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One of the most challenging problems in nuclear astrophysics is answering a question about the origin and abundance of elements. There are numerous sites in the Universe where new elements can be created. In this particular work two types of extreme stellar environments are in focus: Novae and X-ray bursts. The peak temperatures achieved during a Nova explosion (ONe-type) is around 0.4*109 K, while for an X-ray burst this value can be of order of 109 K. Temperature is one of the major factors, which determines how far (by A mass number) the nucleosynthesis can go. While there are numerous reactions to consider in such environments, in this particular study the reaction 34Cl(p,γ)35Ar was investigated. 34Cl has a relatively short halflife (1.5264(14) s) and can quickly decay into 34S. However this process competes with a reaction of proton capture 34Cl(p,γ)35Ar, thus leaving a smaller amount of 34Cl available for the decay to 34S. The latter, on the other hand, is used for determining isotopic abundances in presolar grains that are extracted from meteorites. Typically, these grains have different isotopic ratios for sulphur than the one found in the Solar System. These differences can say something about the places where the pre-solar grains were synthesized. To be able to accurately predict isotopic abundances in Novae or X-ray bursts, the information about the rate of the reaction 34Cl(p,γ)35Ar needs to be evaluated. That, in turn, requires some knowledge about the resonances in 35Ar that lie just above proton threshold separation energy (Sp). The fact that only those resonances are important is dictated by the temperatures of the previously mentioned stellar environments. The Gamow window for a temperature of 0.4*109 K is located around 433 keV above Sp level in 35Ar.An indirect method was used for populating the states above the proton separation energy, Sp, in 35Ar. 35K undergoes β+-decay into 35Ar and the Q-value of this reaction is sufficiently high to populate states above the Sp level. 35Ar then decayed into 34Cl + p. The spectroscopy of the levels decaying by protons was in the main focus of this work. The AstroBoxII is the detector used in this experiment. It allows low background noise measurements with high efficiency. Also HpGe detectors were used in coincidence with the AstroBoxII to be able to accurately distinguish between states in 35Ar that decay either to the ground state or excited states in 34Cl that then undergo a γ-emission. The major result of this thesis is the report of a new resonance at 6348(11) keV in 35Ar. This resonance sits right in the middle of the Gamow window (for T=0.4*109 K) and potentially can have a big impact on the reaction rate of 34Cl(p,γ)35Ar. The estimate for the reaction rate dependence as a function of temperature is given in the Conclusions chapter.

The ClearMind project aims to develop the TOF-PET position sensitive detection module optimized for the time resolution, spatial resolution, and detection efficiency. For this, the ClearMind project uses a large (59 \\(\\times\\) 59 mm\\(^2\\)) monolithic PbWO\\(_4\\) (PWO) scintillating crystal with a bialkali photo-electric layer deposited directly on the crystal. Scintillation and Cherenkov photons result together from the 511 keV gamma-ray interation into the PWO crystal. A micro-channel plate photomultiplier tube (MCP-PMT) encapsulating the PWO crystal amplifies photoelectrons generated at the photocathode, and the corresponding anode signals are collected through the transmission lines read out at both ends and digitized by a SAMPIC module. In this work, we present a realistic Geant4 simulation of the ClearMind prototype detector, including the propagation of the visible photons in the crystal, the modelling of a realistic response of the photocathode and of the PMT, and the propagation of the electrical signals over the transmission lines. The reconstruction of the gamma conversion in the detector volume is performed from the signals registered at both ends of the transmission lines. We compare the reconstruction precision of a statistical algorithm against machine learning algorithms developed using the TMVA package. We expect to reach a spatial resolution down to a few mm\\(^3\\) (FWHM). Finally, we will discuss prospects for the ClearMind detector.

In the present work we describe the design, construction, and testing of the optical prototype developed for the BOLDPET project, with the objective of creating a PET detection module with high spatial and time resolution. The BOLDPET technology uses an innovative detection liquid, trimethylbismuth, for detecting 511 keV \\(\\gamma\\)-quanta resulting from positron annihilation. The optical signal is exclusively produced through the Cherenkov mechanism, and the produced photons are detected using Planacon microchannel-plate photomultiplier. We achieve an excellent time resolution of 150 ps (FWHM) within a sizable detection volume measuring 55 mm x 55 mm x 25 mm. Through detailed Geant4 simulations, we examine the limiting factors affecting time resolution and explore potential avenues for improvement. Furthermore, we demonstrate the feasibility of coarse 2D localization of interactions using the optical signal alone, achieving a precision of about 5-8 mm (FWHM) within the homogeneous detection volume.