Radiation-Induced Colors

Although vitreous silica usually remains colorless following irradiation to very high doses, doped silicas can become colored through the formation of defects associated with impurities. Purple samples, for example, are formed if the glass contains a small amount of aluminum, due to the formation of aluminum-oxygen hole centers (AlOHC). Other impurities, such as germanium or titanium, can also produce colored vitreous silica by formation of defect centers. 

Most common silicate glasses become brown after irradiation. The color is due to formation of many defects, especially hole centers associated with the non-bridging oxygens present in glasses containing alkali or alkaline earth oxides. 

These optical absorptions can be bleached, or thermally annealed, by heating to sufficiently high temperatures. The thermal stability of the defects differs widely, so that the elimination of one defect may occur at room temperature, while the elimination of another requires heating to near the glass transformation temperature of the glass. 

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If we consider NaOH, KOH, and NaF, which act as mineralizers, to be the solvent components, and other minor amounts of elements such as Fe +and Al " to be impurity elements, then the partitioning of these impurity elements is controlled principally by kinetics. The impurity partitioning is related to color, or radiation-induced color, and crystal morphology. 

The radiation-induced color changes in inorganic materials (Ref 145) led to a comprehensive study by Rosenwasser, Dreyfus and Levy (Ref 148) on Na azide, which turns to brownish yellow when subjected to radiation. Subsequently, when mechanically deformed crystals of Na and K azide were irradiated with 107R gamma radiation, Dreyfus and Levy (Ref 69) observed the formation of pyramidal etch pits which occurred mainly in regions where imperfections were located at the surface. These were also evident in ammonium perchlorate crystals (Ref 255)... 

S. Irie, M.-S. Kim, T. Kawai, M. Irie, The radiation-induced coloration of amorphous photochromic dithienylethene films. Bull. Chem. Soc. Jpn., 77, 1037-1040 (2004). 

S. Irie, M. Irie, Radiation-induced coloration of photochromic dithienylethene derivatives in polymer matrices. 

In a more general application, thermoluminescence is used to study mechanisms of defect annealing in crystals. Electron holes and traps, crystal defects, and color-centers are generated in crystals by isotope or X-ray irradiation at low temperatures. Thermoluminescent emission during the warmup can be interpreted in terms of the microenvironments around the various radiation induced defects and the dynamics of the annealing process (117-118). ... 

The colors are clear, yellow, orange, red, blue and green, while the main color centers are radiation induced. Violet and violet-red colors in Cr-containing topaz are generated by two absorption bands in the visible part of the spectriun, which are connected with Cr + substituting for AP". Yellow topaz besides Cr +... 

In 1937 Arnow showed that tyrosine could be converted into DOPA by ultraviolet radiation51 and that the DOPA produced in this manner was subsequently destroyed by further irradiation, the solutions becoming red-brown in color (presumably due to the formation of dopachrome).51 In 1939 Konzett and Weis reported that the blood pressure-raising effect of adrenaline solutions was lost on ultraviolet irradiation and that the solutions became colored and fluorescent the initial red color fades to reddish yellow.62 This phenomenon suggests the initial formation of adrenochrome, followed by its isomerization to adrenolutin, both of these compounds being virtually void of pressor activity. Similarly to the radiation-induced hydroxylation of tyrosine mentioned above, synephrine was first... 

Fig. 5 Smart UV-responsive coating on silica nanoparticles with PNIPAM brushes functionalized with FRET donors, 4-(2-acryloyloxyethylamino)-7-nitro-2,l,3-benzoxadiazole (NBDAE), and photoswitchable acceptors, l -(2-methacryloxyethyl)-3, 3 -dimethyl-6-nitro-spiro(2//-l-benzo-pyran-2,2 -indoline) (SPMA). The UV radiation induces the change from colorless spiropyran derivatives in the outer part of the coating (7) to the fluorescent merocyanine form (2). Thus, FRET with the benzoxadiazole moieties in the inner part of the coating is enabled and the fluorescence color changes from green to red. By variation of the temperature and induction of a collapse of the PNIPAM chains (3), the FRET efficiency can be tuned (4). Reprinted, with permission, from [70], Copyright (2009) American Chemical Society...

Exposure to solar irradiation (sunlight) may alter their chemical integrity and in due course some physical properties such as mechanical strength (e.g. brittleness) or color (e.g. fading) as well. Additives like industrial ultra violet (UV)-absorbers, hindered amine light stabilizers, radical scavengers and antioxidants are incorporated or applied in these products to effectively reduce or delay such radiation induced deteriorations. 

Observations of identical kinetics for radiation-induced exchange and radiation-induced adsorption of D2 on silica gel have been said to support the identity of color and the catalytic site for H2-D2 exchange (91a), but the sites involved under the two ways of doing the experiments (preirradiation or simultaneous irradiation) are not necessarily the same (see Section V,c). 

Radiation-induced discoloration of polypropylene is due mainly to the formation of colored radiolysis by-products from phenolic compounds included as processing aids in commercially availaUe formulations. Embrittlement is initiated through chain scission bringing about the reaction... 

Since the coloration is due to Pb colloids and since diffusion is necessary to produce colloids, it is reasonable that the coloration is suppressed by hydrostatic pressure. Without considering details, it seems possible that the diffusion coefficient is decreased by hydrostatic pressure because of the decreased lattice spacing and attendent increased in the potential barriers for diffusion because of repulsive forces. Contraction by decreasing the temperature might be expected to produce essentially the same increase in potential barriers. For Pb(N3)2 a decrease in temperature from 300 to 78°K produces about the same lattice contraction as applying a hydrostatic pressure of 10 kbar [143-145]. If the suppression of coloration by hydrostatic pressure is due to a decrease of the diffusion coefficient, at least the same decrease should be expected for this decrease in temperature. The data of Figures 16 and 17 indicate that this was not the case, and therefore the decrease in coloration was due to a decreased efficiency of radiation-induced defect production. 

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