A physical approach to the radiation damage mechanisms induced by X-rays in X-ray microscopy and related techniques

A physical approach is used to analyse the various mechanisms induced by the absorption of X-ray photons of energies in the 0.2–20 keV range. At the atomic scale, besides the (Auger and photo) electron transport in the bulk or the ejection into the surrounding media, special attention is devoted to the specific consequences of the initial Auger decay mechanism. At the macroscopic scale, the decisive role of the poor electronic conductivity of the radiation-sensitive materials is outlined and it is shown that the damaging effects occur in irradiated insulators because the lack of conduction electrons prevents the initial charge of the excited atoms being quickly restored. Correlating irradiation conditions and physical properties of the specimen, various expressions are proposed for the first time to quantify these effects. Some are neither dose- nor dose-rate-dependent and the influence of the surrounding medium is also considered. The fundamental mechanisms investigated here hold for a wide variety of specimens or components investigated in X-ray microscopy. Their consequences can be easily transposed to other techniques using transmitted X-rays.     

Methods of detection of radiation damage in electron microscopy

Though the fading of electron diffraction intensities with increasing electron exposure is the most frequently used method for the investigation of radiation damage in electron microscopy, several other in situ methods such as the measurement of transmission and electron energy-loss spectroscopy and X-ray microanalysis can be applied to get more detailed information about what happens during irradiation. Other physical and chemical parameters can be measured on thin films or on larger amounts of material after an electron pre-irradiation. The different methods will be reviewed and discussed with regard to their information yield and their future further capabilities.     

Nanoscopic aspects of radiobiological damage: Fragmentation induced by secondary low-energy electrons

Low-energy electrons (LEEs) are produced in large quantities in any type of material irradiated by high-energy particles. In biological media, these electrons can fragment molecules and lead to the formation of highly reactive radicals and ions. The results of recent experiments performed on biomolecular films bombarded with LEEs under ultra-high vacuum conditions are reviewed in the present article. The major type of experiments, which measure fragments produced in such films as a function of incident electron energy (0.1-45 eV), are briefly described. Examples of the results obtained from DNA films are summarized along with those obtained from the fragmentation of elementary components of the DNA molecule (i.e., thin solid films of H(2)O, DNA bases, sugar analogs, and oligonucleotides) and proteins. By comparing the results of these different experiments, it is possible to determine fundamental mechanisms that are involved in the dissociation of biomolecules and the production of single- and double-strand breaks in DNA, and to show that base damage is dependent on the nature of the bases and on their sequence context. Below 15 eV, electron resonances (i.e., the formation of transient anions) play a dominant role in the fragmentation of all biomolecules investigated. These transient anions fragment molecules by decaying into dissociative electronically excited states or by dissociating into a stable anion and a neutral radical. These fragments usually initiate other reactions with nearby molecules, causing further chemical damage. The damage caused by transient anions is dependent on the molecular environment.     

Quantitative assessment of radiation damage in a thin protein crystal

Radiation damage is a limiting factor for high resolution structural determination of protein crystals. We have used the median and quartile values of N_e exposures and the structural disordering factor (ΔB) to describe the crystalline disordering due to radiation damage in thin crystals of crotoxin complex embedded in glucose at room and low temperatures and in ice embedded crystals. By approximating the radiation damage to follow first order kinetics, we applied a correction to the diffraction intensities from patterns with high accumulated exposures. We used the structural similarity factor (R) to show that, for some data, the effectiveness of the correction can be as good as those in protein X-ray crystallography.     

An in situ field-ion microscope research of radiation damage in tungsten

Results of experiments in which tungsten was irradiated in situ in the field-ion microscope with inert gas ions are described. Tungsten field-ion specimens were irradiated at 78 K with ions in the 0.5–9 keV energy range. Sputtering and displacement of surface atoms which had occurred during irradiation were observed directly in field-ion images. Large scale damage such as vacancy clusters were rarely found. From the ion acceleration voltage and the accommodation coefficient, the displacement energy of tungsten surface atoms in the electric field of about 44 MV/mm is estimated as nearly 22 eV. Damage was observed on the shielded side of the tip apex as well as on the ion incident side, and this fact is discussed in comparison with the channelling of ions in tungsten.     

Characterization of power induced heating and damage in fiber optic probes for near-field scanning optical microscopy

Tip-induced sample heating in near-field scanning optical microscopy (NSOM) is studied for fiber optic probes fabricated using the chemical etching technique. To characterize sample heating from etched NSOM probes, the spectra of a thermochromic polymer sample are measured as a function of probe output power, as was previously reported for pulled NSOM probes. The results reveal that sample heating increases rapidly to approximately 55-60 degrees C as output powers reach approximately 50 nW. At higher output powers, the sample heating remains approximately constant up to the maximum power studied of approximately 450 nW. The sample heating profiles measured for etched NSOM probes are consistent with those previously measured for NSOM probes fabricated using the pulling method. At high powers, both pulled and etched NSOM probes fail as the aluminum coating is damaged. For probes fabricated in our laboratory we find failure occurring at input powers of 3.4+/-1.7 and 20.7+/-6.9 mW for pulled and etched probes, respectively. The larger half-cone angle for etched probes ( approximately 15 degrees for etched and approximately 6 degrees for pulled probes) enables more light delivery and also apparently leads to a different failure mechanism. For pulled NSOM probes, high resolution images of NSOM probes as power is increased reveal the development of stress fractures in the coating at a taper diameter of approximately 6 microm. These stress fractures, arising from the differential heating expansion of the dielectric and the metal coating, eventually lead to coating removal and probe failure. For etched tips, the absence of clear stress fractures and the pooled morphology of the damaged aluminum coating following failure suggest that thermal damage may cause coating failure, although other mechanisms cannot be ruled out.     

Determination of the damage mechanisms in armor structural materials via self-organizing map analysis

Dynamic compression fracture behaviors together with damage mechanisms of materials subjected to a compressive load at a high strain rate were studied by using a Self-organizing map (SOM). The materials used for the analysis were Al5083, Rolled homogeneous armor (RHA) and tungsten heavy alloy (WHA). The deformation behavior and Acoustic emission (AE) signal were acquired through a Split Hopkinson pressure bar (SHPB)-AE coupled test. The self-organization map which is one of the artificial neural network technique was employed to classify the AE energy, amplitude, and peak frequency according to the characteristics of the signal. In addition, distributions of AE signals were represented in stress-strain curves. The correlation between AE characteristics and material damage mechanisms was investigated. Based on the results, it was found that cluster 1 with high AE energy, high amplitude and low frequency was the cluster of the AE signal generated near the yield point of the material. Cluster 3, which has the opposite tendency, was confirmed as a cluster of AE signals that occurred just before a fracture of the material. The change in the measured value can be seen depending on the strain rate and the material, but the overall tendency was similar.     

The application of automatic image analysis in studies of bubble populations produced by radiation damage

This paper describes a method by which automatic image analysis can be applied to the study of size distributions of small (< 5 nm) radiation induced bubbles. Statistically valid results are obtained directly from transmission electron microscope negatives and the optimum imaging conditions for obtaining reliable data are discussed.     

Experimental study of ELNES at grain boundaries in alumina: intergranular radiation damage effects on Al-L23 and O-K edges

The need for understanding the structural and chemical properties of interfaces and grain boundaries in materials is being paralleled by new developments in transmission electron microscopy methods. The extraction of data on grain boundaries has to be carefully evaluated, freed from artefacts and allowing fairly direct interpretation. Besides improvements in data processing, a primary requirement is an improved knowledge of the ELNES fine structures of the relevant absorption edge. For the case of alumina, we first present a compilation of Al-L(23) edges with an energy resolution of 0.5-0.6eV. This allows identification of the ELNES features which are more likely to vary as a function of the aluminium atomic environment, i.e. the excitonic peaks between 77 and 79eV and the near-edge features between 83 and 86eV, which both clearly depend on the Al site environment. Our second investigation concerns the likely occurrence of electron radiation damage at interfaces which may adversely interfere with the identification of new bonding types. The Al-L(23) and O-K ELNES changes associated with several cases of damage are detailed.     

On mechanisms of decreasing friction in coatings of transition metal dichalcogenides due to effect of radiation

Possible mechanisms of the known effect of decreasing friction to anomalously low values in laminar structures of dichalcogenides of transition metals during the irradiation by an ionizing flux are considered. With allowance for the current understanding of complex self-consistent processes of charging dielectrics by particle fluxes, it is shown that the Coulomb repulsion of subsurface layers is the most probable cause of decreasing friction. This effect lasts until an uncompensated surface charge is retained; as the charge drains, it gets weaker.     

Analysis of PCBN tool failure mechanisms in face milling of hardened steel using damage equivalent stress

The International Journal of Advanced Manufacturing Technology - An attempt was made in the present study to apply damage mechanics to investigate the polycrystalline cubic boron nitride (PCBN)...     

An apparatus for in situ spectroscopy of radiation damage of polymers by bombardment with high-energy heavy ions

A new target station providing Fourier transform infrared (FT-IR) spectroscopy and residual gas analysis (RGA) for in situ observation of ion-induced changes in polymers has been installed at the GSI Helmholtz Centre for Heavy Ion Research. The installations as well as first in situ measurements at room temperature are presented here. A foil of polyimide Kapton HN(?) was irradiated with 1.1 GeV Au ions. During irradiation several in situ FT-IR spectra were recorded. Simultaneously outgassing degradation products were detected with the RGA. In the IR spectra nearly all bands decrease due to the degradation of the molecular structure. In the region from 3000 to 2700 cm(-1) vibration bands of saturated hydrocarbons not reported in literature so far became visible. The outgassing experiments show a mixture of C(2)H(4), CO, and N(2) as the main outgassing components of polyimide. The ability to combine both analytical methods and the opportunity to measure a whole fluence series within a single experiment show the efficiency of the new setup.     

Vacuolar sequestration of fluorescent probes in plant cells: a review

The use of fluorescent probes as indicator and tracer molecules is becoming an important aspect of plant cell biology. In many cases the dye, whether introduced directly into the cytosol or sequestered by the cell from its external environment, is preferentially transferred to the vacuole. In the light of increasing evidence for endocytosis in plant cells, the sequestration of high-molecular-weight fluorescent dextrans and the membrane-impermeant dye Lucifer Yellow-CH into the vacuole has been cited as evidence supporting the presence of a fluid-phase endocytic pathway. In this review we consider these recent reports of vacuolar sequestration in the light of new evidence arising on the mechanisms underlying dye uptake.     

Ultrastructural localization of defined sequences of viral RNA and DNA by in situ hybridization of biotinylated DNA probes on sections of herpes simplex virus type 1 infected cells

The influence of fixation and enzymatic digestions on the ability of a denatured double-stranded DNA probe to bind specifically to related sequences of RNA and DNA in sections of Lowicryl embedded cells was investigated. Specificity of the hybridization was assessed using a biotinylated cloned subgenomic herpes simplex virus type 1 DNA fragment to localize viral nucleic acids in sections of infected cells. The probe was detected by anti-biotin antibodies and indirect immunogold labeling. Controls indicated that protease digestion of proteins from the section eliminated non-specific binding of the probe and labeling of endogenous biotin. Both formaldehyde and glutaraldehyde fixation retained viral RNA in protease digested sections. Its labeling was randomly and sparsely distributed over the fibrillo-granular network of the infected nucleus and over the ribosome-rich regions of cytoplasm. Labeling of single-stranded portions of viral DNA in protease-RNase digested sections was infrequent. It was located precisely over nucleoids of a few viral nucleocapsids whatever their location in the cell and their stage of maturation. Labeling of double-stranded viral DNA by denaturation of the DNA in the sections of Lowicryl embedded cells was possible after fixation with formaldehyde but not glutaraldehyde. Among several denaturation protocols, 0.5 N NaOH treatment was best for hybridization of both nonencapsidated and encapsidated viral DNA in protease-RNase digested sections. Free viral genomes were detected exclusively within the virus-replicating region of infected nuclei. Labeling of viral nucleoids was independent of their location in the cell. The high percentage of labeled viral nucleoids suggests that the related viral DNA sequence is not aggregated in the nucleoid but is extended and therefore numerous portions of this defined DNA sequence are accessible at the surface of the section for the binding of the probe.