Thermoluminescence Studies of Proton-Irradiated Cr-, Mg-Codoped β-Ga2O3

Chromium-doped Ga2O3, with intense Cr3+-related red-infrared light emission, is a promising semiconductor material for optical sensors. This work constitutes a comprehensive study of the thermoluminescence properties of Cr-, Mg-codoped β-Ga2O3 single crystals, both prior to and after proton irradiation. The thermoluminescence investigation includes a thorough analysis of measurements with different β- irradiation doses used to populate the trap levels, with preheating steps to disentangle overlapping peaks (TM-TSTOP and initial rise methods) and finally by computationally fitting to a theoretical expression. At least three traps with activation energies of 0.84, 1.0, and 1.1 eV were detected. By comparison with literature reports, they can be assigned to different defect complexes involving oxygen vacancies and/or common contaminants/dopants. Interestingly, the thermoluminescence signal is enhanced by the proton irradiation while the type of traps is maintained. Finally, the pristine glow curve was recovered on the irradiated samples after an annealing step at 923 K for 10 s. These results contribute to a better understanding of the defect levels in Cr-, Mg-codoped β-Ga2O3 and show that electrons released from these traps lead to Cr3+-related light emission that can be exploited in dosimetry applications.

Probing the Cr3+ luminescence sensitization in β-Ga2O3 with ion-beam-induced luminescence and thermoluminescence

Ion-beam-induced luminescence (IBIL) measurements were performed in Cr-doped β-Ga₂O₃ using both protons and helium ions, showing a strong enhancement of the Cr³⁺ luminescence upon ion irradiation. Theoretical modelling of the IBIL intensity curves as a function of the fluence allowed estimating the effective cross-sections associated with the defect-induced IBIL enhancement and quenching processes. The results suggest that sensitizing the Cr³⁺ luminescence is more efficient for H⁺ than for He⁺ irradiation. Thermoluminescence (TL) studies were performed in the pristine sample, with no TL signal being observed in the spectral region corresponding to the Cr³⁺ emission. In agreement with the IBIL study, upon ion irradiation (with either protons or helium ions), this TL emission is activated. Moreover, it can be quenched by annealing at 923 K for 10 s, thus revealing the role played by the defects induced by the irradiation. These results show that the irradiation-induced defects play a major role in the activation of the Cr³⁺ luminescence, a fact that can be exploited for radiation sensing and dosimetry.

Silver and copper as pollution tracers in Neogene to Holocene estuarine sediments from southwestern Spain

Estuaries are very sensitive ecosystems to human activities and the natural evolution of their drainage basins located upstream. Pollution derived from human activities, such as historical mining or recent industrial wastes, can significantly affect their environmental quality. This paper analyzes the silver and copper contents of four cores extracted in two estuaries of SW Spain. Its chronology and vertical evolution allow to differentiate the effects of several pollution episodes (natural, Roman, 19th-20th centuries) on its different sedimentary environments in the last 6 million years. Possible future applications are included in the fields of environmental management or even education.

Naturally occurring radioactive material and risk assessment of tailings of polymetallic and Ra/U mines from legacy sites

Old mine tailings from Northern and Central Portugal were studied in order to perform a radiological and chemical characterization. The evaluation of massic activity of natural radionuclides and concentrations in tailings of polymetallic and Ra/U mines was performed by gamma spectrometry and neutron activation analysis. Iron speciation was carried out by Mössbauer spectroscopy. In polymetallic tailings with physical ore processing (Cumieira and Verdes - exploited for Sn, Nb-Ta) higher contents of Th, ²²⁸Ra and ²²⁶Ra in the coarser materials occur, probably due to their presence in host rock and ore fragments. In finer tailings, washing may explain the lower ²²⁶Ra and ²¹⁰Pb massic activity. In tailings with physical/chemical ore processing (Covas - exploited for W and Sn) high U contents and a tendency for higher ²²⁶Ra and ²¹⁰Pb massic activity in the fine materials is observed, probably due to their incorporation in nano-sized particles of iron oxides. A high variation of the ²¹⁰Pb/²²⁶Ra ratio occurs in polymetallic tailings; a deficit of ²¹⁰Pb can be observed particularly in deposits of settling tanks drained from dumps of chemically treated ore. In Ervideira-Mestras tailings (Ra/U exploitation) where no ore process in situ was performed, a near equilibrium between ²¹⁰Pb and ²²⁶Ra occurs. Dose risk assessment was carried out by calculating external outdoor Annual Effective Dose Rate; the dose rates in air due to terrestrial gamma radiation are low for the polymetallic tailings (<47 nGy/h), and higher for tailings of Ra/U (up to 4130 nGy/h), in the worst scenario.

A relative risk assessment of the open burning of WEEE

Waste electric and electronic equipment (WEEE) represents a potential secondary source of valuable materials, whose recovery is a growing business activity worldwide. In low-income countries, recycling is carried out under poorly controlled conditions resulting in severe environmental pollution. High concentrations of both metallic and organic pollutants have been confirmed in air, soil, water, and sediments in countries with informal recycling areas. The release of these contaminants into the environment presents a risk to the health of the exposed population that has been widely acknowledged but still needs to be quantified. The aim of this work was to evaluate the relative risk from inhalation associated with the open burning of different kinds of WEEE. The shrinking core model was applied to estimate the concentration of the metals which would be released into the environment during the incineration of different types of WEEE. In addition, the potential generation of dioxins during the same informal practice was estimated, based on the plastic content of the WEEE. The results provided for the first time a comparative analysis of the risk posed from the open burning of WEEE components, proposing a methodology to address the absolute risk assessment to workers from the informal recycling of WEEE.

A device-specific prioritization strategy based on the potential for harm to human health in informal WEEE recycling

In developing countries, the recovery of valuable materials from Waste Electrical and Electronic Equipment (WEEE) is carried out via uncontrolled practices, posing potentially severe risks both to human health and the environment. The assessment of the risk, which depends on both the kind and hazardous properties of the substances contained in WEEE, is currently limited as the exposure scenario for the single informal practice cannot be fully characterized for this purpose. In this context, this work proposes and evaluates a strategy to identify the relative potential harm of different kinds of WEEE by their content in metals, selected as the target substances of concern. This was based on the individual metal content, primarily located in the printed circuit boards (PCBs) of the different devices. The metal composition of the individual PCBs was identified and the dominant unregulated metal recovery practices were reviewed to identify the most suitable parameter to express the toxicity of these metals. Based on a mass-normalized cumulative toxicity, via the inhalation route, individual components were assessed from compositional variation found in the literature. The results is a semiquantitative ranking of individual components, revealing significant differences in potential harm posed by different electronic appliances and an opportunity to provide prioritization strategies in future management.

Chemical element accumulation in tree bark grown in volcanic soils of Cape Verde-a first biomonitoring of Fogo Island

Barks from Prosopis juliflora (acacia) were collected in 12 sites of different geological contexts over the volcanic Fogo Island (Cape Verde). Elemental contents of Ba, Br, Co, Cr, Fe, K, Na, Zn and some rare earth elements (REE)-La, Ce, Sm, Eu, Tb, Yb, and Lu, were obtained for biological samples and topsoils by using k ₀-standardized and comparative method of instrumental neutron activation analysis (INAA), aiming the evaluation of chemical elements uptake by acacia bark. This first biomonitoring study of Fogo Island showed that, in general, significant accumulations of trace elements present in high amounts in these soils occur. This can be partially explained by the semi-arid climate with a consequent bioavailability of chemical elements when rain drops fall in this non-polluted environment. REE enrichment factors (EFs) increase with the decrease of ionic radius. Heavy REE (HREE) are significantly enriched in bark, which agrees with their release after the primary minerals breakdown and the formation of more soluble compounds than the other REE, and uptake by plants. Among the potential harmful chemical elements, Cr appears to be partially retained in nanoparticles of iron oxides. The high EFs found in tree barks of Fogo Island are certainly of geogenic origin rather than anthropogenic input since industry and the use of fertilizers is scarce.

Effects on gastric mucosa induced by dental bleaching--an experimental study with 6% hydrogen peroxide in rats

The value of aesthetic dentistry has precipitated several developments in the investigation of dental materials related to this field. The free marketing of these products is a problem and it is subject to various interpretations regarding its legality. There are several techniques for tooth whitening, the most used one being the external bleaching. It is the later version of such technique that poses the greatest danger of ingesting the product. The present study analysed the systemic effect of these products when they are swallowed.

[The effect of the quantitative variation of autologous spongy bone graft applied for bone regeneration in an experimental model of tibia osteotomy]

This study aimed at studying the influence of the quantitative variation of the autologous spongy bone graft on its osteogenic properties by virtue of the fact that its attainment has inconveniences of significant morbidity at local donor level and the limited quantity of grafting able to be obtained. Recourse was made to an osteotomy model for this purpose with the removal of a small 4 mm segment at the mid-diaphysis of the left tibia of twenty one sheep, stabilised by an osteosynthesis plate on which different quantities of autografting spongy bone were applied so that the referred bone defect (1) would not to be completely filled (1.5 g), (2) would be filled without any compression (3 g) and (3) would be filled with an excessive quantity (5 g) (n=5/each group). (4) A control osteotomy (n=6) was also carried out where the bone defect remained empty. Comparison of the evolution of bone regeneration during the postoperative period of 12 weeks was carried out by means of conventional radiographic exams and optical densitometry analysis and by radiological bone densitometry (DEXA--Dual-Energy X-ray Absorptiometry), bone histomorphometry and histological analysis after the animals' euthanasia. Optical density was significantly affected (p<0.0001) by the treatment and by time and with significant differences between the various groups under study over the same time of post-operative period: during immediate postoperative and on the 2nd (p <0.01), 4th (p <0.001), 6th and 8th weeks (p <0.05), but not on the 10th and 12th (p >0.05) post-operative weeks. Bone mineral density (BMD), obtained by DEXA, was 0.4347 +/- 0.3821 g/cm2 in the control group and 0.7482 +/- 0.2327 g/cm2, 0.9517 +/- 0.2292 g/cm2 e 1.0409 +/- 0.0681 g/cm2 in the groups that had received 1.5 g, 3g and 5 g of bone graft, respectively. The BV was 39.2 +/- 24.4% with the control group and 62.0 +/- 14.4%, 76.0 +/- 15.2% and 84.0+/-4.2% in the groups that had received 1.5 g, 3g and 5 g of bone graft, respectively. The BMD and BV were significantly affected (p<0.05 and p <0.01, respectively) by the treatment, nevertheless, there were no significant differences (p >0.05) between the groups that had received the largest volumes of autografting spongy bone on the 12th week of the post-operative period.The conclusion was reached that there was no advantage in excessively filling an osteotomy gap with autografting spongy bone and attainment of only the volume strictly required to fill the osteotomy gap at issue was needed.

Evaluation of microbiological quality of medicinal plants used in natural infusions

Consumption of preparations of medicinal plants has been increasing during the last decades in occidental societies. However, there are no effective sanitary controls of these products. To evaluate the nature and content of microbiological contamination, 62 samples of seven medicinal plants (chamomile, leaves of orange tree, flowers of linden, corn silk, marine alga, pennyroyal mint and garden sage) were studied, using conventional microbiological methods. Practically all samples (96.8%) were contaminated with Bacillus cereus; 19.2% of them with levels higher than 10(3) spores/g. The highest levels were found in corn silk samples (> 10(7) spores/g). Spores of Clostridium perfringens were detected in 83.9% of samples, but only 19.2% had levels greater than 10(3)/g. The mean level of fungal population was 10(5.5) cfu/g. Corn silk samples were the most contaminated, with levels above 10(6) cfu/g. Fusarium spp., Penicillium spp., Aspergillus flavus and Asp. niger were predominant in all samples with the exception of garden sage samples. Many yeasts were found in chamomile, flowers of linden, corn silk, pennyroyal mint and garden sage. Predominant species were Cryptococcus laurentii (28.1%) and Rhodotorula mucilaginosa (22.8%). The mean level of Crypt. laurentii contamination in corn silk was greater than 10(4) cfu/g.