Danny Petschke

The influence of detector setup configuration and scintillator material choice on spectrum quality in Positron Annihilation Lifetime Spectroscopy (PALS) is fundamentally acknowledged primarily by empirical observation. However, this study quantifies the effects of ultra-fast plastic scintillators (BC422Q) within a conventional collinear (180-degree) detector setup used in the laboratory using the Geant4 simulation toolkit. We examine the impact of the scintillator's dimension and geometry (truncated cone vs. cylinder) on the detection efficiency for specific gamma-ray energies (1274 keV and 511 keV) and their proportion of corrupt events such as backscattering or multiple detections. Results indicate that the selection of scintillator dimension and shape significantly enhances detection efficiency albeit with an increase in corrupt events. Furthermore, we investigate the influence on the instrument response function (IRF) of the scintillators, showing how truncated cones offer superior precision and, thus, a narrower IRF compared to cylindrical shapes. Additionally, the effect of the sample material itself, in the case of a truncated cone as the scintillator shape, on the scintillator response was studied. It is observed that with an increasing atomic number of the sample the detection efficiency substantially decreases while the proportion of corrupt events also diminishes. Despite the sample material's influence on the energy of gamma-quanta interacting with the scintillator, no measurable impact on the IRF was detected for the chosen windows of the pulse height spectra. This investigation encourages a profound examination of the influences on spectrum quality in PALS measurements using Geant4 as a simulation tool highlighting the critical balance between detection efficiency and corrupt event frequencies.

Although binary aluminium alloys seem to be uninteresting and well known, some aspects of their precipitation sequence – especially the early stages immediately after quenching – are still not well understood. Since the Al–Cu system is the basis for many ternary and quaternary high-strength alloys with application in the aviation sector, it is important to understand this binary system in detail. This problem is here tackled by a unique combination of differential scanning calorimetry and X-ray absorption fine structure measurements, where relaxed atomic coordinates for simulation of the spectra have been obtained by ab initio calculations. Thereby, it is possible to attribute any exo- or endothermal peak to a certain type of precipitate, even though formation and dissolution regions have a large overlap in this system. This unique combination of experimental and numerical methods allows one to determine the local atomic environment around Cu atoms, thus following the formation and growth of Guinier–Preston zones, i.e. Cu platelets on {100} planes, during the precipitation process.

Adding trace elements (Cd, In, Sn) to Al‐Cu‐based alloys can significantly improve their strength by the growth of small and finely distributed θ′ precipitates. However, the underlying atomic mechanisms of their nucleation are so far only superficially understood. We follow the precipitation process, that is changes in the microstructure, by different methods: differential scanning calorimetry (DSC), giving information on formation and dissolution of precipitates, 3D atom probe tomography (3DAP), giving information on size and density of precipitates and finally, positron annihilation lifetime spectroscopy (PALS), being sensitive especially to quenched‐in vacancies and their interaction with alloying elements. By the use of these complementary methods we obtain information on vacancy binding to the alloying elements and also on structure, kind and distribution of precipitates while correlating this with hardness measurements.