Development of a neutron/gamma coincidence measurement system for the characterization of radionuclide neutron sources

This PhD work is part of sources calibration activities at the LNHB-MA and R&D activities within the SIMRI aimed at developing neutron measurement systems for the CEA and the nuclear industry. The objective of the PhD work is to develop a new measurement system using neutron/gamma coincidences to enable the characterization of the (alpha,n)-type neutron sources. These sources consists of a homogeneous mixture of an alpha particle emitter and the target substance, the nuclei of which emit neutrons via a nuclear reaction. As for example, we can cite for example: AmBe, PuBe, CmBe, or even exotic source of high emissivity and mixing several alpha radionuclides (ex. AmPuBe). For this familly of sources, the emission of neutron by reaction (alpha,n) is in simultaneous cascade with a characteristic gamma at 4.4 MeV. The detection of the neutron and the gamma in coincidence is likely to provide information of interest in the source characterization in terms of emission rate and spectral fluence. The objective is to measure precisely gamma and neutron signatures as well as gamma/neutron intensity ratios resulting from the nuclear reaction. The new measurement device must also be able to measure neutrons emitted by the spontaneous fission reaction or by (n,2n) reaction in beryllium. Others photon emission can be also provide information of interest, ex. the emission of a gamma at 2.2 MeV resulting from the capture on hydrogen. The neutron/gamma coincidence measurements can be also used to improve the evaluation of nuclear data such as cross sections of certain elements, ex. (n,gamma) reaction on oxygen or hydrogen.

Full isotopic fission fragment distribution measurement of 241Pu using inverse kinematics at GANIL with VAMOS and PISTA

The inverse kinematics technique is used at GANIL to produce the so-called in-flight fission. The accelerated fissioning system is excited by a nuclear reaction, and in particular by a nucleon transfer reaction between the beam and the target. Fission fragments are therefore emitted at forward angles in the laboratory frame due to the kinematic boost of the reaction. The VAMOS wide-acceptance magnetic spectrometer is used to identify the mass and nuclear charge of the various fragments, while silicon telescopes are used to characterize the fissioning system by detecting the ejecta emitted by the transfer reaction.
The fission@VAMOS project involves upgrading the silicon detection system used to identify the fissioning system produced by the transfer reaction. The current device is a highly segmented silicon telescope assembly called PISTA. This improves the sensitivity and precision of the fissioning system formation conditions (mass, atomic number, excitation energy).
The subject of this thesis is therefore a detailed multi-parametric study of fission, with a focus on measuring the fission yields of the fissioning system 242Pu (n+241Pu). Finally, a large part of the work will consist of data analysis and interpretation, followed by publication.

Implementation of covariant QRPA to describe deformed atomic nuclei

All other things being equal, what differences can be expected from the choice of a relativistic or non-relativistic interaction in the QRPA description of the excited states of the atomic nucleus? In order to answer it, the student will on one hand use numerical tools to solve non- relativistic interaction QRPA matrix equations and on the other hand use a solver of the finite amplitude method to produce QRPA response functions with relativistic interactions. These numerical tools leverage supercomputers and are widely used for nuclear data and astrophysics issues as well as to conduct academic nuclear structure studies. The relativistic extension of the matrix QRPA solver will make it possible to transfer all the expertise of nuclear data production to the case of interactions from relativistic lagrangians. Thus, an analysis of the respective merits of the two functionals will be conducted and exploited with a view to the development of new generation effective interactions.

Development of an uncertainty propagation method of function-typed input data applied to the decay heat calculation

Characterising the energy released by the disintegration of the radionuclides present in spent nuclear fuel is essential for the design, safety and analysis of storage, transport and disposal systems. Few measurements of this decay heat are available today. In addition, the available experimental values do not cover the wide spectrum of possible combinations between parameters such as discharge burn-up rate, 235U enrichment, cooling time, fuel design parameters, or operating conditions. The estimation of decay heat is therefore mainly based on calculation codes.
The evaluation of the uncertainty associated with the estimation of decay heat is important to achieve reliable predictions. Many efforts have been made to properly evaluate biases and uncertainties coming from nuclear data such as cross sections. The number of studies concerning uncertainties of an epistemic nature (uncertainty in the manufacture of some components, error in reading or adjusting mobile structures, etc…) is comparatively small. Among the latter, while the treatment of complex dependencies of scalar input parameters is well taken into account today, functional-type dependencies, i.e., expressed in the form of a function, are very little explored.
While uncertainties arising from the processing of fixed input parameters, such as fuel manufacturing parameters, independent of time, are quite well covered, the uncertainties coming from the processing of variable (or functional) parameters, such as operating history, evolving during reactor operations, are not. Irradiation history actually brings together several inter-correlated quantities (operating power, absorber movements, core evolution …), subject to modifications over time and influencing the value of numerous observables of interest, including decay heat. The models used today in industrial simulation tools do not make it possible to estimate this impact and to infer a validated uncertainty.

This research work will investigate the impact on decay heat of the uncertainties associated with input parameters having functional dependencies. We will particularly focus on the irradiation history of the reactors (PWR type). A first part of the work will be dedicated to the development of a substitution model for decay heat estimation and quantification of uncertainties of a functional nature. The second part will be devoted to the development of a sensitivity analysis method. Finally, a third part will concern the development of an inverse method for quantifying the uncertainties coming from irradiation modelling.
The doctoral student will be hosted in a reactor physics research unit of the CEA IRESNE located in Cadarache where he will collaborate with other doctoral students and specialists in the field.

Artificial Intelligence for Mass Measurement of Exotic Isotopes

Artificial intelligence opens new perspectives for basic science. It is no exception for nuclear structure studied at the extreme of the nuclear chart by the Super Separator Spectrometer (S3) under construction at GANIL-SPIRAL2. The Piège à Ions Linéaire du Ganil pour la Résolution des Isotopes en Masse (PILGRIM) is a Multi-Reflection time-of-flight Mass Spectrometer (MR-ToF-MS), with state-of-the-art performances that can only be exploited fully thanks to a joint development with the FASTER (http://faster.in2p3.fr/) data acquisition at LPC Caen. The PhD thesis will consist in carrying out this development with the FASTER developers and the physicist in charge of PILGRIM. Machine learning techniques will have to be employed to recognize patterns in the time-of-flight of ions extracted as bunches from the S3 Low Energy Branch. For each individual ion, the time of flight will have to be determined with sub-nanosecond precision, correcting for effects due to pile-up, gain and baseline fluctuations. This development should lead to the determination of masses of exotic nuclei with exquisite precision, enabling tests of nuclear physics models in previously uncharted territories.

INVESTIGATION OF THE NUCLEAR TWO-PHOTON DECAY IN SWIFT FULLY STRIPPED HEAVY IONS

The nuclear two-photon, or double-gamma decay is a rare decay mode in atomic nuclei whereby a nucleus in an excited state emits two gamma rays simultaneously. Even-even nuclei with a first excited 0+ state are favorable cases to search for a double-gamma decay branch, since the emission of a single gamma ray is strictly forbidden for 0+ ? 0+ transitions by angular momentum conservation. The double-gamma decay still remains a very small decay branch (<1E-4) competing with the dominant (first-order) decay modes of atomic internal-conversion electrons (ICE) or internal positron-electron (e+-e-) pair creation (IPC). Therefore we will make use of a new technique to search for the double-gamma decay in bare (fully-stripped) ions, which are available at the GSI facility in Darmstadt, Germany. The basic idea of our experiment is to produce, select and store exotic nuclei in their excited 0+ state in the GSI storage ring (ESR). For neutral atoms the excited 0+ state is a rather short-lived isomeric state with a lifetime of the order of a few tens to hundreds of nanoseconds. At relativistic energies available at GSI, however, all ions are fully stripped of their atomic electrons and decay by ICE emission is hence not possible. If the state of interest is located below the pair creation threshold the IPC process is not possible either. Consequently, bare nuclei are trapped in a long-lived isomeric state, which can only decay by double-gamma emission to the ground state. The decay of the isomers would be identified by so-called time-resolved Schottky Mass Spectroscopy. This method allows to distinguish the isomer and the ground state state by their (very slightly) different revolution time in the ESR, and to observe the disappearance of the isomer peak in the mass spectrum with a characteristic decay time. After a first successful experiment establishing the double-gamma decay in 72Ge a new experiment has been accepted by the GSI Programme Committee and its realization is planned for 2024.

Exploring magicity and nuclear forces in 68Ni

We propose to study the magicity of 68Ni by means of neutron adding and neutron removal transfer reactions (d,p) and (p,d), respectively. This way, we get unique access to the occupancy of the normally occupied orbits and the vacancy of the valence ones. If a sharp transition in occupancy is found, the nucleus is considered as magic, otherwise rather superfluid. Furthermore, this study also allows to study the spin-orbit force, essential to the modeling of atomic nuclei, in a unique manner. 68Ni is produced by means of the LISE spectrometer at GANIL, protons and deuterons produced arising from transfer reactions are detected in the highly-segmented Si array MUST2, gamma-rays with EXOGAM2 and incoming/outgoing nuclei tracks, energy losses and time-of-flights with sets of gas-filled detectors.

Target development and new radioactive beams for SPIRAL1 - GANIL

The Target-Ion-source group at GANIL is seeking to extend the range of re-accelerated radioactive ion beams supplied by the SPIRAL1 facility, in order to increase the attractiveness of existing and future installations. This thesis proposes to produce new beam:
- By producing Fe, Ni, Co beams, by increasing the operating temperature of current TISS (target/ion-source system).
- By producing more intense beams using new targets that need to be designed, produced and tested.

Neutron-star crusts at finite temperature

Neutron stars are among the densest objects in the universe. Born from the explosion of core-collapse supernovae, they are initially very hot and consequently their outer layers (the crust) are made up of a dense liquid composed of various nuclear species immersed in a background “gas” of electrons (and possibly neutrons/protons).
During the doctoral thesis, a theoretical study of the neutron-star crust at finite temperature will be carried out, in particular with regard to the treatment of nuclei in the dense medium characterising the crust. The new model will be employed to calculate the equation of state and the composition of the crust, and applied to predict properties that are important for neutron-star (global) modelling.

Unified theory of nuclear structure and reactions in the open quantum system framework

Light weakly bound or resonant nuclei play an important role in various stellar processes of nucleosynthesis. The comprehensive understanding of these nuclei requires a correct description of the multi-particle continuum. It is proposed to study complex reactions of astrophysical interest and near-threshold narrow resonances which play crucial role in the nucleosynthesis of heavier elements, using Gamow Shell Model in the representation of coupled channels.

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