Using the OSL single-aliquot regenerative-dose protocol with quartz extracted from building materials in retrospective dosimetry

We report on the application of the single-aliquot regenerative-dose (SAR) protocol to the optically stimulated luminescence signal from quartz extracted from fired bricks and unfired mortar in retrospective dosimetry. The samples came from a radioactive materials storage facility, with ambient dose rates of about 0.1 mGy/h. A detailed dose-depth profile was analysed from one brick, and compared with dose records from area TL dosemeters. Small-aliquot dose-distributions were analysed from the mortar samples; one associated with the exposed brick, and one from a remote site exposed only to background radiation. We conclude that unfired materials have considerable potential in retrospective dosimetry.     

Review of optically stimulated luminescence (OSL) instrumental developments for retrospective dosimetry

This review describes 40 years of experience gained at Risø National Laboratory in the development of facilities for irradiation, thermal/optical stimulation and luminescence signal detection. These facilities have mainly been used in luminescence dating and nuclear accident dosimetry. We focus especially on methods for light stimulation and irradiation, and developments of new portable TL/OSL readers for determining doses directly in the field on both Earth and the planet Mars.     

Accident dosimetry using porcelain: TL analysis of low dose samples

Some of the bricks and tiles collected on previous occasions from the region of Chernobyl were found to be contaminated. In addition, in some regions there was a lack of quartz-bearing ceramic building materials. Porcelain fittings, however, are plentiful, both inside and outside dwellings and give a sensitive thermoluminescent signal when using the pre-dose technique. Four types of porcelain domestic fittings, with accumulated doses of less than 200 mGy, were collected from an apartment block in Pripyat which is being modelled and for which accurate TL measurements are required for comparison. The work has concentrated on establishing criteria for the selection of the samples with the best TL properties and on improving the precision of the pre-dose additive dose method for dose evaluation. Important factors for selection of samples in this dose range (<200 mGy) are the ratio S₀/S_n and the sensitivity. The image intensifier proved to be a useful tool for examining the spatial distribution of TL over the porcelain surface and identified one high S₀ sample where the signals from the unactivated and activated sample were emitted from different regions of the surface.     

Absorbed dose calculations for macromolecular crystals: improvements to RADDOSE

Radiation damage is an unwelcome and unavoidable aspect of macromolecular crystallography. In order to quantify the extent of X‐ray‐induced changes, knowledge of the dose (absorbed energy per unit mass) is necessary since it is the obvious metric against which to plot variables such as diffraction intensity loss and B factors. Significant improvements to the program RADDOSE for accurately calculating the dose absorbed by macromolecular crystals are presented here. Specifically, the probability of energy loss through the escape of fluorescent photons from de‐excitation of an atom following photoelectric absorption is now included. For lighter elements, both the probability of fluorescence and of its subsequent escape from the crystal are negligible, but for heavier atoms the chance of fluorescence becomes significant (e.g. 30% as opposed to Auger electron decay from a K‐shell excited iron atom), and this has the effect of reducing the absorbed dose. The effects of this phenomenon on dose calculations are presented for examples of crystals of an iron‐containing protein, 2‐selenomethionine proteins, a uranium derivatised protein, and for a nucleic acid sample. For instance, the inclusion of fluorescent escape results in up to a 27% decrease in the calculated absorbed dose for a typical selenomethionine protein crystal irradiated at the selenium K‐edge.     

The hydrothermal crystal growth of quartz: New developments

Abstract

α-quartz is an important piezoelectric material used in particular for its high frequency stability.

The increase of the power of high frequency oscillators implies α-quartz single crystal with a low concentration of defects.

Due to the low-temperature transition α → β quartz, the hydrothermal synthesis was selected to grow a-quartz single crystals.

On the basis of an analysis of the different factors able to induce physical and chemical defects in this lattice, new solvents and specific experimental (P, T) conditions have been investigated.     

Retrospective dosimetry using Japanese brick quartz: A way forward despite an unstable fast decaying OSL signal

Quartz extracted from heated bricks has been previously suggested for use in dose estimation in accident dosimetry, but this technique has never been applied before to Japanese quartz which often has unusual OSL characteristics. In this study the optically stimulated luminescence (OSL) characteristics of quartz extracted from a Japanese commercial red brick produced by Mishima – Renga – Seizoujyo Co. are studied. These companies are based in the Aichi Prefecture (capital Nagoya), which accounts for about half of the red brick production in Japan. A comparison of TL (thermoluminescence) and OSL signals has been carried out towards identification of common source traps. It is observed that OSL from Japanese brick quartz shows unusual luminescence characteristics; in particular, the initial fast decaying OSL signal contains a dominant (>90%) thermally unstable component related to the 85 °C TL peak, which necessitates a prior heat treatment. A single-aliquot regenerative-dose (SAR) protocol is developed and tested using thermal treatments intended to isolate a stable dosimetric signal. A minimum detection limit of ～65 mGy is then estimated using this protocol. Following irradiation using ⁶⁰Co and ¹³⁷Cs, dose–depth profiles were measured on two different commercial brick types (Mishima – Renga – Seizoujyo Co. and Hase – Renga Co.) with 5 Gy and 10 Gy surface doses. The profiles derived from the two sources were readily distinguishable. It is concluded that the OSL signals from the two types of Japanese brick quartz examined here can be used to derive precise estimates of accident dose, and, possibly to distinguish between sources of gamma radiation in a nuclear accident.To our knowledge, this is the first report on the existence of an unstable fast decaying OSL signal in quartz derived from bricks, and demonstrates a way forward with such samples in retrospective dosimetry.     

New luminescence measurement facilities in retrospective dosimetry

This paper gives a review of recent developments in luminescence measurement facilities on the Risø TL/OSL reader including radio-luminescence (RL), exo-electron and violet stimulation attachments, and a method for characterising and if necessary correcting for beta irradiation source non-uniformity.We first describe improvements to the existing RL option to allow near infra-red detection (NIR) during irradiation by the built-in ⁹⁰Sr/⁹⁰Y beta source. The RL optical signal is collected by a liquid light guide through an F34-901 interference filter and detection is based on a dedicated thermoelectrically cooled NIR sensitive PMT (detection window peak at 855 nm, FWHM 27 nm). Software and electronics have been modified to allow standard TL and OSL measurements in the same sequence as RL measurements. Together with a new bleaching source based on a high-power UV LED (395 nm; 700 mW/cm²), this facility has been used to measure natural doses in feldspar using the decaying NIR RL signal.Secondly, we present a method for mapping radiation field of the built-in ⁹⁰Sr/⁹⁰Y β-source and estimating grain-location specific dose-rates. This is important for the accuracy of single grain results, when radiation field is spatially non-uniform across the sample area. We document the effect of this correction method and further investigate on the effect of lifting the source to achieve a better dose-rate uniformity.Finally we summarise two recently-developed novel facilities to help investigate (i) the time scales involved in OSL processes (time-resolved exo-electron detection) and (ii) extending the age range (violet stimulated signals from deep quartz OSL traps).     

Determination of burial dose in incompletely bleached fluvial samples using single grains of quartz

We determine the burial dose in three known-age incompletely bleached fluvial samples using single grains of quartz. Estimation of burial dose in incompletely bleached samples requires that the characteristics of the well-bleached part of the distribution are known in order to distinguish between well-bleached and poorly bleached grains. It is especially important to investigate if the uncertainties assigned to individual estimates of dose adequately describe the observed variability in well-bleached dose distributions. We investigate this by quantifying the over-dispersion in laboratory-bleached and gamma-irradiated samples. We show that there is a linear relationship between the over-dispersion and the mean dose. Knowing this uncertainty relationship enables us to estimate the burial dose by comparing the predicted uncertainty on the running mean with that calculated from actual dispersion.     

Distinguishing dose populations in sediment mixtures: a test of single-grain optical dating procedures using mixtures of laboratory-dosed quartz

Many natural deposits contain grains that have different burial histories, but reliable procedures to extract the component doses from mixed-dose samples have not been developed in optical dating. Here we present results for synthetic two- and three-component mixtures of data derived from laboratory-dosed sedimentary quartz using a single-aliquot regenerative-dose protocol and statistical models for finite mixtures. Composite sets of data were created from the doses measured for individual grains that had been bleached by sunlight and then given a beta dose of 5, 10 or 20 Gy. We found that the correct number of dose components, corresponding doses, and relative proportions of each component may be estimated if the within-component dispersion is small or is known. We also found, however, that recuperation of the OSL signal in sun-bleached quartz may produce artificially high dose estimates for some grains, an effect that should be taken into account when modelling natural mixtures.     

Radiation-induced modifications of point defects in quartz crystals and their application in radiation dosimetry

Different types of impurity-related point defects in crystalline quartz are known to exhibit various spectroscopic signals. When exposed to ionizing radiations, these defect centers get modified and new species of point defects are formed. This paper presents a study and discussion of radiation-induced modification of point defects in natural as well as cultured quartz, with an emphasis to use this material for radiation dosimetry up to a range of a few Mrad.     

An application of artificial neural intelligence for personal dose assessment using a multi-area OSL dosimetry system

Significant advances have been made in recent years to improve measurement technology and performance of phosphor materials in the fields of optically stimulated luminescence (OSL) dosimetry. Pulsed and continuous wave OSL studies recently carried out on alpha-Al2O3:C have shown that the material seems to be the most promising for routine application of OSL for dosimetric purposes. The main objective of the study is to propose a new personal dosimetry system using alpha-Al2O3:C by taking advantage of its optical properties and energy dependencies. In the process of the study, a new dose assessment algorithm was developed using artificial neural networks in hopes of achieving a higher degree of accuracy and precision in personal OSL dosimetry system. The original hypothesis of this work is that the spectral information of X- and gamma-ray fields may be obtained by the analysis of the response of a multi-element system. In this study, a feedforward neural network using the error back-propagation method with Bayesian optimization was applied for the response unfolding procedure. The validation of the proposed algorithm was investigated by unfolding the 10 measured responses of alpha-Al2O3:C for arbitrarily mixed photon fields which range from 20 to 662 keV.     

Non-monotonic dose dependence of the Ge- and Ti-centres in quartz

The dose response of the Ge- and Ti-centres in quartz is studied over a large dose range. After an initial signal increase in the low dose range, both defects show a pronounced decrease in signal intensities for high doses. The model by Euler and Kahan [1987. Radiation effects and anelastic loss in germanium-doped quartz. Phys. Rev. B 35 (9), 4351–4359], in which the signal drop is explained by an enhanced trapping of holes at the electron trapping site, is critically discussed. A generalization of the model is then developed, following similar considerations by Lawless et al. [2005. A model for non-monotonic dose dependence of thermoluminescence (TL). J. Phys. Condens. Matter 17, 737–753], who explained a signal drop in TL by an enhanced recombination rate with electrons at the recombination centre. Finally, an alternative model for the signal decay is given, based on the competition between single and double electron capture at the electron trapping site. From the critical discussion of the different models it is concluded that the double electron capture mechanism is the most probable effect for the dose response.     

The use of ceramics for retrospective dosimetry in the Chernobyl exclusion zone

The use of luminescence techniques with ceramic materials is playing an increasingly important role in retrospective dosimetry. Thermoluminescence measurements with ceramics at Hiroshima and Nagasaki and in areas downwind of the Nevada Test Site have shown that dose estimates may be obtained which are of value in comparisons with the results of modelling calculations. The Chernobyl accident has provided a recent example where retrospective dosimetry is urgently required in order to advance epidemiological studies of the population. This paper examines some of the approaches which are being used with luminescence techniques to provide dose estimates for samplesfrom Pripyat and how they can be used to contribute to the wider problem of dose reconstruction in the 30 km Exclusion Zone.     

Thermoluminescence in two varieties of jadeite: Irradiation effects and application to high dose dosimetry

The thermoluminescence properties of white (WJ) and green (GJ) mineral jadeite have been investigated with a view to be of use in high dose dosimetry. WJ presented glow curve with 110, 190 and 235 °C peaks. All these peaks grow with radiation dose. The glow curve of GJ the green variety has TL peaks at 140, 210, 250 and 330 °C. We also observed that there is a difference between the TL glow curves for both samples, irradiated with gamma and electron. As expected the green jadeite can be used for measurement of dose as high as 50 kGy.     

Measurements of planar and depth dose distributions using a scintillating fiber-optic image sensor system for dosimetry in radiotherapeutic applications

We developed a scintillating fiber-optic image sensor system (SFISS) using square plastic optical fibers (POFs), a scintillating film, a right-angle mirror, and a complementary metal-oxide semiconductor (CMOS) image camera for dosimetry in radiotherapeutic applications. In this study, the scintillating light images were obtained for measuring two-dimensional planar dose distributions of a 6MV photon beam in a solid-water phantom. We also measured the percentage depth doses (PDDs) of 6 and 15MV photon beams using the SFISS and compared them with those obtained using conventional dosimetry films. The proposed sensor has many advantages, such as real-time readout, high-resolution measurement, and lack of corrections for temperature, pressure, and humidity. From the results of this study, it is expected that a SFISS can be developed to accurately measure the dose distribution in a small beam field for stereotactic radiosurgery (SRS).     

Absorbed dose estimations of 131I for critical organs using the GEANT4 Monte Carlo simulation code

The aim of this study is to compare the absorbed doses of critical organs of ¹³¹I using the MIRD (Medical Internal Radiation Dose) with the corresponding predictions made by GEANT4 simulations. S-values (mean absorbed dose rate per unit activity) and energy deposition per decay for critical organs of ¹³¹I for various ages, using standard cylindrical phantom comprising water and ICRP soft-tissue material, have also been estimated. In this study the effect of volume reduction of thyroid, during radiation therapy, on the calculation of absorbed dose is also being estimated using GEANT4. Photon specific energy deposition in the other organs of the neck, due to ¹³¹I decay in the thyroid organ, has also been estimated. The maximum relative difference of MIRD with the GEANT4 simulated results is 5.64% for an adult's critical organs of ¹³¹I. Excellent agreement was found between the results of water and ICRP soft tissue using the cylindrical model. S-values are tabulated for critical organs of ¹³¹I, using 1, 5, 10, 15 and 18 years (adults) individuals. S-values for a cylindrical thyroid of different sizes, having 3.07% relative differences of GEANT4 with Siegel & Stabin results. Comparison of the experimentally measured values at 0.5 and 1 m away from neck of the ionization chamber with GEANT4 based Monte Carlo simulations results show good agreement. This study shows that GEANT4 code is an important tool for the internal dosimetry calculations.     

Skin dose assessment in unmodulated and intensity-modulated radiation fields with film dosimetry

Superficial dose from 6- and 18-MV photon beams has been studied by measuring surface dose and shallow build-up dose using radiographic film EDR2, radiochromic film EBT2 and plane-parallel chamber. Measurements have been made for intensity- and non-intensity-modulated beams.The results show that the surface dose was found to be 19.8% and 10% of maximum dose in unmodulated fields for 6 and 18 MV photon beams, respectively. The study further showed that intensity modulation decreased surface dose by 1.1% and 0.7% for the same field size at 6 and 18 MV, respectively, and surface dose was dropped by magnetically sweeping contaminating electrons. EDR2 and EBT2 films show in good agreement in shallow build-up region.This study demonstrated the capability of EDR2 film, in addition to radiochromic film, to measure surface and build-up dose in case of treatment planning system uncertainties with regard to skin toxicity or shallow target coverage.     

Infrared (IR) stimulated luminescence from modern bricks in retrospective dosimetry applications

It has frequently been observed that certain roof tiles and bricks, especially from relatively modern European buildings, do not contain enough quartz grains in a suitable grain size range to permit dose reconstruction using thermoluminescence (TL) or optically stimulated luminescence (OSL) methods. In this paper the feasibility of using infrared-stimulated luminescence (IRSL) on the feldspar fraction of such bricks and tiles has been investigated. Appropriate preheating treatments were employed in order to select the most stable signals, and procedures were developed to enhance the signal to noise ratio. The possible effect of anomalous fading under application of these procedures was tested. In the dose range above 100 mGy, it has been demonstrated that using IRSL on the feldspar fraction of such material provides a feasible alternative to the use of green-light-stimulated luminescence (GLSL) on the quartz fraction, for the purposes of retrospective dosimetry. Furthermore, since the use of IRSL as described in this paper involves the measurement of polymineral fine grain fractions of bricks, a technique for the calibration of the built-in β source against the γ source in Secondary Standard Dosimetry facilities for routine use of the technique is described.