Contribution à l’étude de la fission nucléaire : de LOHENGRIN à FIPPS

Nuclear fission consists in splitting a nucleus, in general an actinide, into smaller nuclei. Despite nuclear fission was discovered in 1939 by Hahn and Strassman, fission models cannot predict the fission 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. To reduce uncertainties of the nuclear data used in a nuclear reactor simulation, a validation of the models hypothesis is mandatory. In this work, two features of the nuclear fission were investigated in order to test the resistance of the theories. One aspect is the study of the symmetric fission fragments through the measurement of their yield and kinetic energy distribution. The other aspect is the study of the fission fragment angular momentum. Two techniques are available to assess the angular momentum of a fission fragment. The first one is to look at the properties of the prompt γ. The new spectrometer FIPPS (FIssion Product Prompt γ-ray Spectrometer), is currently under development at the ILL and will combine a fission filter with a large array of γ and neutron detectors in order to respond to these issues. The first part of this work is dedicated to the study of the properties of a Gas Filled Magnet (GFM) which is the type of fission filter considered for the FIPPS project. The second part of this work deals with the measurement of isomeric yields and evaluations of the angular momentum distribution of fission fragments. The study of the spherical nucleus ¹³²Sn shed the light on the current limits of fission models. Finally, the last part of this work is about the measurement of the yields and kinetic energy distributions of symmetric fission fragments. Since models predict the existence of fission modes, the symmetry region is a suitable choice to investigate this kind of prediction. In parallel with all these studies, an emphasis on the development of new methods derived from statistical tools is achieved in order to better control the uncertainties and estimate the biases.La fission nucléaire consiste en la brisure d'un noyau lourd, généralement un actinide, en deux noyaux plus légers (ou trois dans quelques rares cas). Ce phénomène a été découvert par Hahn et Strassman en 1938. Très rapidement Meitner et Frisch proposèrent une explication théorique pour ce processus à l'aide du modèle de la goutte liquide. Depuis les modèles n'ont cessé d'évoluer et de se complexifier à travers l'ajout de nouveaux mécanismes et l'observation de nouveaux phénomènes. L'amélioration des modèles est un enjeu important à la fois pour la compréhension fondamentale du processus de fission mais aussi pour les applications. En effet, le dimensionnement des réacteurs futurs s'appuie de plus en plus sur des simulations numériques. Il devient dès lors primordial de réduire les incertitudes associées aux données utilisées. Cela passe alors par la validation des hypothèses sous-jacentes des modèles de fission nucléaire. Dans le cadre de cette thèse, on s'intéresse à deux aspects de la fission nucléaire qui permettront de tester la robustesse des théories. L'un des aspects concerne l'étude des fragments de fission issus de la région de la symétrie à travers la mesure des rendements et des distributions en énergie cinétique. L'autre aspect étudié est le moment angulaire des fragments de fission. Afin d'accéder au moment angulaire des fragments de fission, l'une des possibilités est d'analyser les propriétés des particules promptes, qui est l'une des ambitions du projet FIPPS (FIssion Product Prompt γ-ray Spectrometer). Une partie de ce travail a été de caractériser les propriétés des spectromètres magnétiques gazeux à travers des mesures expérimentales et le développement d'une simulation Monte Carlo. La seconde partie de ce travail a consisté en la mesure de rapports isomériques et en l'extraction de la distribution du moment angulaire des fragments de fission à l'aide d'un code de désexcitaiton nucléaire. La mesure d'un noyau doublement magique (¹³²Sn) permet de mettre en lumière les limites actuelles des modèles de fission. Enfin la dernière partie de ce travail porte sur la mesure des rendements et des distributions en énergie cinétique des fragments de fission. Certains modèles prédisent l'existence de modes dans la fission nucléaire. La région des masses symétriques est dès lors un lieu de choix pour vérifier la validité de ces affirmations. Il est à noter qu'en parallèle de ces études, un accent fort a été mis sur le développement de méthodes d'analyse s'appuyant sur des outils statistiques afin notamment d'améliorer l'évaluation des incertitudes expérimentales

Nuclear fission studies : from LOHENGRIN to FIPPS

La fission nucléaire consiste en la brisure d'un noyau lourd, généralement un actinide, en deux noyaux plus légers (ou trois dans quelques rares cas). Ce phénomène a été découvert par Hahn et Strassman en 1938. Très rapidement Meitner et Frisch proposèrent une explication théorique pour ce processus à l'aide du modèle de la goutte liquide. Depuis les modèles n'ont cessé d'évoluer et de se complexifier à travers l'ajout de nouveaux mécanismes et l'observation de nouveaux phénomènes. L'amélioration des modèles est un enjeu important à la fois pour la compréhension fondamentale du processus de fission mais aussi pour les applications. En effet, le dimensionnement des réacteurs futurs s'appuie de plus en plus sur des simulations numériques. Il devient dès lors primordial de réduire les incertitudes associées aux données utilisées. Cela passe alors par la validation des hypothèses sous-jacentes des modèles de fission nucléaire. Dans le cadre de cette thèse, on s'intéresse à deux aspects de la fission nucléaire qui permettront de tester la robustesse des théories. L'un des aspects concerne l'étude des fragments de fission issus de la région de la symétrie à travers la mesure des rendements et des distributions en énergie cinétique. L'autre aspect étudié est le moment angulaire des fragments de fission. Afin d'accéder au moment angulaire des fragments de fission, l'une des possibilités est d'analyser les propriétés des particules promptes, qui est l'une des ambitions du projet FIPPS (FIssion Product Prompt γ-ray Spectrometer). Une partie de ce travail a été de caractériser les propriétés des spectromètres magnétiques gazeux à travers des mesures expérimentales et le développement d'une simulation Monte Carlo. La seconde partie de ce travail a consisté en la mesure de rapports isomériques et en l'extraction de la distribution du moment angulaire des fragments de fission à l'aide d'un code de désexcitaiton nucléaire. La mesure d'un noyau doublement magique (¹³²Sn) permet de mettre en lumière les limites actuelles des modèles de fission. Enfin la dernière partie de ce travail porte sur la mesure des rendements et des distributions en énergie cinétique des fragments de fission. Certains modèles prédisent l'existence de modes dans la fission nucléaire. La région des masses symétriques est dès lors un lieu de choix pour vérifier la validité de ces affirmations. Il est à noter qu'en parallèle de ces études, un accent fort a été mis sur le développement de méthodes d'analyse s'appuyant sur des outils statistiques afin notamment d'améliorer l'évaluation des incertitudes expérimentales.Nuclear fission consists in splitting a nucleus, in general an actinide, into smaller nuclei. Despite nuclear fission was discovered in 1939 by Hahn and Strassman, fission models cannot predict the fission 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. To reduce uncertainties of the nuclear data used in a nuclear reactor simulation, a validation of the models hypothesis is mandatory. In this work, two features of the nuclear fission were investigated in order to test the resistance of the theories. One aspect is the study of the symmetric fission fragments through the measurement of their yield and kinetic energy distribution. The other aspect is the study of the fission fragment angular momentum. Two techniques are available to assess the angular momentum of a fission fragment. The first one is to look at the properties of the prompt γ. The new spectrometer FIPPS (FIssion Product Prompt γ-ray Spectrometer), is currently under development at the ILL and will combine a fission filter with a large array of γ and neutron detectors in order to respond to these issues. The first part of this work is dedicated to the study of the properties of a Gas Filled Magnet (GFM) which is the type of fission filter considered for the FIPPS project. The second part of this work deals with the measurement of isomeric yields and evaluations of the angular momentum distribution of fission fragments. The study of the spherical nucleus ¹³²Sn shed the light on the current limits of fission models. Finally, the last part of this work is about the measurement of the yields and kinetic energy distributions of symmetric fission fragments. Since models predict the existence of fission modes, the symmetry region is a suitable choice to investigate this kind of prediction. In parallel with all these studies, an emphasis on the development of new methods derived from statistical tools is achieved in order to better control the uncertainties and estimate the biases

Influence of scission neutrons on the prompt fission neutron spectrum calculations

The calculation of the Prompt Fission Neutron Spectrum (PFNS) was performed using the FIFRELIN Monte Carlo code simulating the de-excitation of the whole fission fragments. This de-excitation is governed by the Hauser-Feshbach statistical model, which has the advantage to take into account the conservation laws for the energy, spin and parity of the initial and final states. In this way, the competition between prompt neutron and prompt gamma emission can be properly accounted for. Assuming that the prompt neutron emission comes only from an evaporation process of the fully accelerated fission fragments, our calculations are not able to reproduce satisfactorily the experimental data. In this context, we have added an additional source of neutrons that may arise during the sudden rupture of the neck (the so-called scission neutrons). Applied in the case of the spontaneous fission of 252Cf, our PFNS calculations show a very good agreement with the Mannhart evaluation by accounting for a 2% scission neutron contribution

Impact of FIFRELIN input parameters on fission observables

Evaluated nuclear data are essential for nuclear reactor studies. In order to significantly improve the precision of nuclear data, more and more fundamental fission models are used in the evaluation processing. Therefore, tests of fission models become a central issue. In this framework, FIFRELIN (FIssion Fragments Evaporation Leading to an Investigation of Nuclear data) is a Monte Carlo code developed in order to modelize fission fragments de-excitation through the emission of neutrons, γ and conversion e-. To be performed, a FIFRELIN calculation relies on several models such as gamma strength function and nuclear level density and of more empirical hypothesis such as total excitation energy repartition or angular momentum given by the fission reaction. Moreover, pre-emission mass yield and kinetic energy distribution per mass are necessary to process the simulation. A set of five free parameters are chosen to reproduce a target observable. Often this observable corresponds to the mean neutron multiplicity for heavy and light fragment. In this work, the impact of the set of parameters on different output observables (neutron emission probability, neutron multiplicity as function of the fission fragment mass) is investigated

Influence of scission neutrons on the prompt fission neutron spectrum calculations

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Prompt particle emission in correlation with fission fragments

The de-excitation process of primary fission fragments can be simulated with the FIFRELIN Monte Carlo code leading to an estimation of prompt fission observables such as neutron/gamma multiplicities and spectra in correlation with fission fragments. De-excitation cascades are simulated using the notion of nuclear realization following Becvar terminology generalized to neutron/gamma coupled emission. A nuclear realization is a random set of nuclear levels (energy, spin, parity) in association with partial widths for neutron, gamma or electron emission. Experimental data related to electromagnetic transitions in the discrete level region are taken from RIPL-3 database. When nuclear level structure is completely unknown (in the continuum region), level density and strength function models are used. In between these regions, our partial knowledge of nuclear structure is completed by models up to a fixed maximum level density. In this way the whole available experimental information is accounted for. FIFRELIN is ruled by five free input parameters driving the excitation energy sharing, the rotational energy and the spin distribution of primary fission fragments. These five free parameters are determined to match a target observable such as the average total prompt neutron multiplicity (ν). Once this procedure is completed, the whole set of fission observables can be compared with experimental results. Obviously the number of observables obtained within this code is higher than what is available from measurements. This code can therefore provide useful insights into the compatibility between models and a whole set of fission observables. In the present work the influence of shell corrections is reported on level densities and prompt fission neutron spectra (PFNS). The impact of the input data such as primary fission fragment total kinetic energy (TKE) is also addressed. Average prompt neutron multiplicity as a function of TKE is also estimated for each mass split and compared to recent measurements. The presence of structures in the calculations (especially for light nuclei) is clearly related to the nuclear level scheme. Various situations occur and an overestimation (or underestimation) of the calculated number of emitted neutrons can be correlated to the light or heavy fragment of a pair and to a restricted energy range. In addition prompt fission gamma spectra (PFGS) are estimated for selected fragment mass ranges and compared to recent measurements. In this way the presence of specific gamma-ray transitions can be established

Prompt particle emission in correlation with fission fragments

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Influence of primary fragment excitation energy and spin distributions on fission observables

International audienceFission observables in the case of 252Cf(sf) are investigated by exploring several models involved in the excitation energy sharing and spin-parity assignment between primary fission fragments. In a first step the parameters used in the FIFRELIN Monte Carlo code “reference route" are presented: two parameters for the mass dependent temperature ratio law and two constant spin cut-off parameters for light and heavy fragment groups respectively. These parameters determine the initial fragment entry zone in excitation energy and spin-parity (E*, Jπ). They are chosen to reproduce the light and heavy average prompt neutron multiplicities. When these target observables are achieved all other fission observables can be predicted. We show here the influence of input parameters on the saw-tooth curve and we discuss the influence of a mass and energy-dependent spin cut-off model on gamma-rays related fission observables. The part of the model involving level densities, neutron transmission coefficients or photon strength functions remains unchanged

Influence of primary fragment excitation energy and spin distributions on fission observables

Fission observables in the case of 252Cf(sf) are investigated by exploring several models involved in the excitation energy sharing and spin-parity assignment between primary fission fragments. In a first step the parameters used in the FIFRELIN Monte Carlo code “reference route" are presented: two parameters for the mass dependent temperature ratio law and two constant spin cut-off parameters for light and heavy fragment groups respectively. These parameters determine the initial fragment entry zone in excitation energy and spin-parity (E*, Jπ). They are chosen to reproduce the light and heavy average prompt neutron multiplicities. When these target observables are achieved all other fission observables can be predicted. We show here the influence of input parameters on the saw-tooth curve and we discuss the influence of a mass and energy-dependent spin cut-off model on gamma-rays related fission observables. The part of the model involving level densities, neutron transmission coefficients or photon strength functions remains unchanged

FIPPS (FIssion Product Prompt γ-ray Spectrometer) and its first experimental campaign

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