High-dose-rate irradiation

The effect of antioxidants such as hindered phenohcs, secondary amine, and thioester on the radiation cross-linking efficiency of LDPE has been reported [260]. Amount of cross-linking at a given dose decreases with aU the antioxidants, the thioester being the most effective. IR absorption spectroscopy has been used to demonstrate dose-rate dependence of trani -vinylene unsaturation in irradiated Marlex 50 PE [261]. When the irradiated polymer is stored in vacuum a decrease is observed in trani-vinylene absorbance over a period of several weeks. After high dose-rate irradiation the decay is preceded by an initial increase. These phenomena have been ascribed to the reaction of trapped radicals. 

The time accelerated aging method for electric wires and cables requires a high dose rate irradiation, but the exposure to polymer in air results only the oxidation of surface when the dose rate is higher. I previously reported the methodology study of time accelerated irradiation of elastomer (2). The study showed two appropriate methods, one was irradiation in pressurized oxygen at room temperature the other was irradiation at 70°C in air. The article studied the effect of higher dose (up to 10 MGy) irradiation on ethylene-propylene-diene elastomer (EPDM) by using the time accelerated method. Irradiation at 70°C in air was chosen as the time accelerated irradiation condition because of the experimental convenience. 

This value is independent of pH above pH 12 and only applies in solutions where the OH and H atom yield are completely converted into hydrated electrons. pH 12 is optimum as lower pH s do not convert all the H and OH yields (2) and higher pH s increase the effects of impurities in the alkali. A hydrogen-saturated solution at pH 12 can be sealed permanently in an optical cell. Such a cell has been used over a period of two years without showing any change in characteristics with an accumulated dose of several megarads. Effects of exposure to high dose rate irradiation (which produces hydrogen peroxide) are removed by exposure to low... 

Fluoropolymers utilizing high molecular weights and copolymerized and alloyed with polyethylene, should be used in most radiation applications. High-dose-rate E-beam processing may reduce oxidative degradation. When irradiated, PTFE and PFA are... 

Akhtar et al. [444] have studied the effect of y-irradiation on NR-PE blend. The high-energy radiation at a high dose rate has been found to cause extensive cross-linking in the bulk. The rupmre energy values increase subsequently in the range of 15-25 Mrad and then decrease, as the absorbed dose increases further. 

High-energy radiation may be classified into photon and particulate radiation. Gamma radiation is utilized for fundamental studies and for low-dose rate irradiations with deep penetration. Radioactive isotopes, particularly cobalt-60, produced by neutron irradiation of naturally occurring cobalt-59 in a nuclear reactor, and caesium-137, which is a fission product of uranium-235, are the main sources of gamma radiation. X-radiation, of lower energy, is produced by electron bombardment of suitable metal targets with electron beams, or in a... 

The main effects of dose rate are due to an increase in temperature of the polymer and depletion of oxygen (for irradiation in air) at high dose rates. It seems unlikely that direct effects of dose rate should occur for electron, gamma and X irradiation, due to the low spatial density of the ionizations and excitations. 

Much research into radiation effects on polymers is done with samples sealed under vacuum. However, polymer materials may, in practical applications, be subjected to irradiation in air. The effect of irradiation is usually substantially different in air, with increased scission at the expense of crosslinking, and the formation of peroxides and other oxygen-containing structures. Diffusion rates control the access of oxygen to radicals produced by the radiation, and at high dose rates, as in electron beams, and with thick samples, the behaviour may be similar to irradiation in vacuum. Surface changes may be quite different from bulk due to the relative availability of oxygen. 

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). 

For example, when the mixed solution of Ag(CN)2 and Au(CN)2 is irradiated by y-radiolysis at increasing dose, the spectrum of pure silver clusters is observed first at 400 nm, because Ag is more noble than Au due to the CN ligand. Then, the spectrum is red-shifted to 500 nm when gold is reduced at the surface of silver clusters in a bilayered structure [102], as when the cluster is formed in a two-step operation [168] (Table 5). However, when the same system is irradiated at a high dose rate with an electron beam, allowing the sudden (out of redox thermodynamics equilibrium) and complete reduction of all the ions prior to the metal displacement, the band maximum of the alloyed clusters is at 420 nm [102]. 

Figure 13 Scheme of the influence of the dose rate on the competition between the inter-metal electron transfer and the coalescence processes during the radiolytic reduction of mixed metal ion solutions. Sudden irradiation at high dose rates favor alloying, whereas low dose rates favor coreshell segregation of the metals because of metal displacement in the clusters. 

In pulse radiolysis studies of Urd and its derivatives (but not with dUrd), spectral changes are observed after the completion of the S04, reaction [k = 3 x 10s s 1 Bothe et al. 1990] that are not typical for S04 reactions with pyrimidines. On the basis of EPR experiments (Hildenbrand 1990 Catterall et al. 1992), these observations can be interpreted by an (overall) intramolecular H-transfer giving rise to a radical at the sugar moiety. This requires that considerable amounts of Ura are released which is indeed observed (Fujita et al. 1988 Aravindakumar et al. 2003 Table 10.4). Chain reactions occur as with the other pyrimidine/peroxodisulfate systems. This increases the Ura yield beyond that expected for a non-chain process, but when corrections are made for this by carrying out experiments at the very high dose rates of electron-beam irradiation, a... 

Table 10.4. G (base release) (unit 10-7 mol J-1) from some pyrimidine nucleosides and 2 -deoxynucleosides induced by the S04 radical [G(S04 ) = 3.3 x 10-7 mol J"1) at different dose rates pulsed electron-beam irradiation ( 6 Gy per 2 ps pulse, high dose rate) and y-irradiation (0.013 Gy s-1, low dose rate Aravindakumar et al. 2003) ...

Subsequently, the absorption spectrum of the transient hydrated electron was observed in pulse radiolysed deaerated water43 44. Its spectrum is shown in Fig. 5. This absorption may also be observed with continuous radiolysis at high dose rates. Thus it has been observed46 with hydrogen-saturated solutions of 10 3 M NaOH when irradiated in a 15,000 curie 60Co source. 

While 7-radiolysis at relatively low dose rates enabled the synthesis of a few alloyed clusters such as Ag-Pd nanoparticles (also chemically synthesized), radiolysis at very high dose rates (by electron beams) led to the synthesis, at room temperature, of a lot of new alloys such as Au-Pd bimetallic nanoparticles/ Like in the case of Ag-Au system, at low dose rate (7-irradiation), bilayered Au -Pd.i,ii nanoparticles were obtained. However, at high dose rates (electron beams), the reduction is faster than the possible inter-metal electron transfer, then alloyed clusters were prepared. Moreover, since the radio-induced reduction of metal ions is faster at high dose rates, the synthesized particles are, in these conditions, always smaller with a narrow distribution in size. 

Alessi et al. (2005) examined the y-irradiation and electron-beam processing of an epoxy-resin system in the presence of a photoinitiator. They showed that increasing the irradiation dose frequency and photoinitiator concentration greatly increases the temperature reached by the samples. The increase in temperature of the system during irradiation is related to the balance between the heat-evolution rate, due to both polymerization reaction and radiation absorption, and the rate of heat release towards the environment. High dose rates and high photoinitiator concentmtions increase the reaction rate and the difference between the heat produced and the heat released to the environment (Alessi et al. (2005). 

Figure 4 shows density data on two EPR samples (B and C of Fig. 1) that were irradiated to similar dose, but at different dose rates. Strongly heterogeneous oxidation is again indicated for the high-dose-rate sample, in contrast to nearly homogeneous oxidation for the low-dose-rate sample. The results correlate well with results obtained by optical examination of polished samples and by relative hardness measurements. 

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