Radiation damage electron

Egerton R F 1976 Measurement of inelastic/elastic scattering ratio for fast electrons and its use in the study of radiation damage Phys. Status Solid a 37 663-8... 

Radiation Damage. It has been known for many years that bombardment of a crystal with energetic (keV to MeV) heavy ions produces regions of lattice disorder. An implanted ion entering a soHd with an initial kinetic energy of 100 keV comes to rest in the time scale of about 10 due to both electronic and nuclear coUisions. As an ion slows down and comes to rest in a crystal, it makes a number of coUisions with the lattice atoms. In these coUisions, sufficient energy may be transferred from the ion to displace an atom from its lattice site. Lattice atoms which are displaced by an incident ion are caUed primary knock-on atoms (PKA). A PKA can in turn displace other atoms, secondary knock-ons, etc. This process creates a cascade of atomic coUisions and is coUectively referred to as the coUision, or displacement, cascade. The disorder can be directiy observed by techniques sensitive to lattice stmcture, such as electron-transmission microscopy, MeV-particle channeling, and electron diffraction. 

Transmission electron microscopes (TEM) with their variants (scanning transmission microscopes, analytical microscopes, high-resolution microscopes, high-voltage microscopes) are now crucial tools in the study of materials crystal defects of all kinds, radiation damage, ofif-stoichiometric compounds, features of atomic order, polyphase microstructures, stages in phase transformations, orientation relationships between phases, recrystallisation, local textures, compositions of phases... there is no end to the features that are today studied by TEM. Newbury and Williams (2000) have surveyed the place of the electron microscope as the materials characterisation tool of the millennium . 

Metals are immune to radiation damage by ionization. This is also a consequence of the free electron structure. Fast charged particles and ionizing rays can knock off electrons from the atoms they encounter. In metals, the positive vacancies so formed are immediately filled up by the electron gas, leaving no sign of damage apart from a small amount of heat. 

Heller, C. and McConnell, H.M. 1960. Radiation damage in organic crystals. II. Electron spin resonance of (C02H)CH2CH(C02H) in P-succinic acid. The Journal of Chemical Physics 32 1535-1539. 

N and 0, in solid material. The second point is that EXELFS is especially suitable for the study of inhomogeneous samples (structurally and compositionally heterogeneous in the sense discussed in section 2.2 above) because the primary electron beam can be focussed to a diameter of ca 20. Other advantages of EXELFS have been discussed elsewhere (60, 61). The limitations of the technique include (i) the need to select an optimal thickness of sample so as to minimize multiple scattering and (ii) the susceptibility of the samples to suffer radiation damage. 

Corbett, J.W. (1966). Electron Radiation Damage in Semiconductors and Metals, Academic press, N.Y. 

In addition to the generation of platelets, hydrogenation of silicon also induces electronic deep levels in the band gap. As in the case of platelet formation, these defects are considered to be unrelated to either plasma or radiation damage because they can be introduced with a remote hydrogen plasma. Comparison of depth distributions and annealing kinetics of the platelets and gap states has been used to a limited extent to probe the relationship among these manifestations of H-induced defects. 

Polymers with chromophores exhibiting mt transitions (e.g., C=0) exhibit weaker UV-absorption and these groups together with unsaturated carbon-carbon bonds which develop during radiation damage can be detected by electronic absorption spectroscopy. 

Electron irradiation causes chain scission and crosslinking in polymers. Both of these phenomena directly affect the glass transition temperature (Tg) of the materials. Thermomechanical (TMA) and dynamic-mechanical analysis (DMA) provide information about the Tg region and its changes due to radiation damage. Therefore, DMA and TMA were performed on all irradiated materials. 

Not every molecular crystal can be resolved at 3A resolution, especially not ones built of aliphatic nonconjugated molecules, which have lower electron densities and are more subject to radiation damage. The final aim of obtaining a direct three-dimensional picture of the chiral molecule itself thus cannot yet be pursued. Assignment of absolute configuration by lattice imaging, however, may be achieved even at lower resolutions (129). 

The effective mass of the electrons changes due to lattice strain, alloy additions, radiation damage, phase transformation, and phase content, directly relates to the ability to use electronic property measurements to assess microstructure phase stability. Electronic properties, such as thermoelectric power coefficients, resistivity and induced resistivity measurements, have a demonstrated correlation to solute and phase content, potential phase transformations, as well as residual strain. 

Although TEM-HREM is a more useful technique as can reveal a picture of the atoms in the materials, however this picture is difficult to interpret. On the other hand, radiation damage from the electron beam is an issue (in HREM observations), as long periods of TEM observation (> 1 min) usually can alter/damage seriously the stmcture of many industrial applications materials like organic chemical compounds, zeolites, polymers, etc. 

Speed of data acquisition is an issue, as radiation damage usually occurs (specially for organic compoimds) for expositions larger than a minute. In fact, in order to resolve successfully a structure in electron crystallography we need to accurately (equal or better than 1 % precision) determine the intensity of all different spots (up to 200) present in an electron diffraction pattern and correct for d5mamical diffraction contribution, specially for strong reflections. 

One important aspect of the electron diffractometer, is that any ED pattern can be captured before scanning by a CCD camera that is placed off-axis in relation with the ED pattern (can be any commercial CCD, even a webcam see fig.l). A dedicated software corrects for any type of optical distortions that may be produced due to the position of viewing angle of the CCD in relation with the ED pattern. This has the big advantage that the user that may observe and define interactively the reflections or the area of the ED pattern that will be scanned, allowing at the same time the main beam to be blanked, in order to avoid radiation damage. 

To solve a crystal structure by direct methods, difficult data are those which are incomplete in the sampling of reciprocal space, have non-atomic i.e. < 1.3A resolution) and are noisy with large (systematic) errors in the data measurements. As we have seen, this definition spans many electron diffraction data sets, but there are some of sufficient quality that they can be solved routinely using conventional direct methods packages. Often these are of inorganic materials or intermetallic compounds that are relatively resistant to radiation damage. 

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