Investigation of PEN as Structural Self Vetoing Material for Cryogenic Low Background Experiments

Polyethylene Naphthalate (PEN) is a plastic material which has recently been shown to scintillate in the deep blue spectrum. The possibility of producing PEN with high radiopurity makes it suitable as an active material in low background experiments such as those searching for the $0\nu\beta\beta$-decay. The main requirement for these experiments is the suppression of the radioactive background that is needed to improve sensitivity; in this context PEN scintillation and light shifting properties can be exploited to assemble a self-vetoing structure. It is known, that the light output of many scintillators improves when they are cooled down to cryogenic temperatures. To investigate whether PEN shows similar behaviour, a test setup has been designed and constructed at the Max-Planck-Institute for physics in Munich. The studied PEN samples have been in the shape of tiles using injection moulding with different machine parameters to optimise the transparency and the light output. They have been evaluated by exciting them with a UV light source at room temperature. Afterwards, a setup has been constructed to repeat this measurements with the tiles cooled down to cryogenic temperature. Knowing the deposited energy and the optical parameters of the tile from simulations, the light yield can be determined. Common plastic scintillators are known to suffer loss of light yield due to radiation-induced damage in an oxygen-rich environment. For PEN, therefore, measurements were made in various configurations of external influences with irradiation of UV light. In this case, a decrease in the light output could also be found.\ In another approach, the structural properties of PEN were investigated by exerting tiles to several stress procedures with and without a liquid nitrogen environment. Afterwards, the change in the light output measured with a spectrometer was evaluated and found to be not significant. As a result, this thesis provides investigations on environmental influences and photon-induced damage to PEN tiles leading to changes in the scintillation light output. Furthermore, a setup is designed and constructed able to determine fundamental parameters of PEN to contribute those to its investigation as a structural self-vetoing material for cryogenic low background experiments