Yelisieieva О. Development of ways to increase the radiation resistance of a plastic scintillator

Thesis for obtaining the scientific degree of Doctor of Philosophy in specialty 132 – Materials Science (field of knowledge 13 – Mechanical Engineering). – Institute for Scintillation Materials of the National Academy of Sciences of Ukraine, Kharkiv, 2025. The dissertation is devoted to solving the issues of increasing the radiation resistance of polystyrene-based plastic scintillators. In modern physical experiments at the Large Hadron Collider, radiation loads on the scintillator can reach tens of Mrad, but traditional polystyrene-based plastic scintillators usually have a low radiation resistance threshold. Thus, in known commercially available plastic scintillators of the type SCSN-81T (Kuraray, Japan), BC-408 (Bicron, USA), UPS-923A and UPS-98RH (Amcrys-H, Ukraine) the radiation resistance is only 1–5 Mrad. Therefore, the development of approaches to increase the radiation resistance of a plastic scintillator is an important task. The work investigated the main ways to increase the radiation resistance of a plastic scintillator. The first way is the use of luminescent additives with a large Stokes shift. It is known that under the influence of ionizing radiation, traps are formed in traditional polymer bases of plastic scintillator, which effectively absorb energy in the range from 300 nm to 480 nm. To create an effective radiation-resistant material, it is necessary to bypass these traps, which is what additives with a large Stokes shift provide. Within the framework of this approach, a number of scintillation additives with a large Stokes shift were synthesized - fluoro-, phenyl- and fluorophenyl derivatives of 3-hydroxyflavone, containing up to four fluorine atoms in different positions of the molecule. It was shown that the introduction of fluorine atoms into the molecules of 3-hydroxyflavone and 4ʹ-phenyl-3-hydroxyflavone does not significantly affect their spectral properties. The obtained additives were used as activators of the polymer base of a plastic scintillator. Their use made it possible to increase the radiation resistance of the material and obtain a scintillator with a half-attenuation dose of the light output D1/2 of 49 Mrad. The second way is based on the introduction of diffusion enhancers into the plastic scintillator, which increase the mobility of free radicals and thereby reduce damage to the material. This method improves the radiation resistance of the plastic scintillator, but leads to significant degradation of mechanical properties and a decrease of long-term stability. To overcome these shortcomings, a crosslinking agent is introduced into the polymer scintillation composition. The work shows that the simultaneous use of a crosslinking agent and a diffusion enhancer in a plastic scintillator increases its radiation resistance while maintaining the light output and mechanical properties at a level suitable for functional use. Thus, plastic scintillators containing diffusion enhancers and crosslinking agents were obtained, the D1/2 of which reaches 9.7 Mrad. A scintillation composition was also created, which simultaneously contains a diffusion enhancer, a crosslinking agent and a 3-hydroxyflavone derivative. This combination allowed to significantly increase the radiation resistance of the material (D1/2 = 37 Mrad). The third path is based on the use of more radiation-resistant polymer bases and luminescent additives to create plastic scintillators. The work investigated the possibility of creating a radiation-resistant scintillator on a polysiloxane basis. Thus, a number of scintillators on polysiloxane bases with different contents of phenyl groups were obtained and studied. A scintillator on mixed polystyrene-polyphenylmethylsiloxane bases was also developed, the D1/2 of which is 22 Mrad. In addition, a search and synthesis of activators suitable for use in a polysiloxane basis were carried out. A new activator for a plastic scintillator has been obtained - 2,4,4ʹʹ-tri-tert-butyl-1,1ʹ:4ʹ,1ʹʹ-terphenyl. Polysiloxane scintillation compositions have been developed, the light output half-attenuation dose of which reaches 60 Mrad. Scientific results that were obtained for the first time: 1. A number of new luminescent additives with a large Stokes shift were developed and obtained - derivatives of 3-hydroxyflavone. 2. It is shown that the introduction of one or two fluorine atoms into any part of the 3-hydroxyflavone molecule and its 4ʹ-phenyl derivative does not affect the character of the luminescence spectra. The luminescence maximum undergoes a bathoflor shift of 5–10 nm only after the replacement of four hydrogen atoms with fluorine. The time characteristics of the scintillation pulse of plastic scintillators activated by fluorine derivatives of 3-hydroxyflavone containing one, two or four fluorine atoms do not change, while the decay time for all samples is 7.6 ns.