The precise experimental characterization of non-unique forbidden beta transitions is both an important and very difficult subject and, indeed, very few reliable studies exist in the literature. Indeed, the continuous energy spectrum of these transitions is difficult to measure precisely for various reasons: high diffusivity of electrons in matter and the non-linearity of the detection system; availability of radionuclides and the presence of impurities; long half-lives and the complexity of the decay schemes; etc. Realistic theoretical predictions are also difficult because of the necessity to couple accurate models of both atomic and nuclear structures through the weak interaction, in a fully relativistic formalism. However, improving our knowledge of the non-unique forbidden beta transitions is essential in radioactivity metrology to define the becquerel SI unit in the case of pure beta emitters. It can have a large impact in nuclear medicine (micro-dosimetry, internal vectorised radiotherapy) and for the nuclear industry (calculation of the residual power of reactors, nuclear waste management). Recent studies have also shown the importance of these beta transitions for some fundamental physics topics, such as the search for dark matter and reactor neutrino physics.
Reactor antineutrinos emitted during the beta minus decay of nuclear fuel fission products, due to their exceptionally high production rate, have become a key probe in particle physics. Over the last decades, two significant anomalies have been highlighted with respect to predictions: a systematic deficit in flux with distance from the reactor, and an excess of antineutrinos in the energy spectrum around 6 MeV. The most recent experimental analyses question the validity of the so-called summation predictions, based on a set of nuclear data, reactor simulations and theoretical calculations of beta transitions. As a result of a recent thesis between IRFU/DPhP and LIST/LNHB, these predictions have been improved, establishing for the first time a complete modelling of uncertainties. Non-unique forbidden beta transitions have been identified as one of the major uncertainty components because of their approximate modelling that does not account for the nuclear structure of the nuclei involved, on which they strongly depend.
The main objective of the thesis proposed is to improve the modelling of forbidden transitions and to establish the associated uncertainties. To do this, several beta transitions for nuclei with structures similar to those of fission products will be measured very precisely at LIST/LNHB. These measurements will require an improvement of an existing apparatus (detectors, numerical acquisition, coincidences, radioactive sources), very accurate Monte Carlo simulations, spectral deconvolution to eliminate detection distortions (scattering, bremsstrahlung), and the development of a robust methodology to establish the measurement uncertainties including correlations. The results will be compared to accurate theoretical predictions developed at LIST/LNHB, including a realistic nuclear structure component, which will allow the assumptions of the formalism used to be refined. The forbidden beta transitions contributing the most to the total antineutrino spectrum will then be determined according to this new modelling and will be used to improve the summation predictions of reactor antineutrino spectra with the tools and models developed at IRFU/DPhP.
This very complete subject will allow the PhD student to acquire experimental, theoretical and analytical skills that will open many career opportunities. The candidate should have good knowledge in nuclear instrumentation, programming and Monte Carlo simulations, as well as a solid theoretical background.