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PubMed2002

Characterisation of thermoluminescent LiF:Mg,Ti (MTS-N) detectors in the 85-550 K temperature range.

Miniature LiF:Mg,Ti (MTS-N) pellets, specially designed for dosimetry in proton radiotherapy, were studied in the 85-550 K temperature range. TL glow curves for differently Mg- or Ti-doped detectors were recorded, showing the preponderant influence of Mg concentration on the intensity of peak 5. The presence of a three-peak complex was also confirmed near 140 K; spectral emission of this peak consists of two bands whose maxima occur at 290 and 420 nm. TSEE glow curves above 300 K similar to those of TL were observed. The behaviour of TSEE and TL peak 5 following an X ray exposure from 33 to 500 Gy was studied: its intensity is a linear function of air kerma in both cases.

Fluorides↗
PubMed2002

Analysis of the glow curves obtained from LiF:Mg,Cu,Na,Si TL material using the general order kinetics model.

Three-dimensional thermoluminescence (TL) spectra based on temperature, wavelength and intensity for newly developed LiF:Mg,Cu,Na,Si TL material at the Korea Atomic Energy Research Institute (KAERI) were measured and analysed. The glow curves were obtained by integration of luminescence intensity over all wavelengths at each temperature, and various trapping parameters related to the traps were determined by analysing these curves. A computerised glow curve deconvolution (CGCD) method which was based on the general order kinetics (GOK) model was used for the glow curve analysis. The glow curves of LiF:Mg,Cu,Na,Si TL material were deconvoluted to six isolated glow curves which have peak temperatures at 333, 374, 426, 466, 483 and 516 K. The main glow peak of peak temperature at 466 K had activation energy of 2.06 eV and a kinetic order of 1.05. This TL material was also found to have three recombination centres, 1.80 eV, 2.88 eV and 3.27 eV by analysis of the TL spectra.

Copper↗
PubMed2002

Comparative study of trapping parameters and repeatability of LiF:Mg,Cu,P (GR-200A) from different production batches.

Distinct differences in repeatability between two different production batches had been observed. The results are presented of an investigation into the change of sensitivity of LiF:Mg,Cu,P originating from 7 different production batches prepared during 1994 to 2000 after repeated usage. Computerised glow curve analysis has been used to determine the trapping parameters of these thermoluminescent materials. The sensitivity in LiF:Mg,Cu,P for all investigated batches remains stable after repeated usage. The maxima of glow peaks 2 to 4 are found at the same temperature within very small limits. The activation energy and frequency of glow peaks 2 to 4 vary little from batch to batch. For all investigated peaks, no correlation has been found between glow sensitivity and trapping parameters. The peak areas of glow peaks 3 and 4 originating from 4 batches prepared during 1996 to 2000 are significantly larger than that of the other three batches. The GR-200A LiF:Mg,Cu,P has been improved dramatically in recent years.

Copper↗
PubMed2002

Influence of readout parameters on TL response, re-usability and residual signal in LiF:Mg,Cu,P.

It has recently been recommended that heating rates do not exceed 10 K.s(-1) and that the maximum temperature of readout should not exceed 265 degrees C for LiF:Mg,Cu,P. In some cases, a decrease of sensitivity in this material in the first of several re-use cycles had been reported. Influence of heating rates up to 30 K.s(-1), duration time up to 40s and maximum readout temperatures up to 270 degrees C on TL response, re-usability and residual signal was investigated. It was found that the maximum readout temperatures above 240 degrees C may lead to the thermoluminscent response decrease in the first several re-use cycles. The readout parameters can be optimised to minimise the residual signal (less than 0.4%) and to retain a constant sensitivity at the same time at high heating rates up to 30 K.s(-1) in a short time (less than 1 min per TL chip) without the necessity of heating above 240 degrees C. A concept of 'efficient residual signal' was put forward to quantify more accurately the real residual signal which affects the precision of the next measurement.

Copper↗
PubMed2002

New advances in LiF:Mg,Cu,P TLDs (GR-200A).

In order to volume-produce LiF:Mg,Cu,P TLDs, increase the repeatability of preparation technology and further improve the features of TLDs, especially the moisture resistant property and the accuracy in ultra-low dose, the prescription and preparation techniques have been investigated further in recent years. The main features of GR-200A have been improved dramatically by adding a chemical additive and adopting a particular preparation technology. Recent improvements are presented including prescription, preparation technology, TL sensitivity, signal-to-noise rate, detection threshold, residual signal, the moisture resistant property and the uncertainty in low dose measurements. The sensitivity of GR-200A has been increased to 65 times as high as TLD-100 from the original 29 times. The residual signal has been decreased to less than 1% from the original of about 2.5% following 240 degrees C readout. The detection threshold decreases and the signal-to-noise ratio increases. The uncertainty in low dose measurements is reduced significantly. The resistance to humidity has been improved remarkably. The preparation technology to volume-produce GR-200A is stable.

Copper↗
PubMed2002

Investigation of the emission spectra of LiF:Mg,Ti (TLD-100) during thermoluminescence.

The thermoluminescence (TL) of LiF:Mg,Ti has been studied using a high resolution CCD spectrophotometer, which simultaneously measures the emission intensity and the emission spectrum as a function of sample temperature. Computerised deconvolution was used to resolve the emission spectrum into individual Gaussian bands and the TL glow curve into glow peaks. Correction for spectral response and non-linear changes in dispersion were taken into account in the evaluation of the emission spectra. The emission spectra of all TL glow peaks following 200 Gy beta irradiation were found to consist of a common set of five emission bands at 2.49 eV, 2.64 eV, 2.84 eV, 3.1 eV and 3.59 eV, of which the 2.95 eV and 3.59 eV bands have not been previously observed at these dose levels. The band characteristics (E, deltaE) are not sample temperature or dose dependent indicating that the same LCs or the same single centre with several excited states are common to all the glow peaks. The relative intensities of the emission bands do. however, change as a function of readout temperature and dose, a behaviour that could be connected to a temperature and dose dependent perturbation between spatially coupled TCs and LCs.

Beta Particles↗
PubMed2002

A radioluminescence study of spectral and dose characteristics of common luminophors.

Many synthetic materials are used as thermoluminescence dosemeters for the measurement of the absorbed dose from ionising radiation sources. A part of the absorbed energy leads to a prompt luminescence (radioluminescence, abbreviated RL) which dose behaviour mainly corresponds with the densitity of charge carriers in the respective traps or recombination sites. The RL reported in this study was stimulated using two 137Cs sources with activities of 3.7 MBq (spectral measurements) or 5 MBq (dosimetry studies), respectively, and was recorded steadily during stimulation. This presentation gives a comprehensive survey of the spectral and dose dependent RL properties of a number of luminescent materials like LiF:Mg,Ti, Al2O3:C, CaSO4:Dy, CaF2:Mn, Li2B4O7:Mn, BeO and ZnS:Ag. The spectral and dose dependent results were compared with thermoluminescence as well as other RL studies.

Aluminum Oxide↗
PubMed2002

Main dosimetric characteristics of some tissue-equivalent TL detectors.

The aim of this work was to determine important dosimetric characteristics of several types of the most interesting tissue-equivalent thermoluminescent detectors (TLDs). Special attention was given to the determination of energy dependence for medium and low energy X rays. The following types of TLDs were investigated: (a) two new types based on lithium borate: Li2B4O7:Cu,In and Li,B4O7:Cu,In,Ag; (b) two types of the recently developed highly sensitive LiF:Mg,Cu,P material: TLD-700H and GR 200A and (c) two well known types of LiF:Mg,Ti detectors: TLD-100 and TLD-700. In order to determine their photon energy response characteristics, TLDs previously calibrated with 137Cs gamma rays were simultaneously irradiated with X ray beams in the range of effective energies between 33 and 116 keV. Measured energy responses (relative to air), normalised to those to 137Cs photons were compared with calculated data. Although the deviations of the measured data from the 'theoretical' predictions are different for all the investigated TLDs, there is no large difference in 'tissue-equivalency' between them.

Borates↗
PubMed2002

TL emission spectra from differently doped LiF:Mg detectors.

There are two widely applied types of thermoluminescent detectors based on LiF:Mg luminophor: Lif:Mg,Ti and highly sensitive LiF:Mg,Cu,P. The role of luminescence centres in these materials is usually attributed to defects connected with, respectively, titanium and phosphorus dopants. In order to check how composition of dopants introduced into the LiF lattice influences emission spectra, measurements on a series of variously doped LiF:Mg samples were performed. Apart from LiF:Mg,Cu,P and LiF:Mg,Ti detectors with different concentration of activators, an experimental sample being a kind of a 'hybrid' between both standard materials was also prepared. It was synthesised with concentrations of magnesium and copper identical to those used for LiF:Mg,Cu,P preparation. but instead of phosphorus it was doped with titanium (LiF:Mg,Cu,Ti). The measurements of the emission spectra were performed by using a liquid nitrogen cooled CCD 1024E detector with an SP150 spectrograph. During the measurements the samples were placed inside a cryostat in a vacuum. Resulting data were numerically deconvoluted for individual peaks with respect to the wavelength and the temperature. The glow curve shape of this material resembles that of LiF:Mg,Cu,P, while sensitivity is at the level of LiF:Mg,Ti. Preliminary results indicate that emission of the LiF:Mg,Cu,Ti sample is similar to that of LiF:Mg,Cu,P rather than to LiF:Mg,Ti, showing a maximum for wavelengths well below 400 nm.

Copper↗
PubMed2002

Optically stimulated luminescence of some thermoluminescent detectors as an indicator of absorbed radiation dose.

Stimulation spectra of several TLD materials in the short-wave spectral region are measured using the optically stimulated afterglow (OSA) method for determination of absorbed dose. Optical stimulation spectra are studied in the region of wavelengths lower than those of emission spectra. The effective optical stimulation hands have been found for examined materials in the regions of wavelengths which overlap with fluorescence excitation bands. Application of short-wave OSA bands for determination of absorbed dose is analysed.

Aluminum Oxide↗
PubMed2002

Thermoluminescence properties of LiF:Mg,Cu,Na,Si pellets in radiation dosimetry.

Sintered LiF:Mg,Cu,Na,Si thermoluminescence (TL) pellets have been developed for application in radiation dosimetry. LiF:M,Cu,Na,Si TL pellets were made from TL powders using a sintering process, that is, pressing and heat treatment. These pellets have a diameter of 4.5 mm, and a thickness of 0.8 mm are blue in colour and have a mass of 28 mg each. After 400 pellets had been produced they were irradiated with 137Cs gamma radiation and samples having a sensitivity within a +/-5% standard deviation were selected for experimental use. In the present study, the physical and dosimetric properties of LiF:Mg,Cu,Na,Si TL pellets were investigated for their emission spectrum, dose response, energy response and fading characteristics. Photon irradiation for the experiments was carried out using X ray beams and a 137Cs gamma source at the Korea Atomic Energy Research Institute (KAERI). The average energies and the dose were in the range of 20-662 keV and 10(-6) - 10(2) Gy respectively. The glow curves were measured with a manual type thermoluminescence dosimetry reader (system 310, Teledyne) at a constant nitrogen flux and a linear heating rate. For a constant heating rate of 5 degrees C.s(-1). the main dosimetric peak of the glow curve appeared at 234 degrees C, its activation energy was 2.34 eV and the frequency factor was 1.00 x 10(23). The TL emission spectrum appeared at the blue region centred at 410 nm. A linearity of photon dose response was maintained up to 100 Gy. The photon energy responses relative to the 137Cs response were within +/-20% in the overall photon energy region. No fading of the TL sensitivity of the pellets stored at room temperature was found over the course of a year. Therefore LiF:Mg,Cu,Na,Si TL pellets can be used for personal dosimetry, but more research is needed to improve the characteristics for repeated use.

Copper↗
PubMed2002

Investigation of radiation doses in open space using TLD detectors.

The low energy component of the cosmic radiation field is strongly modified by the shielding of the spacecraft and it is time and location dependent. Thermoluminescent lithium fluoride detectors have been applied to determine the radiation doses inside the ESA-Facility BIOPAN. The BIOPAN facility was mounted outside and launched on a Foton spacecraft and opened to space to allow exposure of several experiments to open space. Standard TLD-600. TLD-700 chips, two layers MTS-Ns sintered pellets with different effective thickness of the sensitive layer and MTS-N of different thickness have been exposed with different shielding thicknesses in front of them. The measured TL signal in the 0.1 mm thick detector just shielded by an aluminised Kapton foil of 25 microm thickness in front yielded a dose of 29.8 Gy (calibrated with 137Cs gamma rays) for an exposure time of 12.7 days: after 2.5 g.cm(-2) shielding the doses dropped to 3 mGy. The monitoring of radiation doses and its depth dose distribution outside the spacecraft are of great interest for radiation protection of astronauts working in open space. The knowledge of depth-dose distribution is a prerequisite to determine the organ doses an astronaut will receive during an extravehicular activity (EVA). The BIOPAN experiments are to be continued in the future.

Aerospace Medicine↗
PubMed2002

Relative thermoluminescence efficiency of TLD-600 and TLD-700 dosemeters irradiated with 59.8 MeV per nucleon krypton-86 ions.

A batch of LiF thermoluminescence dosemeters (TLDs), each containing five TLD-600 and TLD-700 thermoluminescence dosemeter chips, was irradiated with 59.85 MeV per nucleon 86Kr20+ ions from the K1200 superconducting cyclotron at the National Superconducting Cyclotron Laboratory (NSCL). Michigan State University, USA. The average linear energy transfer of the accelerated 86Kr ions and the resulting dose imparted to the TLD chips were calculated to be 3343 keV.microm(-1) per ion and 1.68 Gy respectively. A similar batch of TLD chips was irradiated with 1.3 MeV gamma rays from a 60Co source to 1.0 Gy. The TLD chips were evaluated at a ramp heating rate of 10 degrees C.s(-1) to 400 degrees C using a hot-finger type TLD reader. The thermoluminescence efficiency of the TLD-600 and TLD-700 dosemeters, relative to 60Co gamma rays was calculated to be 0.0025 and 0.0027 respectively

Fluorides↗
PubMed2002

Advantages of passive detectors for the determination of the cosmic ray induced neutron environment.

Due to the pronounced energy dependence of the neutron quality factor, accurate assessment of the biologically relevant dose requires knowledge of the spectral neutron fluence rate. Bonner sphere spectrometers (BSSs) are the only instruments which provide a sufficient response over practically the whole energy range of the cosmic ray induced neutron component. Measurements in a 62 MeV proton beam at Paul Scherrer Institute, Switzerland, and in the CERN-EU high-energy reference field led to the assumption that conventional active devices for the detection of thermal neutrons inside the BSS, e.g. 6Lil(Eu) scintillators, also respond to charged particles when used in high-energy mixed radiation fields. The effects of these particles cannot be suppressed by amplitude discrimination and are subsequently misinterpreted as neutron radiation. In contrast, paired TLD-600 and TLD-700 thermoluminescence dosemeters allow the determination of a net thermal neutron signal.

Aerospace Medicine↗
PubMed2002

Response of TL dosemeters to cosmic radiation on board passenger aircraft.

Measurements were performed with various LiF based TLDs on board seven Polish aircraft, flying long-distance or middle-distance routes. All of the 7LiF detectors used (various types of 7LiF:Mg,Ti and 7LiF:Mg,Cu,P detectors), which measure the non-neutron component of the radiation field, produced consistent results. It was found that the characteristics of the TLD response (ratio of different detector responses, glow curve shapes) after doses of radiation at flying altitudes differ from those obtained after exposure at the CERN facility (CERF), suggesting a lower contribution of densely ionising radiation. The neutron induced TL signal was also more affected by the thickness of the holder, suggesting the presence of a softer neutron energy spectrum at flight altitudes. Further in-flight and CERF exposures of detectors are planned to resolve these issues.

Aerospace Medicine↗
PubMed2002

On the quasi-equilibrium problem in thermally stimulated luminescence and conductivity.

Strong doubts havc been expressed about the validity of the quasi-equilibrium (QE) assumption used in the derivation of the analytical expressions of thermoluminescence (TL). So far there is no established method available to check if QE actually prevails during the emission of an experimental TL signal. The present study shows that the level of QE changes with a change in the heating rate beta. The change in the level of QE in its turn gets reflected in a change in peak shape when the system turns to a non-QE condition. This property is used as the first ever experimental method to test whether or not the emission of a given glow peak occurs under the QE condition. An essential condition for holding the QE condition is found to be T(R)/taum> or = 10(-3) where T(R) and taum are the glow peak recording duration and the maximum value of the free carrier lifetime, respectively. This relation between T(R) and taum is useful in finding the approximate value of taum. The value of taum being a function of the concentration and cross section of the TL related centres, one may be able to assess these basic parameters from the study of TL glow curves. The theoretical results are discussed in the perspective of LiF (TLD-100).

Fluorides↗
PubMed2002

On the correct measurement of relative heavy charged particles to gamma thermoluminescent efficiencies.

In order to better understand the most important experimental aspects for performing correct measurements of relative thermoluminescent (TL) efficiencies, an investigation has been carried out to quantify the effect of using different experimental procedures in the evaluation of 3 MeV proton-to-gamma relative efficiency (etap,gamma) of LiF:Mg,Ti. Variations in batch, presentation, annealing and reader have been studied. When the same protocol is used to measure proton and gamma TL response, efficiency values obtained range from 0.36 to 0.59 for peak 5 and from 0.44 to 0.79 for the total signal. The use of different annealings and different batches leads to 20% and 10% differences in etap,gamma respectively. Large differences (40%) are found between efficiency values measured with TLD-100 chips and those obtained using TLD-100 microcubes. When 'mixed' procedures are used to measure the proton and the gamma response, differences in etap,gamma may increase even more. The main conclusion of this work is to stress the importance of measuring an entire series of experiments in the same laboratory with a carefully defined protocol and using dosemeters from the same batch to obtain heavy charged particle TL response and gamma TL response with identical annealing and readout procedures.

Fluorides↗