Beta, gamma and neutron attenuation properties of PMMA/colemanite composites exposed to space radiation in low earth orbit

Abstract

Polymers are an important material group that has gained a prominent place in various fields of industry. The usage of polymers has been expanded in a wide range of products ranging from automotive to aerospace industry apart from plenty of various promising scientific research. It is inevitable to evaluate that with a comprehensive perspective, polymers have gained a place that is difficult to change in today's world, thanks to its several properties such as low cost, easy to process and suitability for composite production. However, it becomes clear that some polymer- based materials are more suitable than other material options for a number of sophisticated research areas. Poly (methyl methacrylate) (PMMA) is a polymer-based synthetic material that is widely used in today's world. It is possible to state PMMA is an important thermoplastic material and stands out with its mechanical properties such as transparency and hardness. The applications of PMMA can be intrinsically diversified due to the opportunity of using as matrix in a composite structure apart from its utilization solely based on some physical and chemical properties. In other words, there is no such an obstacle to improve the any properties of PMMA and creating the new material that is more favorable than its base form with reinforcement voluminous elements. The reinforcement elements must be determined by taking into account a number of purposes since introducing agents directly affects the properties of the composite to be produced. However, Colemanite (CMT) is a mineral that can be considered as a candidate reinforcement element for use in the field of radiation shielding due to its chemical constituent. It is a foreseeable fact that introducing the CMT as a reinforcement agent along with PMMA that has been used for radiation attenuation applications within the scope of various researches can improve the attenuation ability of base PMMA against different types of radiation. On the other hand, it is an extremely critical point that for a developed material to maintain the attenuation properties in long-term use and to prevent various operational restrictions in terms of functionality. For this reason, the targeted properties must also be preserved during using of the material in addition to achieving the cost and desired properties in the initial synthesis process. There is a huge amount of radiation dose for a matter due to the components of space environment such as galactic cosmic rays and solar energetic particles. It is possible to foresee that there may be a series of changes in the structure of material groups in an exceedingly harsh radiation environment of space. Thereby, the materials used in space studies must be able to ensure effective radiation shielding for long term uses even after even after receiving high doses of radiation. Moreover, it is clear that long-term use of material in space can be considered as enormously favorable by taking into account cost and operational difficulty. In this study, base PMMA, PMMA/CMT 5 wt.%, PMMA/CMT 15 wt.% and PMMA/CMT 5 wt.% with microspheres were produced by the atom transfer radical polymerization (ATRP) method. Synthesized samples have been exposed to space radiation on the International Space Station for approximately a year. Afterwards, attenuation properties of the samples against various types of radiation were examined within the scope of the ground-based experiments. Experimental studies on the exposure to space radiation were performed in cooperation with the Japan Aerospace Exploration Agency (JAXA). The launch and landing operations were carried out by the Space-X company. The samples were directed to JAXA's Tsukuba Space Center (TKSC) after landing on Earth. Launching was carried out with the Falcon-9 rocket and samples were placed in the 'ExHAM' facility which is connected to the outside of the Japanese experiment module on the International Space Station. In addition, the absorbed dose amount for the samples that interacted with high-energy electrons and positrons along with galactic cosmic radiation and solar energetic particles was determined as thereabout 0.02 kGy. Within the scope of the ground-based experiments, Sr-90 radioisotope was used as the beta source while Co-60 and Cs-137 radioisotopes were used as the gamma source. Apart from radioisotopes, 239Pu-Be neutron howitzer has preferred as the neutron source. Beta, gamma and neutron attenuation properties of PMMA/CMT composites that exposed to space radiation were examined. Furthermore, PMMA/CMT composites 15 wt. % concentrations were investigated for different radiation types to obtain the attenuation properties for both unirradiated and irradiated state in each experimental setup. Hence, the effects of both CMT reinforcement and radiation environment in low earth orbit on the beta, gamma and neutron attenuation properties were determined. Studies have shown that the increase in CMT ratio in the composite structure improves the attenuation properties of PMMA/CMT composites that exposed to space radiation. In addition, linear attenuation coefficients of the samples were determined through the different thicknesses and compared with theoretical values. PMMA/CMT 15 wt. % samples in both irradiated and unirradiated state were examined in the same experimental setup to understand the effects of open space radiation. It can be stated that there is no decrease in the attenuation properties of the irradiated PMMA/CMT 15 wt. %. It is possible to predict that the samples have meet a series of structural changes as they were exposed to radiation in low earth orbit. The ATRP technique in the synthesis of a material was preferred by considering the space radiation environment. It is considered that new reactive end groups and free radicals formed due to the high amount of radiation dose taken during the space experiments. The formation of molecular cross-links within the polymer matrix can be occurred as a result of these newly formed end groups. However, the network structure of the relevant composite can be strengthened via the creation of cross-links. It has been inferred that the strengthening and density increase of the composite structure can be deducted with the formation of cross-links.