Optical absorption coefficient of 7--ray irradiated PMMA and copolymers MMA-AN increases with dosage first linearly up to about 4 x 105r, then assumes transient saturation in about 6×10
6γ_??_1×10
7γ region, after which it resumes the increase slowly and parabolically reaching its final saturation. The region of initial linear increase can be used for measuring high doses of radiation. Copolymers MMA-AN in particular give rise to very strong absorption bands of small fading coeffi-cient in visible region and are therefore convenient materials for devising handy dosimeters.
Irradiation induces in the copolymers two kinds of absorption simultaneously, one is due to conjugated double bonds, the other is due to color centers and/or trapped radicals. The former appears in ultraviolet region and is irreversible, while the latter is in visible region, partly vanish-ing suddenly by heating above glass transition temperature, partly producing conjugated double bonds by atom rearrangement. Below glass transition temperature, however, the color centers and/or the trapped radicals, the chromophores, are firmly captured by the lattices and do not recom-bine, therefore are stable, but in free air they are completely bleached by oxygen leaving a distinct mark of boundary below the surface of the plastics suggesting that the oxygen molecules adsorbed on the surface penetrate it according to the law of diffusion and react on the chromophores. Penetration rate depends on dosage, composition of the plastics and temperature.
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