5.2 Evidence of the Non-Existence of the LET Effect in the Mechanical Properties of Polymer Materials


Fig. 5-3 Elongation at break by tensile test for ion beam irradiated polyethylene films

Irradiation induces cross-linking reactions between polymer molecules resulting in decreased elongation. Here, it is seen that the degree of the radiation effect depends only on the absorbed dose, irrespective of the different types of radiation, though the LET value varies considerably from 0.2 eV/nm (2 MeV electron) to 287 eV/nm (350 MeV Ne8+).


Fig. 5-4 Formation of double bonds in polyethylene irradiated by various ions and gamma-rays

The yield for double bond formation in polyethylene has been measured by IR spectrum analysis. The two solid curves in the figure represent the yield with gamma-ray irradiation at room temperature and at 100°C. The yield depends on the absorbed dose, and not on the types of radiation, and with a slight dependence on the irradiation temperature.



Energy transfer occurs along the path of radiation traversing matter. The density of the transferred energy is mainly determined by the charge and velocity of the traversing energetic particles. In the case of X- or gamma-rays the relevant particles are assumed to be electrons. Changes of mechanical properties or chemical yields in irradiated materials, which are generally studied based on the absorbed energy in units of Gy (= J/kg), are discussed in terms of Linear Energy Transfer (LET = Energy transferred while the particle travels a unit distance in matter) to clarify effects caused by different types of radiation. In particular, it is important to know the LET effects of materials to be used in space technology in order to examine their durability when exposed to different types of radiation.
Polymer films of 100 mm x 100 mm with 100 mm thickness were irradiated uniformly with various different forms of radiation, (gamma-rays and various ion beams) and then submitted to measurements of their mechanical properties (Fig. 5-3). We also studied chemical changes in the polymer by examining the formation of double bonds as a result of hydrogen molecule emission, and the formation of new chemical bonds resulting from cross-linking between polymers (Fig. 5-4). In our experiment, no LET dependence was observed, the change in mechanical properties as well as the yield of formation of new bonds depended only on the absorbed dose. These results show that the same local density of energy is deposited along the path of any single ion, even if the ion has a different LET as in the case of gamma-rays.


Reference
T. Seguchi et al., Ion Beam Irradiation Effect on Polymers. LET Dependence on the Chemical Reactions and Change of Mechanical Properties, Nucl. Instrum. Methods Phys. Res., B, 151, 154 (1999).

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