Effects of radiation damage on the image

In a favorable case the final composition may not be much different from the starting composition. Polystyrene, of elemental composition (CIi)x loses only 15-20% of its total mass before stabilizing at a composition near (CHo.8)a . This allows useful quantitative measurement of mass 

Loss of crystallinity causes all diffraction contrast features in the TEM image to fade away. Moire fringes, lattice fringes, bend contours and the like will all lose contrast during irradiation [115]. Features which depend on orientation such as bend contours or dislocation strain field images will become smeared out, as directions in imperfect or very small crystals are less well defined (the reciprocal lattice spot increases in size). During irradiation, new contrast features-radiation artifacts-can appear temporarily and then fade with the rest. 

Large dimensional changes can be induced by radiation which distorts the object and changes the image permanently. This is a serious effect in polymer microscopy because it can be accidentally overlooked. Loss of mass is difficult to miss and the transient crystallographic contrast details require the operator to be concerned about radiation damage. But the distortion may be instantaneous under the viewing conditions normally used and the result is a stable and maybe mislead- 

In thin samples, low molecular weight fragments will rapidly diffuse to the surface and evaporate. Bubbles may form at high dose rates in thick specimens when volatile products are trapped, and this is often incorrectly taken to imply that the specimen temperature is high. In any case, the mass of the specimen decreases during observation. Polymers that degrade will lose a lot of mass, and those that crosslink will lose less. 

In TEM the bright field transmitted intensity increases as mass is lost. This increase of intensity with dose has been measured [213, 214]. Calibration with films of known thickness allows the mass loss to be calculated if the film composition does not change too drastically. In the SEM the unirradiated bulk of the sample always acts to stabilize the form of the irradiated surface. Mass loss may cause a uniform depression of the sample surface or holes and cracks may appear [215]. 

Mass loss destroys the sample to a greater or lesser extent, but it can be regarded positively as an etching process using the electron beam, and put to use. Two polymers in a blend or copolymer may have little contrast initially, but one phase loses more mass and the contrast increases or reverses [220]. Even in a homopolymer, the cracks and voids that appear can sometimes be related to the microstructure, making it more visible. For example, cracks form in molded poly(oxymethylene) samples during irradiation in the SEM [215], which follow spherulite radii and show up the oriented skin (e.g., see Fig. 4.34). 

The dose required to change the diffraction pattern into diffuse rings, J, has often been used to determine the radiation sensitivity of materials in the microscope [10, 207, 221, 222]. If high resolution of the crystalline structure is required, this is an overestimate, and the decay of the relevant diffraction spots is more 

The change in the diffraction pattern takes place in two ways, seen most clearly when single crystals are used to give an initial sharp spot pattern (Fig. 3.15). In some polymers the sharp 

Changes are seen, similar to those in the TEM, but on a fine scale, less than the penetration depth of the electrons. In spherulitic polyethylene this causes initially smooth surfaces to become full of fine structure-related detail [122, 126-128]. 

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Enhancement is the procedure used to increase the signal-to-noise (S/N) ratio of images by averaging. For all types of specimen, there will be local variations in the thickness of the ice film, in concentration of the buffer salts and other contaminants and impurities such as denatured proteins. Random noise variation also arises from the support film and the effects of radiation damage on the ensemble. If... 

Low specimen temperature reduces the damage rate and crystalline material undergoes chemical change more slowly than amorphous material. Both effects can be explained by the reduced mobility of the molecules. Therefore it is to be expected that specimen coating, a physical restraint on the escape of material from the specimen should have a beneficial effect. The restraint normally used is a thin film of carbon on both sides of the specimen. This combines good physical properties with a comparatively small loss of contrast in the image. There are few data available on this procedure for the enhancement of radiation resistance. 

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