Apparent radiation stability

The important additional reaction in determining the apparent radiation stability of C02 is rapid oxidation of CO by a negative ion species (say 02"). This reaction becomes important once a small concentration of molecular 02 has accumulated in irradiated C02, and it is the addition of this process to reactions involving the C03 radical which accounts for the difference between the radiolysis and ultraviolet photolysis of gaseous... 

Furthermore, one may think of radiation influencing the chemical stability of the solid. There is no experimental evidence for any of these effects [H2]. In all test procedures a long-term dose has been simulated in a short time, which will probably rather enhance the effects. The apparent radiation stability should be not too surprising, bearing in mind that an alpha dose of 10 /cm over 100 years is a rather modest one compared to the fast-neutron doses to which materials in a nuclear reactor are exposed. 

The relatively stable azides of the strongly electropositive metals undergo controlled (i.e. non-explosive or "slow") decomposition. For this reason, sodium and barium azides are probably the most completely studied salts in the azide group [12], Their decompositions are characterized by relatively large apparent activation energies and well-defined induction periods to onset of reaction [10], Mechanistic aspects of azide decompositions have been reviewed by Tompkins [17], The radiation stabilities of the Group lA metal azides do not correspond to the sequence of thermal stabilities [10]. The catalytic decomposition of HNj has also been investigated [12]. 

On the cautionary side it was noted that it is usually not possible to predict the radiation stability of pharmaceuiicaliy active substances from previous knowledge of other related substances, as minor differences in chemical structure can apparently have significant bearing on radiation stability and on therapeutic activity. 

Perhaps Perrin s continuing commitment is explained by the fact that his radiation hypothesis was an update of Marie and Pierre Curie s original explanation of radioactivity, that the apparently spontaneous emission of radiations and charged particles from molecules is activated by immanent radiations. The Curies supposed that radiations in the atmosphere or in the ether disturb the stability of the naturally radioactive elements. This hypothesis, like the radiation hypothesis of chemical activation, eventually succumbed to an explanation using electron configurations, adumbrated by the new quantum interpretation of matter and energy. 

Although this picture of nuclear atoms served to describe the alpha-paiticle scattering experiments, it still left many questions unsolved. One of these questions referred to the apparent stability of the atoms. An electron moving around the nucleus would lend to emit radiation, to lose its energy, and thereby to spiral into the nucleus. Why did it not do so Why did tlie atoms all seem to be quite stable, and all to be of approximately the same size, even though some contain 90 or more electrons, while hydrogen con lams only one ... 

Atherton et al. (153) have extended the calculations to include diffuse-gray radiation between components of the enclosure and reached essentially the same conclusions regarding the stability of the process. However, they discovered a new mechanism for the damped oscillation of the crystal radius caused by the radiative interaction between the crystal surface just above the melt level and the hot crucible wall. These oscillations are especially apparent when the vertical temperature gradient in the crystal is low, so that radiative heat transport has a dominating influence. 

Preliminary experiments showed that when thermally stabilized thin films (80 p thickness) of vinyl chloride-methyl vinyl ketone copolymers were subject to UV radiation they became brittle without color formation. When the ketone concentration in the ci olymer was approximately 6 mol-%, the time to brittleness in the Accelerometer was less than 20 hr, whereas 40 to 50 hr were required if the ketone concentration was only 2%. Samples of PVC homcpolymer, identically stabilized, were photolyzed along with the copolymers. No color or chemical change was apparent in these samples, even after 100 hr photolysis. 

Copolymers of vinyl chloride and methyl vinyl ketone undergo chain scission with concomitant rapid decreases in tensile strength and elongation when exposed to near ultraviolet li t and solar radiation. Free radicals formed by the homol3rtic scission of the acyl group apparently deplete the stabilizers used and lead to rapid discoloration of the polymer, presumably by the usual radical chain reaction involving the production of HCl and conjugated double bonds. 

As shown in several of the previous examples, the entities set free from the surface by radiation may appear in the gas phase and, in a flow system or one continuously pumped, may not return. There is considerable likelihood that fragments formed by decomposition will remain on the surface, and this has been observed for H atoms formed by y-irradiation of silica gel and silica-alumina 72, 74, 75,151), and apparently stabilized by being adsorbed in microcracks 151). 

Irradiation of polymers with y- or electron beams is an attractive alternative to chemical sterilization because of its speed, ease of control, and the absence of residue. Radiation treatment of polypropylene, however, also initiates chemical changes which lead ultimately to embrittlement. These changes in physical properties may not become apparent until some time after the treatment. The ability of antioxidants to prevent radiation damage does not always follow the trends observed in thermal oxidation, which has stimulated efforts to develop new stabilizers or optimized combinations of existing ones. 

In contrast, the presence of Cl based on low-molecular amines of NDA or G-2 type hampers gas-penetrability of the polymer films as a result of atmospheric aging [114]. Vapor-penetrability of PE films increases by 1.5-2 times after exposure to UV radiation for 200 hours or to climatic factors for two months. Under the same conditions, the vapor-penetrability of inhibited films grows, but by only 10-20%. Apparently, Cl of the named type function in this case like a stabilizer that hampers photo and thermal-oxidative aging of PE. 

All elements of pristine TPP and metalloporphyrins were found in XPS spectra (photon energy 1486.69 eV (A1KJ and 100-600 eV of synchrotron radiation) after moderate annealing in UHV. Yb-TPPBrj apparently partially decomposed even after annealing at 100°C in UHV. XPS and UPS studies have also demonstrated higher thermal stability of Yb-TPP in UHV as compared with brominated compounds. 

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