Numerous measurements of fission product yields were performed since the discovery of the nuclear fission process. However, more precise and reliable fission product yields are requested. Lack of covariance matrices make difficult to use it for specific application purposes such as the propagation uncertainty of decay heat. In this work, we propose to measure independently the fission product mass yields for the whole heavy peak (including the symmetric mass region) for the $$^{233}$$ U $$(\text {n}_{\text {th}},\text {f})$$ reaction. Both average values and experimental covariance is provided. The fission product mass yields are measured with the LOHENGRIN recoil mass spectrometer of the ILL using an ionization chamber located at the focal plane. A new procedure of data taking has been developed in order to minimize the biases. Concretely several ionic charges and kinetic energy distributions have been measured for each mass. Particular attention has been considered in the monitoring of the target time evolution. Additional corrections were necessary in the symmetry mass region due to contaminants coming from the LOHENGRIN recoil mass spectrometer. A complex Monte Carlo analysis has been developed in order to better propagate all the uncertainties. The fission product mass yields of the $$^{233}$$ U $$(\text {n}_{\text {th}},\text {f})$$ and its associated covariance matrix has been produced. An overall good agreement has been observed with ENDF/B-VIII.0 in contrast with the JEFF-3.3 evaluation. A precision around 2% for the heavy peak has been measured. The experimental covariance matrix was also computed. In the symmetry mass region, two components were observed in the kinetic energy distribution. One of this component was considered as an artifact and was ruled-out.

The Fission Product Prompt γ-ray Spectrometer (FIPPS) is a new instrument of the Institut Laue-Langevin (ILL) for the spectroscopy of nuclei produced after neutron induced reactions. The future upgrade of the instrument aims to explore the neutron-rich region of the nuclear chart with higher selectivity by identifying the fragments produced after neutron induced fission. The Gas-Filled-Magnet (GFM) technique has been chosen in order to obtain a good mass separation (<4 amu at A = 150) and a large geometrical and momentum acceptance (>50 msr and Δ P/P > 10%). A GFM design consisting of a 1/r magnetic field index and Thales circle-shaped entrance and exit magnet edges is proposed. The characteristic of the magnet is presented with realistic magnetic field calculations and Monte-Carlo simulations.

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.

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.

FIPPS is the new nuclear physics instrument of ILL for the spectroscopy of nuclei produced in neutron-induced reactions. The performance of the first implementation of the setup will be shown, together with an overview of the first experimental campaign (December 2016-March 2017). Future perspectives and physics opportunities will then be discussed.

The main goal of this work is the determination of spins and parities of excited states in Zr-97, among others the 2264.3-keV level, which had previously been tentatively reported as the 11/2(-) excitation corresponding to the h(11/2) neutron orbital. Low-spin excited states in Zr-97 were populated via the cold-neutron capture reaction. The gamma rays emitted following the reaction were measured using the highly efficient array of high-purity germanium detectors, EXILL, at the Institut-Laue-Langevin (ILL), Grenoble. The primary gamma rays deexciting the capture state in( 97)Zr were identified for the first time. The new, precise neutron binding energy in Zr-97 of 5569.15(4) keV differs significantly from the value reported in the literature. The previously reported level scheme has been amended by several new low-spin levels. The (n, gamma) data were complemented by a measurement of the beta(-) decay of Y-97 at the Lohengrin fission-fragment separator of the ILL, and the observation of the beta(-) decay of Y-97 populated in neutron-induced fission of U-235 and measured using the EXILL Ge array. The combined data allowed unique spin-parity assignments for many levels in Zr-97. The 2264.3-keV level, with a new spin-parity assignment of 9/2(+), may correspond to the vg(9)(/2) extruder orbital.

Delayed gamma-ray and conversion-electron spectroscopy is performed on A = 152 fission fragments, at the Lohengrin spectrometer of the Institut Laue-Langevin, providing a new decay scheme for Pr-152. The quasiparticle phonon model, combined with the particle-rotor model, which allows octupole correlations and Coriolis mixing to be taken into account, is applied to analyze its low-energy structure. The main configurations are found to be (pi 3/2[422] circle times nu 5/2[642])1(+) for the isomer and (pi 3/2[541] circle times nu 3/2[521])3(+) for the ground state.

In this article we report on the observation of new yrast and non-yrast states in I-136, populated in the prompt-y spectroscopy EXILL campaign at the ILL using both U-235(n,f) and Pu-241(n,f) reactions. We propose an extension of the level scheme and interpretation of the new spectroscopic results in comparison to state-of-the-art shell-model calculations. We discuss the role of the proton d(5/2) orbital in the structure of this nucleus with Z = 53 and the energy split between the pi d(5/2) and pi g(7/2) orbitals. We also provide a complete overview of all experimental information about this nucleus and include new data from Cf-252, Cm-248 fission and beta decay of Te-136 to I-136. We give a new interpretation of I-136 and its decay to Xe-136 in terms of the influence of the neutron h(9/2) orbital and the Gamow-Teller strength for N = 83.

Isomeric ratios are a powerful observable to investigate fission fragment total angular momenta. A recent experimental campaign achieved at the LOHENGRIN spectrometer, shows a kinetic energy dependence of μs isomeric ratios from fission fragments populated in neutron induced fission of 235U. For the first time, this dependence was measured for the isomeric ratio of the doubly magic 132Sn. A Bayesian assessment of the angular momentum distribution of 132Sn is proposed according to calculations performed with the FIFRELIN code and interpreted with spin generation models.

Fission yields are essential for nuclear reactor studies (decay heat, fuel inventory...) and constitute also one of the main observables needed to improve our understanding of the fission process. The symmetric mass region is of particular interest due to various intriguing properties of the fission fragments already reported in the literature : inversion of the nuclear charge polarization, large width of the fission fragment kinetic energy distribution, strong change of the prompt neutron multiplicity, etc. Recently, measurements of fission yields and kinetic energy distributions in the symmetric mass region were achieved at the LOHENGRIN mass spectrometer of the Institut Laue-Langevin (ILL). This experimental work is challenging due to the low counting rate and the appearance of contaminant masses, leading to pronounced components in the fission fragment kinetic energy distribution. Despite removing the undesirable contributions, the fission fragment kinetic energy distributions still show two components, indicating that the fission process could be modal. To go further and better characterize these components a comparison between our experimental data and Monte Carlo calculations (FIFRELIN code) simulating the de-excitation of the fission fragments for different fission channels will be presented and discussed.

The study of fission yields has a major impact on the characterization and understanding of the fission process and is mandatory for reactor applications. In the past with the LOHENGRIN spectrometer of the ILL, priority has been given for the studies in the light fission fragment mass range. The LPSC in collaboration with ILL and CEA has developed a measurement program on symmetric and heavy mass fission fragment distributions. The combination of measurements with ionisation chamber and Ge detectors is necessary to describe precisely the heavy fission fragment region in mass and charge. Recently, new measurements of fission yields and kinetic energy distributions are has been made on the 233U(nth,f) reaction. The focus of this work has been on the new optical and statistical methodology and the self-normalization of the data to provide new absolute measurements, independently of any libraries, and the associated experimental covariance matrix.

The fission fragment yield distribution has been measured in the Pu-239(n(th), f) reaction in the mass region of A = 126 to 150 using the Lohengrin recoil-mass spectrometer. Three independent experimental campaigns were performed, allowing a significant reduction of the uncertainties compared to evaluated nuclear data libraries. The long-standing discrepancy of around 10% for the relative yield of A = 134 reported in JEF-2.2 and JEFF-3.1.1 data libraries is finally solved. Moreover, the measured mass distribution in thermal neutron-induced fission does not show any significant dip around the shell closure (A = 136) as seen in heavy-ion fission data of Pb-208(O-18, f) and U-238(O-18, f) reactions. Lastly, comparisons between our experimental data and the predictions from Monte Carlo codes (GEF and FIFRELIN) are presented and discussed.

Excited states in Y-98, populated in neutron-induced fission of U-235 and in spontaneous fission of Cm-248 and Cf-252, have been studied by means of gamma spectroscopy using the Lohengrin fission-fragment separator at ILL Grenoble and the EXILL, Eurogam2, and Gammasphere Ge arrays. Two new isomers have been found in Y-98: a deformed one with T-1/2 = 180(7) ns and a rotational band on top of it, and a spherical one with T-1/2 = 0.45(15) mu s, analogous to the 8(+) isomer in Y-96, corresponding to the (nu g(7/2), pi g(9/2))(8+) spherical configuration. Using the JYFLTRAP Penning trap, an accurate excitation energy of 465.7(7) keV has been determined for the 2.36-s isomer in Y-98. This result and the studies of excited levels in Zr-98, populated in beta-decay of the isomer, indicate a new spin-parity, I-pi = (7)(+) for the isomer. The high spin and the decay properties of this isomer suggest the presence of the 9/2(+)[ 404] neutron extruder orbital in its structure. This is consistent with the large deformation of the isomer, reported recently. The present work does not provide arguments to support the special role of the nu g(7/2)-pi g(9/2) interaction (the spin-orbit-partner, or SOP, mechanism).

Nuclear fuel devices of Pressurised Water Reactors are composed of uranium oxide pellets which are enclosed in zircaloy cylinders. During reactor operation different processes occur. In the contact with the fuel the zircaloy oxidises nonuniformely in depths of order of some micrometers. Further on, energy deposition of fission recoils leads to sputtering of uranium onto the inner surface of the cladding material. Thus, sputtered uranium ions start to migrate outwards. This paper presents first the results of experiments performed on the Lohengrin spectrometer in order to simulate the alteration of cladding tubes in contact with the nuclear fuel during irradiation. The energy loss of selected fission products is correlated to an oxygen mass gain. From these experiments, we deduce the oxidation kinetics constant under irradiation at a mean temperature of 400°C. These results are compared with those of a thermal oxidation. Following oxidation the fission product kinetic energy is stabilised, but still a broadening of the energy distribution is observed, which is characteristic of actinide diffusion inside the zirconia target. Using the Fick model, an actinide diffusion coefficient into zirconia under irradiation is deduced.

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.

Calorimetric low temperature detectors (CLTD’s) for heavy-ion detection have been combined with the LOHENGRIN recoil separator at the ILL Grenoble for the determination of nuclear charge distributions of fission fragments produced by thermal neutron-induced fission of \(^{235}\)U. The LOHENGRIN spectrometer separates fission fragments according to their mass-to-ionic-charge ratio and their kinetic energy, but has no selectivity with respect to nuclear charges Z. For the separation of the nuclear charges, one can exploit the nuclear charge-dependent energy loss of the fragments passing through an energy degrader foil (absorber method). This separation requires detector systems with high energy resolution and negligible pulse height defect, as well as degrader foils which are optimized with respect to thickness, homogeneity, and energy loss straggling. In the present, contribution results of test measurements at the Maier Leibnitz tandem accelerator facility in Munich with \(^{109}\)Ag and \(^{127}\)I beams with the aim to determine the most suitable degrader material, as well as measurements at the Institut Laue–Langevin will be presented. These include a systematic study of the quality of Z-separation of fission fragments in the mass range \(82\le A \le 132\) and a systematic measurement of \(^{92}\)Rb fission yields, as well as investigations of fission yields toward the symmetry region.

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 Fission Product Prompt γ-ray Spectrometer, FIPPS, is under development to enable prompt γ-ray spectroscopy correlated with fission fragment identification. This will open new possibilities in the study of fission and of nuclear structure of neutron rich nuclei. FIPPS will consist of an array of γ and neutron detectors coupled with a fission fragment filter. The chosen solution for the filter is a Gas Filled Magnet (GFM). Both experimental and modeling work was performed in order to extract the key parameters of such a device and design the future GFM of the FIPPS project. Experiments performed with a GFM behind the LOHENGRIN spectrometer demonstrated the capability of additional beam purification.

Nuclear fission consists of splitting a nucleus into smaller nuclei. Several observables are available to study the fission process such as fission yields or fission fragment angular momentum. Currently, fission models cannot predict all the observables with an acceptable accuracy for nuclear fuel cycle studies for instance. Improvement of fission models is an important issue for the knowledge of the process itself and for the applications. In this work, we take an interest in fission fragment angular momentum distribution. Isomeric ratios (IRs) are a common observable giving access to investigate these distributions. We measured accurate IRs for 88Br, 132Sn and 132Te with the fission fragment separator LOHENGRIN and developed a new analysis method to assess the mean value and uncertainty of the IR. An evaluation of the angular momentum distribution of 132Sn was also performed with the FIFFRELIN code.