Chemical aspects of clustered DNA damage induction by ionising radiation

Ionising radiation induces a variety of chemical modifications to DNA, ranging from simple, isolated lesions to clustered DNA damage, in which two or more lesions are formed within a few tens of base pairs by a single radiation track. Multiple lesions, e.g. tandem lesions and amplification of damage, may also be induced in DNA by reaction with a single hydroxyl radical. It has been proposed from biophysical modelling that clustered DNA damage is less repairable and therefore contributes to the biological severity of ionising radiation. In this review, some evidence is presented which indicates that non-double strand break (non-DSB) clustered DNA damage is induced in significant yield, relative to that of DSBs, in mammalian cells. Enzymatic processing of clustered DNA damage in synthetic oligonucleotides has been shown to be compromised, depending on the nature of the lesions present. The role of clustered DNA damage in the early stages of the development of radiation-induced carcinogenesis remains to be addressed.     

Radiation quality of tritium

Tritium occurs from both natural and manufactured processes. In the environment, tritium can exist in the form of tritiated water (HTO) and in an organic form known as organically-bound tritium (OBT). Although, the concentrations of environmental OBT are relatively low, there is concern that current risk factors may underestimate the risk from OBT. Because tritium poses an internal hazard at cellular levels, microdosimetric techniques provide suitable tools for the study of radiation quality of tritium. In this study, microdosimetric simulations are performed for tritium uniformly distributed in a medium, and for tritium bound to biologically critical sites of dimensions from 10 nm to 2 microm. Results of local energy density are different for these two cases in microscopic regions. Based on the spatial distribution of energy deposition, dose mean lineal energies are calculated for tritium in forms of HTO and OBT. The dose mean lineal energies of OBT are about a factor of 1.7 higher than those of HTO in a wide range of target dimensions of biological interest. The results are consistent with radiobiological findings that OBT is about twice as effective as that of HTO.     

Radiation damage in solar cells

A review is given of the various aspects of solar-cell degradation in space. By way of introduction, defect creation in a solid by energetic particles is outlined, and the basic results of solar-cell theory are presented. The identification of the minority-carrier lifetime as the principal quantity of concern in solar-cell degradation then paves the way for the discussion of specific materials. The radiation damage in silicon, gallium arsenide and indium phosphide solar cells is discussed in some detail, paying particular attention to microscopic defects and their interaction with impurities.     

Radiation damage in negative-differential resistance devices

Tunnel diodes made with silicon and gallium arsenide have been tested in both neutron- and gamma-radiation environments. Experimental data show that failure usually occurs in the range 10¹⁴–10¹⁸ n cm^–2 or 50-270 Mrad range. The primary failure mechanism for neutron-irradiated ] samples is an increase in the valley current (from 0.10 mA to 0.58 mA and from 1.5 A to 30 A for silicon and GaAs diodes, respectively). In the case of gamma-irradiated silicon samples, the valley current reaches a value of 0.48 mA, at 260.8 Mrad, although their initial values are 0.1 mA. As a result, the peak-to-valley current ratios of the irradiated devices were shown to decrease severely. Both the valley and forward peak voltage values were shown to decrease with radiation. Values of 0.18 and 0.25 V for silicon samples were measured after exposure to 5 × 10¹⁶ n cm^–2 although their initial values were 0.42 and 0.80 V, respectively. As a result, the devices' output power were shown to be affected seriously. Finally, silicon devices irradiated for 48h in the ET-RR-1 research reactor, Egypt, for up to 1.872 × 1011 n cm^–2 or to gamma doses up to 2.6 × 10⁸ rad, were greatly influenced and they lost their main feature as PN-junctions.     

Radiation damage in silicon strip detectors

Silicon strip detectors with 5 μm spatial resolution have been used during 1982–1985 in the ACCMOR spectrometer at CERN. After a local beam flux of about 10¹⁴ minimum ionizing particles per cm² we observe a significant increase in dark current and systematic distortions in the measured coordinates which are explained in terms of a decrease in the effective donor concentration.     

Radiation damage test of silicon multistrip detectors

Radiation damage test of silicon multistrip detectors were performed using an 800 GeV proton beam. The local proton fluence was up to 10¹⁴/cm². The observed prominent changes were the proportional increase of the leakage current with the integrated beam intensity and the change of the effective impurity density. The effective impurity density decreases with fluence up to ≈4×10¹³/cm² but for greater fluences, it increases. This may indicate the type conversion of the bulk silicon. We have also observed the change of the carrier collection properties, which may be caused by the synergistic effect of the charge-up of surface SiO₂ and the decrease of the effective impurity density in bulk silicon.     

Radiation damage test of barium fluoride scintillator

Small samples of BaF₂ scintillator have been irradiated in a high-energy proton beam with doses of up to 1.3 × 10⁷ rad. To the precision of our measurement there was no effect in the scintillation output and only a slight decrease in transmission was observed.     

Radiation-induced DNA damage responses

The amazing feature of ionising radiation (IR) as a DNA damaging agent is the range of lesions it induces. Such lesions include base damage, single strand breaks (SSBs), double strand breaks (DSBs) of varying complexity and DNA cross links. A range of DNA damage response mechanisms operate to help maintain genomic stability in the face of such damage. Such mechanisms include pathways of DNA repair and signal transduction mechanisms. Increasing evidence suggests that these pathways operate co-operatively. In addition, the relative impact of one mechanism over another most probably depends upon the cell cycle phase and tissue type. Here, the distinct damage response pathways are reviewed and the current understanding of the interplay between them is considered. Since DNA DSBs are the major lethal lesion induced by IR, the focus lies in the mechanisms responding to direct or indirectly induced DSBs.     

Compositional dependence of damage buildup in Ar-ion bombarded AlGaN

Results are presented for the RBS/channeling study of the structural defect behavior in ion bombarded Al_xGa_1 ? xN (for x = 0; x = 0.44; x = 1) compounds at RT. Epitaxial layers of approximately 1 μm thickness were grown by the MOVPE technique on sapphire substrates. Experiments consisted of implantation with 320 keV Ar ions to fluences ranging from 5 × 10¹² to 1 × 10¹⁷ at/cm² followed by RBS/channeling measurements using 1.7 MeV ⁴He ions. Damage distributions were evaluated using the McChasy Monte Carlo simulation code assuming that they consist of randomly displaced lattice atoms. Detailed dependence of damage buildup on the alloy composition was determined and evaluated in the frame of the Multi-step Damage Accumulation model.     

Radiation damage of SiC Schottky diodes by electron irradiation

The impact of radiation damage on the device performance of 4H-SiC Schottky diodes, which are irradiated at room temperature with 2-MeV electrons is studied. After irradiation the reverse current increases, while the forward current and the capacitance decrease with barrier height and carrier density. The decrease of the barrier height is mainly responsible for the increase of the reverse current, while the decrease of the forward current for a high fluence is caused by the increase of the resistance in the bulk of the crystal. Although no electron capture levels are observed before irradiation, three electron capture levels (E₁, E₂, and E₃) are induced after irradiation. It is noted that the decrease in carrier density is partly caused by the contribution of non-observed electron capture level in the DLT spectrum, which compensates the free carriers.     

Radiation damage of N-MOSFETS fabricated in a BiCMOS process

Results are presented of a study on the radiation damage and its recovery behavior resulting from thermal annealing of n-MOSFETs fabricated in a 0.8-m single-well BiCMOS process, subjected to -rays, 1-MeV electrons and 1-MeV neutrons. After irradiation, the base (substrate) current and interface trap density normally increase with increasing fluence. This result points out that both ionization damage in the gate oxide and lattice defects in the p-well are induced by the irradiation. The interface trap density recovers by 85% for -ray irradiation with a fluence of 1×10⁸ rad, after a 300 °C annealing.     

Temperature dependence of capacitance on DNA solutions

We have measured the temperature dependence of the capacitance on the poly(dA), poly(dA)–poly(dT), and DNA solutions with both A–T and G–C base pairs to investigate the relation between the capacitance and the DNA structure. As the temperature increases, the capacitance of the single-stranded DNA solution increases, whereas that of the double-stranded one decreases with increasing the temperature and exhibits an upturn when DNA molecules are denatured. Moreover, the temperature dependence of the capacitance is found to be different depending on the DNA sequence. These results suggest that the conformational change of DNA molecules can be detected through the capacitance measurements that do not require any labeling and immobilization on a solid support.     

Radiation damage and high resolution electron microscopy of polydiacetylene crystals

The relationship between radiation damage and the application of high resolution electron microscopy (HREM) to polydiacetylene single crystals has been examined. The damage is seen to occur by the decay of electron diffraction patterns and the loss of contrast in bright- and dark-field micrographs. The rate of damage was found to be different for the two polydiacetylene derivatives studied and to take place probably by cross-linking. It was found that lattice images from planes parallel to the chain direction with a spacing of 0.9±0.1 nm could be obtained from the most resistant derivative. Images of chain-end dislocations were obtained for the first time in a polymer crystal. The problems of applying HREM to polymers and the conditions for imaging individual molecules are discussed.     

Numerical analysis of high-frequency induction heating includingtemperature dependence of material characteristics

We have carried out a numerical analysis of the magnetic field on high-frequency induction heating. This analysis includes the dependence of various magnetic properties on temperature. The required characteristics are obtained experimentally. We compare the experimental results with the theoretical values obtained by approximations. Until now, the current density inside the exciting coil on this kind of problem has been assumed to be uniform, which is different from actual phenomena. We propose a new method which takes the inhomogeneous distribution of exciting current into account. In this analysis, the eddy current of the exciting coil is also taken into account