Neutron-capture Studies on (235)U and (238)U via AMS

Major nuclear data libraries exhibit some discrepancies for the neutron capture reactions (235)U(n,gamma) and (238)U(n,gamma) at keV energies. These differences reflect also the scatter of experimental data that are primarily based on time-of-flight measurements (TOF) with detection of prompt gamma-rays. We report here on an independent approach for studying such reactions: Neutron activations with subsequent accelerator mass spectrometry (AMS) measurement represent an independent technique, where interference from fission is completely excluded. Activations of natural uranium samples were performed with cold neutrons (Budapest), thermal (Vienna) and with neutrons of 25 and 500 keV (Karlsruhe) for the measurement of (235)U(n,gamma) and (238)U(n,gamma). The produced long-lived (236)U and the decay product of (239)U, (239)Pu, respectively, were subsequently counted by AMS at the Vienna Environmental Research Accelerator. This method for measuring the neutron capture cross section has the advantage that the involved systematic uncertainties are in no way correlated with those inherent to the TOF technique. Preliminary data for these neutron energies indicate good agreement with evaluations and seem to support in the keV energy range the reported lower cross-section values for both capture reactions

An Update of the Nuclear Data Standards Activities

To allow new experimental data and improvements in evaluation procedure to be used in a timely manner in producing standards evaluations, an IAEA Data Development Project was initiated. For this project, it was decided that, in addition to the traditional activities related to standards, studies should be done on the possibility of extending the energy ranges of the standards and whether to include "reference data" that are not as well known as the standards but can be very useful in the measurements of certain types of cross sections. Activities include; an update of the experimental database for the traditional standards; improvements in smoothing procedures for capture cross sections, a review of the status of (252)Cf spontaneous fission neutron spectrum measurements; new work on the (235)U thermal neutron induced fission neutron spectrum that is not a standard but is important for reactor applications; improved calculations for fission neutron spectra; consideration of the (197)Au capture reaction as a reference cross section for capture cross-section measurements at energies of importance to astrophysics; and a study of reference cross sections for prompt gamma-ray production in fast neutron-induced reactions. A review of the progress made by this project will be given

Study of Photon Strength Function of Actinides : the Case of U-235, Np-238 and Pu-241

The decay from excited levels in medium and heavy nuclei can be described in a statistical approach by means of Photon Strength Functions and Level Density distributions. The study of electromagnetic cascades following neutron capture based on the use of high efficiency detectors has been shown to be well suited for probing the properties of the Photon Strength Function of heavy (high level density) and/or radioactive (high background) nuclei. In this work we have investigated for the first time the validity of the recommended PSF of actinides, in particular U-235, Np-238 and Pu-241. Our study includes the search for resonance structures in the PSF below S-n and draws conclusions regarding their existence and their characteristics in terms of energy, width and electromagnetic nature.Peer reviewe

Neutron Capture Measurements on Minor Actinides at the n_TOF Facility at CERN : Past, Present and Future

The successful development of advanced nuclear systems for sustainable energy production and nuclear waste management depends on high quality nuclear data libraries. Recent sensitivity studies and reports [1-3] have identified the need for substantially improving the accuracy of neutron cross-section data for minor actinides. The n_TOF collaboration has initiated an ambitious experimental program for the measurement of neutron capture cross sections of minor actinides. Two experimental setups have been constructed for this purpose: a Total Absorption Calorimeter (TAC) [4] for measuring neutron capture cross-sections of low-mass and/or radioactive samples and a set of two low neutron sensitivity C(6)D(6) detectors for the less radioactive materials.Peer reviewe

The Role of Fe and Ni for S-process Nucleosynthesis and Innovative Nuclear Technologies

The accurate measurement of neutron capture cross sections of all Fe and Ni isotopes is important for disentangling the contribution of the s-process and the r-process to the stellar nucleosynthesis of elements in the mass range 60 < A < 120. At the same time, Fe and Ni are important components of structural materials and improved neutron cross section data is relevant in the design of new nuclear systems. With the aim of obtaining improved capture data on all stable iron and nickel isotopes, a program of measurements has been launched at the CERN Neutron Time of Flight Facility n_TOF

Characterization of the New n TOF Neutron Beam: Fluence, Profile and Resolution

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Improved Neutron Capture Cross Section Measurements with the n_TOF Total Absorption Calorimeter

The n_TOF collaboration operates a Total Absorption Calorimeter (TAC) [1] for measuring neutron capture cross-sections of low-mass and/or radioactive samples. The results obtained with the TAC have led to a substantial improvement of the capture cross sections of (237)Np and (240)Pu [2]. The experience acquired during the first measurements has allowed us to optimize the performance of the TAC and to improve the capture signal to background ratio, thus opening the way to more complex and demanding measurements on rare radioactive materials. The new design has been reached by a series of detailed Monte Carlo simulations of complete experiments and dedicated test measurements. The new capture setup will be presented and the main achievements highlighted