75 research outputs found
Neutron-induced fission cross sections of Th 232 and U 233 up to 1 GeV using parallel plate avalanche counters at the CERN n_TOF facility
This work was partially supported by the European Com-
mission under Contract No. FIKW-CT-2000-00107 and by the
funding agencies of the participating institutes. D.T. acknowl-
edges support from the Spanish Ministerio de Educación
under Grant No. FPU-AP2007-04542.The neutron-induced fission cross sections of Th232 and U233 were measured relative to U235 in a wide neutron energy range up to 1 GeV (and from fission threshold in the case of Th232, and from 0.7 eV in case of U233), using the white-spectrum neutron source at the CERN Neutron Time-of-Flight (n_TOF) facility. Parallel plate avalanche counters (PPACs) were used, installed at the Experimental Area 1 (EAR1), which is located at 185 m from the neutron spallation target. The anisotropic emission of fission fragments were taken into account in the detection efficiency by using, in the case of U233, previous results available in EXFOR, whereas in the case of Th232 these data were obtained from our measurement, using PPACs and targets tilted 45° with respect to the neutron beam direction. Finally, the obtained results are compared with past measurements and major evaluated nuclear data libraries. Calculations using the high-energy reaction models INCL++ and ABLA07 were performed and some of their parameters were modified to reproduce the experimental results. At high energies, where no other neutron data exist, our results are compared with experimental data on proton-induced fission. Moreover, the dependence of the fission cross section at 1 GeV with the fissility parameter of the target nucleus is studied by combining those (p,f) data with our (n,f) data on Th232 and U233 and on other isotopes studied earlier at n_TOF using the same experimental setup.European Commission
FIKW-CT-2000-00107 ECMinisterio de Educación, Cultura y Deporte
FPU-AP2007-04542 MEC
Two absolute integral measurements of the Au-197(n,γ) stellar cross-section and solution of the historic discrepancies
This work has been supported by the Spanish projects FIS2015-69941-C2-1-P and FPA2016-77689-C2-1-R, (MINECO-FEDER,EU),A-FQM-371-UGR18 (Programa Operativo FEDER Andalucia 2014-2020), and the Spanish Association Against Cancer (AECC) (Grant No. PS16163811PORR).The Maxwellian averaged cross section (MACS) for Au-197(n,gamma) is used in neutron capture cross section measurements as a reference for reactions important for astrophysics, reactor and dosimetry applications. The traditionally adopted value for this reference cross section, in the energy range relevant for astrophysical (3 < E (n) < 200 keV), was obtained by Ratynski and Kappeler in 1988. However, the MACS calculated using the 2006 standards evaluation is approximately 6 % above the Ratynski and Kappeler (R&K) evaluation. Because of this discrepancy new experiments and reanalyses were done in an attempt to resolve the problem. In 2011 we started as well a series of integral experiments (activation) for determining the MACS -30 (kT=30 keV) of Au with two different Maxwellian neutron spectra: i) QMNS-25 (as R&K) and ii) MNS-30 (new method). Our results agree with those obtained with the standard evaluation. At present (2018), the updated MACS -30 has been included as standard Here we present the results of our measurements and the reasons for the lower value of the R&K measurement.Spanish projects (MINECO-FEDER,EU)
FIS2015-69941-C2-1-P
FPA2016-77689-C2-1-RSpanish Association Against Cancer (AECC)
PS16163811PORRPrograma Operativo FEDER Andalucia 2014-2020
A-FQM-371-UGR1
92Zr(n, γ) and (n,tot) measurements at the GELINA and n_TOF facilities
We are indebted to the anonymous referees for a careful reading of the manuscript and constructive remarks. This research was funded by the European Community Seventh Framework Programme FP7/2007-2011 under the Project CHANDA (Grant No. 605203) , by the European Commission within HORIZON2020 via the EURATOM project EUFRAT for transnational access and by the funding agencies of the participating institutes.Background: Stellar nucleosynthesis of elements heavier than iron is driven by neutron capture processes. Zr-92 is positioned at a strategic point along the slow nucleosynthesis path, given its proximity to the neutron magic number N = 50 and its position at the matching region between the weak and main slow processes.
Purpose: In parallel with recent improved astronomical data, the extraction of accurate Maxwellian averaged cross sections (MACSs) derived from a more complete and accurate set of resonance parameters should allow for a better understanding of the stellar conditions at which nucleosynthesis takes place.
Methods: Transmission and capture cross section measurements using enriched Zr-92 metallic samples were performed at the time-of flight facilities GELINA of JRC-Geel (BE) and n_TOF of CERN (CH). The neutron beam passing through the samples was investigated in transmission measurements at GELINA using a Li-glass scintillator. The gamma rays emitted during the neutron capture reactions were detected by C6D6 detectors at both GELINA and n_TOF.
Results: Resonance parameters of individual resonances up to 81 keV were extracted from a combined resonance shape analysis of experimental transmissions and capture yields. For the majority of the resonances the parity was determined from an analysis of the transmission data obtained with different sample thicknesses. Average resonance parameters were calculated.
Conclusions: Maxwellian averaged cross sections were extracted from resonances observed up to 81 keV. The MACS for kT = 30 keV is fully consistent with experimental data reported in the literature. The MACSs for kT less than or similar to 15 keV are in good agreement with those derived from the ENDF/B-VIII.0 library and recommended in the KADoNTS database. For kT higher than 30 keV differences are observed. A comparison with MACSs obtained with the cross sections recommended in the JEFF-3.3 and JENDL-4.0 libraries shows discrepancies even for kT less than or similar to 15 keV.European Commission 605203European Commission
European Commission Joint Research Centr
Neutron beam line for TOF measurements at the Spanish National Accelerator Lab (CNA)
The authors acknowledge financial support from the Spanish MINECO funds (FPA2013-47327-C2-I-R, BES-2014-068808, and FPA2016-77689-C2-1-R), AECC (PS16163811PORR), Spanish Fundacion ACS, Capitan Antonio, and FEDER Andalucia (US-1261006).A few years ago, the Spanish National Accelerator Lab (CNA) developed the first accelerator-based neutron facility in Spain called HiSPANoS (HiSPAlis Neutron Source). The first applications of the line were related to integral measurements applied to nuclear astrophysics, dosimetry and single event effects produced by neutrons in electronic devices. The successful of HiSPANoS pushed the enhancement of the facility. In collaboration with the NEC (R) Company, two devices were designed for pulsing ion beams (Chopper) and for compressing in time (Buncher) the pulsed beams. The Chopper-Buncher system has been already installed and commissioned. Proton and deuteron beams are delivered with repetition rates from 62.5 kHz to 2 MHz and 1 ns pulse width. In addition, a new line of the 3 MV Tandem Pelletron accelerator was designed for neutron time of-flight (TOF) experiments. Conventional devices and a dedicated Pick-Up for timing measurements form the new line. In order to check the performance of the whole TOF system, we have carried out the measurement of the neutron spectrum produced by Li-7(p,n)Be-7 reaction at E-p = 1912 keV. Such spectrum has been measured by the TOF technique few times and it can be considered a standard neutron field, in particular in nuclear astrophysics. The first result of such experiment performed at CNA is shown in some detail. The excellent performance of the accelerator, the Chopper-Buncher system and the acquisition system allow us to offer the TOF line at HiSPANoS-CNA to the neutron community.Spanish MINECO
FPA2013-47327-C2-I-R
BES-2014-068808
FPA2016-77689-C2-1-RAECC
PS16163811PORRSpanish Fundacion ACSCapitan AntonioFEDER Andalucia
US-126100
The Stellar 72Ge(n, γ) Cross Section for weak s-process: A First Measurement at n_TOF
In line with the principles that apply to scientific publishing and the CERN policy in matters of scientific publications, the n_TOF Collaboration recognises the work of V. Furman and P. Sedyshev (JINR, Russia), who have contributed to the experiment used to obtain the results described in this paper. This work was supported by the European Research Council ERC-2015-STG Nr. 677497, the Austrian Science Fund FWF (J 3503), the Science and Technology Facilities Council UK (ST/M006085/1), the Adolf Messer Foundation, the Croatian Science Foundation under the project 8570, the MSMT of the Czech Republic, the Charles University UNCE/SCI/013 project and by the funding agencies of the participating institutes.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 Ge-72(n, gamma) Maxwellian-Averaged Cross Section (MACS) has an important influence on the production of isotopes between Ge and Zr in the weak s-process in massive stars and so far only theoretical estimations are available. An experiment was carried out at the neutron time-of-flight facility n_TOF at CERN to measure the Ge-72(n, gamma) reaction for the first time at stellar neutron energies. The capture measurement was performed using an enriched (GeO2)-Ge-72 sample at a flight path length of 184 m, which provided high neutron energy resolution. The prompt gamma rays produced after neutron capture were detected with a set of liquid scintillation detectors (C6D6). The neutron capture yield is derived from the counting spectra taking into account the neutron flux and the gamma-ray detection efficiency using the Pulse Height Weighting Technique. Over 70 new neutron resonances were identified, providing an improved resolved reaction cross section to calculate experimental MACS values for the first time. The experiment, data analysis and the new MACS results will be presented including their impact on stellar nucleosynthesis, which was investigated using the post-processing nucleosynthesis code mppnp for a 25 solar mass model.European Research Council (ERC)
Spanish Government
677497Austrian Science Fund (FWF)
J 3503UK Research & Innovation (UKRI)
Science & Technology Facilities Council (STFC)
ST/M006085/1Adolf Messer FoundationCroatian Science Foundation
8570Ministry of Education, Youth & Sports - Czech RepublicCharles University
UNCE/SCI/01
Preliminary results on the 233U capture cross section and alpha ratio measured at n_TOF (CERN) with the fission tagging technique
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
High accuracy, high resolution U-235(n,f) cross section from n_TOF (CERN) from 18 meV to 10 keV
The 235U(n,f) cross section was measured in a
wide energy range (18 meV–170 keV) at the n_TOF facility
at CERN, relative to 6Li(n,t) and 10B(n,α) standard reactions,
with high resolution and accuracy, with a setup based on a
stack of six samples and six silicon detectors placed in the
neutron beam. In this paper we report on the results in the
region between 18 meV and 10 keV neutron energy. A resonance
analysis has been performed up to 200 eV, with the
code SAMMY. The resulting fission kernels are compared
with the ones extracted on the basis of the resonance parameters
of the most recent major evaluated data libraries.Acomparison
of the n_TOF datawith the evaluated cross sections is
also performed from thermal to 10 keV neutron energy for the
energy-averaged cross section in energy groups of suitably
chosen width. A good agreement, within 0.5%, is found on
average between the new results and the latest evaluated data
files ENDF/B-VIII.0 and JEFF-3.3, as well as with respect to
the broad group average fission cross section established in
the framework of the standard working group of IAEA (the
so-called reference file). However, some discrepancies, of up
to 4%, are still present in some specific energy regions. The
newdataset here presented, characterized by a unique combination
of high resolution and accuracy, low background and
wide energy range, can help to improve the evaluations from
the Resolved Resonance Region up to 10 keV, also reducing
the uncertainties that affect this region.CERN (European Organization for Nuclear Research
Study of the photon strength functions and level density in the gamma decay of the n + 234U reaction
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 4Ï€ 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
Measurement of the 244Cm and 246Cm neutron-induced capture cross sections at the n_TOF facility
The neutron capture reactions of the 244Cm and 246Cm isotopes open
the path for the formation of heavier Cm isotopes and heavier elements such as
Bk and Cf in a nuclear reactor. In addition, both isotopes belong to the minor
actinides with a large contribution to the decay heat and to the neutron emission
in irradiated fuels. There are only two previous 244Cm and 246Cm capture cross
section measurements: one in 1969 using a nuclear explosion [1] and the most
recent data measured at J-PARC in 2010 [2]. The data for both isotopes are very
scarce due to the di culties in performing the measurements: high intrinsic
activity of the samples and limited facilities capable of providing isotopically
enriched samples.
We have measured both neutron capture cross sections at the n_TOF Experimental
Area 2 (EAR-2) with three C6D6 detectors and also at Area 1 (EAR-1)
with the TAC. Preliminary results assessing the quality and limitations (background
subtraction, measurement technique and counting statistics) of this new
experimental datasets are presented and discussed
The neutron time-of-flight facility n-TOF at CERN: Phase II
Neutron-induced reactions are studied at the neutron time-of-flight facility n-TOF at CERN. The facility uses 6∼ns wide pulses of 20 GeV/c protons impinging on a lead spallation target. The large neutron energy range and the high instantaneous neutron flux combined with high resolution are among the key characteristics of the facility. After a first phase of data taking during the period 2001-2004, the facility has been refurbished with an upgraded spallation target and cooling system for a second phase of data taking which started in 2009. Since 2010, the experimental area at 185 m where the neutron beam arrives, has been modified into a worksector of type A, allowing the extension of the physics program to include neutron-induced reactions on radioactive isotopes
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