Critical Dose Rate Research Articles

Exploring Functional Materials by Understanding Beam‐Sample Interactions

AbstractUltra‐low‐dose electron diffraction is performed with a double metal cyanide catalyst (DMC) to understand how electron irradiation stimulates structural alterations in functional materials. The commonly fading diffraction patterns with dose accumulation depend on the irradiated area and the beam current even when below 50 femto Amperes. Heat generation is observed and modeled by statistical, inelastic scattering events to describe how phonon excitations modulate radiation hardness. Specifically, the characteristic 1/e‐decay of Bragg intensities from DMC is delayed from 6 to 30 eÅ−2 at room temperature, which is comparable to the effect of embedding radiation soft matter in ice. DMC's radiation hardness is enhanced by a latency dose that forms during a phase transformation. This unifying model predicts that a critical dose rate exists for any material that varies between 0.1 and 104 eÅ−2s−1 because of a material dependent competition of heat generation and spread. It shows that Brillouin scattering causes time dependent perturbations in electron irradiated solids that trigger time‐temperature‐transformations on a time scale of nanoseconds to microseconds at room temperature, which is not included in traditional models describing the decay of Bragg intensities by radiolysis.

Dose-rate-dependent damage of cerium dioxide in the scanning transmission electron microscope

Beam damage caused by energetic electrons in the transmission electron microscope is a fundamental constraint limiting the collection of artifact-free information. Through understanding the influence of the electron beam, experimental routines may be adjusted to improve the data collection process. Investigations of CeO2 indicate that there is not a critical dose required for the accumulation of electron beam damage. Instead, measurements using annular dark field scanning transmission electron microscopy and electron energy loss spectroscopy demonstrate that the onset of measurable damage occurs when a critical dose rate is exceeded. The mechanism behind this phenomenon is that oxygen vacancies created by exposure to a 300keV electron beam are actively annihilated as the sample re-oxidizes in the microscope environment. As a result, only when the rate of vacancy creation exceeds the recovery rate will beam damage begin to accumulate. This observation suggests that dose-intensive experiments can be accomplished without disrupting the native structure of the sample when executed using dose rates below the appropriate threshold. Furthermore, the presence of an encapsulating carbonaceous layer inhibits processes that cause beam damage, markedly increasing the dose rate threshold for the accumulation of damage.

Influence of γ -irradiation and temperature on the mechanical properties of EPDM cable insulation

The mechanical properties of EPDM polymers, degraded as a result of extensive thermal and radiochemical aging treatment, are studied. The focus is given to dose rate effects in polymer insulation materials extracted from industrial cables in use in Belgian nuclear power plants. All studied mechanical characteristics such as the ultimate tensile stress, the Young's modulus, and the total elongation (or elongation at break) are found to be strongly affected by the irradiation dose. The ultimate tensile stress and Young's modulus are clearly exhibiting the dose rate effect, which originated from oxidation mediated interplay of polymer cross-linking and chain scission processes. The change of crossover between these two processes is found to be gradual, without critical dose rate or temperature values. On the contrary, the total elongation is observed not to be sensitive neither to irradiation temperature nor to the dose rate. Both cross-linking and chain scission seem to affect the total elongation in a similar way by reducing the average polymers chain length. This idea is confirmed by the model which shows that all total elongation data as a function of irradiation time can be reproduced by varying a single parameter, the pre-exponential factor of the irradiation rate constant.

Quantitative Modeling of Microbial Population Responses to Chronic Irradiation Combined with Other Stressors.

Microbial population responses to combined effects of chronic irradiation and other stressors (chemical contaminants, other sub-optimal conditions) are important for ecosystem functioning and bioremediation in radionuclide-contaminated areas. Quantitative mathematical modeling can improve our understanding of these phenomena. To identify general patterns of microbial responses to multiple stressors in radioactive environments, we analyzed three data sets on: (1) bacteria isolated from soil contaminated by nuclear waste at the Hanford site (USA); (2) fungi isolated from the Chernobyl nuclear-power plant (Ukraine) buildings after the accident; (3) yeast subjected to continuous γ-irradiation in the laboratory, where radiation dose rate and cell removal rate were independently varied. We applied generalized linear mixed-effects models to describe the first two data sets, whereas the third data set was amenable to mechanistic modeling using differential equations. Machine learning and information-theoretic approaches were used to select the best-supported formalism(s) among biologically-plausible alternatives. Our analysis suggests the following: (1) Both radionuclides and co-occurring chemical contaminants (e.g. NO2) are important for explaining microbial responses to radioactive contamination. (2) Radionuclides may produce non-monotonic dose responses: stimulation of microbial growth at low concentrations vs. inhibition at higher ones. (3) The extinction-defining critical radiation dose rate is dramatically lowered by additional stressors. (4) Reproduction suppression by radiation can be more important for determining the critical dose rate, than radiation-induced cell mortality. In conclusion, the modeling approaches used here on three diverse data sets provide insight into explaining and predicting multi-stressor effects on microbial communities: (1) the most severe effects (e.g. extinction) on microbial populations may occur when unfavorable environmental conditions (e.g. fluctuations of temperature and/or nutrient levels) coincide with radioactive contamination; (2) an organism’s radioresistance and bioremediation efficiency in rich laboratory media may be insufficient to carry out radionuclide bioremediation in the field—robustness against multiple stressors is needed.

Maintaining the genuine structure of 2D materials and catalytic nanoparticles at atomic resolution

The recent development of atomic resolution, low dose-rate electron microscopy allows investigating 2D materials as well as catalytic nano particles without compromising their structural integrity. For graphene and a variety of nanoparticle compositions, it is shown that a critical dose rate exists of <100e−/Å2s at 80keV of electron acceleration that allows maintaining the genuine object structures including their surfaces and edges even if particles are only 3nm large or smaller. Moreover, it is demonstrated that electron beam-induced phonon excitation from outside the field of view contributes to a contrast degradation in recorded images. These degradation effects can be eliminated by delivering electrons onto the imaged area, only, by using a Nilsonian illumination scheme in combination with a suitable aperture at the electron gun/monochromator assembly.

Global View on Dose Rate Effects in Silica-Based Fibers and Devices Damaged by Radiation-Induced Carrier Trapping

We give a novel and very general treatment to a standard model describing dose rate effects in systems damaged by carrier trapping. This model is well adapted for optical fibers, but the lessons we draw may also be helpful to discuss dose rate effects (DRE) in electronic devices. By highlighting the few determinant ratios of physical parameters that govern the system behavior, we clarify when, how and how much dose rate effects affect trap filling and radiation-induced degradation. Critical dose rate, marking the demarcation between low and high dose rate regimes, is also estimated as a function of these parameters. Taking this step back is important to enlighten contradictory results reported on DRE behaviors. The dose and dose rate dependencies of trap filling measured on silica optical fibers, as well as the critical dose rate, are successfully reproduced with a single set of a few adjustable physical parameters.

Constant-pitch microprism-array optical device for beam condensers in hard x-ray synchrotron radiation beamlines

A constant-pitch microprism-array optical device has been developed. It is a modified version of the previously reported quasi-Fresnel lens [Kagoshima et al., Appl. Phys. Lett. 101, 163102 (2012)]. The modification eases the fabrication of the lens, although it degrades some of the focusing performance. It consists of twenty 50-μm-wide right-angle microprisms, whose slope angles gradually increase to 67.8° at the outer side. By inclining the array, the aspect ratio of the microprisms becomes large enough for X-ray use. The effective slope angle can be enlarged to 86.4° at an inclination angle of 80.5°. A 10-keV synchrotron beam with a size of 680 μm (V) × 660 μm (H) was two-dimensionally condensed to 130 μm (V) × 380 μm (H) with a photon flux density gain of ∼3. We also show that the microprism array is suitable for practical use below the critical radiation dose rate.

Impact of Intelligent Intravenous Infusion Pumps on Directing Care Toward Evidence-Based Standards: A Retrospective Data Analysis

Introduction: Published literature has successfully demonstrated the impact of intravenous (IV) infusion pump safety software on improving the quality of health care delivery. Much of this literature has focused solely on the ability of these devices to prevent potential medication errors, while overlooking the devices' additional valuable advantages. One non-reported benefit is the ability of IV infusion pump safety software to consistently administer doses of IV medication, which are based on evidence. This article describes the process undertaken to implement and evaluate the impact of IV infusion pump safety software on driving care toward evidence-based standards. Methods: An advisory group of expert users was convened for a 2-day session to develop consensus recommendations of best practices for IV infusion pump safety software. Using these recommendations, administrative data were collected from a community hospital to assess the endpoints identified by the advisory panel. Results: Data analysis of rescue agents (ie, flumazenil, glucagon, and protamine sulfate) showed reductions in utilization in the post-implementation period of the safety software. The decreased requirement for blood transfusions in patients receiving heparin infusions suggests that heparin infusions were more safely administered in the post-implementation period. The decreased length of stay and mortality rate observed in patients with complex respiratory infections during the post-implementation period suggests that by correctly infusing antibiotics consistently, patient outcomes may be improved. Additionally, alert and edit data from the pumps demonstrated that the IV infusion pump safety software alerted to and influenced edits on many critical dose rate errors for benzodiazepines, heparin, and several antibiotics. Conclusion: Intravenous infusion pump safety software improves clinical outcomes through consistent application of evidence-based standards of dose rates for IV drugs.

Radiation damage to a protein solution, detected by synchrotron X-ray small-angle scattering: dose-related considerations and suppression by cryoprotectants

In small-angle X-ray scattering experiments at high-brilliant synchrotron sources, protein aggregation results from radiation damage. The radiation-induced aggregation of lysozyme in solution was qualitatively evaluated based on forward scattering and radii of gyration. The scattering did not change below 400 Gy and increased exponentially above this dose. The aggregation is only seen beyond the critical dose rate, and the 'dilution effect' known in radiology was also observed. Mass spectroscopy of the lysozyme solution exposed to a monochromatic X-ray beam did not show any cleavage of the polypeptide chain. Small-angle X-ray scattering patterns suggested that the radiation-induced aggregation should be a non-specific association of intact lysozyme, without substantial alterations of the folding topologies. It was found that the addition of small amounts of cryoprotectants, such as glycerol, ethylene glycol and sucrose, effectively reduced the radiation damage. Glycerol and ethylene glycol were identically effective in the 100 mM concentration range. A similar effective concentration was observed for sucrose. The damage reduction by the cryoprotectants was mainly ascribed to changes in the protein-protein interactions, and rarely to decreases in the diffusion rates of activated species.

Estimation of radiation doses from 137Cs to frogs in a wetland ecosystem

Currently, there is no established methodology to estimate radiation doses to non-human biota. Therefore, in this study, various dose models were used to estimate radiation doses to moor frogs (Rana arvalis) in a wetland ecosystem contaminated with 137Cs. External dose estimations were based on activity concentrations of 137Cs in soil and water, considering changes in habitat over a life-cycle. Internal doses were calculated from the activity concentrations of 137Cs measured in moor frogs. Depending on the dose model used, the results varied substantially. External dose rates ranged from 21 to 160 mGy/y, and internal dose rates varied between 1 and 14 mGy/y. Maximum total dose rates to frogs were below the expected safe level for terrestrial populations, but close to the suggested critical dose rate for amphibians. The results show that realistic assumptions in dose models are particularly important at high levels of contamination.

Estimation of radiation doses from 137Cs to frogs in a wetland ecosystem

Currently, there is no established methodology to estimate radiation doses to non-human biota. Therefore, in this study, various dose models were used to estimate radiation doses to moor frogs ( Rana arvalis) in a wetland ecosystem contaminated with 137Cs. External dose estimations were based on activity concentrations of 137Cs in soil and water, considering changes in habitat over a life-cycle. Internal doses were calculated from the activity concentrations of 137Cs measured in moor frogs. Depending on the dose model used, the results varied substantially. External dose rates ranged from 21 to 160 mGy/y, and internal dose rates varied between 1 and 14 mGy/y. Maximum total dose rates to frogs were below the expected safe level for terrestrial populations, but close to the suggested critical dose rate for amphibians. The results show that realistic assumptions in dose models are particularly important at high levels of contamination.

Relaxation process of ion irradiation defects in IV-semiconductors

Relaxation process of ion irradiation defects in IV-semiconductors (Si, Si 0.85Ge 0.15, Ge, and heavily P-doped (∼10 20 cm −3) n +-Si) was investigated. The IV-semiconductors were irradiated with 25 keV Ar + ions (dose: 1×10 13–1×10 16 cm −2, dose rate: 3×10 11–6×10 12 cm −2 s −1, temperature: 25–250 °C). Amorphicity was comprehensively evaluated by using spectroscopic ellipsometry. For samples irradiated at temperatures around 100 °C, dose rate dependence of amorphicity was remarkably observed. The Arrhenius plot of critical dose rate, at which defect generation and annihilation rates were balanced, showed that activation energies for defect relaxation process were 0.43, 0.51, 0.88, and 0.18 eV for Si, Si 0.85Ge 0.15, Ge, and n +-Si, respectively. These results suggest that defect relaxation process is governed by migration of V 0 in Si, Si 0.85Ge 0.15, and Ge, while V 2− in n +-Si. The migration energy of V 0 in Si 0.85Ge 0.15 obeys the Vegard's law.

Behavior of radiation-induced defects and amorphization in silicon crystal

We have investigated the dose rate dependence of the lateral amorphization of silicon crystals irradiated with 40 keV Si 2+ focused ion beams (FIB) as a function of sample temperature. The recovery time of point defects, τ, and the extent of their distribution, d, around the collision cascades produced by impinging ions were evaluated. The amorphous line-width was measured with scanning electron microscopy (SEM) after a selective etching. We have obtained a critical dose rate 1 (τd 2) of 1.0 × 10 15 cm −2s −1 for radiation at 100°C. The temperature dependence of the critical dose rate suggests that the lateral amorphization is controlled by a simple kinetics of the defects with an activation energy of 0.85 eV. From the value of the activation energy, we speculate that the recovery process of radiation-induced defects is controlled by the migration of interstitial Si atoms.

Theory of diffusion-controlled defect aggregation under irradiation: A comparative study of three basic approaches

Abstract Recent advances achieved within the framework of three basic theoretical approaches to the diffusion-controlled clustering of similar defects during irradiation are compared. These approaches (which can be identified as respectively macroscopic, mesoscopic and microscopic formalisms) have in common that they predict the existence of a critical dose rate for aggregation which increases with temperature, and a bell-shaped temperature dependency of the aggregation rate. Mesoscopic and microscopic approaches demonstrate that the elastic interaction between radiation defects plays a decisive rôle in the aggregation process. Results which are obtained via the different approaches are illustrated for NaCl crystals.

Aggregation of Frenkel defects under irradiation: a mesoscopic approach

The radiation-induced aggregation of Frenkel defects in solids is studied in terms of a mesoscopic approach. The asymmetry in elastic interactions between mobile interstitials (I-I) and between interstitials and vacancies (I-V) plays a decisive rôle in the aggregation of similar defects. The conditions for defect aggregation are studied in detail for NaCl crystals. The critical dose rate for aggregation has been calculated as a function of the temperature as well as the aggregation rate as a function of temperature and dose rate. Furthermore, the role of deep traps (like impurities and di-vacancies), reducing the mobility of interstitials, and the rôle of dislocations serving as sinks for interstitials, are studied. The aggregation appears to reach a maximum at a distinct temperature which is in agreement both with experiment and the Jain-Lidiard theory. The model also predicts a shift of this maximum towards lower temperatures if the dose rate is decreased. The consequences of the model for the disposal of nuclear waste in rock salt formations are briefly discussed.

Amorphization kinetics of Zr 3Fe under electron irradiation

Previous investigations using 40Ar-ion bombardments have revealed that Zr 3Fe, which has an orthorhombic crystal structure, undergoes an irradiation-induced transformation from the crystalline to the amorphous state. In the present investigation, 0.9 MeV electron irradiations were performed at 28–220 K in a high-voltage electron microscope (HVEM). By measuring the onset, spread and final size of the amorphous region, factoring in the Gaussian distribution of the beam, a kinetic description of the amorphization in terms of dose, dose rate and temperature was obtained. The critical temperature for amorphization by electron irradiation was found to be ~ 220 K, compared with 570–625 K for 40Ar-ion irradiation. Also, the dose-to-amorphization increased exponentially with temperature. Results indicated that the rate of growth of the amorphous region under the electron beam decreased with increasing temperature and the dose-to-amorphization decreased with increasing dose rate. The size of the amorphous region saturated after a given dose, the final size decreasing with increasing temperature, and it is argued that this is related to the existence of a critical dose rate, which increases with temperature, and below which no amorphization occurs.

Relationship between Mitotic Delay and the Minimum Dose Rate of X Irradiation Required to Stop Cell Proliferation

When cells are subjected to irradiation, their progression through the cell cycle can be arrested. If the arrested cells are subjected to additional damage, their period of arrest is prolonged. Under continuous low-dose-rate irradiation, the cumulative nature of arrest time leads to a geometric increase in the arrest time as a function of the dose rate. Above a certain cell-line-specific dose rate, the arrest duration becomes infinite and cell proliferation ceases. We find that the lowest dose rate (critical dose rate) required to stop cell proliferation during continuous irradiation is the reciprocal of the mitotic delay per gray of high-dose-rate irradiation. The calculated critical dose rates for X rays agree with those measured by Mitchell et al. (Radiat. Res. 79, 537-551, 1979), provided that the critical dose rate is identified with the minimum dose rate which allows no more than one population doubling. This specific identification of the critical dose rate is explained on the basis of cell cycle kinetics. A kinetic mechanism for cell cycle arrest and recovery leads to the critical dose rate as the reciprocal sensitivity for G2 arrest.

Area Monitoring of Photons and Neutrons from Medical Electron Accelerators Using Tissue-Equivalent Proportional Counters

A low pressure tissue-equivalent proportional counter (TEPC) was used to determine absorbed dose and dose equivalent of photons and neutrons in the control room of a medical linear accelerator. It is shown that a TEPC based area monitor can be used for routine dosimetry in the pulsed radiation fields outside the treatment rooms of medical electron accelerators. Corrections due to pile-up are negligible if a critical photon dose rate is not exceeded. This limit depends on the time structure of the radiation and on the detector size and is about 10 µSv.h-1 for a repetition rate of 160 s-1 and a TEPC volume of 100 cm3.

The Competition between Ion Beam Induced Epitaxial Crystallization and Amorphization in Silicon: The Role of the Divacancy

AbstractThe transition from ion induced epitaxial crystallization to planar amorphization of a preexisting amorphous layer in silicon has been investigated. The conditions for dynamic equilibrium at the transition were determined for different ion species as a function of dose rate and temperature. The critical dose rate for equilibrium varies exponentially with 1/T, exhibiting an activation energy of ∼1.2 eV. Furthermore, for different ions, the critical dose rate is inversely proportional to the square of the linear displacement density created by individual ions. This second order defect production process and the activation energy, which is characteristic of divacancy dissociation, suggest that the accumulation of divacancies at the amorphous/crystalline interface controls the balance between crystallization and amorphization.

The Role of Time-factor and Rbe for the Induction of Osteosarcomas by Incorporated Short-lived Bone-seekers

In a large series of experiments, fractionate-d injections of short-lived bone-seekers have been shown in many cases to cause a remarkable increase of the osteosarcoma incidence compared with a single administration of the same total skeletal dose. This effect has been observed with both alpha- and beta-emitters. In addition the latency period was shortened by protracting the dose. The total skeletal doses investigated ranged between 0.9 and 20 Gy for alpha-emitters (224Ra and 227Th) and between 28 and 112 Gy for the beta-emitter (177Lu). In all cases the protracted dose had higher or at least equal effects when compared with a single application. Reference experiments with long-lived alpha- and a-emitting bone-seeking nuclides (226Ra and 90Sr) showed that the incidence of osteosarcomas per Gy was sometimes lower than that observed when the same skeletal dose was applied by protraction of short-lived radionuclides. The dependence of osteosarcoma incidence on dose-time distribution, duration of internal irradiation, and radiation quality is discussed. In this context the possibility that the critical initial dose rate may be related to the initiating event within the multi-stage hypothesis of carcinogenesis is considered.