Post-irradiation Heat Treatment Research Articles

Effect of Ge doping on the creation of luminescent radiation defects in MBE Si

We report on the observation of new luminescence centres in irradiated Ge-doped Si (Si:Ge). Molecular beam epitaxy grown Si:Ge samples with Ge content in the range 0–1.25 at.% were irradiated with protons at room temperature and subjected to post-irradiation heat treatments at temperatures from 100 to 650 °C. For each annealing step, the samples were investigated by means of photoluminescence (PL) spectroscopy at temperatures between 4 and 300 K. We have found that the PL spectra of Ge-containing samples are different from those measured in Ge-free samples and from those previously reported in irradiated bulk Si:Ge. Several new lines have been found in the near band edge (NBE) region of the spectrum, whereas many lines typically measured in Ge-free irradiated Si have not been observed. The dependence of the intensity of the new NBE lines on the measurement temperature has been studied. With increasing Ge content, these lines shift to lower energies following the band gap shrinkage. However, they are broader than expected, when only a purely statistical variation of the band gap energy produced by local fluctuations in the alloy composition is taken into account.

Defects investigation in neutron irradiated reactor steels by positron annihilation

Positron annihilation spectroscopy (PAS) based on positron lifetime measurements using the pulsed low energy positron system (PLEPS) were applied for the investigation of defects of irradiated and thermally treated reactor pressure vessel (RPV) steels. PLEPS results showed that the changes in microstructure of the RPV-steel properties caused by neutron irradiation and post-irradiation heat treatment can be well detected. From the lifetime measurements in the near-surface region (20–550 nm) the defect density in Russian types of RPV-steels was calculated using the diffusion-trapping model. The post-irradiation heat treatment studies performed on non-irradiated specimens are also presented.

Large deformation compression induced crystallinity degradation of conventional and highly crosslinked UHMWPEs

The effect of a large compressive plastic deformation on the melt temperature ( T m), lamellar thickness, crystallinity, and density of four UHMWPEs (two conventional and two highly crosslinked) was examined. The materials were prepared from a single batch of medical grade GUR 1050 resin (Ticona, Bayport, TX, USA). The two conventional UHMWPEs were as-received (virgin) and gamma radiation sterilized at 30 kGy in a nitrogen atmosphere (radiation sterilized). The two highly crosslinked UHMWPEs were each irradiated at 100 kGy and then post-processed with one of either two thermal treatments: annealing, which was done below the melt transition temperature ( T m), at 110 °C for 2 h (110 °C-annealed); and, remelting, which was done above T m, at 150 °C (150 °C-remelted). Differences in changes upon compression between the materials were examined using ANCOVA analyses. The 150 °C-remelted material showed a significant change in T m and lamellar thickness upon compressive plastic deformation whereas the other three UHMWPE materials did not. However, all of the materials showed significantly decreased crystallinity and density upon compressive deformation. The findings of this study support that microstructural evolution during compressive deformation is a function of UHMWPE formulation, as affected by irradiation and post-irradiation heat treatment.

Modeling tensile response and flow localization effects in selected copper alloys

Radiation-induced defect structures are known to elevate material yield strength and reduce material ductility. Together, these changes substantially reduce uniform elongation compared to the unirradiated material condition so that the small strains induce plastic instability. This process, commonly known as flow localization, is examined here for selected copper alloys and compared to similar response in 316SS. It is found that uniform elongation levels are limited by a critical material strength which is independent of the irradiation damage state. This result establishes that the details of the post-yield flow and strain hardening processes are less important than the critical stress for controlling plastic instability. In the case of OFHC Cu, post-irradiation heat treatment restores some initial ductility, but also reduces the critical stress for incipient flow localization.

Investigation of defect distributions in neutron-irradiated and thermally treated reactor steels by positron annihilation

Positron annihilation spectroscopy (PAS) based on positron lifetime (PL) measurements using the pulsed low-energy positron system (PLEPS) was applied for the investigation of defects of irradiated and thermally treated reactor pressure vessel (RPV) steels. PLEPS results showed that the changes in the micro structure of the RPV-steel properties caused by neutron-irradiation and post-irradiation heat treatment can be well detected. From the lifetime measurements in the near-surface region (20–550 nm) the defect density in the Russian types of RPV-steels was calculated using the diffusion trapping model. The post-irradiation heat treatment studies performed on non-irradiated specimens are also presented.

Metastable phase stability in the ternary Zr–Fe–Cr system

As the existing literature reports, when Zircaloy-2 and Zircaloy-4 are irradiated at intermediate temperatures with high-energy neutrons, a crystalline to amorphous phase transition occurs, coincidental with a preferential depletion of Fe at the periphery of the Zr(Fe,Cr)2 intermetallic particles. In order to describe these phenomena, relative phase stability criteria combined with a balance between the rate of irradiation damage and irradiation enhanced diffusion leading back to a modified or new state should be considered. Aiming at this suggested study, in this work we calculate the metastable free-energy diagram of the ternary Zr–Fe–Cr system. The free energies of formation of the metastable amorphous and solid solution phases and of the stable Zr(Fe,Cr)2 compound have been calculated using Miedema's model. By solving equations of analytical geometry, regions of metastable equilibria and their tie-lines are defined, linking the hcp-Zr-rich solid solution with the Zr–Fe–Cr amorphous phase, and the latter with the irradiation-energized compound. Composition profiles, reported in the literature, were measured on partially and completely amorphized particles, and also in the case when the latter experienced microcrystallization with solute reversion, that is, with the Fe concentration increased during post-irradiation heat treatments. Those composition profiles define a path along a calculated tie-line, deviating near the crystalline core/amorphous crown interface toward low Fe concentrations. The analysis in the present work suggests that the microstructure in irradiated Zircaloy tends to reach a new radiation-induced metastable co-existence of phases.

Microstructure alterations in the base material, heat affected zone and weld metal of a 440-VVER-reactor pressure vessel caused by high fluence irradiation during long term operation; material: 15 Ch2MFA ≈0.15 C–2.5 Cr–0.7 Mo–0.3 V

Within the scope of the EC research project Tacis ’91 (‘RPV-Embrittlement’), trepans were taken from the highly irradiated circumferential RPV-weld of the Novovoronesh power plant unit-2 of the type VVER-440/230. The cumulated fast fluence level in this position reaches up to 6.5×10 19/cm 2 ( E>0.5 MeV). In a joint research work, the mechanical properties, the chemical composition, and the microstructure of the base material, the heat affected zone (HAZ), and the weld metal have been investigated in order to study the influence of irradiation, and of post irradiation heat treatment (475°C, 560°C) on the properties. The examination of the microstructure performed by analytical transmission electron microscopy (200 kV) shows the existence of dislocation loops (‘black dots’), irradiation induced precipitates, and segregation of copper in the carbides. These changes in microstructure, which are due to service affection (neutron irradiation, temperature) have occurred more pronounced in the weld metal and the HAZ than in the base material.

The Broad Midgap Deep-Level Transient Spectroscopy Band in Proton (65 MeV) and Fast Neutron-Irradiated n-GaAs

DLTS measurements in H + (65 MeV)-irradiated n-GaAs reveal a broad band near 250 to 340 K which is similar to the band in neutron-irradiated samples. DLTS evaluation and the modification of this band due to post-irradiation heat treatment indicates that it is formed by two subbands superposed on each other, so the combined band cannot be resolved fully in as-irradiated samples. The temperature position of the peak and the shape of this band are sensitive to electric fields in the compensation depletion layers of the damage clusters produced upon bombardment. The damage cluster model and the related analytical expression for the response have been extended and simulated DLTS spectra have been presented for the samples investigated.

Protein Synthesis Modulates the Biological Effectiveness of the Combined Action of Hyperthermia and High-LET Radiation

The combined effects of heavy-ion radiation and hyperthermia on the survival of CHO-SC1 cells and its temperature-sensitive (ts) mutant tsH1 cells were studied using accelerated neon ions followed by mild heating at 41.5 degrees C. The sequence of application of heat and high-LET radiation is significant to cell-killing effects. Heat applied to cells prior to irradiation with neon plateau ions (LET = 32 keV/microns) was less effective than heat applied immediately after irradiation. The ability of cells to synthesize new proteins plays a key role in this sequence-dependent thermal sensitization. When protein synthesis was shut down in tsH1 cells, the thermal enhancement of cell killing by high-LET radiation was the same regardless of the sequence. The thermal enhancement of radiation-induced cell killing was LET-dependent for the SC1 cells, but this was not clearly demonstrated in the tsH1 cells. Furthermore, the RBE of heated SC1 cells varied with LET and reached a maximum of greater than 3 at 80 keV/microns. In the absence of protein synthesis, the maximum RBE value was reduced to 2.6. These results suggest that the accumulation of cellular damage caused by exposure to densely ionizing particles with increasing LETs can be potentiated with active protein synthesis during postirradiation heat treatment.

Modification of recovery from radiation damage by hyperthermia in yeast

Recovery from radiation damage was studied in the diploid yeast Saccharomyces cerevisiae BZ 34 subjected to pre-irradiation, post-irradiation or simultaneous hyperthermic treatment. Pre-irradiation heat treatment at 51 degrees C for 20 m min sensitized the cells to lethal damage; however, these cells could efficiently recover from potentially lethal damage (PLD) on liquid holding. In contrast, post-irradiation hyperthermia (51 degrees C for 20 min or 52 degrees C for 15 min) partially inhibited recovery from PLD. For treatments at 40 and 45 degrees C the inhibition of recovery was restricted to the duration of heat treatment. As soon as the temperature of the cells was reduced to 30 degrees C for 48 h, they showed a significant recovery from PLD. The kinetics of the liquid holding recovery of cells exposed to 600 and 900 Gy of radiation followed by heat treatment at 51 degrees C, 20 min or 52 degrees C, 15 min was studied. Post-irradiation heat treatment inhibits PLD repair (PLDR) during the first 3 h. Subsequent liquid holding for longer durations results in recovery from PLD, reaching a maximum of nearly 50% of that obtained in cells exposed to radiation alone. When the cells were held for 0-48 h between radiation and heat treatment, they showed recovery from radiation damage such that sensitization by subsequent heat treatment was reduced significantly over the 48 h interval. The inhibition of PLDR by hyperthermia also decreases progressively with an increase in the time interval between the two treatments. When both heat and radiation treatments were carried out simultaneously by irradiating the cells at 48 degrees C, the inhibition of PLDR was partial. The recovery factor, calculated as the ratio of survival after recovery to that on immediate plating (without PLDR), was 2.6 +/- 0.9 for simultaneous heat and radiation treatment compared to 11.9 +/- 2.7 for radiation alone.

Experience with neutron irradiated reactor pressure vessel steels-a Mössbauer study

The influence of neutron irradiation and post-irradiation heat treatment on the Mossbauer spectra of reactor pressure vessel steels is studied. Substantial changes in the materials are detected, but a final interpretation turned out to be impossible without performing further investigations.

ChemInform Abstract: Thermal Neutron Irradiation of Y‐Ba‐Cu‐O Superconductor

Abstractis performed at 360 K with a well‐thermalized flux of 1.82·1017 n/(m2·s).

Thermal Neutron Irradiation of Y‐Ba‐Cu‐O Superconductor

A Y‐Ba‐Cu‐O superconductor was irradiated at 360 K in the NRX reactor, with a well‐thermalized flux of 1.82 × 1017 n/(m2·s) (E < 1 MeV). Superconducting characteristics at 90 K disappeared by a fluence of 6.6 × 1021 n/m2 (10 h) but were recovered after a postirradiation heat treatment at 760 K in flowing oxygen. The results suggest that suitable shielding will be required for any application which exposes the superconductor to neutron irradiation.

Single-strand breaks can lead to complex configurations of plasmid DNA in vitro.

DNA species which migrate extremely slowly in agarose gel electrophoresis may be formed from plasmid DNA containing radiation-induced single-strand breaks (ssbs). Postirradiation heat treatment in low ionic strength buffer and subsequent incubation with Mg2+ strongly enhanced the formation of these species. Electron micrographs taken after such treatment show numerous complex configurations containing DNA material from several plasmid molecules. Less extreme formation of slowly migrating DNA occurred without postirradiation heat treatment or Mg2+ incubation when the DNA was co-precipitated with calcium phosphate in a physiologically balanced buffer and incubated under conditions used for DNA-mediated gene transfer. The data suggest that homologous pairing between single-stranded regions formed in relation to ssbs may contribute to cohesion between different molecules. The significance of the cohesion process for gene transfer experiments and cellular radiation effects is discussed.

Oxygen Precipitation in Oxygen-Ion Implanted Silicon

Heat treatment of oxygen-ion implanted CZ-silicon may be expected to induce precipitation of oxygen-rich precipitates when the peak total oxygen concentration is above the oxygen solubility limit, but not far enough above to result in a “buried” SiOx layer. We report here first results of an investigation of oxygen-rich precipitates formed in oxygenimplanted CZ-silicon given a multistep post-irradiation heat treatment with maximum temperatures of 1050°C. The implantation energy and dose were 400kV and 3xl015cm-2 respectively, yielding a peak oxygen concentration of 6.8x1019 cm-3 at a projected range of 1μm, based on LSS statistics. This concentration is about one order of magnitude above the oxygen solubility limit and three orders of magnitude below the stoicheometric oxide concentration.

Effect of Hypoxia on Recovery from Damage Induced by Heat and Radiation in Plateau-Phase CHO Cells

The effect of hypoxia on the induction of and recovery from damage by radiation alone and in combination with heat has been investigated using plateau-phase Chinese hamster ovary (CHO) cells. Postirradiation hypoxia reduced the potentially lethal damage recovery (PLDR) in cells irradiated under an euoxic state and completely eliminated PLDR in cells irradiated under hypoxia. Cells which were maintained under hypoxia during both irradiation and a 4-hr recovery period and then incubated for a further period of 4 hr under euoxic conditions showed PLDR, suggesting that the inhibition of PLDR by hypoxia is reversible. Oligomycin, an inhibitor of energy metabolism, completely eliminated PLDR when present at a concentration of 1 microM during the postirradiation period. Pre- or postirradiation heat treatment at 42.5 degrees C for 30 min appreciably sensitized the cells to the induction of lethality. Thermal enhancement ratio (TER) was 1.7 for cells irradiated and heat treated under hypoxic conditions. The same heat treatment reduced the oxygen enhancement ratio (OER) associated with gamma radiation from 3.1 to 2.5. Cells subjected to this postirradiation heat treatment showed a small extent of PLDR, whereas the pre-heat-treated cells showed as much recovery as non-heat-treated cells. When hypoxic conditions prevailed during the post-treatment incubation period, PLDR was reduced in preheated cells and completely eliminated in postheated cells. The kinetics of interaction between heat and radiation damage were studied by introducing a time gap of 4 hr between the treatments. Cells maintained under euoxic conditions between the treatments showed an appreciable decrease in interaction, suggesting recovery from damage induced by the first treatment. Hypoxic conditions intervening the two treatments largely inhibited the loss of sensitization. Analysis of the results suggests that cells fail to recover from sublethal heat damage when held for 4 hr under hypoxic conditions. Cells held under hypoxic conditions partly recover from the radiation damage which subsequently interacts with sublethal heat damage, resulting in cell lethality.

Heat-sensitive radiation damage in single-stranded DNA irradiated in a bacterial extract.

The effect of heat treatment (45 degrees C) on gamma-irradiated biologically active single-stranded phi X174 DNA, dissolved in a bacterial extract, was studied. The results show that under these circumstances heat-sensitive damage is found, which is absent after irradiation in pure buffer. The damage is non-lethal and probably becomes an apurinic/apyrimidinic site, which is lethal, upon the post-irradiation heat treatment. Prolonged heating converts it into a break. The amount of the initial damage depends on the conditions of irradiation.

Effects of hyperthermia at 42 degrees C on DNA lesion and cell inactivation caused by X-ray.

The hyperthermic modification of radiation effect on cell inactivation should be important role for cancer therapy. This work has been done to obtain the DNA-scision and -repair relevant to the cell death and to recovery of clonogenesity after preirradiation heating at 42°C where heat effect on cell death has not been severe. Radiation inactivation of human melanoma cultured cells (HMV-I) has been synergetically enhanced by the preirradiation heating at 42°C. The DNA lesion has been detected by the alkaline elution method. The amount of initial lesion for cells heated before irradiation seemed to be less than those for X-ray alone irradiated cells. This result may be relevant to the crosslinking or aggregation introduced in DNA strand after the heat treatment. Recovery capacity of cell clonogenic damage after irradiation was considerably reduced by post-irradiation heat treatment. Time course of DNA damage repair after irradiation was similar to that of recovery of clonogenesity. These results indicate that mild hyperthermic treatment inhibit the repair of DNA damage caused by irradiation.

Significance of Selected Residual Elements to the Radiation Sensitivity of A302-B Steels

The effects of three levels of copper content and phosphorus content and two levels of sulfur content on radiation sensitivity and postirradiation heat treatment response were explored for a reactor pressure vessel steel, Type A302-B. Test plates for the investigation were produced from 182-kg (400-lb) laboratory melts. The contributions of individual elements were assessed from Charpy-V (CV) notch ductility changes with 288/sup 0/C (550/sup 0/F) irradiation and with a 343/sup 0/C (650/sup 0/F), 168-h postirradiation heat treatment. Limited studies of properties recovery by postirradiation 399/sup 0/C (750/sup 0/F) heat treatment were also made. Radiation embrittlement sensitivity, as shown by CV transition temperature elevation and CV upper shelf reduction, generally increased with increased copper and phosphorus content and with decreased sulfur content. Certain ranges of phosphorus and copper content were found to be more critical than others. Response to 343/sup 0/C (650/sup 0/F) postirradiation heat treatment, as evidenced by transition temperature recovery in degrees Celsius, appeared to be independent of copper, phosphorus, and sulfur content for the ranges investigated. Response to heat treatment also appeared to be independent of the magnitude of the prior transition temperature elevation by irradiation. On the other hand, a dependence of percentage recovery on impurity elementmore » content was observed. A dependence of upper shelf recovery on copper content was also found. Six of the eight plate compositions exhibited full upper shelf recovery but only small transition temperature recovery after 343/sup 0/C (650/sup 0/F) heat treatment.« less

Effect of ionizing radiation on the chemical composition, crystalline content and structure, and flow properties of polytetrafluoroethylene

AbstractThis article describes a study of ionizing radiation‐induced changes in the chemical composition, crystalline content and structure, and flow properties in polytetrafluoroethylene (PTFE). Irradiatins conducted in the presence of oxygen cause acid fluoride end groups to be formed, which on exposure to water vapor hydrolyze to form carboxylic acid end groups. Analyses by infrared (IR) spectroscopy indicate that when irradiated in a vacuum PTFE exhibits defect absorption bands which have been attributed to branch and crosslink formation. The crystalline content of PTFE which increases after exposure to radiation was monitored by IR spectroscopy, density, x‐ray diffraction on unoriented samples, and differential scanning calorimetry (DSC) as the measurement probes. The melt viscosity of PTFE exposed to various radiation doses in air decreases dramatically after irradiation. Between 2.5 and 5 Mrd an increase in viscosity is attributed to the formation of branches and crosslinks. The effects of preirradiation crystallinity and postirradiation heat treatment were studied. A model is presented to explain the mechanism of the observed radiation effects.