Phototransferred Thermoluminescence Research Articles

Ultraviolet Radiation Measurement with Neutron Pre-Irradiated Thermoluminescence Dosemeters

The characteristics of TL 6LiF:Mg,Ti (TLD-600) to be used as an ultraviolet (UV) radiation dosemeter based on the phototransfer thermoluminescence (PTTL) method are considered. Thermoluminescence dosemeters were pre-irradiated with D2O moderated 252Cf neutrons instead of gamma rays in order to increase the growth efficiency of residual thermoluminescence to be transferred by UV radiation. The optimal pre-annealing condition before UV irradiation was investigated. It depends on the pre-irradiation neutron fluence. Area variation of the glow curve was studied in detail and it is concluded that no new charge carriers were created in the PTTL process. The lower limit of detection for UV is dependent on the pre-irradiation neutron fluence and is about 10 to 103 J.m-2 for neutron fluences of 1013-1011 m-2. The upper limit may be at least 106 J.m-2.

Deep traps in highly sensitive α-Al 2O 3:C TLD crystals

The influence of deep traps on dosimetric characteristics of anion-defective α-Al 2O 3:C TLD 500 K detectors is studied. Optical absorption, thermoluminescent and phototransferred thermoluminescence responses of crystals are investigated after isochronous annealing in air at 450–1350°C. At least three main types of charge carrier traps exist which participate in the formation of the following dosimetric characteristics: (1) main dosimetric hole traps with a delocalization temperature of 190°C, (2) deep hole traps, annealed at 450–550°C, and (3) deep electron traps with delocalization temperatures of 800–900°C.

Application of Glow Curve Analysis Methods to Improve TLD-100 Dose Reassessment Performance

This paper presents a method of improving the TLD-100 dose reassessment performance. This method consists of applying numerical analysis techniques for evaluating the TLD-100 phototransferred thermoluminescence (PTTL) glow curve. From this analysis, a simple procedure for estimating the ultraviolet background components usually present in phototransferred thermoluminescence (TL) signals has been established. This procedure has been implemented in a computer program which performs the automatic evaluation of the glow curves and extracts the dose information contained in the PTTL curves. The use of this computer-aided evaluational method has enabled the extension of the working range of estimated absorbed dose down to 0.2-0.5 mGy with very adequate operational quality for doses even below the conventionally admitted lower reestimation limit (approximately 2 mGy). Because TL readout is a destructive process, the ability to reestimate doses can be important in any kind of dosimetric activity, such as operational dosimetry programs. The other commonly used dosimeter, film, uses a nondestructive readout and, therefore, presents some advantages over TLD when dose reassessment is necessary. With the reported improvements in the TLD-100 dose reassessment performance, the full range of absorbed doses covered by film dosimetry can now be reliably reassessed using TLD-100 dosimeters.

Ultraviolet-induced photo-transfer thermoluminescence of TLD-6LiF: Mg, Ti pre-exposed to neutrons

Abstract This paper deals with the characteristics of TLD-6LiF: Mg, Ti (TLD-600) to be used as an ultraviolet radiation dosimeter based on the photo-transfer thermoluminescence method. Thermoluminescent dosimeters were pre-irradiated with D2O-moderated 252Cf neutrons instead of gamma rays in order to increase the growth efficiency of residual thermoluminescence to be transferred to UV irradiation. Thermoluminescent parameters, trap depth (E), frequency factor ([sgrave]) and kinetic order for neutron-irradiated TLD were studied by both glow curve shape analysis and variable heating rate methods. For the 203°C peak, the E values are 1.36 and 1.31 eV and [sgrave] values are 1.5 × 1013 and 4.2 × 1012 s−1 respectively for these two methods. For the 263°C peak, the E values are 1.58 and 1.76 eV and [sgrave] values are 3.8 × 1013 and 2.1 × 1015 s−1 respectively. Kinetic orders of both peaks were close to unity. The parameters obtained for peak 3 are comparable with that obtained by other investigators. The metho...

Phototransferred thermoluminescence in quartz annealed at 1000°C

Three phototransferred thermoluminescence (PTTL) peaks appear at 100, 155, and 200°C from alpha and beta irradiated fine-grain samples of quartz annealed at 1000°C after first read-out up to 210°C and illumination with 340 nm light. Linear dose response of PTTL was measured up to 50 Gy of beta dose and up to 470 Gy of alpha dose. The efficiency of the phototransfer process depends on illumination time, is different after alpha and beta irradiations, and increases with quartz annealing temperature.

Isothermal Decay of Peak V in the PTTL of LiF(TLD-100)

Abstract Recent controversy about the order of kinetics of peak V in the TL of LiF TLD-100 centres on whether the departure from a single exponential behaviour of the 165 oC isothermal decay (ID) is due to the peak V decay, this peak not following first order kinetics, or to some remaining peak IV, not completely removed by the pretreatment employed. Results obtained for the isothermal decay of peak V detected in the TLD-100 photo-transferred thermoluminescence (PTTL) glow curve are presented. The PTTL curve is very convenient for this kind of measurement, as it is known that peak IV is not excited by UV light, so that the ID recorded at 165 oC can be unambiguously associated with peak V, free from possible interference coming from peak IV when the the direct TL emissions are employed for the ID measurements. The results obtained confirm the intrinsic complexity of the 165 oC decay found previously, showing the existence of a weak rapid initial stage. As in the case of the ID for TL, this initial stage is erased by adequate thermal treatment, producing the purely exponential decay typical of first order kinetics. The origin of the perturbing rapid initial component is discussed in the light of the differences found between ID recorded for TL or PTTL emissions.

Comparative PITL and PTTL Investigations on TL Detectors

The photo-induced intrinsic thermoluminescence (PITL) and phototransferred thermoluminescence (PTTL) characteristics of various detectors - TLD-100 (Harshaw), LiF:Mg,Ti (Poland), CaF2:Mn (Yugoslavia) and two types of Al2O3:Mg,Y (Hungary) - were examined to determine the possibility of their re-estimation. The re-estimation limit by PTTL was found to be 1.5 mGy, 2 mGy, 4 mGy and 12 mGy for CaF2, LiF(P), TLD-100 and Al2O3 (D-3), respectively. The PITL response of Al2O3:Mg,Y depends on the grain size of the polycrystal and on the oxidative/reductive character of the atmosphere during preparation.

Comparative PITL and PTTL Investigations on TL Detectors

Abstract The photo-induced intrinsic thermoluminescence (PITL) and phototransferred thermoluminescence (PTTL) characteristics of various detectors - TLD-100 (Harshaw), LiF:Mg,Ti (Poland), CaF2:Mn (Yugoslavia) and two types of Al2O3:Mg,Y (Hungary) - were examined to determine the possibility of their re-estimation. The re-estimation limit by PTTL was found to be 1.5 mGy, 2 mGy, 4 mGy and 12 mGy for CaF2, LiF(P), TLD-100 and Al2O3 (D-3), respectively. The PITL response of Al2O3:Mg,Y depends on the grain size of the polycrystal and on the oxidative/reductive character of the atmosphere during preparation.

Studies of PTTL and OSL in TLD-400

Abstract We present data on the mechanisms of phototransferred thermoluminescence (PTTL) and optically stimulated luminescence (OSL) in single crystal CaF2:Mn (3%) and in TLD-400 chips. The TL peak under study is the main dosimetry peak near 275 oC. From measurements of the excitation spectra for PTTL it is found that the maximum transfer rate occurs when the wavelength of the illuminating light is 280 nm. This corresponds to the wavelength of an absorption peak which remains in the sample following initial irradiation and heating to 400 oC. This wavelength also corresponds to a complex peak which is seen in the OSL excitation spectra of irradiated samples. Possible models based on optical stimulation of electrons and diffusion of excited F centres are discussed.

Ultraviolet sensitivity and photo-transferred thermoluminescence of the Harshaw and Panasonic used TLDs—a comparison

The intrinsic u.v. sensitivity and the photo-transferred thermoluminescence (PTTL) of three Harshaw (LiF:Mg, Ti, CaF 2:Dy and CaF 2:Tm) and two Panasonic (Li 2B 4O 7:Cu and CaSO 4:Tm) TL phosphors were evaluated. It was found that all the TLDs, except Li 2B 4O 7:Cu, have a low intrinsic u.v. sensitivity. The CaSO 4:Tm was found to possess PTTL, being one order of magnitude greater than the intrinsic u.v. and nearly independent of previous γ-irradiations ranging between 5 and 100 mgGy.

On the mechanism of phototransfer thermoluminescence in LiF TLD-100

LiF TLD-100 exposed to 25.8 C kg-1 and then annealed at 350 degrees C for 15 minutes contains defects responsible for the 225 nm optical absorption band and peak XII. Other defects have not been identified in this sample. Consequently, phototransfer thermoluminescence (PTTL) has been assumed to originate from peak XII. Results are presented that show that PTTL is not as simple a process as the transfer of charges from peak XII traps to the traps corresponding to peaks at lower temperatures. The mechanism is much more involved and the defects responsible for absorption at 225 nm play only a catalytic role in PTTL.

Phototransfer-thermoluminescence (PTTL) characteristics of mineral epidote

Abstract An investigation of phototransfer-thermoluminescence (PTTL) in epidote crystals has been carried out, using a broad-band u.v. source. The correlation between PTTL and possible anomalous fading of the high temperature (> 500°C) TL glow intensity has been studied by measuring the PTTL-induced low-temperature (> 500°C) peak height in samples which had been stored at room temperature for varying lengths of time following thermal drainage of TL from traps up to 500°C and before u.v. exposure. The results indicate that there is no noticeable fading of latent TL from deep traps for storage times of up to 3 months prior to the u.v. exposure. It may therefore be possible to employ the PTTL technique for purposes such as dating, instead of using the main TL peak (∼270°) in epidote, which is subject to anomalous fading.

Dose Re-assessment of LiF:Mg,Cu,P

An experimental investigation is described of the possibility of dose re-assessment by photo-transferred thermoluminescence in the recently developed material LiF:Mg,Cu,P. Using the same photo-transfer parameters of wavelength and irradiance of ultraviolet radiation as used routinely with LiF:Mg,Ti (TLD-100), the percentage re-assessed signals are 3 orders of magnitude lower with LiF:Mg,Cu,P than those obtained with TLD-100 for primary absorbed doses in the range 200 to 1500 mGy.

Effect of annealing on the phototransfer-thermoluminescence (PTTL) properties of epidote and sphene

Phototransfer-thermoluminescence (PTTL) properties of mineral epidote and sphene have been investigated. It is found that these properties are altered by thermal treatment of the crystals prior to UV exposure. Heating the epidote or sphene at 600°C for 1 h decreases the amount of UV bleaching and enhances the PTTL efficiency of the sample. This sensitization of PTTL by heat treatment is in line with the observed enhancement of TL sensitivity (i.e. TL output per unit test dose) by preannealing of the crystals for 1 h at the above temperature. These effects may be caused by the diffusion of defects or luminescence centres resulting in an enhanced probability of radiative transitions.

On the phototransferred thermoluminescence in MgB 4O 7:Dy

On the phototransferred thermoluminescence in MgB 4O 7:Dy

Multiple Dose Re-Assessment in Thermoluminescent Materials

Abstract Thermoluminescence dosemeters are widely used in a number of applications and the techique of phototransferred thermoluminescence by ultraviolet radiation at elevated temperatures is used for re-assessment of absorbed dose. Work has been done to investigate phototransfer processes in lithium fluorde, beryllium oxide, and calcium sulphate doped with dysprosium and with thulium and to gain information on the multiple re-assessment of absorbed dose. The latter enables high primary absorbed doses to be assessed within the normal sensitivity range of thermoluminescence detecting systems.

Thermoluminescence mechanism in LiF (TLD-100) from 90 to 300K

The thermoluminescence (TL) of LiF (TLD-100) has been studied over the temperature range 90 to 300K. A total of nine glow peaks are observed in this temperature interval. The most intense peak is the one at 137K. Besides this, three other peaks at 149, 254 and 275K are well resolved and their growth with exposure could be followed. No supralinearity could be found in the response of any of these peaks. Similarly, a sensitisation effect is also not observed for any of the glow peaks. Emission spectra were recorded at several temperatures and found to be changing. Up to about 150K, three bands at approximately 250, approximately 305 and approximately 420 nm were observed. The 250 nm band then gradually disappeared and the 420 nm band gained in prominence. At temperatures above about 200K, the emission spectrum contained only the 420 nm band. Phototransferred TL (PTTL) could be obtained in samples irradiated at 90K, warmed up to 300K and then subjected to 254 nm light. The PTTL glow curve showed the repopulation of only two peaks, namely at 149 and at 275K. The emission spectrum of the 149K PTTL peak consists of two bands at approximately 315 and approximately 440 nm and that of the 275K peak only one band at approximately 450 nm. These observations show that the simultaneous recombination of modified VK mobile holes at two different sites is indicated and their recombination should result in 305/315 nm and 420/440 nm emissions. Optical repopulation studies further show that the two strongest low-temperature peaks have a distinctly different behaviour and may be associated with holes.

Re-Estimation of Low Level Gamma Ray Doses Detected by Lithium Fluoride Thermoluminescence Dosemeters

An experimental method has been described for the re-estimation of low level gamma ray doses detected by standard Harshaw TLD cards, consisting of two extruded ribbon type LiF thermoluminescence dosemeters covered with thin Teflon foils. After the first readout at a temperature plateau of 300 oC for 10 s the dosemeters were exposed to 254 nm UV light for 25 min at a temperature of 110 oC and re-evaluated. A primary gamma ray dose as low as 2 mSv was re-assessed with an accuracy of ±25% from the second evaluation of the glow peak, resulting from the UV induced phototransferred thermoluminescence.

Evolution of TL and PTTL glow curves in LiF TLD-100

Reports studies of the variation in thermoluminescence (TL) glow curves with gamma exposure and in photo-transfer thermoluminescence (PTTL) glow curves after monochromatic UV exposure at 254 nm. It is shown that although the TL glow curves are better described by the recently proposed mobile-interstitial model, PTTL is not explained by this model. A revision of the mobile-interstitial model is suggested which will be useful in explaining PTTL. However, there are certain features of PTTL which at present cannot be explained satisfactorily by any of the existing models.

Using radiation damage to obtain a large quantity of matched-sensitivity thermoluminescent dosimeters.

We used radiation damage to reduce the sensitivity of thermoluminescent dosimeters (TLD) to match that of the working stock in our Dosimetry Laboratory. The mean sensitivity of the TLDs we altered fell within 1% of the stock mean and the distribution we obtained was much narrower than the +/- 10% limits on the working stock. By rejecting 1.5% of the test material, limits of +/- 5% could be established, thereby improving the precision of dosimetry measurements. The shapes of glow curves after irradiation remained unchanged. Our phototransferred thermoluminescence dose-reestimation procedure was still viable; however, the necessity for removing the TLDs' prior dose history became apparent, and we describe a procedure to handle this problem.