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

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

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.

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

The s-process in the Nd-Pm-Sm region: Neutron activation of 147Pm

The Nd-Pm-Sm branching is of interest for the study of the s-process, related to the production of heavy elements in stars. As 148Sm and 150Sm are s-only isotopes, the understanding of the branching allows constraining the s-process neutron density. In this context the key physics input needed is the cross section of the three unstable nuclides in the region: 147Nd (10.98 d half-life), 147Pm (2.62 yr) and 148Pm (5.37 d). This paper reports on the activation measurement of 147Pm, the longest-lived of the three nuclides. The cross section measurement has been carried out by activation at the SARAF LiLiT facility using a 56(2) μg target. Compared to the single previous measurement of 147Pm, the measurement presented herein benefits from a target 2000 times more massive. The resulting Maxwellian Averaged Cross Section (MACS) to the ground and metastable states in 148Pm are 469(50) mb and 357(27) mb. These values are 41% higher (to the ground state) and 15% lower (to the metastable state) than the values reported so far, leading however to a total cross section of 826(107) mb consistent within uncertainties with the previous result and hence leaving unchanged the previous calculation of the s-process neutron density.University of Seville [FPA2013-45083P, FPA2014-53290-C2-2-P, FPA2016-77689-C2-1-R]EC FP7 projects NeutAndalus [334315]CHANDA [605203

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.Croatian Science Foundation - Project No. 168

Preparation and characterization of 33-S samples for 33-S(n,alpha)30-Si cross-section measurements at the n_TOF facility at CERN

Thin 33S samples for the study of the 33S(n,a)30Si cross-section at the n_TOF facility at CERN were made by thermal evaporation of 33S powder onto a dedicated substrate made of kapton covered with thin layers of copper, chromium and titanium. This method has provided for the first time bare sulfur samples a few centimeters in diameter. The samples have shown an excellent adherence with no mass loss after few years and no sublimation in vacuum at room temperature. The determination of the mass thickness of 33S has been performed by means of Rutherford backscattering spectrometry. The samples have been successfully tested under neutron irradiation.Ministerio de Economía y Competitividad de España-FPA2013-47327- C2-1-R, FPA2014-53290-C2-2-P, FPA2016-77689-C2-1-RJunta de Andalucía-P11-FQM-8229Ministerio de Economía y Empresa de España (Fondos FEDER)-FIS2015-69941-C2-1-PAECC (Asociación Española Contra el Cáncer)-PS16163811POR

Thermal (n, γ) cross section and resonance integral of 171Tm

Background: About 50% of the heavy elements are produced in stars during the slow neutron capture process. The analysis of branching points allows us to set constraints on the temperature and the neutron density in the interior of stars. Purpose: The temperature dependence of the branch point 171Tm is weak. Hence, the 171Tm neutron capture cross section can be used to constrain the neutron density during the main component of the s process in thermally pulsing asymptotic giant branch (TP-AGB) stars. Methods: A 171Tm sample produced at the ILL was activated with thermal and epithermal neutrons at the TRIGA research reactor at the Johannes Gutenberg-Universität Mainz. Results: The thermal neutron capture cross section and the resonance integral have been measured for the first time to be σth = 9.9 ± 0.9 b and σRI = 193 ± 14 b. Conclusions: Based on our results, new estimations of the direct capture components’ impact on the Maxwellian-nAveraged cross sections (MACS) are possible.European Unions’s Seventh Framework Programme (FP/2007-2013

A compact fission detector for fission-tagging neutron capture experiments with radioactive fissile isotopes

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 ProjectEuropean Commission within HORIZON2020 via the EURATOM Project EUFRA

Recent results in nuclear astrophysics at the n_TOF facility at CERN

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