On-line ultrasonic monitoring and modelling of radiation-induced structural changes in polymethyl methacrylate
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Structural changes in polymethyl methacrylate (PMMA, monomer formula C2H2CH3COOCH3) induced by60Co radiation have been detected by means of on-line monitoring of increases in the attenuation of 10 MHz longitudinal ultrasound. Attenuation changes first became noticeable at a dose of 15 kGy and had increased by 75% at the maximum dose of 36.5 kGy. A theoretical upper bound to structural relaxation loss induced by radiation has been calculated. As a consequence it was then possible to show that at the dose levels encountered, the additional loss was attributable mainly to collective motions involving many atoms and of low attempt frequencies, rather than to the relaxation of individual atoms or structural groupings. The theory proposes an attenuation change proportional to the 5/3th power of the dose, which is in excellent agreement with experiment. It is suggested that on-line monitoring of ultrasound loss could be a sensitive diagnostic test of the onset of unwanted structural change during the practice of food preservation by the use of irradiation.
KeywordsAttenuation Structural Change PMMA Polymethyl Methacrylate Dose Level
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