Absorbed dose measurements of a handheld 50 kVP X-ray source in water with thermoluminescence dosemeters

Absorbed dose rate measurements of a 50 kV(p) handheld X-ray probe source in a water phantom are described. The X-ray generator is capable of currents of up to 40 microA, and is designed for cranial brachytherapy and intraoperative applications with applicators. The measurements were performed in a computer-controlled water phantom in which both the source and the detectors are mounted. Two different LiF thermoluminescence dosemeter (TLD) phosphors were employed for the measurements, MTS-N (LiF:Mg,Ti) and MCP-N (LiF:Mg,Cu,P). Two small ionisation chambers (0.02 and 0.0053 cm(3)) were also employed. The TLDs and chambers were positioned in watertight mounts made of water-equivalent plastic. The chambers were calibrated in terms of air-kerma rate, and conventional protocols were used to convert the measurements to absorbed dose rate. The TLDs were calibrated at National Institute of Standards and Technology (NIST) in terms of absorbed dose rate using a (60)Co teletherapy beam and narrow-spectrum X-ray beams. For the latter, absorbed dose was inferred from air-kerma rate using calculated air-kerma-to-dose conversion factors. The reference points of the various detectors were taken as the center of the TLD volumes and the entrance windows of the ionisation chambers. Measurements were made at distances of 3-45 mm from the detector reference point to the source center. In addition, energy dependence of response measurements of the TLDs used was made using NIST reference narrow spectrum X-ray beams. Measurement results showed reasonable agreement in absorbed dose rate determined from the energy dependence corrected TLD readings and from the ionisation chambers. Volume averaging effects of the TLDs at very close distances to the source were also evident.     

Miniature thermoluminescent detectors for dosimetry in radiotherapy

At the Institute of Nuclear Physics in Kraków (INP), in collaboration with the Centre of Oncology in Kraków, several types of miniature thermoluminescent LiF:Mg,Ti and LiF:Mg,Cu,P detectors specially designed for clinical dosimetry in radiotherapy have been developed. The detectors are manufactured in the form of solid pellets of diameter down to 1 mm and typical thickness 0.5 mm, in the form of rods with a diameter of 0.5 mm and a length of a few mm, and as two-layer detectors with a thin (in the range of 0.065 mm) active layer of high-sensitive LiF:Mg,Cu,P. All three types of newly developed detectors have already been applied in proton beam dosimetry, surface dosimetry of eye-plaque brachytherapy applicators, phantom dosimetry for vascular brachytherapy and in vivo dosimetry in interstitial brachytherapy. These detectors were found to be very useful for dose measurements in high dose gradients, where spatial resolution better than 1 mm is required.     

The use of enriched 6Li and 7Li Lif:Mg,Cu,P glass-rod thermoluminescent dosemeters for linear accelerator out-of-field radiation dose measurements

(6)LiF:Mg,Cu,P and (7)LiF:Mg,Cu,P glass-rod thermoluminescent dosemeters (TLDs) were used for measurements of out-of-field photon and neutron doses produced by Varian iX linear accelerator. Both TLDs were calibrated using 18-MV X-ray beam to investigate their dose-response sensitivity and linearity. CR-39 etch-track detectors (Luxel+, Landauer) were employed to provide neutron dose data to calibrate (6)LiF:Mg,Cu,P TLDs at various distances from the isocentre. With cadmium filters employed, slow neutrons (<0.5 eV) were distinguished from fast neutrons. The average in-air photon dose equivalents per monitor unit (MU) ranged from 1.5±0.4 to 215.5±94.6 μSv at 100 and 15 cm from the isocentre, respectively. Based on the cross-calibration factors obtained with CR-39 etch-track detectors, the average in-air fast neutron dose equivalents per MU range from 10.6±3.8 to 59.1±49.9 μSv at 100 and 15 cm from the isocentre, respectively. Contribution of thermal neutrons to total neutron dose equivalent was small: 3.1±7.2 μSv per MU at 15 cm from the isocentre.     

Comparative study of LiF:Mg,Cu,Na,Si and Li2B4O7:Cu,Ag,P TL detectors

Recently, two new types of 'tissue equivalent' thermoluminescent detectors (TLDs) have aroused attention: LiF:Mg,Cu,Na,Si and Li2B4O7:Cu,Ag,P. In this work the characteristics of both detectors were compared with the characteristics of the well-known type LiF:Mg,Ti detector, TLD-100. The following properties were investigated: the glow curve structures, relative sensitivity, batch homogeneity and uniformity, detection threshold, reproducibility of the response, linearity in the wide dose range and fading. Also, the energy dependence for medium and low energy X rays was determined in the range of mean energies between 33 and 116 keV. The results confirmed 'tissue equivalency' of both new types in the investigated range of photon energies. LiF:Mg,Cu,Na,Si detector has very high sensitivity (approximately 75 times higher than that of TLD-100) and is convenient for use in a very low range of doses. Li2B4O7:Cu,Ag,P detector shows some improvements in comparison with the previously prepared types of lithium borate. The most important is the five times higher sensitivity than that of TLD-100. This detector is also very promising, especially in medical dosimetry.     

Energy dependence of new thermoluminescent detectors in terms of HP(10) values

The aim of this work was to determine energy dependence characteristics in terms of values of the personal dose equivalent, HP(10). The following types of thermoluminescent detectors (TLDs) were investigated: (a) two new types based on lithium borate, Li2B4O7:Cu,In and Li2B4O7:Cu,In,Ag; (b) two types based on the highly sensitive material LiF:Mg,Cu,P, TLD-700H and GR 200A; (c) two well-known types of LiF:Mg,Ti detector, TLD-100 and TLD-700 and (d) highly sensitive Al2O3:C detectors. TLDs previously calibrated with 137Cs gamma rays were simultaneously irradiated with X ray beams in the range of mean energies between 33 and 116 keV. The irradiations were performed with detectors in polymethyl methacrylate (PMMA) holders placed on a 30 cm x 30 cm x 15 cm water phantom with PMMA walls (ISO phantom). Measured energy responses were compared with calculated data for HP(10) values. The results confirmed the satisfactory tissue equivalent characteristics of all investigated TLDs except Al2O3:C, which (due to its large energy dependence) is suitable for personal dosimetry only with an appropriate filter.     

Evaluation of the performance of two LiF:Mg,Ti and LiF:Mg,Cu,P dosemeters for extremity monitoring

In this paper, the results aimed at assessing the performance of two varieties of LiF detectors (LiF:Mg,Ti and LiF:Mg,Cu,P) in photon fields relatively to reproducibility, detection threshold and angular dependence as defined in the ISO 12794 standard are presented. The fading properties and the limit of detection were also investigated for both materials. The results suggest that both LiF varieties are well suited for extremity monitoring. However, better fading properties of LiF:Mg,Cu,P when compared with LiF:Mg,Ti, combined with previous results relatively to energy dependence suggests that LiF:Mg,Cu,P dosemeters are better suited for extremity monitoring.     

Energy response of LiF and Mg2SiO4 TLDs to 10-150 keV monoenergetic photons

Energy response of LiF:Mg,Ti, LiF:Mg,Cu,P and Mg2SiO4:Tb thermoluminescence dosemeters (TLDs) was measured in the range 10-150 keV for monoenergetic photons at SPring-8 of an 8-GeV synchrotron radiation facility. The photon beam was monitored by a parallel-plate free-air ionisation chamber calibrated with an uncertainty of 3%. Owing to the small dimension of the beam, a rotating holder was designed in order to irradiate TLDs uniformly. The measured responses of LiF to energy were approximately in agreement with the calculated dose absorption dependence in the soft tissue. However, two types of LiF TLDs presented the different luminescent responses to the photon energy. The response of LiF:Mg,Ti had a smooth curve, and that of LiF:Mg,Cu,P presented a local maximum at 30 keV and a local minimum at 100 keV. The Mg2SiO4:Tb response was nearly bone equivalent. Linearity of dose responses was also confirmed up to 2 Gy on each TL material.     

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.     

Dosimetry of densely ionising radiation with three LiF phosphors for space applications

A new method of thermoluminescence dosimetry of densely ionising radiation based on the ratio of different efficiency-LET functions of three thermoluminescent detectors (TLDs) has been developed. The applied TLD types are: MTS-7 ((7)LiF:Mg,Ti), MCP-7 ((7)LiF:Mg,Cu,P) and MTT-7 (a newly developed (7)LiF:Mg,Ti with modified activator composition and increased response to high-LET radiation). The tests of this method, performed with high-energy ion beams at the HIMAC accelerator within the ICCHIBAN project, proved that good agreement with the true dose values may be achieved even in very complicated mixed fields. The proposed method will be applied for analysis of several thousand TLDs used for the determination of organ doses in an anthropomorphic phantom orbiting outside the International Space Station within the MATROSHKA experiment.     

A system for rapid large-area monitoring of gamma dose rates in the environment based on MCP-N (LiF:Mg,Cu,P) TL detectors

One lesson learned from the Chernobyl accident was that the spatial distribution of far-field contamination was strongly non-uniform due to local variation of atmospheric conditions, such as wind direction, rain etc. An environmental monitoring system using highly sensitive thermoluminescent LiF:Mg,Cu,P (MCP-N) detectors has been completed and field-tested. The system consists of 3000 MCP-N detectors in 1000 TLD cards (three TLDs per card), two Mikrolab automatic TL readers, heating ovens, and specially developed software which includes a database for rapid evaluation of results. The main dosimetric parameters of MCP-N dosemeters, such as thermally-induced fading, light sensitivity, minimum detectable dose, self-dose, zero-dose, energy response up to 6-7 MeV, influence of annealing and readout conditions on detector stability, have been tested. About 100 locations over an area of about 15,000 km2 in the south of Poland were selected for measurements lasting from 4 days to 3 months. The kerma rates measured over a 4 day screening period agree well with kerma rates determined over a 75 day monitoring period. Results from short- and long-term exposure periods agree well with those performed using MTS-N (LiF:Mg,Ti) over southern Poland in 1985, before the Chernobyl accident. Thus, using the system based on MCP-N detectors, one is able simultaneously to monitor environmental radiation kerma rates at a large number of locations over periods of four days or less. Provided natural background kerma rates at selected monitoring points are available prior to the accident, the system can be applied to assess kerma rates rapidly in the environment, following a nuclear accident.     

Dosimetry at the Portuguese research reactor using thermoluminescence measurements and Monte Carlo calculations

This work presents an extensive study on Monte Carlo radiation transport simulation and thermoluminescent (TL) dosimetry for characterising mixed radiation fields (neutrons and photons) occurring in nuclear reactors. The feasibility of these methods is investigated for radiation fields at various locations of the Portuguese Research Reactor (RPI). The performance of the approaches developed in this work is compared with dosimetric techniques already existing at RPI. The Monte Carlo MCNP-4C code was used for a detailed modelling of the reactor core, the fast neutron beam and the thermal column of RPI. Simulations using these models allow to reproduce the energy and spatial distributions of the neutron field very well (agreement better than 80%). In the case of the photon field, the agreement improves with decreasing intensity of the component related to fission and activation products. (7)LiF:Mg,Ti, (7)LiF:Mg,Cu,P and Al(2)O(3):Mg,Y TL detectors (TLDs) with low neutron sensitivity are able to determine photon dose and dose profiles with high spatial resolution. On the other hand, (nat)LiF:Mg,Ti TLDs with increased neutron sensitivity show a remarkable loss of sensitivity and a high supralinearity in high-intensity fields hampering their application at nuclear reactors.     

Thermoluminescent detectors applied in individual monitoring of radiation workers in Europe--a review based on the EURADOS questionnaire

Among the activities of EURADOS Working Group 2 formed by experts from several European countries is the harmonisation of individual monitoring as part of radiation protection of occupationally exposed persons. Here, we provide information about thermoluminescent detectors (TLDs) applied by the European dosimetric services and the dosimetric characteristics of dosemeters in which these detectors are applied. Among 91 services from 29 countries which responded to the EURADOS questionnaire, 61 apply dosemeters with TLDs for the determination of personal dose equivalent H(p)(10) for photons and beta radiation, and 16 services use TLDs for neutron albedo dosemeters. Those most frequently used are standard lithium fluoride TLDs (mainly TLD-100, TLD-700, Polish MTS-N and MTS-7, Russian DTG-4), high-sensitive lithium fluoride (GR-200, MCP-N) and lithium borate TLDs. Some services use calcium sulphate and calcium fluoride detectors. For neutron dosimetry, most services apply pairs of LiF:Mg,Ti TLDs with (6)Li and (7)Li. The characteristics (energy response) of individual dosemeters are mainly related to the energy response of the detectors and filters applied. The construction of filters in dosemeters applied for measurements of H(p)(10) and their energy response are also reviewed.     

Characterisation of MCP-600D and MCP-700D thermoluminescence detectors and their applicability for photoneutron detection

This paper presents the characteristics of two high-sensitive LiF:Mg,Cu,P thermoluminescence detectors (TLDs) named MCP-600D and MCP-700D [thermoluminescence detector (TLD) Poland]. Furthermore, the applicability of both detectors used as a paired system for photoneutron detection in a high-energy photon field at a linear accelerator is shown. For MCP-600D and MCP-700D, the batch homogeneity is within 22 and 14%, respectively (2 SD). Correction for the individual response of each TLD leads to a reproducibility of 5 and 4%, respectively Both TLD types reveal a linear detector response to dose up to 4 Gy. The energy dependence for both is within 2% for 4 and 6 MV photons. For a 15 MV photon beam, the MCP-600D shows a higher response (10%); compared with the MCP-700D (2%). The MCP-600D is capable of detecting extra doses due to photoneutrons in a 15 MV photon exposure; however, the signal for an open field of the used linear accelerator is in the order of the reproducibility. Using a kind of albedo technique allows detection of photoneutrons in the open photon field anyhow. The neutron detection limit is 10 microGy neutron dose per 1 Gy photon dose. Reproducibility of the TLDs, however, requires more than 10 detectors to determine results with an uncertainty of <5%.     

Comparison of the effects of exposure to light in Harshaw LiF:Mg,Ti and LiF:Mg,Cu,P

The response of thermoluminescence dosemeters (TLDs) to light, in various conditions, has been studied. TLD cards containing both conventional lithium fluoride (LiF:Mg,Ti) and the high-sensitivity material LiF:Mg,Cu,P were available, so permitting a comparison between the two types. Also available for the tests were Harshaw(TM) extremity EXT-RAD (LiF:Mg,Cu,P) dosemeters. The LiF:Mg,Ti body TLD cards and the EXT-RAD extremity dosemeters both showed some response to fluorescent light, while the LiF:Mg,Cu,P cards showed no significant response. It is therefore concluded that LiF:Mg,Cu,P body cards need no special precautions to protect them from the effects of light. For LiF:Mg,Ti cards and extremity dosemeters, effects are small, but steps to avoid excessive light exposure should be considered.     

A TLD-based personal dosemeter system for aircrew monitoring

Evaluation of LiF:Mg,Ti thermoluminescence dosemeters (TLDs) according to the high-temperature ratio (HTR) method enables the determination of the dose-average linear energy transfer (LET), the mean quality factor and the dose equivalent in mixed radiation fields of unknown composition. The neutron contribution is assessed by the Extended Pair method calibrated in the CERN-EU High-Energy Reference Field (CERF). The advantages of the small passive detectors as an easy-to-handle monitoring system for in-flight surveillance are demonstrated by measurements on-board north-bound and trans-equatorial flights. The experimental results are compared with calculations by the well-established CARI code.     

The right choice: extremity dosemeter for different radiation fields

Measurements of weakly penetrating radiation in personal dosimetry present problems in the design of suitable detectors and in the interpretation of their readings. For the measurement of the individual beta radiation dose, personal dosemeters for the fingers/tips are required. LiF:Mg,Cu,P is a promising thermoluminescent (TL) material which allows the production of thin detectors with sufficient sensitivity. Dosimetric properties of two different types of extremity dosemeters, designed to measure the personal dose equivalent Hp(0.07), have been compared: LiF:Mg,Ti (TLD100) and LiF:Mg,Cu,P (TLD700H). A type test for energy response for photon and beta radiation according to ISO 4037-3 and ISO-6980 was carried out and the results for both dosemeters were compared. Simultaneous measurements with both types of dosemeters were performed at workplaces, where radiopharmaceuticals containing different radioisotopes are prepared and applied. Practices in these fields are characterized by handling of high activities at very small distances between source and skin. The results from the comparison of the two-dosemeter types are presented and analysed with respect to different radiation fields. Experiments showed a satisfactory sensitivity for the thinner dosemeter (TLD 700H) for detecting beta radiation at protection levels and a good energy response.     

Determination of LiF:Mg,Ti and LiF:Mg,Cu,P TL efficiency for X-rays and their application to Monte Carlo simulations of dosemeter response

The simulation of response of a new passive area dosemeter for measuring ambient dose equivalent H*(10) for photons has been performed using the Monte Carlo code MCNP and experimentally determined responses of LiF:Mg,Ti and LiF:Mg,Cu,P thermoluminescent (TL) detectors for hard-filtered X-ray spectra from 20 to 300 keV and for 137Cs and 60Co gamma radiation. Relative TL efficiency for both types of detectors, determined in experiments with bare detectors and similar Monte Carlo simulations, compared favourably with prediction of microdosimetric models for proposed microdosimetric target sizes in the range of 20-40 nm. The concluding verification experiment showed small deviations between measured and simulated dosemeter energy response values in the range of a few percent.     

Thermoluminescence dosimetry of a thermal neutron field and comparison with Monte Carlo calculations

The characteristics of thermoluminescence dosemeters (TLDs) regarding the determination of photon and neutron absorbed doses were investigated in a thermal neutron beam. Harshaw TLD-100 (LiF:Mg,Ti) and TLD-700 (7LiF:Mg,Ti) were compared with similar materials from Solid Dosimetric Detector and Method Laboratory (People's Republic of China). Harshaw TLD-700H (7LiF:Mg,Cu,P) and aluminium oxide (Al2O3:Mg,Y) from Hungary were also considered for photon dose measurement. The neutron sensitivity of the investigated materials was measured and found to be consistent with values reported by other authors. A comparison was made between the TL dose measurements and results obtained via conventional methods. An agreement within 20% was obtained, which demonstrates the ability of TLD for measuring neutron and photon doses in a mixed field, using careful calibration procedures and determining the neutron sensitivity for the usage conditions.     

Study of real time temperature profiles in routine TLD read out--influences of detector thickness and heating rate on glow curve shape

This paper reports the results of a study using a commercial routine read out system with non-contact hot nitrogen heating and linear heating gas profiles. Glow curves of LiF:Mg,Ti as well as LiF:Mg,Cu,P were analysed for different linear heating rates beta from 1 to 30 degrees K s(-1). Different thermoluminescent detectors (TLDs) of different thicknesses (0.38-0.90 mm) were studied and compared. By means of the application of CGCD program considering kinetic parameters of the used TL-material the analysis of the peak temperature of the individual TL peaks lead to the approximation of the real heating profile T(chip)(t) in the TL chip. The real heating profile deviates strongly from linearity and can be characterised by the solution of a differential equation T(chip)(t) = F [T(gas)(t)]. The model of this equation is discussed in the paper. The difference between gas and chip temperatures are heating rate and chip thickness dependent and reach values of up to 100 degrees C (for thick detectors and fast heating rates). Especially for LiF:Cu,P, knowledge of the real chip temperature is essential, since read out shall be performed at the highest possible temperature, without destroying the dosimetric properties of the material. On the basis of this work, an optimisation of the readout parameters for LiF:Cu,P is possible.     

The implementation in routine of the ENEA new personal photon dosemeter

The ENEA photon dosemeter, introduced in 1995, consisting of two differently filtrated LiF(Mg,Cu,P) detectors, has been modified recently. The ABS (acrylonitrile butadiene styrene) plastic support has been replaced by a new aluminium card supporting the same two detectors (LiF(Mg,Cu,P) GR200). The new card, fully developed at the ENEA-Radiation Protection Institute (which is going to be patented), can now be processed through a Harshaw Model 6600 Automated TLD Reader, a hot gas reader. This paper reports the results of the individual calibration of approximately 60,000 LiF(Mg,Cu,P) GR200 detectors inserted on the new aluminium cards. Before the implementation in routine of the new cards, the reader has been characterised. Steps and tests to be made to use the card in routine (i.e. reader stability, linearity, reproducibility, etc.) are reported. The whole dosimetric system now combines the very good performances of the Harshaw Model 6600 reader and that of LiF(Mg,Cu,P) thermoluminescent material.