The dissertation is dedicated to the development of fabrication methods and characterizing crystals of mixed garnets grown from melt in reducing and inert environments for the next generation of granular particle detectors. A method for obtaining long YAG:Ce fibers with improved attenuation length grown by the μ-PD method in an inert atmosphere has been developed, as well as crystals of solid solutions LuxY3-xAl5O12 with improved characteristics were grown by the Czochralski method in a reducing atmosphere using cheap tungsten crucibles.
To date, the search for new types of detectors for the next generation of experiments in high-energy physics remains a relevant problem. One promising option for such a detector is a granular detector, which consists of a large number of fibers, signals from which are recorded separately. Fibers in such a detector are divided into 2 types - activated scintillating fibers for registering scintillation light, and non-activated fibers for registering Cherenkov radiation. While non-activated fibers only need to be transparent in the sensitivity range of the photodetector and the Cherenkov light spectrum, the requirements for activated scintillating fibers are much more stringent: they must have a length of more than 20 cm, a sufficiently large light output (> 15000 photons/MeV), and an attenuation length (a measure of the optical transparency of the fiber) of over 20 cm. The first prototypes of granular detectors based on Lu3Al5O12 fibers grown by the μ-PD method and Gd3Ga3Al2O12 fibers cut from boules grown by the Czochralski method were tested but did not yield the desired results. Therefore, the search for the optimal scintillating material that will serve as the basis for a new type of high-energy physics detectors was continued.
In Chapter 1, the analysis of publications suggests that scintillating crystals such as bismuth germanate (BGO) and lead tungstate (PWO) have been used for a long time in particle accelerator detectors, including at CERN. The development of tomography detectors and the increase in particle collision frequency at accelerators lead to the need for new scintillating materials with controlled parameters. This task is addressed by controlling the energy structure of crystals to prevent the formation of charge carrier traps and to control their transport to luminescent centers (intrinsic or dopant).
Within this concept, crystals such as Gd3Al2Ga3O12:Ce with a light output of 50000-60000 photons/MeV, Y3Al2Ga3O12:Ce with a decay time of 20 ns, (Lu,Y)2SiO5:Ce, and other materials with improved properties were developed. It was concluded that to develop a material that would meet the necessary requirements of high-energy physics experiments, attention should be focused on crystals based on Y3Al5O12 and Lu3Al5O12 , and accordingly, on a solid solution of them - (Lu,Y)3Al5O12, taking into account the possibility of growing them by several methods, namely the μ-PD method for obtaining fibers, or the Czochralski method using inexpensive W crucibles. These works were carried out at the Institute for Scintillation Materials of the National Academy of Sciences of Ukraine in collaboration with the Institute of Light and Matter, CNRS, Lyon, France, and CERN.
In the second chapter, detailed descriptions of the experimental procedures for raw material preparation, features of growth chamber construction, growth monitoring methods, and post-growth processing of crystals for both μ-PD and Czochralski methods are provided. The chapter also outlines the methods of fabricating experimental samples and the characterization procedures of their scintillation and optical properties.
In Chapter 3, the methodologies for growing single-crystal YAG and GAGG fibers by the μ-PD method and their impact on the optical and scintillation properties of the crystals, as well as their defect structure, are described in detail.
The first step was the selection of optimal raw materials for growing YAG fibers. A series of fibers was grown using sintered Y2O3 and Al2O3 powders mixed in a stoichiometric ratio. The obtained fibers had a significant number of structural defects, leading to their cracking. This was attributed to the feature of EFG and μ-PD methods where crystallization occurs from a thin melt meniscus and there is no segregation of impurities into the main melt volume. Such methods are more sensitive to the purity of the raw material and deviation of the melt composition from stoichiometry towards one of the components.
In subsequent experiments, fragments of YAG and YAG:Ce crystals grown by the Czochralski method at the Institute of Scintillation Materials for the National Academy of Sciences of Ukraine were used as raw materials. The fibers grown in this way contained less defects, but their attenuation length was below the required threshold of 20 cm.
