Sibilieva T. Manufacturing of polystyrene-based scintillators by 3D printing

The thesis is devoted to 3D-printing obtaining plastic scintillators based on polystyrene with a reflector on the surface, in particular, in the form of fine-granularity scintillators. Recently essential progress was achieved in the development of new three-dimensional plastic fine-granularity detectors (FGD) for imaging electromagnetic and hadron fluxes, as well as neutrino interactions, for example in experiments such as T2K and DUNE. FGD with possible event tracking consist of scintillation elements that are optically isolated from each other by reflector, and the scintillation light is collected in three planes by wavelength shifting (WLS) fibers. The smaller the size of the scintillation element, the higher the spatial resolution during event tracking. The performance requirements of a FGD include: high light output, fine segmentation combined with optical isolation, long-term stability and short emission time. For example, the new FGD developed in the SuperFGD project, which is used in the modernization of the near-field detector in the T2K experiment, deploys three readout planes to provide isotropic three-dimensional reconstruction of neutrino interactions. Currently, neutrino detector upgrade projects require between two and ten million 10 mm × 10 mm × 10 mm cubes made of scintillation material on a vinyl aromatic polymer base (polystyrene or polyvinyltoluene) with a reflector on the surface, as well as orthogonal holes for WLS fibers. The complexity of the geometry of such a detector requires several manufacturing steps, with specialized equipment at each step, including the fabrication of each individual plastic scintillator cube, the creation of optical isolation by chemically etching the surface of each cube, and the drilling of holes in three planes to accommodate WLS fibers. In addition, the assembly of several million cubes requires significant effort and requires a robust support structure that mechanically supports the structural integrity of the entire detector. The thesis proposes the use of 3D-printing for the production of scintillation detectors with complex geometry, which allows simplifying the process of manufacturing multi-element finely segmented scintillators by creating larger elementary blocks, the so-called "Supercubes". Due to the possibility of simultaneous printing with several materials, the use of 3D-printing creates the prospect of simultaneously manufacturing a scintillator and a reflector, and holes for the WLS fibers can be formed during printing without the need to drill them. That is, 3D-printing can make it possible to create scintillators with complex geometry, avoiding multi-stage production, post-processing, as well as avoiding difficulties and additional supporting structures during the detector assembly process. Therefore, the current task of modern scintillation materials science is the development of scintillation and reflective materials for 3D-printing and the development of technological approaches to the manufacture of multi-element finely segmented detectors, in particular by 3D-printing methods, as well as the study of the optical and scintillation properties of the obtained samples. In the thesis, scintillation and reflective filaments were developed for the manufacture of scintillation elements based on polystyrene with a reflector on the surface using the 3D-printing methods. The obtained filaments allow the manufacture of multi-element finely segmented scintillators in one production cycle without additional post-processing. The effect of introducing plasticizers into the composition of the scintillation filament material on the optical and scintillation characteristics was studied. The manufactured filament has a minimum bending radius of 65 mm, which allows 3D-printing of plastic scintillators without cracking the filament in the extrusion system of a 3D-printer operating using the fused deposition modeling (FDM) technology. The effect of 3D-printing modes on the optical and scintillation characteristics of the obtained samples of plastic scintillators based on polystyrene was determined. It is shown that the optimized printing mode allows for the production of transparent plastic scintillator samples with a technical attenuation length of up to 20 cm and scintillation characteristics comparable to samples manufactured using traditional technologies (cast polymerization, extrusion, and injection molding). The influence of various pigments and polymer binders in the composition of reflective filaments on the reflectivity of printed reflector samples was investigated, and the dependence of the reflection and transmission coefficients on the concentration of pigments and the thickness of the samples was also revealed.