Thèse

Vendredi 20 Novembre 2026 à 10h00.

Development of Ultrafast and Radiation-Hard Scintillators for the Next Generation of High-Energy Physics Calorimeters

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Directeur de thèse / thesis director :
Christophe Dujardin

Membres du jury / jury members :
MIH.KOV. Eva (FZU, Académie des Sciences Tchèque, Prague)
MOREL Christian (Professeur des Universités, Aix-Marseille Université)
AUGIER Corinne (Professeure des Universités, Université Lyon 1)
BOUDOUL Ga.lle (Chercheuse CNRS, IP2I Lyon)- Invitée
PAUWELS Kristof (ESRF, Grenoble)
AUFFRAY Etiennette (CERN, Genève) Co-directrice de thèse
DUJARDIN Christophe (Professeur des Universités)

Résumé / Abstract :
The performance of a light-based calorimeter in high-energy physics rests on the properties of its scintillating material: light yield, scintillation kinetics, and radiation hardness. The High-Luminosity LHC will impose particle fluxes and radiation levels well beyond those of present detectors, while demanding more precise energy and time reconstruction to resolve the resulting dense and pile-uprich environments. Its innermost calorimeter regions must keep their response under an absorbed dose of the order of 1 MGy, a condition that no future electronpositron collider imposes on the same scale. These conditions call for faster and more radiation-tolerant scintillators. This thesis addresses that need through the optimization of ultrafast and radiation-hard scintillators for a spaghetti calorimeter(SpaCal) with crystal fibers, the candidate technology for the innermost region of the LHCb Upgrade II electromagnetic calorimeter, with particular emphasis on cerium-doped garnets such as gadolinium aluminum gallium garnet (GAGG, Gd3Al2Ga3O12:Ce).
The work follows three complementary and interconnected lines. The first is a systematic laboratory characterization of GAGG bulk crystals and fibers, which quantifies the trade-off of cerium and magnesium co-doping: the scintillation is accelerated from an effective decay time of about 42 ns down to the sub-nanosecond range, at the cost of a fall in light yield from about 30 000 to a few thousand photons per MeV, while the coincidence time resolution stays close to that of standard GAGG scintillators. The radiation tolerance is assessed under a 24 GeV proton irradiation reaching about 1 MGy and under reactor neutron irradiation. The second line comprises testbeam campaigns at the DESY II facility and the CERN SPS, where tungsten-GAGG SpaCal prototypes reach a time resolution of about 12 ps at 100 GeV, and where standalone scintillator pixels are used as minimum-ionizing-particle timing detectors. The third line is a Geant4 simulation program that translates these results into quantitative material specifications, setting the tolerances on light-yield inhomogeneity, optical attenuation length, and scintillation decay uniformity required to keep the energy resolution within its design target of a 10% sampling term and a 1% constant term.
Taken together, these results establish a coherent set of material requirements for the scintillators of next-generation high-energy physics calorimeters, connecting the optical and scintillation properties of the crystals to the macroscopic performance of the detector, and provide direct, quantitative feedback to the crystal producers engaged in the development of ultrafast and radiation-hard GAGG.

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