22 research outputs found
Medida de la probabilidad de emisiĂłn de neutrones retardados de âžâ¶As y âčÂčBr
Tesis inĂ©dita de la Universidad Complutense de Madrid, Facultad de Ciencias FĂsicas, leĂda el 15-12-2020Nuclear technology developments and nuclear astrophysics studies impose high data requirements on neutron-rich nuclei in terms of amount and quality. Radioactive decay data on -delayed neutron emitters are of particular relevance given the important role delayed neutrons play in both fields. In 2013, the International Atomic Energy Agency (IAEA) started Coordinated Research Project (CRP) on a reference database for beta-delayed neutron emission evaluation [170]. The database, which is in the process of publication [168], includes -delayed neutron emission probabilities for 309 nuclei, about a hundred more than Evaluated Nuclear Structure Data File (ENSDF) to date (as of July, 2020) [7,8]. But there is a long road ahead to meet the delayed neutron data requirements of nowadays nuclear science and technology. The recent large-scale theoretical evaluation by Marketinet al. [6] on 5409 neutron-rich nuclei predicts that 90% of them are -delayed neutron emitters...El desarrollo de tecnologĂa nuclear y los estudios de astrofĂsica nuclear imponen altos requerimientos en tĂ©rminos de cantidad y calidad a los datos nucleares de nĂșcleos ricos en neutrones. Los datos sobre el decaimiento radioactivo de nĂșcleos emisores de neutrones retardados son de particular relevancia dado el importante papel que estos neutrones juegan en ambos campos. En 2003, la OrganizaciĂłn Internacional de la EnergĂa AtĂłmica (OIEA)comenzĂł un Proyecto de InvestigaciĂłn Coordinado (CRP) para la creaciĂłn de una base de datos de referencia para evaluaciones de la emisiĂłn de neutrones retardados tras el decaimiento beta. La base de datos, que estĂĄ en proceso de publicaciĂłn [168], incluye probabilidades de emisiĂłn de neutrones retardados para 309 nĂșcleos, aproximadamente cien mĂĄs que el Evaluated Nuclear Structure Data File (ENSDF) a dĂa de hoy (a Julio, 2020) [7,8].Sin embargo, queda un camino muy largo por recorrer para satisfacer los requerimientos de la ciencia y la tecnologĂa nuclear actuales. La reciente evaluaciĂłn teĂłrica a gran escala de Marketin y col. [6] sobre 5409 nĂșcleos ricos en neutrones predice que el 90% de ellos emiten neutrones retardados tras el decaimiento beta...Fac. de Ciencias FĂsicasTRUEunpu
Measurements of neutron cross sections for advanced nuclear energy systems at n_TOF (CERN)
The n_TOF facility operates at CERN with the aim of addressing the request of high accuracy nuclear data for advanced nuclear energy systems as well as for nuclear astrophysics. Thanks to the features of the neutron beam, important results have been obtained on neutron induced fission and capture cross sections of U, Pu and minor actinides. Recently the construction of another beam line has started; the new line will be complementary to the first one, allowing to further extend the experimental program foreseen for next measurement campaigns
Neutron cross-sections for advanced nuclear systems: the n_TOF project at CERN
The study of neutron-induced reactions is of high relevance in a wide variety of fields, ranging from stellar nucleosynthesis and fundamental nuclear physics to applications of nuclear technology. In nuclear energy, high accuracy neutron data are needed for the development of Generation IV fast reactors and accelerator driven systems, these last aimed specifically at nuclear waste incineration, as well as for research on innovative fuel cycles. In this context, a high luminosity Neutron Time Of Flight facility, n_TOF, is operating at CERN since more than a decade, with the aim of providing new, high accuracy and high resolution neutron cross-sections. Thanks to the features of the neutron beam, a rich experimental program relevant to nuclear technology has been carried out so far. The program will be further expanded in the near future, thanks in particular to a new high-flux experimental area, now under construction
Neutron capture cross section measurement of 238U at the CERN n_TOF facility in the energy region from 1 eV to 700 keV
The aim of this work is to provide a precise and accurate measurement of the 238U(n,Îł) 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 1â3% 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,Îł) measurements performed at the n_TOF CERN facility are presented in this work. The Îł-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
Experimental neutron capture data of 58Ni from the CERN n_TOF facility
The neutron capture cross section of 58Ni was measured at the neutron time of flight facility n_TOF at CERN, from 27 meV to 400 keV neutron energy. Special care has been taken to identify all the possible sources of background, with the so-called neutron background obtained for the first time using high-precision GEANT4 simulations. The energy range up to 122 keV was treated as the resolved resonance region, where 51 resonances were identified and analyzed by a multilevel R-matrix code SAMMY. Above 122 keV the code SESH was used in analyzing the unresolved resonance region of the capture yield. Maxwellian averaged cross sections were calculated in the temperature range of kT = 5 â 100 keV, and their astrophysical implications were investigated
GEANT4 simulation of the neutron background of the C6D6 set-up for capture studies at n_TOF
The neutron sensitivity of the C6D6 detector setup used at n_TOF facility for capture measurements has been studied by means of detailed GEANT4 simulations. A realistic software replica of the entire n_TOF experimental hall, including the neutron beam line, sample, detector supports and the walls of the experimental area has been implemented in the simulations. The simulations have been analyzed in the same manner as experimental data, in particular by applying the Pulse Height Weighting Technique. The simulations have been validated against a measurement of the neutron background performed with a natC sample, showing an excellent agreement above 1 keV. At lower energies, an additional component in the measured natC yield has been discovered, which prevents the use of natC data for neutron background estimates at neutron energies below a few hundred eV. The origin and time structure of the neutron background have been derived from the simulations. Examples of the neutron background for two different samples are demonstrating the important role of accurate simulations of the neutron background in capture cross-section measurements
Fission fragment angular distribution measurements of 235U and 238U at CERN n_TOF facility
Neutron-induced fission cross sections of 238U and 235U are used as standards in the fast neutron region up to 200 MeV. A high accuracy of the standards is relevant to experimentally determine other neutron reaction cross sections. Therefore, the detection effciency should be corrected by using the angular distribution of the fission fragments (FFAD), which are barely known above 20 MeV. In addition, the angular distribution of the fragments produced in the fission of highly excited and deformed nuclei is an important observable to investigate the nuclear fission process.
In order to measure the FFAD of neutron-induced reactions, a fission detection setup based on parallel-plate avalanche counters (PPACs) has been developed and successfully used at the CERN-n_TOF facility. In this work, we present the preliminary results on the analysis of new 235U(n,f) and 238U(n,f) data in the extended energy range up to 200 MeV compared to the existing experimental data
High accuracy 234U(n,f) cross section in the resonance energy region
New results are presented of the 234U neutron-induced fission cross section, obtained with high accuracy in the resonance region by means of two methods using the 235U(n,f) as reference. The recent evaluation of the 235U(n,f) obtained with SAMMY by L. C. Leal et al. (these Proceedings), based on previous n_TOF data [1], has been used to calculate the 234U(n,f) cross section through the 234U/235U ratio, being here compared with the results obtained by using the n_TOF neutron flux
High precision measurement of the radiative capture cross section of 238U at the n_TOF CERN facility
The importance of improving the accuracy on the capture cross-section of 238U has been addressed by the Nuclear Energy Agency, since its uncertainty significantly affects the uncertainties of key design parameters for both fast and thermal nuclear reactors. Within the 7th framework programme ANDES of the European Commission three different measurements have been carried out with the aim of providing the 238U(n,Îł) cross-section with an accuracy which varies from 1 to 5%, depending on the energy range. Hereby the final results of the measurement performed at the n_TOF CERN facility in a wide energy range from 1âeV to 700âkeV will be presented