Measurements of the capture cross sections of natural silver in the resonance range with the time of flight technique

Neutron capture cross section measurements have been performed at the time-of-flight facility GELINA of the EC-JRC-Geel. Prompt gamma rays, originating from a natural silver sample, were detected by a pair of C6D6 liquid scintillation detectors. The total energy detection principle in combination with the pulse height weighting technique has been used. In this contribution the experimental details together with the data reduction process are described. In addition, first results of calculations with REFIT are presented to verify the quality of recommended cross section data in the resolved resonance region

Measurement of the 14^{14}N(n,p)14^{14}C cross section at the CERN n_TOF facility from sub-thermal energy to 800 keV

Background: The $^{14}$N(n,p)$^{14}$C reaction is of interest in neutron capture therapy, where nitrogen-related dose is the main component due to low-energy neutrons, and in astrophysics, where 14N acts as a neutron poison in the s-process. Several discrepancies remain between the existing data obtained in partial energy ranges: thermal energy, keV region and resonance region. Purpose: Measuring the 14N(n,p)14C cross section from thermal to the resonance region in a single measurement for the first time, including characterization of the first resonances, and providing calculations of Maxwellian averaged cross sections (MACS). Method: Time-of-flight technique. Experimental Area 2 (EAR-2) of the neutron time-of-flight (n_TOF) facility at CERN. $^{10}$B(n,${\alpha}$)$^7$Li and $^{235}$U(n,f) reactions as references. Two detection systems running simultaneously, one on-beam and another off-beam. Description of the resonances with the R-matrix code sammy. Results: The cross section has been measured from sub-thermal energy to 800 keV resolving the two first resonances (at 492.7 and 644 keV). A thermal cross-section (1.809$\pm$0.045 b) lower than the two most recent measurements by slightly more than one standard deviation, but in line with the ENDF/B-VIII.0 and JEFF-3.3 evaluations has been obtained. A 1/v energy dependence of the cross section has been confirmed up to tens of keV neutron energy. The low energy tail of the first resonance at 492.7 keV is lower than suggested by evaluated values, while the overall resonance strength agrees with evaluations. Conclusions: Our measurement has allowed to determine the $^{14}$N(n,p) cross-section over a wide energy range for the first time. We have obtained cross-sections with high accuracy (2.5 %) from sub-thermal energy to 800 keV and used these data to calculate the MACS for kT = 5 to kT = 100 keV.Comment: 18 pages, 15 figures, 4 table

Neutron capture cross section measurements of Am-241 at the n_TOF facility

Neutron capture on Am-241 plays an important role in the nuclear energy production and also provides valuable information for the improvement of nuclear models and the statistical interpretation of the nuclear properties. A new experiment to measure the Am-241(n,gamma) cross section in the thermal region and the first few resonances below 10 eV has been carried out at EAR2 of the n_TOF facility at CERN. Three neutron-insensitive C6D6 detectors have been used to measure the neutron-capture gamma cascade as a function of the neutron time of flight, and then deduce the neutron capture yield. Preliminary results will be presented and compared with previously obtained results at the same facility in EAR1. In EAR1 the gamma-ray background at thermal energies was about 90% of the signal while in EAR2 is up to a 25 factor much more favorable signal to noise ratio. We also extended the low energy limit down to subthermal energies. This measurement will allow a comparison with neutron capture measurements conducted at reactors and using a different experimental technique

Influence of the delayed neutron group parameters on reactivity estimation by rod drop analysis

Recent efforts have been undertaken in the reevaluation of the delayed neutron group parameters, namely, the half-life periods and the delayed neutron fractions Indeed, 8-group parameters are now proposed instead of the standard Tuttle 6-group. Moreover, several laboratories among the nuclear engineering community are conducting researches to provide a standard set of group time constants that can suit a lot of fissile isotopes. Since, delayed neutron parameters are widely used in order to estimate reactivity by conducting dynamic experiments, it is worth investigating the influence of the group model (i.e. the number of groups and the number of fitted parameters) on reactivity estimates. This paper focuses on two issues. First, various sets of parameters obtained from JEFF-3.1 precursors data fits are compared. Using simulated rod drop transients, we discuss the uncertainty and bias on reactivity estimation. It is shown that the sets of parameters with fewer fitted parameters and with a greater number of groups give very satisfactory results. Second, the influence of the neutron spectrum is quantified. Biases up to 14% are obtained. Standard statistical tests are done in order to identify such biases on rod drop reactivity estimate

towards a comprehensive physical modelling of fission chambers.

International audienceFission chambers are neutron detectors used in different types of nuclear reactors. The outgoing pulse shape and the detector response are important parameters from the design point of view. This article presents an overall organization of a new multi-physic, multiobjective fission chamber simulation code. Diverse aspects of the detector physics are implemented in an object-oriented Monte Carlo code, based on Geant4. This simulation code, called Lion, combines stochastic description of several phenomena: the neutron impact, neutron induced fission, fission product and heavy ion penetration, electron transport as well as particle interaction with matter. In addition, the code can calculate transport of charge carriers (free-electrons and ions), the electric field propagation and the acquisition system response. Thanks to the modular design, Lion allows for future extensions in form of new physics modules or new detector types. The article presents a few novel solutions which were not used in previous fission chamber simulation codes (e.g. new models of the fission fragment penetration or a new scheme for space charge modelling). To validate the code, a series of irradiation experiments will be conducted in Minerve reactor in CEA Cadarache and their outcome will be compared to the simulation results

Sensitivity of power spectral density noise techniques to numerical parameters in analyzing neutron noise experiments

International audiencePower spectral density (PSD) methods are well-known and widely used for the analysis of neutron noise experiments and obtaining the reactor's integral kinetic parameters, i.e., the effective delayed neutron fraction βeff and the prompt neutron generation time . Many uncertainties are usually associated with PSD methods, e.g., statistical fluctuations in the neutron flux, power drifts, uncertainties in the Diven factor, the integral fission rate, and in the reactivity value. However, the uncertainty associated with the numerical parameters used in the power spectrum calculation procedure is hardly discussed in the literature and generally overlooked.The aim of this paper is to study the uncertainties in the kinetic parameters of a reactor core, obtained by PSD methods, which are associated with the numerical parameters of the method. A comprehensive estimation of the kinetic parameters, including all other uncertainties, is not pursued. In this paper, PSD methods are implemented to analyze critical and subcritical configurations of the MINERVE zero power reactor in order to measure its integral kinetic parameters and . Both cross and auto power spectral densities are calculated and the kinetic parameters are obtained via Lorentzian curve fitting over the calculated PSD. The sensitivity of the obtained kinetic parameters to the choice of numerical parameters used for spectrum calculations is studied and found to be significant with respect to other uncertainties. A novel methodology is proposed for analyzing the kinetic parameters' sensitivity to the PSD calculations and for quantifying the associated uncertainties

Experimental study of columnar recombination in fission chambers

International audienceIn this paper, we present experimental saturation curves of a small gap miniature fission chamber obtained in the MINERVE reactor. The chamber is filled with argon at various pressures, and the fissile material can be coated on the anode, cathode, or both. For analyzing the recombination regime, we consider a model of columnar recombination and discuss its applicability to our chamber. By applying this model to the data, it is possible to estimate the ratio between the recombination coefficient k and an effective column radius b, appearing in the model. From these results, a routine measurement of the recombination regime is proposed in order to detect gas leakage. This online diagnosis would be beneficial in terms of lifetime and reliability of the neutron instrumentation of nuclear reactors

Thermal neutron activation experiments on Ag, In, Cs, Eu, V, Mo, Zn, Sn and Zr in the MINERVE facility

International audienceThe MAESTRO experimental program has been designed to improve nuclear data uncertainty on a large range of materials used for detection, absorption, moderation and structures in LWRs. It consists of pile-oscillation and neutron activation experiments, carried out in the MINERVE low power facility. For this program, the core configuration has been designed to be representative of HZP (Hot Zero Power) conditions of a typical PWR. Samples of high purity elements have been manufactured with severe technological constraints to reach a target accuracy of $\pm$2% (1$\sigma$) on the measurement. This paper presents a preliminary analysis of activation experiments, based on TRIPOLI4 Monte-Carlo calculations and various nuclear data libraries

Innovative local pile oscillator technique in the Minerve reactor: comparison between open loop and closed loop

International audiencePile oscillator techniques are powerful methods to measure small reactivity worth of isotopes of interest for nuclear data improvement. The principle is to create, in a well-known neutron flux, a weak and localized perturbation, the characteristics of which (level, spectrum) can be connected to physical parameters of the source (i.e. cross sections). Techniques are called open loop if the reactor power is free to drift. On the contrary, it is called closed loop if a power control system is activated. Such pile oscillator experiments have long been held in the Minerve reactor, operated by CEA at Cadarache. A closed loop reactivity oscillator method is implemented which is based on measuring the global reactivity perturbation associated to samples motion in the core center. The technique has recently been extended by introducing CEA-made miniature fission chambers close to the irradiation channel in order to measure local perturbations of the neutron flux. New detectors and associated acquisition systems were developed by CEA for that purpose. A measurement campaign, called MAESTRO-SL, took place in 2015 in Minerve. Its objective was to assess the feasibility of the method and investigate the possibility to disentangle global and local perturbation. Main experimental results are presented and discussed in this paper, which focus on comparing data processing methods of open vs closed loop setups.First, it is demonstrated that the open loop is equivalent to the closed loop setup. Uncertainty management and methods reproducibility are discussed. Second, results show that measuring local flux depression around the oscillated sample provides indeed valuable information regarding neutron cross sections. The technique is found to be very sensitivity to the capture cross section at the expense of scattering, making it very useful to measure small capture effects of highly scattering samples

Innovative hybrid pile oscillator technique in the Minerve reactor open loop vs. closed loop

International audiencePile oscillator techniques are powerful methods to measure small reactivity worth of isotopes of interest for nuclear data improvement. This kind of experiments has long been implemented in the Mineve experimental reactor, operated by CEA Cadarache. A hybrid technique, mixing reactivity worth estimation and measurement of small changes around test samples is presented here. It was made possible after the development of high sensitivity miniature fission chambers introduced next to the irradiation channel. A test campaign, called MAESTRO-SL, took place in 2015. Its objective was to assess the feasibility of the hybrid method and investigate the possibility to separate mixed neutron effects, such as fission/capture or scattering/capture. Experimental results are presented and discussed in this paper, which focus on comparing two measurements setups, one using a power control system (closed loop) and another one where the power is free to drift (open loop). First, it is demonstrated that open loop is equivalent to closed loop. Uncertainty management and methods reproducibility are discussed. Second, results show that measuring the flux depression around oscillated samples provides valuable information regarding partial neutron cross sections. The technique is found to be very sensitive to the capture cross section at the expense of scattering, making it very useful to measure small capture effects of highly scattering samples