Identification of paramagnetic centers and the dating of cave dripstones by electron paramagnetic resonance

Dating of samples of cave dripstones collected from the Jaskinia Czarna (the Polish Tatra Mountains) and fossil bones was carried out by means of electron paramagnetic resonance (EPR). By this method the age of the sample is found as the ratio of the total dose (TD) to the annual dose rate (D _a). TD is established on the basis of EPR spectra, whileD _a is the sum of the internal dose rate, calculated on the basis of the concentration and activity of radioactive nuclides in the sample, and the external dose rate measured at the site of the sample collection by a calibrated γ-radiation probe. Two models of uranium accumulation were used: the model of linear accumulation and the model on the basis of the assumption of a disturbed radiation equilibrium in the uranium series. The results were compared with the age determined by the uranium-thorium method. Moreover, by the computer resolution enhancement method the dynamics of paramagnetic centers in calcite annealed in nitrogen atmosphere was studied in the range of 283–873 K, and the results were compared with earlier results for calcite annealed in the air. On the basis of the study of paramagnetic center dynamics in dripstone calcite it was established that the radicals responsible for the EPR signals were CO₂⁻ in different symmetries of the crystalline field, CO₃³⁻ in the axial symmetry, CO₃⁻ in the axial and isotropic symmetry and HCO₃²⁻. The results implied that samples to be subjected to EPR dating should be kept at room temperature.     

Fiber-optic-coupled RbMgF3:Eu for remote radiation dosimetry

A portable fiber optic dosimeter has been developed that incorporates RbMgF₃:Eu²⁺ at the end of a multimode polymer fiber. It uses two stimulation sources and takes advantage of the range of stimulable traps in this compound. We observe radioluminescence (RL) during gamma ray irradiation and show that the low-dose RL increases linearly with increasing dose rate where the minimum detectible dose rate is ∼0.015 μSv/s. We show that pulsed infrared-stimulated (940 nm) optically stimulated luminescence (OSL) can be used for real time dose monitoring. The cumulative dose can be readout after irradiation where a linear OSL dose response was observed when stimulating at 505 nm and the minimum detectable dose is ∼50 μSv.     

Mineral zircon: A novel thermoluminescence geochronometer

Mineral zircon contains trace amounts (typically 10-1000 v ppm) of the f -emitters uranium and thorium, which irradiate this mineral internally. This outstanding feature of zircon turns out to be extremely useful when this mineral is applied as a thermoluminescence (TL) dating medium, because the build-up of the age-dependent luminescence is dominated by the presence of well-defined internal radioactive sources and the contributions to the dose from external radiation sources are two orders of magnitude smaller. The results presented in this paper have led us to the conclusion that for zircon dating it is necessary to carefully select the best and homogeneous zircon grains of the highest optical quality. For successful dating experiments on very young and historically well-defined coastal dune sands, selection of the most stable luminescence component by means of narrow band interference filters is needed. Our results suggest that ultimately optical zircon dating will allow us to determine the age of extremely young samples ( e.g. 12 months!).     

Na-rich feldspar as a luminescence dosimeter in infrared stimulated luminescence (IRSL) dating

One of the challenges in dating rock surfaces is the choice of the luminescence mineral. Although quartz is the preferred dosimeter in sediment dating, it is often not sufficiently sensitive when extracted from solid rocks. The intensity of signals from feldspars tends to be much less dependent on geological origin and erosion history, but the dosimetry of K-rich feldspar grains extracted from rocks is complicated because the internal dose rate is very dependent on the original feldspar grain size. The in situ grain size information is lost during the crushing process used to separate the grains for measurement. This latter problem does not apply to Na-rich feldspar because of the absence of internal radioactivity.The potential application of Na-rich feldspar as a luminescence dosimeter for the IRSL dating of rock surfaces is investigated using a variety of sediment samples from different geological settings for which independent age control is available. The blue and yellow luminescence emissions are measured for IR stimulation at 50 °C (IR₅₀), and post-IR IR stimulation at 290 °C (pIRIR₂₉₀). Thermal stability experiments imply that the corresponding signals in both emissions have comparable thermal stabilities and that all signals have similar recombination kinetics and are thermally stable over geological timescales. The IR₅₀ doses measured using blue and yellow emissions are similar to or lower than quartz doses while pIRIR₂₉₀ blue doses are higher than those from yellow emission and quartz doses. The fading rates measured for the IR₅₀ signals are ∼3%/decade larger than those measured for the pIRIR₂₉₀ signals in both yellow and blue emissions. Furthermore the average fading rates of both yellow signals are ∼3%/decade higher than the corresponding fading rates of the blue signals. However, there is no detectable correlation between fading rates and the measured D_e values. The residual doses measured from the laboratory-bleached samples and a modern analogue suggest that the IR₅₀ signals in both blue and yellow emissions bleach to the same degree, as do the corresponding pIRIR₂₉₀ signals, and that there is no significant naturally-unbleachable residual dose observed using these signals. Neither anomalous fading nor incomplete bleaching explains the observed dose discrepancy between the two emissions. Eight uncorrected and fading-corrected ages are calculated for each sample based on all four signals, using the dose rate relevant to Na-rich feldspar extracts (i.e. ∼3% K). The IR₅₀ and pIRIR₂₉₀ blue ages were also calculated assuming a dose rate based on 12.5% internal K (i.e. assuming that the blue signals were mainly derived from contamination by K-rich feldspar). The latter pIRIR₂₉₀ blue ages are in agreement with the expected age control, raising the possibility that this signal originates mainly from K-rich feldspar contamination in our Na-rich fractions, and thus is not so useful in the luminescence dating of rock surfaces. On the other hand, the pIRIR₂₉₀ fading-corrected ages based on the yellow emission are consistent with the independent age controls; higher preheat and stimulation temperatures may result in more stable yellow signals from Na-rich feldspar extracts from rocks, and so reduce the size of the fading correction. We conclude that, because this signal avoids the dosimetry difficulties of K-rich feldspar extracts, it has considerable potential in the IRSL dating of rock surfaces.     

The slow component of quartz optically stimulated luminescence

The slow component of quartz OSL displays a number of properties that clearly distinguish it from the main (‘rapidly bleachable’) part of the quartz OSL signal traditionally used for dating. These properties include an extremely high thermal stability, dose saturation level and a charge concentration dependence in both signal form and decay rate. The physical mechanism responsible for the slow component is thought at present to involve a direct donor–acceptor recombination component, possibly associated with competing pathways below, and possibly up to, the conduction band. The thermal stability and high dose saturation characteristics of the slow component suggest much potential for long-range dating exists although at present it is uncertain whether difficulties associated with partial resetting may preclude routine use of the slow component for dating sedimentary deposits. A single-aliquot additive dose method was however used to obtain an estimate of D_e from the slow component for an Egyptian quartz sample that was in broad agreement with previous estimates based on the standard multiple-aliquot additive dose method.The slow component is often small in magnitude compared to the initial portion of the quartz OSL decay. However, this is not always the case and for some samples significant inaccuracies in D_e estimation may occur when deriving ages from the initial ‘rapidly bleaching’ portion of the OSL decay if the effect of the slow component is ignored or taken to be constant.     

静电场作用下α-碘酸锂单晶中子衍射的增强(Ⅱ)

在以前工作的基础上,继续作了以下的实验:(1)在低温条件下测定了α-碘酸锂单晶在静电场中的衍射情况,观察到在～180K以下出现“冻结”现象,即加上电场并不能使衍射增强,撤去原在室温所加的电场后衍射强度也并不减弱;(2)用狭束中子探测了加静电场后晶体的不同部位,观察到衍射增强是体效应而不是表面层效应;(3)测定了低频交变电场对晶体中子衍射强度的影响,衍射束增强的程度随频率的下降而加大,频率在1500Hz时衍射束的增强已不明显。     

Optically stimulated luminescence dating of Holocene alluvial fans,East Anatolian Fault System,Turkey

In this study, the optically stimulated luminescence dating technique was used to determine the time of deposition of alluvial sediment samples from the Türko?lu-Antakya segment of the East Anatolian Fault System (EAFS) in Turkey. The double-single aliquot regenerative dose protocol on fine grain samples was used to estimate equivalent doses (D_e). Annual dose rate was computed using elemental concentration of uranium (U) and thorium (Th) determined by using thick-source alpha counting and potassium (K) concentrations using X-ray fluorescence and/or atomic absorption spectroscopy. The environmental dose was measured in situ using α-Al₂O₃:C chips inside plastic tubes for a year. The two different bulk sediment samples collected from the Islahiye trench yielded ages of 4.54?±?0.28 and 2.91?±?0.23?ka. We also obtained a 2.60?±?0.18?ka age for the alluvial deposit in the K?ranyurdu trench and 2.31?±?0.14?ka age for an excavation area called Malzeme Oca??. These ages were consistent with the corresponding calibrated Carbon-14 (¹⁴C) ages of the region. The differences between the determined ages were insufficient to clearly distinguish the disturbance event from the effects of bioturbation, biological mixing, or other sources of D_e variation in the region. They provide a record of alluvial aggradation in the region and may determine undocumented historical earthquake events.     

Environmental gamma-ray dose rate measurement by using ultra-high sensitive LiF:Mg,Cu,Si TLD

Environmental gamma-ray dose rates were measured by using ultra-high sensitive LiF:Mg,Cu,Si thermoluminescence dosimeters (TLDs) during short-term burial. The resultant gamma-ray dose rates were compared with those measured by using optically stimulated luminescence (OSL) of Al₂O₃:C and assessed by using HPGe spectrometer. Regardless of the short-term burial and the change of environmental conditions as well as the mismatch of the effective atomic number (Z_eff) with the soil (mainly SiO₂), good agreement was observed between the doses measured by TLDs and OSL dosimeters or the assessed doses. LiF:Mg,Cu,Si TLD has been found to be an effective alternative for the measurement of environmental gamma-ray annual dose rates for use in optical dating.     

Study of chemisorption on copper low-index and stepped surfaces

Adsorption of CHCl₃, O₂, and hydrocarbons has been studied on Cu(111) and stepped surfaces using LEED, AES, and UPS at room temperature. We find that ordered Cl overlayers form upon Cu(111), Cu[3(111) × (100)], and Cu[5(111) × (100)] surfaces upon exposure to CHCl₃. Exposure to O₂ results in rearrangement of the Cu[5(111) × (100)] surface to hill-and-valley regions with large (111) areas, whereas Cu[2(111) × (100)] is stable for the same exposure. The photoemission spectra show new energy levels due to C1 above and below the Cu d band region and a small splitting of the halogen p orbitals. Effects consistent with interaction with the Cu d band are observed. Similar effects are observed with oxygen adsorption. The initial rate of Cl or O₂ chemisorption as measured by photoemission is proportional to the density of steps on these surfaces. Apparently, structural effects play an important role in chemisorption on metals (such as copper) with low density of states at the Fermi energy.     

Thermoluminescence dosimetry in evaluation of liquid 32P sources for intravascular brachytherapy

Several new types of miniature (1 and 2 mm diameter) thermoluminescence (TL) detectors were elaborated and tested aiming at dosimetry of liquid [³²P] sodium phosphate sources suggested for intravascular brachytherapy. They were prepared using LiF:Mg,Cu,P and LiF:Mg,Ti TL materials in a two-layer technique with the effective thickness below 0.1 mm. Dosimetric measurements were carried out in tissue-equivalent polymethyl methacrylate (PMMA) phantoms, with TL detectors placed at various distances around angioplasty balloons filled with aqueous solution of Na₂H³²PO₄. Of particular interest was the response of detectors placed at shortest distances from the balloon surface. All types of the TL detectors used in trials showed very good spatial resolution and dose readings were consistent for all detector types, even at the steepest parts of the dose–distance curve. In our opinion, the new TL detectors can be recommended for measurements of dose distribution around liquid radiation sources.     

Medical dosimetry using a RL/OSL prototype

A portable and robust instrument has been developed for the routine assessment of patient exposure to ionizing radiation during radiotherapy treatments. The design principles of hardware and software are described, along with preliminary measurements that illustrate the operation of the system and its capabilities. In this study the authors used radioluminescence (RL) and Optically Stimulated Luminescence (OSL) from Al₂O₃:C detectors coupled to a PMMA optical fibre to acquire dose in medical dosimetry. The RL/OSL prototype can provide two independent dose estimates from the same in vivo treatment: one integrated dose estimate (OSL) and one real-time dose estimate (RL), which can be compared to one another. The authors first characterized the dose–response to a calibration source (¹³⁷Cs), analysing the OSL and the RL signal to doses from 0.5 to 3 Gy. Later the percentage dose depth from RL is presented for two gamma (6 and 15 MV) and two electron (6 and 12 MeV) medical beams.     

High-pressure Mössbauer spectroscopy using synchrotron radiation and radioactive sources

A high-pressure ⁵⁷Fe Mössbauer study of SrFeO₃ up to 74 GPa has been performed with diamond-anvil-cell (DAC) using synchrotron radiation and a radioactive point source of ⁵⁷Co in Rh. SrFeO₃ is known as a typical cubic perovskite with a high-valence state of Fe⁴⁺ and shows metallic conductivity at 0.1 MPa down to 4.2 K. Applying an external high pressure, SrFeO₃ has not shown any structural transformation up to 74 GPa keeping an Fe⁴⁺ state but the Néel temperature increases up to 300 K at 18 GPa. The external high pressure may induce the ferromagnetism in SrFeO₃ by a decrease of the interatomic distance of Fe or an increase of the d-band width. ⁵⁷Fe Mössbauer measurements under externally applied longitudinal magnetic field using radioactive ⁵⁷Co in Rh source and also nuclear forward scattering measurements with a linearly polarized synchrotron radiation under external magnetic field indicate the existence of the pressure induced ferromagnetism in SrFeO₃. In this work we compare high-pressure Mössbauer spectroscopy using synchrotron and radioactive sources and summarize the advantages and disadvantages of each method.     

Temperature dependence of a Ce3+ doped SiO2 radioluminescent dosimeter for in vivo dose measurements in HDR brachytherapy

Fiber-optic-coupled scintillation dosimeters are characterized by their small active volume if compared to other existing systems, and are therefore particularly suited for internal in vivo dosimetry. Due to possible differences between calibration conditions (i.e., room temperature) and conditions of clinical application (i.e., body temperature), their temperature dependence should be accurately studied. In this work, the temperature dependence of a Ce³⁺ doped SiO₂ scintillation detector coupled to a SiO₂ optical fibre was investigated for high dose rate brachytherapy applications. To this aim, two sets of irradiations with two different Ir-192 sources were performed in a water bath phantom at water temperatures ranging between 17 °C and 40.4 °C (Experiment 1). The relative response of the dosimeter was collected and analyzed. The same experiment was repeated with a second optical fibre which was designed without the active Ce³⁺ doped part at its end (Experiment 2) as well as by changing the length of the passive fibre inserted in water (Experiment 3). The two sets of measurements of experiment 1 were in accordance, indicating a linear increase with temperature of the scintillator sensitivity, with an average increase of 0.27 ± 0.2%/°C. In experiment 2, a 0.5%/°C increase of the collected signal resulted for the passive fibre. No significant difference of the temperature coefficient was found by changing the length of the fibre inserted in water (experiment 3). The obtained results show that a temperature-specific correction factor should be adopted at temperatures different than room temperature (e.g. for in vivo internal dosimetry). Further studies are required to understand the observations.     

Radiophotosynthesis of some 14C-labelled Amino Acids using the Unicellular Green Alga Scenedesmus acutus

Radiophotosynthesis has been carried out using the unicellular green algea Scenedesmus acutus grown, as a substrate for preparing some carbon-14 labelled amino acids. Gaseous ¹⁴CO₂, in an air tight photosynthesis chamber or NaH¹⁴CO₃ solution, in an ordinary photosynthesis chamber, were used as radioactive carbon sources. The yields, radiochemical yields and specific activities of the formed radioactive products are reported in both cases. The results obtained clearly showed the advantages of using the NaH¹⁴CO₃ method. In that case the process was by far less tedious. Moreover, the chemical and radiochemical yields of the formed amino acids were relatively much more higher than the values obtained on using ¹⁴CO₂ in the tight photosynthesis chamber.     

Mössbauer effect in α-Fe2O3 following the reaction56Fe(d,p)57Fe

Mössbauer spectra in α-Fe₂O₃ were measured below and above the Morin temperature following the reaction⁵⁶Fe(d, p)⁵⁷Fe. Within high accuracy, shift, quadrupole splitting, and Debye-Waller-factor are found to be the same as corresponding radioactive source data, whereas the magnetic field is lower by 10 kG. It follows that 10^?7 s after the nuclear reaction essentially all excited recoils are at normal lattice sites with nearest neighbors being undisturbed. The deviation in magnetic field cannot be explained by a rise in temperature.     

The estimation of basic experimental parameters in the fine-grained quartz multiple-aliquot regenerative-dose OSL dating of Chinese loess

The sensitivity-corrected multiple-aliquot regenerative-dose (MAR) protocol provides a reliable approach for fine-grained quartz optically stimulated luminescence (OSL) dating. For reliable estimation of the equivalent dose (D_e), we investigated certain basic experimental parameters in the fine-grained quartz MAR OSL dating of Chinese loess. (1) For suitable bleaching of the natural OSL signal of the regenerative-dose aliquots, the effect of bleaching duration using sunlight, SOL2 and blue LEDs on D_e was studied, and it is found that the appropriate method is a short-duration SOL2 (e.g. 5 min) or blue LEDs (e.g. 60 s) bleaching. (2) To select the appropriate test dose, the relationship between the test dose and D_e was investigated based on three samples having D_e values of approximately 11, 31 and 137 Gy respectively. It is suggested that the test dose for sensitivity correction may be limited to less than approximately 10–20 Gy. (3) Three commonly used fitting modes for quartz OSL growth curve were compared at three regenerative-dose scales. The results indicate that the mode of two saturating exponential functions plus a constant is appropriate and universal. (4) The comparison of D_e values derived using OSL approach with those obtained using the recuperated OSL (ReOSL) protocol shows that the reliable D_e estimation in the fine-grained quartz MAR OSL dating of Chinese loess may be limited to less than approximately 300 Gy. (5) The comparison of growth curves for 18 samples from the Weinan, Xifeng and Jingyuan sites shows that it is feasible to construct a standardized growth curve (SGC) for fine-grained quartz OSL signal in the Chinese Loess Plateau (CLP).     

Measurements of thermal electron attachment rate coefficients to molecules using an electron swarm technique

An existing electron swarm apparatus has been redesigned and upgraded. In particular, the new design incorporates a novel planar radioactive foil to form an integral part of the drift tube, allowing us to overcome inherent problems present in our earlier system which used a cylindrical radioactive source. In addition to this, substantial upgrades have been made to improve the gating and amplification electronics and the data acquisition system. This has resulted in a much greater signal to noise ratio and improved accuracy. This paper describes the upgraded apparatus and its use in obtaining thermal (300 K) attachment rate coefficients to a number of molecules. The quality of the measurements and data are illustrated through the measurement of the thermal attachment rate coefficient for SF₆ (k_th(SF₆) = (2.38 ±0.15 ) ×10 ^-7 cm³ s^-1). Thermal electron attachment rate coefficients for four other molecules are presented, namely for two derivatives of SF₆, SF₅CF₃ and SF₅Cl, and two perfluorocarbons, c-C₄F₈ and 2-C₄F₈.     

μSR Investigation of magnetic phases in R1−xSrxCoO3 oxides (R=Pr, Nd)

We report results of our muon spin relaxation measurements in the series of polycrystalline compounds Pr_1−xSr_xCoO₃ (x=0.3, 0.4 and 0.5) and Nd_1−xSr_xCoO₃ (x=0.3 and 0.5). For the Pr-based samples our data clearly indicate the existence of two magnetic transitions, as also inferred from macroscopic measurements. While the high temperature transition is typical for cobaltites (∼200 K), the low temperature one is unusual. In our experiments it occurs below about 120 K and it manifests itself as a change in the slope of the temperature variation of the muon spin depolarization rate λ(T). For the Nd-based samples we found an increase of the muon spin depolarization rate below 45 K, temperature at which the sample is ferrimagnetic, when the Nd sublattice coupled antiparallel to the Co lattice. No phase separation could be evidenced in our samples by μSR experiments.     

On the initial-occupancy dependence of some luminescence phenomena under the one-trap-one-recombination-center (OTOR) model

We discuss the expected dependence of the maximum signal of a number of luminescence phenomena, used in dosimetry and archaeological and geological dating, on the initial occupancy of the relevant traps, n₀, within the OTOR (also called General One Trap (GOT)) model. This, in turn, has important bearing on the dose dependence of these phenomena. We discuss the dependence of the linearly-modulated optically-stimulated-luminescence (LM-OSL) as well as the non-linear optical stimulation (NL-OSL) on the initial concentration n₀ of trapped carriers. We also consider the behavior of CW-OSL and phosphorescence in a transformed form, as well as TL measured under hyperbolic and linear heating rates. Using an appropriate presentation, the maximum of the signal is seen to depend nearly linearly on n₀, which in many cases means nearly linear dependence on the dose, a property important for dosimetry and dating.     

Bleaching of the post-IR IRSL signal from individual grains of K-feldspar: Implications for single-grain dating

Post-IR IRSL (pIRIR) signals from K-feldspar grains measured at elevated temperatures are increasingly being used for dating sediments. Unfortunately the pIRIR signal from K-feldspars bleaches more slowly than other signals (e.g. OSL from quartz) upon exposure to daylight, leading to concerns about residual signals remaining at deposition. However, earlier studies have not assessed whether the pIRIR signal bleaches at the same rate in all feldspar grains. In this study laboratory bleaching experiments have been conducted and for the first time the results show that the rate at which the pIRIR signal from individual K-feldspar grains bleach varies. To determine whether grain-to-grain variability in bleaching rate has a dominant control on equivalent dose (D_e) distributions determined using single grains, analysis was undertaken on three samples with independent age control from different depositional environments (two aeolian and one glaciofluvial). The D_e value determined from each grain was compared with the rate at which the pIRIR₂₂₅ signal from the grain bleaches. The bleaching rate of each grain was assessed by giving a 52 Gy dose and measuring the residual D_e after bleaching for an hour in a solar simulator. There is no clear relationship between the rate at which the pIRIR₂₂₅ signal of an individual grain bleaches and the magnitude of its D_e. It is concluded that variability in the bleaching rate of the pIRIR₂₂₅ signal from one grain to another does not appear to be a dominant control on single grain D_e distributions.