Quantum mechanical view on ultrafast scintillators

Groep: Lumilab

Promotor: dr. Jonas Joos

Supervision: dr. Jonas Joos

Scintillator crystals are essential building blocks of radiation detectors that are used in e.g. positron emission tomography (PET), or high-energy physics experiments. They emit visible light following an excitation process caused by the absorption of high-energy X-ray or gamma photons in a process called radioluminescence. A crucial parameter that determines their performance is the characteristic lifetime of the emitting state: the faster a scintillator decays, the better the timing resolution of the detector. For this reason, scintillators based on luminescent impurities with a nanosecond spin- and parity-allowed decay such as trivalent cerium (Ce3+) are currently hot topic.

A recently proposed strategy to further improve the timing of Ce3+ based scintillators is to deliberately convert a part of the luminescent Ce3+ ions to their tetravalent counterpart, Ce4+. It has been empirically verified that this indeed decreases the excited state lifetime, but unfortunately also leads to a lowering of the scintillator yield, i.e. the number of emitted photons per MeV of incident radiation. The underlying quenching mechanism is unclear, but is found to depend highly on the Ce3+/Ce4+ ratio. Accurately assessing this ratio is in fact another important practical issue, as this requires expensive experimental facilities such as a synchrotron.

In this thesis, Ce doped garnet scintillators will be theoretically investigated to explain the quenching mechanism. Additionally it is the goal to find a methodology to assess the amount of Ce4+ from optical spectroscopy, as a low-cost alternative for the synchrotron measurements,  by calculating how Ce4+ induces new absorption bands. To achieve this, state-of-the-art quantum mechanical techniques based on wave function theory are addressed using a high-performance computer. As Ce is a genuine multivalent heavy element with 4f valence electrons, special relativistic effects are important and will be accurately accounted for via relativistic Hamiltonians. Although this project is predominantly theoretical, experimental facilities are readily available that allow to confront theoretical results with experiment, thus increasing the impact of new findings.

Quantum mechanical view on ultrafast scintillators

Opmerkingen:

  • Dit thesisonderwerp kan ook opgenomen worden in het kader van de Educatieve Master Fysica en Sterrenkunde.
  • Aan dit onderwerp is geen mobiliteitsaspect verbonden. Er kan met de promotoren wel overlegd worden over gerelateerde stage-activiteiten en/of vakken aan andere universiteiten.