Radiation resistance electron irradiation

Ke)Tvvords radiation resistance, electron accelerators, gamma radiation, peroxy radicals, absorbed dose, irradiation-induced degradation, molecular weight distribution (MWD), crystallinity, stabilization to gamma radiation die molecules. 

Irradiation. Although no irradiation systems for pasteurization have been approved by the U.S. Food and Dmg Administration, milk can be pasteurized or sterilized by P tays produced by an electron accelerator or y-rays produced by cobalt-60. Bacteria and enzymes in milk are more resistant to irradiation than higher life forms. For pasteurization, 5000—7500 Gy (500,000—750,000 tad) are requited, and for inactivating enzymes at least 20,000 Gy (2,000,000 rad). Much lower radiation, about 70 Gy (7000 tad), causes an off-flavor. A combination of heat treatment and irradiation may prove to be the most acceptable approach. 

The increase in the modulus for Bis A PSF and Hq/Bp PSF with irradiation indicated that crosslinking predominated for both polymers and that the crosslink structures were probably basically similar. Hq/Bp(50) PSF was considerably more radiation resistant than Bis-A PSF, as shown by the rate of decrease in the elongation at failure. For both polymers, there was an initial rapid decrease in the elongation at failure followed by a slower decrease. This effect was also demonstrated by the variation in the fracture toughness (KI(.) with irradiation for Bis-A PSF. This work with cobalt-60 gamma radiation complements earlier studies of these materials using high dose rate electron beam irradiation (6). 

Although the irradiation of 200 kGy decomposes about 80% of polystyrene in toluene by the dissociative electron attachment, the yield of the decomposition is only 20% for solid toluene. Because of its low efficiency of scission, the coupled polystyrene may not be a polymer suitable as a radiation resist. However, the present study has shown that a polymer that can be decomposed into two equivalent skeletons by ionizing radiation is possible to be... 

The radiation resistance of zirconolite has been tested with 238Pu- and 244Cm-doped and ion-irradiated samples (Weber et aL 1998 Weber Ewing 2002). Amorphization of zirconolite occurred at doses corresponding to 0.3-0.5 dpa at room temperature. Dose-age relationships have been determined by analytical transmission electron microscopy for the onset dose and critical amorphization dose of a suite of natural zirconolites (Lumpkin etal. 1994,1998b). 

Evans et al. [43] carried out 4 MeV electron irradiations of 14 different epoxy resins at 77 K which were selected from a large number of resin systems after screening tests on thermal shock at cryogenic temperatures [44]. The results of flexural tests show that most of these irradiated resins possess only moderate resistance to radiation. Takamura and Kato [45] tried to irradiate the bisphenol-A type epoxy resins with various hardeners at 5 K in a fission reactor and reported that the compressive strength of these epoxy resins decreased sharply after a combined neutron and y-ray irradiation equivalent to a dose of about 107 Gy. 

Molecular Crystals Fullerites. Organic crystals are usually prone to ionization damage and decompose very rapidly under electron irradiation they can thus be studied for only a short time (a few. seconds) and only with a very low electron beam intensity. Transmission electron microscopy has, therefore, seldom been applied to organic crystals. However, the all-carbon molecules Qo, C70. etc., (fullerenes) discovered at the end of the 1980s resist electron radiation fairly well. Early structural studies on the crystalline phases of ftil-lerenes (fullerites) were performed mainly by electron microscopy because only small quantities of sufficiently pure material were available. At room... 

El-Naggar et al. [84] have studied the effect of electron-beam irradiation on the thermal and mechanical properties of PPS. The dose rate was 50Mrads/sec and the maximum dose was 4000Mrads. They observed a decrease in crystallinity of PPS with increasing dose. Also, it was observed that the effect was more pronounced for the initially amorphous PPS than initially semicrystalline PPS, suggesting a lower radiation resistance of the amorphous phase. 

Figure 5.128 Radiation resistance ofPOiJm thick polyethylene films irradiation in vacuum at 5000 kGy/h with 1 MeV electrons from a van de Graaf generator and irradiation in air at WkGy/h with y-radiation from a °Co source [711]...

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