high-energy photons; charged pions participate in ~40% of the integral neutron yield. Nevertheless, photon and neutron yields are shown to be relatively insensitive to large changes of the average pion multiplicity in the individual spallation reactions. We characterize this robustness as a peculiar property of hadronic cascades in thick targets.Comment: 17 pages, 14 figures. Submitted to Eur. Phys. J.

Experimental setup and procedure for the measurement of the 7Be(n,p)7Li reaction at n_TOF

Following the completion of the second neutron beam line and the related experimental area (EAR2) at the n_TOF spallation neutron source at CERN, several experiments were planned and performed. The high instantaneous neutron flux available in EAR2 allows to investigate neutron induced reactions with charged particles in the exit channel even employing targets made out of small amounts of short-lived radioactive isotopes. After the successful measurement of the 7Be(n,) cross section, the 7Be(n,p)7Li reaction was studied in order to provide still missing cross section data of relevance for Big Bang Nucleosynthesis (BBN), in an attempt to find a solution to the cosmological Lithium abundance problem. This paper describes the experimental setup employed in such a measurement and its characterization.Séptimo Programa Marco de la Comunidad Europea de la Energía Atómica (Euratom)-Proyecto CHANDA (No. 605203)Narodowe Centrum Nauki (NCN)-UMO-2012/04/M/ST2/00700-UMO-2016/22/M/ST2/00183Croatian Science Foundation-HRZZ 168

First tests of the applicability of γ\gamma-ray imaging for background discrimination in time-of-flight neutron capture measurements

In this work we explore for the first time the applicability of using $\gamma$-ray imaging in neutron capture measurements to identify and suppress spatially localized background. For this aim, a pinhole gamma camera is assembled, tested and characterized in terms of energy and spatial performance. It consists of a monolithic CeBr$_3$ scintillating crystal coupled to a position-sensitive photomultiplier and readout through an integrated circuit AMIC2GR. The pinhole collimator is a massive carven block of lead. A series of dedicated measurements with calibrated sources and with a neutron beam incident on a $^{197}$Au sample have been carried out at n_TOF, achieving an enhancement of a factor of two in the signal-to-background ratio when selecting only those events coming from the direction of the sample.Comment: Preprint submitted to Nucl. Instr. and Meth.

New detection systems for an enhanced sensitivity in key stellar (n,γ) measurements

Neutron capture cross-section measurements are fundamental in the study of astrophysical phenomena, such as the slow neutron capture (s-) process of nucleosynthesis operating in red-giant and massive stars. However, neutron capture measurements via the time-of-flight (TOF) technique on key s-process nuclei are often challenging. Difficulties arise from the limited mass (∼mg) available and the high sample-related background in the case of the unstable s-process branching points. Measurements on neutron magic nuclei, that act as s-process bottlenecks, are affected by low (n,γ) cross sections and a dominant neutron scattering background. Overcoming these experimental challenges requires the combination of facilities with high instantaneous flux, such as n_TOFEAR2, with detection systems with an enhanced detection sensitivity and high counting rate capabilities. This contribution reviews some of the latest detector developments in detection systems for (n,γ) measurements at n_TOF, such as i-TED, an innovative detection system which exploits the Compton imaging technique to reduce the dominant neutron scattering background and s-TED, a highly segmented total energy detector intended for high flux facilities. The discussion will be illustrated with results of the first measurement of key the s-process branching-point reaction 79Se(n,γ).European Research Council (ERC)European Union’s Horizon 2020 research and innovation programme (ERC Consolidator Grant project HYMNS, with grant agreement No. 681740)FJC2020-044688-IICJ220-045122-I funded by MCIN/AEI/ 10.13039/501100011033European Union NextGenerationEU/PRTRSpanish Ministerio de Ciencia e Innovación under grants PID2019- 104714GB-C21FPA2017-83946-C2-1-P, FIS2015-71688-ERCPIE-201750I26CERN policy in matters of scientific publications, the n_TOF Collaboratio

Pulse processing routines for neutron time-of-flight data

A pulse shape analysis framework is described, which was developed for n_TOF-Phase3, the third phase in the operation of the n_TOF facility at CERN. The most notable feature of this new framework is the adoption of generic pulse shape analysis routines, characterized by a minimal number of explicit assumptions about the nature of pulses. The aim of these routines is to be applicable to a wide variety of detectors, thus facilitating the introduction of the new detectors or types of detectors into the analysis framework. The operational details of the routines are suited to the specific requirements of particular detectors by adjusting the set of external input parameters. Pulse recognition, baseline calculation and the pulse shape fitting procedure are described. Special emphasis is put on their computational efficiency, since the most basic implementations of these conceptually simple methods are often computationally inefficient.Comment: 13 pages, 10 figures, 5 table

New perspectives for neutron capture measurements in the upgraded CERN-n_TOF Facility

This work has been carried out in the framework of a project funded by the European Research Council (ERC) under the European Union ' s Horizon 2020 research and innovation programme (ERC Consolidator Grant project HYMNS, with grant agreement No. 681740). This work was supported by grant FJC2020-044688-I funded by MCIN/AEI/10.13039/501100011033 and by European Union NextGenerationEU/PRTR. The authors acknowledge support from the Spanish Ministerio de Ciencia e Innovacion under grants PID2019-104714GB-C21, FPA2017-83946-C2-1-P, FIS2015-71688-ERC, CSIC for funding PIE-201750I26.The n_TOF facility has just undergone in 2021 a major upgrade with the installation of its third generation spallation target that has been designed to optimize the performance of the two n_TOF time-of-flight lines. This contribution describes the key features and limitations for capture measurements in the two beam lines prior to the target upgrade and presents first results of (n,gamma) measurements carried out as part of the commissioning of the upgraded facility. In particular, the energy resolution, a key factor for both increasing the signal-to background ratio and obtaining accurate resonance parameters, has been clearly improved for the 20 m long vertical beam-line with the new target design while keeping the remarkably high resolution of the long beamline n_TOF-EAR1. The improvements in the n_TOF neutron beam-lines need to be accompanied by improvements in the instrumentation. A review is given on recent detector R&D projects aimed at tackling the existing challenges and further improving the capabilities of this facility.European Research Council (ERC)European Union's Horizon 2020 research and innovation programme HYMNS 681740MCIN/AEI FJC2020-044688-IEuropean Union (EU)Instituto de Salud Carlos III Spanish Government PID2019-104714GB-C21, FPA2017-83946-C2-1-P, FIS2015-71688-ERCCSIC PIE-201750I2

The CERN n_TOF facility: a unique tool for nuclear data measurement

CNR*15 - 5th International Workshop on Compound-Nuclear Reactions and Related TopicsThe study of the resonant structures in neutron-nucleus cross-sections, and therefore of the compound-nucleus reaction mechanism, requires spectroscopic measurements to determine with high accuracy the energy of the neutron interacting with the material under study. To this purpose, the neutron time-of-flight facility n_TOF has been operating since 2001 at CERN. Its characteristics, such as the high intensity instantaneous neutron flux, the wide energy range from thermal to few GeV, and the very good energy resolution, are perfectly suited to perform high-quality measurements of neutron-induced reaction cross sections. The precise and accurate knowledge of these cross sections plays a fundamental role in nuclear technologies, nuclear astrophysics and nuclear physics. Two different measuring stations are available at the n_TOF facility, called EAR1 and EAR2, with different characteristics of intensity of the neutron flux and energy resolution. These experimental areas, combined with advanced detection systems lead to a great flexibility in performing challenging measurement of high precision and accuracy, and allow the investigation isotopes with very low cross sections, or available only in small quantities, or with very high specific activity. The characteristics and performances of the two experimental areas of the n_TOF facility will be presented, together with the most important measurements performed to date and their physics case. In addition, the significant upcoming measurements will be introduce