Thermoluminescence Of CaSO4 Research Articles

Studies of blue light induced phototransferred thermoluminescence in CaSO4:Mg

In this work, phototransferred thermoluminescence (PTTL) was observed in polycrystalline powder samples of Mg-doped CaSO4 (CaSO4:Mg) which were produced via the precipitation technique. The thermoluminescence (TL) glow curve showed the two separate main peaks. Composition of the peaks was at 90, 145, 180 and 235 °C and on the high temperature side, at 355, 410 and 475 °C with a 5 °C/s heating rate after delivering 100 Gy beta dose. The PTTL induced in CaSO4:Mg using blue light (470 nm) was reported and some PTTL characteristics were investigated. The PTTL intensity as a function of duration and temperature of illumination was recorded. As a function of increasing beta dose, a linear dose–response was obtained for the sample at 100 °C within the dose range from 1 to 150 Gy. The decaying of the PTTL signals of the previously irradiated sample was examined through successive blue light stimulation cycles. PTTL was used to determine dark fading of the material which was found stable with 2% loss of charge in the deep traps accessed by the PTTL measurement procedure. The thermal activation energy of the source traps involved in the PTTL was evaluated as 1.90 eV using the pulse annealing technique. Although these results showed that CaSO4:Mg presented a good PTTL performance and the blue light-induced PTTL signal can be used for dosimetry, it needs further study to understand the physical mechanisms associated with the transfer of charge from the deep traps probed by the PTTL measurements.

Effect of rare earth ions on TL of CaSO4:Dy, P phosphor

Effect of different rare earths co-activated in CaSO4:Dy, P phosphor prepared by acid distillation method are reported in this paper. For characterization of phosphors scanning electron microscopy (SEM), photoluminescence (PL) and thermoluminescence (TL) spectra were carried out. The SEM characterization shows that the surface morophology resulted after preparation has rod type structures with particle size in micron range. Under photoluminescence study the incorporation of different co-dopants with their characteristic photoluminescence spectra in CaSO4:Dy, P host is confirmed. Thermoluminescence (TL) property of as prepared phosphors is comparatively good though certain variations have been observed in case of some phosphors. The TL intensity of phosphors prepared and reported in this paper is nearly 1.11 times greater than the standard CaSO4:Dy TLD phosphor.

Defect centres and thermoluminescence in CaSO4:Dy,Ag phosphor

The defect centres formed in the TL phosphor CaSO4:Dy,Ag are studied using the technique of Electron Spin Resonance. The Ag co-doped phosphor exhibits three glow peaks around 130, 220 and 375 degrees C in contrast with the two glow peaks observed in the CaSO4:Dy phosphor at 130 and 220 degrees C at a gamma ray dose of 1Gy. ESR studies show that the additional peak at 375 degrees C correlates with a Ag2+ centre formed due to gamma irradiation and observable only below -170 degrees C. The Ag2+ centre is characterised by an axial g-tensor with principal values g(parallel) = 2.38 and g(perpendicular) = 2.41. ESR studies further indicate that the precursor to a centre observable at low temperature (-170 degrees C) appears to act as the recombination centre for the TL peak at 375 degrees C; this radical is characterised by the g-values g(parallel) = 2.0023 and g(perpendicular) = 2.0038 and is assigned to SO3- radical. It is observed that there is more incorporation of Ag in the CaSO4:Dy system as compared with that in pure CaSO4 system.

Studies of UV induced phototransferred thermoluminescence in CaSO4:Dy pellets.

The ultraviolet (UV) radiation response of the CaSO4:Dy pellets produced at IPEN/SP was studied using phototransferred thermoluminescence (PTTL). The PTTL characteristics of three different types of CaSO4:Dy pellets were investigated. The main parameters studied were: thermal treatments, post UV exposure storage time, PTTL response as a function of gamma dose, exposure time, radiant energy response and lower detection limits.

High Dose and Phototransferred Thermoluminescence in CaSO4, CaSO4:Dy, and CaSO4:Tm

An investigation is presented to test the feasibility of extending the usefulness of the CaSO 4 phosphor doped with Tm or Dy into the high dose/high temperature regime. The doses studied ranged from 1 kGy to 100 kGy obtained from γ ray and flash electron beam sources. Intregrated glow curve signals in the 'dosimetry range' (∼ 100-350°C) and the high temperature range (∼ 350-600°C) are measured as a function of dose. Integrated UV phototransferred thermoluminescence signals (∼ 100-300°C) are similarly considered. For powdered samples doped to 0.1 mol.%, the onset of saturation occurs near 10 kGy. This establishes that thermoluminescence dosimetry for industrial food processing (pasteurisation, disinfestation, radicidation and radurisation) is a possibility.

Role of Dy in the Thermoluminescence of CaSO4:Dy

Abstract Building on recent models of the thermoluminescence (TL) process in CaSO4:Dy, work is undertaken to delineate more specifically the role of the Dy impurity in the TL process. Thermally stimulated currents observations show that the recombination mechanism for peak 3 is bimolecular while that for peak 1 is monomolecular. TL results clearly show that the absence of Dy removes the dosimetric peak 3 near 220 oC in this material. Apparently, only Dy impurity develops the appropriate SO4 --VCa centres needed for observation of these peaks. The peak-3-related centres are shown to be quite different in behaviour from that of the structurally similar peak-1 - related SO4 --VCa centres. These results emphasise the importance of Dy impurities in stabilising the TL process needed to observe the dosimetric peak in this system.

Role of Dy in the Thermoluminescence of CaSO4:Dy

Building on recent models of the thermoluminescence (TL) process in CaSO4:Dy, work is undertaken to delineate more specifically the role of the Dy impurity in the TL process. Thermally stimulated currents observations show that the recombination mechanism for peak 3 is bimolecular while that for peak 1 is monomolecular. TL results clearly show that the absence of Dy removes the dosimetric peak 3 near 220 oC in this material. Apparently, only Dy impurity develops the appropriate SO4--VCa centres needed for observation of these peaks. The peak-3-related centres are shown to be quite different in behaviour from that of the structurally similar peak-1 - related SO4--VCa centres. These results emphasise the importance of Dy impurities in stabilising the TL process needed to observe the dosimetric peak in this system.

Dose dependence of photo-transfer thermoluminescence in CaSO4:Dy

Photo-transfer thermoluminescence (PTTL) has been found to be highly dependent on gamma ray dose. The PTTL and the reduction in residual thermoluminescence intensity ( Delta RTL) suggest that transfer originates not only from the residual thermoluminescent (RTL) traps but also from other radiation-induced nonluminescent centres.

Photo-stimulated luminescence and thermoluminescence in CaSO4: Dy

Photo-stimulated luminescence provides a method of radiation dosimetry in which repeated readouts from an irradiated sample are possible without erasing the initial dose information. It is found that in CaSO4:Dy, photo-stimulated luminescence as well as thermoluminescence processes are both originating from the same types of electron and hole traps produced by ionizing radiations. The holes released by photo-stimulation recombine at luminescent centres via shallow traps to cause phosphorescence. In thermoluminescence with increase in temperature the charge carriers are released and their recombination at luminescent centres results in instantaneous light emission. Photoillumination releases energy partially for short illumination periods from the phosphor grains and the readout output decreases with each readout in repeated readout measurements and this provides a method in which the same sample can be read again and again. Photostimulierte Lumineszenz liefert eine Methode der Strahlungsdosimetrie, bei der wiederholtes Auslesen einer bestrahlten Probe moglich ist, ohne das die Information der Ursprungsdose geloscht wird. Es wird gefunden, das in CaSO4,:Dy sowohl die photostimulierte Lumineszenz als auch die Thermolumineszenzprozesse von denselben Typen der Elektron- und Loch-Traps herruhren, die von ionisierenden Strahlungen erzeugt wurden. Die durch Photostimulierung befreiten Locher rekombinieren an Lumineszenzzentren uber flache Traps und erzeugen Phosphoreszenz. Bei der Thermolumineszenz werden mit steigender Temperatur Ladungstrager befreit und ihre Rekombination an Lumineszenzzentren ergibt eine instantane Lichtemission. Lichtbestrahlung macht teilweise Energie in kurzen Bestrahlungsperioden frei von den Phosphorkornern, und der Auslesungsausgangswert nimmt mit jedem Auslesevorgang bei wiederholten Auslesungsmessungen ab und dies liefert eine Methode, mit der dieselbe Probe immer wieder gelesen werden kann.

Thermoluminescence processes in CaSO4:Dy. Dependence on stopping power

Thermoluminescence of CaSO4:Dy has been investigated for radiations with widely different stopping powers. The lower stopping power radiations produce wide variations in the glow curve and a substantial increase in TL response. With increase in stopping power the extent of such variations reduces and the TL response shows a decrease in supralinearity. A physical model based on 'quasi-continuous' distribution of trap depths is proposed which qualitatively explains the stopping-power dependent TL behaviour of CaSO4:Dy.

Thermoluminescence of CaSO4doped with rare earths

The influence of rare-earth (RE) impurities as well as the host ions in the thermoluminescence (TL) of CaSO4 phosphors has been studied in detail by recording the TL glow curves, TL emission spectra and electron spin resonance (ESR) spectra of a number of gamma-irradiated CaSO4 samples doped individually with different elements of the lanthanide series. It is concluded that: (i) Dy and Tm are the most efficient activators of TL in CaSO4 and follow the well known concentration quenching law; (ii) radicals like SO4-, SO3-, O3-, etc., produced by gamma irradiation, form stable trapping sites of varying activation energies which are responsible for the multipeak TL glow curves exhibited by CaSO4 phosphors; (iii) the temperatures of occurrence of the various glow peaks remain remarkably constant irrespective of the RE dopant and are directly proportional to the activation energies of the respective traps; and (iv) the TL emission spectra correspond to the fluorescence emission of RE3+ ions. A possible model for the process of TL in CaSO4 (RE) is also discussed.