M.T. Jose

Luminescence in CaSO4 : Dy phosphor - dependence on grain agglomeration, sintering temperature, sieving and washing

In the recently developed high-sensitive CaSO4 : Dy phosphor, sieving before the high-temperature sintering treatment has successfully eliminated particle agglomeration during subsequent sintering, and has further enhanced its thermostimulated luminescence (TSL) sensitivity to γ-rays. The reduction in TSL sensitivity of higher sized grains observed earlier following the procedure of sieving after sintering has also more or less vanished. Maximum TSL sensitivity is seen after sintering around 700°C, whereas maximum photoluminescent (PL) sensitivity is seen after sintering around 325°C. While the observed increase in TSL sensitivity (by 30%) with increasing sintering temperature in the range 325-700°C is explained on the basis of diffusion of Dy3+ ions from the surface to the whole volume of the grains (0-75 µm), the drastic decrease (by a factor of 3) in PL sensitivity with increasing sintering temperature is explained on the basis of change in the Dy3+ environment on the grain surface perhaps due to oxygen incorporation. Washing with water and acetone, which affect mainly the surface traps, enhances the PL sensitivity of CaSO4 : Dy slightly; however, it does not influence TSL sensitivity very significantly. Grinding reduces PL in general, but no such trend was noticed in TSL which supports the conclusion that PL originates mainly from surface traps since grinding affects mainly the grain surface. However, the sharp reduction in TSL and PL sensitivities observed at 400°C indicates that an unusual process takes place near that sintering temperature.

Role of sediment characteristics on natural radiation level of the Vaigai river sediment, Tamilnadu, India

The sediment characteristics such as granulometric contents (sand, silt and clay), organic matter, magnetic susceptibility (low and high frequency) and weight percentage of magnetic minerals and the natural radionuclide ((238)U, (232)Th and (40)K) contents have been analyzed for the sediment samples of Vaigai river with an aim of evaluating the radiation hazard nature and assessing characterization of sediment. Granulometric analysis confirmed that the sand is major content. The organic matter content is ranged from 0.30 to 8.62% and comparison shows that the present river has high organic content. The magnetic measurement results indicated that the sites S16, S18 and S25 may be affected anthropogenically. Frequency dependence magnetic susceptibility is calculated to know the contribution of grains to magnetic susceptibility. Average of activity concentrations (except (40)K) and all calculated radiological parameters are within the recommended level. Multivariate statistical analyses (Pearson correlation, cluster and factor analysis) dictated that the role of sediment characteristics on the level of radioactivity in the river sediments. The content of organic matter and clay, and magnetic parameters are positively correlated with important radioactive variables. The measurement of organic matter and magnetic susceptibility in various granulometric contents show some higher values in silt and clay fractions. Radioactivity level was also measured for the samples (after removing silt and clay fractions from bulk samples) and the results show decrease in radioactive variables. The present study stated that the lower grain sized fractions have the ability to absorb the contents such as organic content and magnetic minerals as cations on their surface and raise the level of radioactivity. Percentage of decrease in the natural radionuclides of (238)U, (232)Th, (40)K and absorbed dose rate of the samples (after removing the silt and clay fractions from bulk samples) to the bulk samples are 13.59, 33.37, 20.52 and 26.13% respectively. Thus, it is concluded that the radiation effect does not pose to the public those who are using these sediments.

SYNTHESIS AND CHARACTERISATION OF BaSO4:Eu THERMOLUMINESCENCE PHOSPHOR

Polycrystalline powder samples of BaSO(4) doped with Eu(2+) were prepared by solid-state reaction in different reducing atmospheres. Photoluminescence (PL), thermoluminescence (TL), TL kinetic and dosimetric studies have been carried out in this phosphor. The TL glow curve of BaSO(4):Eu(2+) showed only a single peak at 513 K unlike other phosphors and the TL intensity is about three to four times higher than that of CaSO(4):Dy, which is currently used as the radiation dosemeter for personnel monitoring in India. The TL dose response of the phosphor was found to be linear up to the dose range of 10(3) Gy beyond which saturation sets in. PL and TL spectra showed the characteristic emission of Eu(2+) ion. The TL parameters such as trap depth (E) or the energy required to release the electron or hole from the trap, frequency factor (s) and the order of kinetics (b) are determined by different methods such as isothermal decay, initial rise and variable heating rate.

Function of minerals in the natural radioactivity level of Vaigai River sediments, Tamilnadu, India – Spectroscopical approach

Using Gamma ray and Fourier Transform Infrared (FTIR) spectroscopic techniques, level of natural radioactivity ((238)U, (232)Th and (40)K) and mineralogical characterization of Vaigai River sediments have been analyzed with the view of evaluating the radiation risk and its relation to available minerals. Different radiological parameters are calculated to know the entire radiological characterization. The average of activity concentrations and all radiological parameters are lower than the recommended safety limit. However, some sites are having higher radioactivity values than the safety limit. From the FTIR spectroscopic technique, the minerals such as quartz, microcline feldspar, orthoclase feldspar, kaolinite, gibbsite, calcite, montmorillonite and organic carbon are identified and they are characterized. The extinction co-efficient values are calculated to know the relative distribution of major minerals such as quartz, microcline feldspar, orthoclase feldspar and kaolinite. The calculated values indicate that the amount of quartz is higher than orthoclase feldspar, microcline feldspar and much higher than kaolinite. Crystallinity index is calculated to know the crystalline nature of quartz and the result indicates that the presence of ordered crystalline quartz in the present sediment. The role of minerals in the level of radioactivity is assessed by multivariate statistical analysis (Pearsons correlation and Cluster analysis). The statistical analysis confirms that the clay mineral kaolinite is the major factor than other major minerals to induce the important radioactivity variables such as absorbed dose rate and concentrations of (232)Th and (238)U.

Soil radioactivity measurements and estimation of radon/thoron exhalation rate in soil samples from Kalpakkam residential complex

The objective of this study is to compute the primordial radionuclides activity in soil samples and estimate the radon/thoron exhalation rates. A total of 25 locations were chosen for the study at Kalpakkam. Ambient radiation levels were monitored prior to collection of samples, which were subjected to gamma spectrometry. While (238)U concentration was found to be below detectable limit, the activities of (232)Th and (40)K were varying from 34.53 to 1093.11 Bq kg(-1) and 36.6 to 570.08 Bq kg(-1), respectively. The radium equivalent activities (Raeq) were in the range of 83-1574 Bq kg(-1). There was no appreciable radon exhalation, and the thoron surface exhalation rate varied from 942 to 7720 Bq m(-2) h(-1). The annual effective dose was ranging from 0.05 to 0.81 mSv y(-1). Good correlation was observed between (232)Th content and thoron exhalation rate. The details of the study are presented in this article.

Thermoluminescence mechanism in rare-earth-doped magnesium tetra borate phosphors

Magnesium tetra borate (MTB) doped with rare earths (REs) was prepared by the solid state sintering technique. Among the different RE dopants studied in this phosphor, gadolinium-doped phosphors resulted in a dosimetric peak at a relatively higher temperature. The thermoluminescence (TL) emission spectra of RE-doped MTB showed characteristic RE 3+ emissions. Electron paramagnetic resonance measurements were carried out in these phosphors to identify the defect centers formed during gamma irradiation and to establish a mechanism for the TL process. Signals corresponding to (BO 3)2−, O v− were seen upon irradiation which vanished on annealing at 250 °C, showing the role of these centers in the TL process. The thermal activation energies calculated based on the decay of these signals matched well with those calculated on the basis of the usual conventional method showing the validity of the mechanism of TL.

Role of monovalent co-dopants on the PL emission properties of YAl3(BO3)4:Ce3+ phosphor

Abstract The cerium (Ce 3+ ) doped yttrium aluminium borate (YAB) phosphor was synthesized by modified solid state reaction. The phosphors phase purity and its emission properties were studied using powder X-ray diffraction pattern and photoluminescence spectroscopy. The synthesized YAB had rhomobohedral crystal structure. The phosphor had two different excitation and emission spectra. By 325 nm excitation, the phosphor had emission at 373 nm and with 363 nm excitation; the phosphor gave violet-blue emission at 418 nm. The UV emission of the phosphor originated due to Ce 3+ ions at the yttrium site and violet-blue emission owing to Ce 3+ ions at non-regular sites viz., Al 3+ and interstitial sites. The emission intensity of the phosphor was enhanced when monovalent ions (K + , Na + , and F − ) were added as co-dopants. The crucial role of ionic radii of monovalent co-dopants on the emission enhancement of the YAB:Ce 3+ phosphor was reported. Thermogravimetric study showed that the YAB possessed high thermal stability at up to 900 °C.

Influence of (group IIB) codopants on the thermostimulated luminescence sensitivity of CaSO4:Dy/Tm

Thermostimulated luminescence (TSL) phosphors based on CaSO4:Dy, Zn and CaSO4:Tm, Cd have been studied. These phosphors show an increase in the 220 °C TSL peak intensity by a factor of about 50% compared to CaSO4:Dy, as there is no change in the position of the peak temperature. Their dose response characteristics to 60Co gamma radiation is similar to that of CaSO4:Dy. However, there are very minor changes in their photoluminescence intensity and excitation spectra compared with those of CaSO4:Dy.

Charge compensation mechanism in X-ray image intensifying phosphor BaFBr:Eu

Abstract Photoluminescence and diffuse reflectance spectral studies reveal that the commercially available X-ray image intensifying phosphor BaFBr:Eu contains both Eu 2+ and Eu 3+ in contrast to the X-ray image storage phosphor BaFBr:Eu which contains only Eu 2+ . In the former, photostimulated luminscence (PSL) at room temperature (RT) and thermostimulated luminescence (TSL) above RT are quenched. The latter, however, exhibits intense PSL and TSL. The results are explained on the basis that the presence of Eu 3+ removes the anion vacancies created by the incorporation of ubiquitous oxygen impurity at fluorine ion sites.

Radiation-induced defects in manganese-doped lithium tetraborate phosphor

Lithium tetraborate doped with manganese synthesised by solid-state sintering technique exhibits a dosimetric peak at 280°C. The high-temperature glow curve results in no fading for three months. The sensitivity of Li2B4O7:Mn is determined to be 0.9 times that of TLD-100. The infrared spectrum of this phosphor indicates the presence of bond vibrations corresponding to BO4 tetrahedral and BO3 triangles. The mechanism for thermoluminescence in this phosphor was proposed based on the thermoluminescence (TL) emission spectra, kinetic analysis of TL glow curves and electron paramagnetic resonance (EPR) measurements on non-irradiated and gamma-irradiated phosphors. It was identified that oxygen vacancies and Boron oxygen hole centre (BOHC) are the electron and hole trap centres for TL in this phosphor. When the phosphor is heated, the electrons are released from the electron trap and recombine with the trapped holes. The excitation energy during the recombination is transferred to the nearby Mn(2+) ions, which emit light at 580 nm.