88 research outputs found
paramagnetic centers in irradiated sedimentary quartz
No full textThe effect of thermal excitation on paramagnetic defects in natural sedimentary quartz irradiated with different doses of gamma radiation was studied using electron spin resonance (ESR) spectroscopy. We report a variation in the activation energy and the frequency factor for [AlO4/h+]0 and [TiO4/M+]0 paramagnetic defects with the gamma dose, for a dose range investigated between ∼100 Gy and ∼40 000 Gy. Our results indicate that both [AlO4/h+]0 and [TiO4/M+]0 defects are less thermally stable above 1 kGy–2 kGy than below this dose range. The correlation between the two kinetic parameters (activation energy and frequency factor) satisfies the Meyer–Neldel rule. A linear correlation was found between the amplitude of the ESR signals of [AlO4/h+]0 and [TiO4/M+]0 paramagnetic defects corresponding to different doses after the application of thermal treatments in the pulse annealing procedure. We propose a mechanism involving the exchange of the cation, assigned mainly to Li+ here, between the two defects. Under irradiation, the cation is removed from [AlO4/M+]0 (forming [AlO4/h+]0) to [TiO4]− (forming [TiO4/M+]0), while under heating, the reverse mechanism takes place.The migration energy of the cation from one defect to another was found to be about 51 meV, corresponding to a temperature of
about 325 ○C
Dating Sediments by EPR Using Al-h Centre: A Comparison between the Properties of Fine (4–11 µm) and Coarse (>63 µm) Quartz Grains
No full textThe possibility of EPR dating for sediments using Al-h signals of fine (4–11 μm) grains of quartz has not been previously discussed. Here, the Al-h and peroxy EPR spectra of fine (4–11 μm) and coarse (63–90, 125–180 μm) sedimentary quartz from thoroughly investigated loess sites in Eastern Europe were examined. By comparing experimental spectra with a simulated signal, we evaluated the overestimation observed when using the standard approach established by Toyoda and Falguères to measure Al-h intensity for different doses of radiation, up to 40,000 Gy. This overestimation, caused by the presence of peroxy signals, was much more pronounced for fine grains. Fine grains exhibited some additional dose-dependent signals, which, for some samples, caused a complete distortion of the Al-h spectra at high doses, making it impossible to measure the standard amplitude. We propose a new approach to measuring Al-h signal intensity, focusing on the peak-to-baseline amplitude of the part of the signal at g ≈ 2.0603, which is not affected by the peroxy signals and therefore has the potential of providing more accurate results. The shapes of dose response curves constructed for coarse and fine grains using the new approach show considerable similarity, suggesting that Al-h centre formation in fine and coarse grains upon artificial radiation at room temperature follows the same pattern
Element concentrations of loess-paleosol sequence Orlovat
No full textElement concentrations of loess-paleosol sequence Orlova
Calcium carbonate concentrations of loess-paleosol sequence Orlovat
No full textCalcium carbonate concentrations of loess-paleosol sequence Orlova
On the dose dependence prior and after stimulation with visible light of E' and Al-hole centres in sedimentary quartz: correlation and mechanisms
No full textE’ and Al-hole centres are some of the most common and abundant paramagnetic defects in sedimentary quartz. Here we investigate the dose dependence of these defects before and after exposure to light by electron spin resonance (ESR). Unlike the Al-hole centre, known to have only radiation-induced formation mechanisms, the E’ seems to possess a response to gamma dose characterised by predominantly radiation-induced annihilation at lower doses (about 1000 Gy) and a predominantly radiation-induced formation at higher doses, at least in our investigated dose range (up to 40 kGy). We propose these dose response mechanisms to be governed by electron trapping by E’ itself and by hole trapping by the oxygen deficiency centre (ODC), known to be the main precursor of E’. We show that the ESR signals of both defects are linearly correlated during their formation as well as during their dissociation under both irradiation and sunlight exposure. We further show that there is a clear correlation between the light sensitive Al-hole centres (also known as the Al-hole bleachable part), and the amount of E’ produced after prolonged light exposure. This indicates a correlation between the holes released from Al-hole and those trapped by one of the two electrons in the ODCs producing E’. As such, the origin of the unbleachable part of the Al-hole signal resides in the availability of oxygen deficiency centres which are also dependent on the accumulated gamma dose
Grain size analyses of loess-paleosol sequence Orlovat
No full textGrain size analyses of loess-paleosol sequence Orlova
Thermoluminescence properties of 30Y 2 O 3 ·30P 2 O 5 ·40SiO 2 vitroceramics in mixed neutron-gamma fields
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