Radiative capture on 242Pu for MOX fuel reactors

Proposal: Radiative capture on 242Pu for MOX fuel reactorsThe use of MOX fuel (mixed-oxide fuel made of UO2 and PuO2) in nuclear reactors allows substituting a large fraction of the enriched Uranium by Plutonium reprocessed from spent fuel. Indeed around 66% of the plutonium from spent fuel is made of 239Pu and 241Pu, which are fissile in thermal reactors. A typical reactor of this type uses a fuel with 7% reprocessed Pu and 93% depleted U, thus profiting from both the spent fuel and the remaining 238U following the 235U enrichment. With the use of such new fuel compositions rich in Pu the better knowledge of the capture and fission cross sections of the Pu isotopes becomes very important. This is clearly stated in the recent OECD NEA’s “High Priority Request List” and in the WPEC-26 “Uncertainty and target accuracy assessment for innovative systems using recent covariance data evaluations” report. In particular, a new series of cross section evaluations have been recently carried out jointly by the European (JEFF) and United States (ENDF) nuclear data agencies. As the new evaluations on 240Pu and 241Am have been already completed, 242Pu is the next to be reevaluated, and the scarceness of capture data (only two TOF measurements from 1973 and 1976 are available and disagree with each other) calls for a new time-of flight capture cross section measurement. This will be the first measurement in 40 years and, with the use of more advanced techniques, shall provide a more reliable and accurate result. We propose to measure the capture cross section of 242Pu in the region from thermal up to at least 60 keV, aiming for a high energy limit of 500 keV. The experiment would make use of an array of 4 low neutron sensitivity C6D6 detectors and be carried out at the n_TOF EAR-1 (185 m flight path) measuring station. Compared to the current uncertainty of 35%, this measurement aims at an improved accuracy between 7% and 12% depending on the energy region.Preprin

Measurement of the neutron capture cross-section of 236U

Proposal: Measurement of the neutron capture cross-section of 236UPreprin

Fission fragment angular distribution measurements of U-235 and U-238 at CERN n_TOF facility

Neutron-induced fission cross sections of U-238 and U-235 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 efficiency 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 U-235(n,f) and U-238(n,f) data in the extended energy range up to 200 MeV compared to the existing experimental data.Postprint (published version

Approaching the precursor nuclei of the third r-process peak with RIBs

The rapid neutron nucleosynthesis process involves an enormous amount of very exotic neutron-rich nuclei, which represent a theoretical and experimental challenge. Two of the main decay properties that affect the final abundance distribution the most are half-lives and neutron branching ratios. Using fragmentation of a primary 238U beam at GSI we were able to measure such properties for several neutron-rich nuclei from 208Hg to 218Pb. This contribution provides a short update on the status of the data analysis of this experiment, together with a compilation of the latest results published in this mass region, both experimental and theoretical. The impact of the uncertainties connected with the eta-decay rates and with beta-delayed neutron emission is illustrated on the basis of r-process network calculations. In order to obtain a reasonable reproduction of the third r-process peak, it is expected that both half-lives and neutron branching ratios are substantially smaller, than those based on FRDM+QRPA, commonly used in r-process model calculations. Further measurements around N 126 are required for a reliable modelling of the underlying nuclear structure, and for performing more realistic r-process abundance calculations.Postprint (published version

Commissionning of the n_TOF-Ph2 facility

The white spectrum neutron time-of-flight facility n_TOF is operating at CERN since 2001. The neutron beam has a very high instantaneous flux and high resolution in energy and it is delivered in the experimental area located 187 m downstream from the spallation target. The intense neutron fluence per proton burst results in a much enhanced signal to background ratio for neutron capture and fission reactions on radioactive isotopes, thus making the facility well suited for accurate measurement of neutron-induced reaction cross-sections. This is especially true to highly radioactive targets which are of major importance in new nuclear energy system such as Gen-IV reactors, especially those with fast spectrum. Combined with state-of-the-art detectors and with advanced data acquisition systems, the innovative characteristics of the n_TOF neutron beam allow collecting data on a variety of stable and radioactive isotopes of interest for nuclear astrophysics and for applications to advanced reactor technologies.Peer ReviewedPostprint (published version

Nuclear data measurements at the upgraded neutron time-of-flight facility n-TOF at CERN

Applications of nuclear data like neutron-induced reaction cross sections are related to research fields as stellar nucleosynthesis, the study of nuclear level densities and strength functions, and also play a key role in the safety and criticality assessment of existing and future nuclear reactors, in areas concerning radiation dosimetry, medical applications, transmutation of nuclear waste, accelerator-driven systems and fuel cycle investigations. The evaluations in nuclear data libraries are based both on experimental data and theoretical models. 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 its scientific measurement programme in 2002. 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.Postprint (published version

The n_TOF facility at CERN: performances and first physics results

The neutron Time of Flight (n_TOF) facility at CERN is a source of a wide-range (1 eV<En<250 MeV) flux of neutrons, generated by spallation of 20-GeV/c protons onto a solid lead target. The goal of the n_TOF is to provide unprecedented precision in neutron kinetic energy determination, which will in turn bring the much-needed precision in neutron-induced cross-section measurements. The unique features of the n_TOF facility (instantaneously very intense neutron flux, low duty cycle, high resolution, and low background) make possible the measurement of highly radioactive isotopes usually available in small quantities. Such measurements are vital for a range of studies in fields as diverse as nuclear technology, astrophysics, and fundamental nuclear physics. In this paper, the characteristics of the n_TOF facility will be described, together with the main features of the highperformance detectors and acquisition system used for cross-section measurements, with a summary of the results and experience acquired during the first years of operation.Postprint (published version

Measurement of the 240,242Pu(n,f) cross section at the CERN n_TOF facility

Knowledge of neutron cross sections of various plutonium isotopes and other minor actinides is crucial for the design of advanced nuclear systems. The 240, 242Pu(n,f) cross sections were measured at the CERN n_TOF facility, taking advantage of the wide energy range (from thermal to GeV) and the high instantaneous flux of the neutron beam. In this work, preliminary results for 242Pu are presented along with a theoretical cross section calculation performed with the EMPIRE code.Postprint (published version

Measurements of 90,91,92,93,94,96-Zr neutron capture cross sections at the n_TOF facility at CERN

Neutron capture cross sections for zirconium isotopes have important implications in the field of nuclear astrophysics. In particular they play a key role in ascertaining the neutron density in the He burning zone of the Red Giant stars. Neutron capture cross sections of 90,91,92,93,94,96Zr have been measured over the energy range from 1 eV to 1 MeV at the spallation neutron facility n_TOF at CERN. Based on these data, capture resonance strengths and Maxwellian-averaged cross sections were determined with much improved accuracy.Postprint (published version

Measurement of the 197-Au(n,gamma ) cross section at n_TOF, - a step forward towards a new standard for capture cross section measurements

The accurate knowledge of the 197Au(n,g ) reaction cross section is of great importance, since this reaction if often used as a reference in capture cross section measurements relevant to Nuclear Astrophysics, as well as for neutron flux determination in nuclear power reactors. With the aim of improving the accuracy of the neutron capture cross section on 197Au, extensive measurements were performed at the n_TOF facility at CERN with two detection systems: a total absorption calorimeter and a set of C6D6 detectors. The capture yield and the resonance parameters have been determined in the energy range from 1 eV to 5 keV and compared with evaluated data files. The present capture kernels are on average in good agreement with tabulated data, although sizable differences are observed for several resonances. A few new resonances are also reported.Postprint (published version