High energy radiations

The amount of energy set free in the processes shown in Table 4.4 is, for example, around 3 J/year for 1 g 238U (in equilibrium with its daughter product). 235U emits 18 J/g/year, 232Th 0.8 J/g/year. The average amount of the three elements present in granite and volcanic rocks is shown in Table 4.5. 

It is assumed that the average heat generation due to the radioactive processes described above was around 8x 10-6 J/g of rock (Birch, 1954). If this value is extrapolated to the Earth s crust (to a depth of 35 km), the result is 53 J/cm2/year. It seems clear that the crust of the primeval Earth contained about four times as much 40K as today. The higher half-life of 238U means that the amount of this isotope present about four billion years ago was around twice today s value. The corresponding factor for 235U, with a half-life of 7x 108 years, is 64. Thus, the value of 

53 J/cm2/year calculated for the Earth today is much lower than that available four billion years ago (197 J/cm2/year). Most of this energy was converted to heat, and some simply radiated into space. 

High-energy radiation from the sun and the cosmos also reaches Earth. It is clear that the young Earth was subjected to such radiation however, no exact information is available on the radiation intensity four billion years ago, and thus on the possible consequences for chemical evolution we rely on estimates. 

Vacuum ultraviolet photolysis of acetylene results in formation of triplet C2, as evidenced by its characteristic emission.139 Presumably, triplet acetylene is first formed and decomposes to C2 and H2. Saturated hydrocarbons undergo radiolytic reactions, but the relative importance of excited states versus ionized states has not yet been established with any certainty. 

---

A unique high-energy radiation laboratory is functioning at the Institute, in which studies Can be performed and thick items can be controlled using powerful X-ray units and betatrons. 

In comparison with most other analytical techniques, radiochemical methods are usually more expensive and require more time to complete an analysis. Radiochemical methods also are subject to significant safety concerns due to the analyst s potential exposure to high-energy radiation and the need to safely dispose of radioactive waste. 

Manufactured PVDF parts can be cross-linked using high energy radiation to produce high temperature wire insulation, and heat-shrinkable tubing or film. 

Cross-linking of polyethylene can be accomphshed either chemically or by high energy radiation. Radiation cross-linking is usually accomphshed by x-rays (44) or electrons (45,46). Chemical cross-linking of polyethylene is accomphshed with dicumyl peroxide (47), d4-tert-huty peroxide (48), or other peroxides. Radiation cross-linking (49) is preferred for thin foams, and chemical cross-linking for the thicker foams. 

A unique process for chemical stabili2ation of a ceUular elastomer upon extmsion has been shown for ethylene—propylene mbber the expanded mbber obtained by extmsion is exposed to high energy radiation to cross-link or vulcani2e the mbber and give dimensional stabUity (9). EPDM is also made continuously through extmsion and a combination of hot air and microwaves or radio frequency waves which both activate the blow and accelerate the cure. 

Initiation of radical reactions with uv radiation is widely used in industrial processes (85). In contrast to high energy radiation processes where the energy of the radiation alone is sufficient to initiate reactions, initiation by uv irradiation usually requires the presence of a photoinitiator, ie, a chemical compound or compounds that generate initiating radicals when subjected to uv radiation. There are two types of photoinitiator systems those that produce initiator radicals by intermolecular hydrogen abstraction and those that produce initiator radicals by photocleavage (86—91). 

Another use is of minor proportions of polyfunctional aHyl esters, eg, diaHyl maleate, ttiaHyl cyanurate, and ttiaHyl isocyanurate, for cross-linking or curing preformed vinyl-type polymers such as polyethylene and vinyl chlotide copolymers. These reactions ate examples of graft copolymerization in which specific added peroxides or high energy radiation achieve optimum cross-linking (see Copolymers). 

Radiation sensitive cast polymers from DADC are also used in resists for microelectronic circuitry. Relief images result from differential rates of solution in alkali induced by exposure to high energy radiations. 

DiaHyl esters find Htde appHcation in lenses. However, DAP and DAIP can be polymerized by high energy radiation in lens molds (74). Coatings of sihca and alumina by vaporization give antiglare, scratch-resistant lenses. 

Like almost all synthetic polymers, styrene plastics ate susceptible to degradation by heat, oxidation, uv radiation, high energy radiation, and shear, although... 

EFFECTS OF THERMAL, PHOTOCHEMICAL AND HIGH-ENERGY RADIATION... 

Ejfects of Thermal, Photochemical and High-energy Radiation 97 Table 5.9 Thermal degradation of selected polymers (Ref, 7)... 

In analogy with thermal and light radiations, high-energy radiation may also lead to scission and cross-linking. The relative stabilities of various polymer stmctures are shown in Figure Whilst some materials cross-link others... 

Table 5.10 Behaviour of polymers subjected to high-energy radiation"...

Such structural changes are a consequence of chemical reactions of which the most common are oxidation, ozone attack, dehydrochlorination and ultraviolet attack. (Reactions due to high-energy radiation or to high temperature are not considered here as causing natural aging.) Over the years many materials have been introduced as antioxidants, antiozonants, dehydrochlorination stabilisers and ultraviolet absorbers—originally on an empirical basis but today more and more as the result of fundamental studies. Each of these additive types will be eonsidered in turn. 

The polymers also have excellent resistance to oxidative degradation, most chemicals other than strong bases and high-energy radiation. Exposure for 1500 hours to a radiation of about 10 rads at 175°C led to embrittlement but the sample retained form stability. 

---