| Abstract [eng] |
Polymer structures are well known materials that are changing their properties due to irradiation by high energy particles and photons. Highly radiation sensitive structures made basically of monomer, oxygen scavenger, gelatin and water are known as hydrogels. When ionizing radiation is applied the polymerization process triggers and the amount of polymerized material is a function of absorbed radiation dose. It is known that radiation induce water radiolysis and net structure of gel starts its formation. The density of the cross linked net is a potential property enabling evaluation of polymerized structures. Evaluation might be performed using different methods including UV-VIS spectrometry, since radiation induced polymerization in hydrogels is responsible for optical density changes in these gels. It is also known that polymer composites have outstanding feature to attenuate radiation due to dose absorption in material which depends on material composition. This feature is used in radiation protection equipment. The behavior of polymeric structures under irradiation is very important due to radiation induced chemical and structural changes, however the majority of performed investigations is related to the evaluation of physical properties of materials after irradiation to high doses >10 Gy. The research presented in this dissertation was focused on the investigations of dose sensitivity of polymer composites irradiated to low doses up to 10 Gy. Performed investigations were addressed to two objectives: development of dose gels with enhanced sensitivity to low dose irradiation for 3D dosimetry in radiotherapy and development of lead free optically transparent polymer composites as filling material in radiation protection screens. Specific aims of the study included the following: 1. Preparation of reference normoxic gels, their irradiation by high-energy photons and various particle beams, and assessment of optical characteristic changes due to low-dose induced polymerization processes in the irradiated dose gels. 2. Development of advanced polymerized dose gels that are sensitive to varying composition of low-dose photon irradiation and concentrations of gel constituents. 3. Development of photo-scanning method and instrumentation for dose gel read out. 4. Development of lead-free transparent polymeric composites with effective X-ray radiation shielding properties. Most of the experimental work was carried out in Kaunas University of Technology: Faculty of Mathematics and Natural Sciences Department of Physics and Institute of Materials Science. Proton irradiation of the samples was performed in OncoRay Clinic in Dresden, Germany. Samples electron and photon irradiation was performed in Lithuanian Health Science University Kaunas Clinics and Oncology hospital. Samples neutron irradiation was performed in Center for Physical Sciences and Technology (FTMC) in Vilnius. The analysis of the optical characteristics of dose gels that are irradiated to low doses (<10 Gy) has shown that the sensitivity of gels was directly dependent on the type of irradiation that is responsible for the gel’s polymerization and chemical content of gels. It was found that the standard nMAG gel was most sensitive to all types of irradiation as compared to nPAG and VIPET gels. The sensitivity parameter of nMAG gel to 15 MeV X-rays was 0.068 (a.u.), to gamma photons (60Co source) — 0.099 (a.u.), to electrons — 0.11 (a.u.), to protons — 0.22 (a.u.). By varying concentrations of the nMAG gel components, the dose gel of advanced composition was developed, which recorded the enhanced by 30 % sensitivity to high-energy photons. The composition of the advanced nMAG gel was as follows: 8 % w/w of methacrylic acid as a monomer, 6 % w/w of gelatin, 86 % w/w of purified water, and 15 mM of hydroxymethyl phosphonium chloride, as an oxygen scavenger. A new photo scanning dosimetry instrumentation for dose evaluation in the irradiated gels was developed, constructed, and implemented for the experimental measurements. The constructed system allowed the scanning of irradiated polymer gel samples with a spatial resolution of 0.20 mm. It was shown that the polymerization spread out in the irradiated dose gels was dependent on the proliferation of the radiation produced reactive radicals within the gel. The polymerization spread out was smooth and varied from 2 mm (0.5 Gy) to 6 mm (5 Gy) away from the irradiated volume. The development and investigation of different lead-free polymeric composites, recording X-ray attenuation properties that are similar to those of lead, revealed that most promising composites contained water, 1 % (C3H5NO)n (polyacrylamide), and different concentrations of (NH4)6H2W12O40xH2O. It was shown that (NH4)6H2W12O40xH2O concentration of 60 % in the composite was sufficient enough for its application in the radiation protection equipment, since the estimated lead equivalent of the composite was 0.63 mmPb, and UV-VIS transparency was 89 %. Experience obtained developing photo-scanning method was used constructing prototype of catheter based gel dosimetry system BrachyDOSE which is aimed for dose measurement and verification in high dose rate brachytherapy. For the commercialization of the proposed dosimetry system SME “Šeši partneriai“ was established, which has won the international competition and got financial support from EU structural funds in the frame of Horizon 2020 project for SMEs, Phase - 1 for the feasibility study related to the commercialization of the product. The author of this dissertation is the main project manager and investigator in the company. The developed lead free polymer composite was used as a filling for the radiation protection screen. A new type of radiation protection screen prototype was tested in clinical environment and protected under LT patent Nr. B6292. “Radiation protecting material composition, screen for radiation protection and method of making the same”. |
