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In nuclear reactors operating in nominal conditions, delayed neutrons (DN) drive the kinetic behavior of the neutron population. Predicting this behavior requires therefore the knowledge of the DN yield per fission, average lifetime, and kinetic of their emission, which is described by precursors groups abundances. Characterizing the uncertainties of these macroscopic data is essential, especially as they are used to design the safety margins of reactors. Even for major fissile isotopes such as 235U and 239Pu, discrepancies still appear between the main nuclear data library. In 2018, CEA launched the ALDEN project (Average Lifetime of DElayed Neutrons), gathering several laboratories (CEA/DES and DRF, CNRS/LPSC, LP2i and LPC Caen, ENSICAEN, and University of Caen). This collaboration aims at producing new precise DN macroscopic data for reactor applications. Three experimental campaigns were conducted in 2018, 2019 and 2021 on the thermal fission of 233U, 235U and 239Pu at Institut Laue Langevin in Grenoble (France), giving high quality results. In February 2023, a new campaign was performed at the PTB Ion Accelerator Facility in Braunschweig (Germany), focusing on the fast fission of 238U. In particular, the DN yield was measured for eight energies between 1.5 and 19 MeV. This paper presents the experimental setup, that was adapted to the new facility and neutron spectrum. The methodology of analysis is also detailed, and some preliminary delayed neutron decay curves are showed.

The study of nuclear fission yields has a major impact on the characterization and understanding of the fission process and is mandatory for reactor applications. In the framework of a collaboration between the CEA, the LPSC and the ILL, a program of actinide fission yield measurements has been ongoing for several years at the LOHENGRIN spectrometer. However, the measurement of very low fission yields in the symmetry region and the heavy wing of the distributions is difficult to achieve due to the strong contamination by other masses with much higher yields and requires the development of a new experimental setup. This paper will first present the results of a new absolute measurement of the 235U(nth,f) mass yields using an ionization chamber placed at the exit of the spectrometer. Although very well documented in the literature, these yields show uncertainties lying from 3% to 10% with large discrepancies between libraries and a lack of correlation matrices. New experimental data obtained at the LOHENGRIN spectrometer will be detailed, along with the measurement method and the production of the experimental covariance matrix. The second part will show the development of a Time of Flight (ToF) line in order to improve the background rejection in the mass yield measurements. In the symmetry region, the precision of the measurement is limited by the background estimation due to the charge exchanges with the residual gas of the separator. We plan to analyze the events using a triple coincidence (∆E x E) x ToF, whereas today only (∆E x E) selection is available. The new ToF line is built using Si3N4 foils and secondary electron detectors (SED) for the start and stop detectors. We will present the choices made for the SED technology along with the progress achieved on the ToF line characterization.

Fission yields are of major interest for the nuclear industry and the study of the fission process. Fission yields are determined by evaluation process based on the analysis of experimental datasets and completed by phenomenological models. The aim is to provide the best estimation of independent yields of 235U(nth,f) reaction. In this paper, we will focus on the methodology of assessment of the nuclear charge distribution per mass. In the JEFF-3.1.1, the Wahl’s parametrization of the Zp-model is used to complete the experimental knowledge. Nevertheless, the lack of this parametrization consists in the absence of uncertainty information. Consequently, it is impossible to generate the components of independent yield correlation matrix from the Zp-model. In this context, we propose to use the Zp model with a new parametrization to assess the charge distributions. Two different calculations have been developed: the first one is based on a Wahl-like linear approximation of the parameters as a function of mass. The second approach consists to apply a “direct-Zp model” without linear assumptions. This analysis is achieved using the Markov-Chain Monte-Carlo (MCMC) method to adjust the model parameters and then to deduce the covariance of charge distributions. Moreover, with our development, we have directly and consistently described the pre-neutron and post-neutron charge distributions.

AbstractThe study of fission yields has a major impact on the characterization and understanding of the fission process and its applications. For the latter, it is crucial to provide fission yields with their associated correlation matrix in order to estimate precisely the uncertainties of crucial quantities, such as the reactivity loss. In the last decade, different works have been proposed to estimate the correlations of the independent fission yields satisfying the consistency of the cumulative yield evaluations. In these previous works, only model parameters and conservation laws have driven the correlations.The novelty of the present work consists of new complete and consistent evaluations of 235U(nth,f) independent and cumulative yields and their correlation matrices, starting from experimental data. The characterization of the probability density functions of fission yields validates the multivariate Gaussian assumption and constitutes a major issue in the validation of the uncertainty propagation tools of the applications. In addition, the new 235U(nth,f) fission yield covariance data have resulted in a decrease in nuclear data related to uncertainty in associated burnup calculations. [ABSTRACT FROM AUTHOR]

Nuclear fission yields are key data for reactor studies, such as spent fuel inventory or decay heat, and for understanding fission process. Despite a significant effort allocated to measure fission yields during the last decades, the recent evaluated libraries still need improvements in particular in the reduction of the uncertainties. Moreover, some discrepancies between these libraries must be explained. Additional measurements provide complementary information and estimations of experimental correlations, and new kinds of measurements enable to test the models used during the nuclear data evaluation process. A common effort by the CEA, the LPSC and the ILL aims at tackling these issues by providing precise measurements of isotopic and isobaric fission yields with the related variance-covariance matrices. Additionally, the experimental program involves a large range of observables requested by the evaluations, such as kinetic energy dependency of isotopic yields and odd-even effect in order to test the sharing of total excitation energy and the spin generation mechanism. Another example is the complete range of isotopic distribution per mass that allows the determination of the charge polarization, which has to be consistent for complementary masses (pre-neutron emission). For instance, this information is the key observable for the evaluation of isotopic yields. Finally, ionic charge distributions are indirect measurements of nanosecond isomeric ratios as a probe of the nuclear de-excitation path in the (E*, J, π) representation. Measurements for thermal neutron induced fission of 241 Pu have been carried out at the ILL in Grenoble, using the LOHENGRIN mass spectrometer. Methods, results and comparison to models calculations will be presented corresponding to a status on fission fragments observables reachable with this facility.

The CEA in collaboration with ILL and LPSC has developed a measurement program on symmetric and heavy mass fission product distributions. The combination of measurements with ionisation chamber and Ge detectors is necessary to describe precisely the heavy fission product region in mass and charge. Recently, new measurements of fission yields and kinetic energy distributions, for different fissioning systems (233,235 U(nth, f),241 Am(2nth, f) and 239,241 Pu(nth, f), were performed with recoil spectrometer LOHENGRIN. The focus has been done on the self-normalization of the data to provide new absolute measurements, independently of any libraries along with the experimental covariance matrix. To reach precise measurements, a new experimental procedure was developed along with a new analysis method.

Absolute measurements of neutron fluence are an essential prerequisite of neutron-induced cross section measurements, dosimetric investigations and neutron beam lines characterisation. Independent and precise neutron flux measurements can be performed with respect to the H(n,p) elastic cross section. However, the use of silicon proton recoil detectors is not straightforward below incident neutron energy of 1 MeV, due to a high background in the detected proton spectrum. A new gaseous proton-recoil detector has been designed to answer the challenge. The detector is described in details and results of the commissioning tests are presented.

Nuclear fission yields are key parameters to evaluate reactor physics observables, such as fuel inventory, decay heat, spent fuel radiotoxicity, criticality but also for understanding the fission process. Despite a significant effort allocated to measure fission yields during the last decades, the recent evaluated libraries still need improvements in particular in the description of the uncertainties with the associated correlations. Additional kinds of measurements provide complementary information in order to test the models used in the nuclear data evaluation. Moreover, some discrepancies between these libraries must be explained. A common effort by the CEA, the LPSC and the ILL aims at tackling these issues by providing precise measurement of isotopic and isobaric fission yields with the related variance-covariance matrices. Nevertheless, the experimental program represents itself a large range of observables requested by the evaluations: isotopic yields, nuclear charge polarization, odd-even effect, isomeric ratio and their dependency with fission fragment kinetic energy as a probe of the nuclear de-excitation path in the (E*, Jπ) representation. Measurements for thermal neutron induced fission of 241Pu have been carried out at the Institut Laue Langevin using the LOHENGRIN mass spectrometer. Experimental program, observables reachable, results and comparison to model calculations are shown.

As part of the design studies conducted at CEA for future power and research nuclear reactors, the validation of neutron and photon calculation schemes related to nuclear heating prediction are strongly dependent on the implementation of nuclear heating measurements. Such measurements are usually performed in low-power reactors, whose core dimensions are accurately known and where irradiation conditions (power, flux and temperature) are entirely controlled. Due to the very low operating power of such reactors (of the order of 100 W), nuclear heating is assessed by using dosimetry techniques such as thermoluminescent dosimeters (TLDs). However, although they are highly sensitive to gamma radiation, such dosimeters are also, to a lesser extent, sensitive to neutrons. The neutron dose depends strongly on the TLD composition, typically contributing to 10-30% of the total measured dose in a mixed neutron/gamma field. The experimental determination of the neutron correction appears therefore to be crucial to a better interpretation of doses measured in reactor with reduced uncertainties. A promising approach based on the use of two types of LiF TLDs respectively enriched with lithium-6 and lithium-7, precalibrated both in photon and neutron fields, has been recently developed at INFN (Milan, Italy) for medical purposes. The CANDELLE experiment is dedicated to the implementation of a pure neutron field “calibration” of TLDs by using the GENEPI-2 neutron source of LPSC (Grenoble, France). Those irradiation conditions allowed providing an early assessment of the neutron components of doses measured in EOLE reactor at CEA Cadarache with 10% uncertainty at 1σ.

Nuclear fission yields data measurements for thermal neutron induced fission of 241Pu have been carried out at the ILL in Grenoble, using the Lohengrin mass spectrometer. The relative isotopic yields for the masses 137 up to 141 have been derived with the associated experimental covariance matrices. Moreover, from preliminary results for the masses 92, 138 and 139, a clear evolution over fission product kinetic energy of the isotopic total count rate is observed.

We present the results of two experiments where we have measured for the first time simultaneously the fission and gamma-decay probabilities induced by different surrogate reactions. In particular, we have investigated the 238U(d,p), 238U(3He,t) and 238U(3He,4He) reactions as surrogates for the neutron-induced n + 238U, n + 237Np and n + 236U reactions, respectively. In the region where gamma emission, neutron emission and fission compete, our results for the fission probabilities agree fairly well with the neutron-induced data, whereas our gamma-decay probabilities are significantly higher than the neutron-induced data. The interpretation of these results is not obvious and is discussed within the framework of the statistical model with preliminary results for calculated spin-parity distributions populated in surrogate reactions. We also present future plans for surrogate-reaction studies in inverse kinematics with radioactive-ion beams at storage rings.

The study of fission yields has a major impact on the characterization and understanding of the fission process and is mandatory for reactor applications. The LPSC in collaboration with ILL and CEA has developed a measurement program on fission fragment distributions at the Lohengrin spectrometer of the ILL, with a special focus on the masses constituting the heavy peak. We will present in this paper our measurement of the very low fission yields in the symmetry mass region and the heavy mass wing of the distribution for 233U thermal neutron induced fission. The difficulty due to the strong contamination by other masses with much higher yields will be addressed in the form of a new analysis method featuring the required contaminant correction. The apparition of structures in the kinetic energy distributions and possible interpretations will be discussed, such as a possible evidence of fission modes.

The modelling of γ heating and neutron damage inside a nuclear reactor is essential to design the next generation of nuclear reactors. The determination of the fission fragment momentum is a key element to perform accurate calculations of the γ heating. One way to assess this information is to look at the isomeric ratio of different nuclei. According to the lifetime of the isomeric state, different experimental techniques were developed at the LOHENGRIN spectrometer. A focus on the measurement of isomeric ratios of 136I in neutron induced fission of 241Pu is presented. A discussion with the current assumptions used in the evaluation process for isomeric ratio is also shown.

Within the EXILL campaign a large and efficient cluster of Ge-detectors was installed around a very well collimated neutron beam. This has allowed to carry out rather complete spectroscopic studies close to the line of stability using the (n,γ) reaction. Neutron rich isotopes were produced by neutron induced fission and prompt spectroscopy was carried out. The isotope selection in this setup was based on a partially known level scheme and the use of triple coincidences. The latter is limiting the statistical sensitivity in the case of weak production yields. Based on the experiences of these campaigns we are currently developing a new setup: FIPPS (FIssion Product Prompt Spectroscopy). This setup combines a collimated neutron beam, a highly efficient cluster of Ge detectors, a gas filled magnet and auxiliary detectors. The presence of the gas filled magnet will allow us to identify fission products directly and should give access to a new quality of studies if compared to the EXILL campaign. The EXILL campaign and the FIPPS project are presented.

The spectroscopy of the prompt gamma decay from fission products gives information on the entry states, e.g. distribution functions for excitation energy and spin, and therefore a direct link to the fission process itself. This type of spectroscopy is, however, only possible when a filter can be constructed which allows setting a gate to the gamma-spectrum in a narrow region in mass and nuclear charge, as well as on the total excitation energy of the fragment split under investigation. A possible configuration of a prompt gamma-ray spectrometer consist of a gamma-ray array composed of high resolution germanium detectors, coupled to a gas filled magnet. We will outline the principles for a gas filled magnetic spectrometer for fission product spectroscopy. In particular the focusing characteristics of such a device, which are valid for particles in the velocity regime of E/A< 1MeV/amu, will be addressed. First experiments on the LOHENGRIN spectrometer in Grenoble investigating on the behavior of fission products in gas filled magnets have been performed, and have validated the experimental approach to the nuclear fission process with such a device.

A new experimental set-up for a simultaneous measurement of neutron induced capture and fission cross sections was designed, assembled and optimized. The measurements will be performed at GEel LINear Accelerator (GELINA) neutron time-of-flight facility in Belgium, where neutron cross sections can be measured over a wide energy range with high energy resolution. The fission events detector consists of a dedicated multi-plate high efficiency fission ionization chamber (IC). The γ-rays produced in capture reaction are detected by an efficient array of C6D6 scintillators. Fission γ-rays events are distinguished from capture events by the anticoincidence signals from the IC and the C6D6 detectors. For the undetected fission events a correction has to be applied with respect to the efficiency of the IC that should be high and known with a high precision. Another important issue is the good separation between fission-fragment (FF) and the high alpha pile-up. The performances of the IC during test experiments are presented, focusing in particular on the detection efficiency.

The study of fission yields has a major impact on the characterization and understanding of the fission process and is mandatory for reactor applications. While the yields are known for the major actinides (235U, 239Pu) in the thermal neutron-induced fission, only few measurements have been performed on 242Am. This paper presents the results of a measurement at the Lohengrin mass spectrometer (ILL, France) on the reaction 241Am(2nth,f): a total of 41 mass yields in the light and the heavy peaks have been measured and compared with the fission process simulation code GEF. Modus operandi and first results of a second experiment performed in May 2013 on the same reaction but with the goal of extracting the isotopic yields are presented as well: 8 mass yields were re-measured and 18 isotopic yields have been investigated and are being analyzed. Results concerning the kinetic energy and its comparison with the GEF Code are also presented in this paper.

Excited states in the nuclei 83As and 84,86Se have been studied via prompt γ-ray spectroscopy. The nuclei were produced by the proton-induced fission of a 238U target, at the accelerator of the University of Jyväskylä. The JUROGAM-II array was used to detect prompt γ-rays and a triple-γ coincidence analysis performed. A comparison of the N = 50 nuclei with shell-model calculations reproduces the low-lying states in 83As and 84Se well. The inclusion of particle-hole excitations is necessary to correctly describe the states above ∼ 3.5 MeV.

Prompt γ-ray spectroscopy of neutron-rich cadmium isotopes has been performed. The nuclei of interest have been populated via a 25-MeV, proton-induced fission of the 238U thick target and prompt γ-rays measured using the multi-detector HPGe array JUROGAM II. New high-spin decays have been observed and placed in the level scheme using triple coincidence gates. The experimental results are compared to shell-model calculations and show good agreement.

We review the statistical model and its application for the process of nuclear fission. The expressions for excitation energy and spin distributions for the individual fission fragments are given. We will finally emphasize the importance of measuring prompt gamma decay to further test the statistical model in nuclear fission with the FIPPS project.

The study of fission yields has a major impact on the characterization and understanding of the fission process and is mandatory for reactor applications. While the yields are known for the major actinides (235U, 239Pu) in the thermal neutron-induced fission, only few measurements have been performed on 242Am. The interest of 242Am concerns the reduction of radiotoxicity of 241Am in nuclear wastes using transmutation reactions. This paper presents the measurement of the fission mass yields from the reaction 241Am(2nth,f) performed at the Lohengrin mass spectrometer (ILL, France) for both the light and the heavy peaks: a total of 41 mass yields have been measured. The experiment was also meant to determine whether there is a difference in mass yields between the isomeric state and the ground state as it exists in fission and capture cross sections. The method used to address this question is based on a repeated measurement of a set of fission mass yields as a function of the ratio between the 242gAm and the 242mAm fission rates. The presented experiment is also a first step towards the measurement of the isotopic fission yields of 242Am.

The accurate knowledge of the fission of actinides is necessary for studies of innovative nuclear reactor concepts. The fission yields have a direct influence on the evaluation of the fuel inventory or the reactor residual power after shutdown. A collaboration between the ILL, LPSC and CEA has developed a measurement program on fission fragment distributions at ILL in order to measure the isotopic and isomeric yields. The method is illustrated using the 233U(n,f)98Y reaction. However, the extracted beam from the Lohengrin spectrometer is not isobaric ions which limits the low yield measurements. Presently, the coupling of the Lohengrin spectrometer with a Gas Filled Magnet (GFM) is studied at the ILL in order to define and validate the enhanced purification of the extracted beam. This work will present the results of the spectrometer characterisation, along with a comparison with a dedicated Monte Carlo simulation especially developed for this purpose.

In the framework of studies concerning new fuel cycles and nuclear wastes incineration experimental data of the α ratio between capture and fission cross sections of 233U reactions play an important role in the Th/U cycle. The safety evaluation and the detailed performance assessment for the generation IV nuclear-energy system based on 232Th cycle strongly depend on this ratio. Since the current data are scarce and sometimes contradictory, new experimental studies are required. The measurement will take place at the neutron time-of-flight facility GELINA at Geel, designed to perform neutron cross section measurements with high incident neutron-energy resolution. A dedicated high efficiency fission ionization chamber (IC) as fission fragment detector and six C6D6 liquid scintilators sensitive to γ-rays and neutrons will be used. The method, based on the IC energy response study, allowing to distinguish between gammas originating from fission and capture, in the resonance region, will be presented.

A new integral measurement of the 7Li neutron induced elastic scattering cross section was determined in a wide neutron energy range. The measurement was performed on the LPSC-PEREN experimental facility using a heterogeneous graphite-LiF slowing-down time spectrometer coupled with an intense pulsed neutron generator (GENEPI-2). This method allows the measurement of the integral elastic scattering cross section in a slowing-down neutron spectrum. A Bayesian approach coupled to Monte Carlo calculations was applied to extract naturalC, 19F and 7Li elastic scattering cross sections.

We present a review of the fission cross section measurements made by the CENBG collaboration over the last years using the surrogate reaction method. For example the neutron-induced fission cross sections of 233Pa(T1/2 = 27 d), 242Cm(T1/2 d) and 243Cm(T1/2 y) have been obtained by our group with this technique. The advantages and the difficulties of the surrogate method are discussed. Special attention is paid to the comparison between cross sections measured with the surrogate method and those obtained directly with neutrons at low energies. This comparison provides information on possible differences between the spin-parity distributions achieved in the two methods.