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Advantages and disadvantages of luminescence dosimetry
Radiation Measurements 45 (2010) 506–511
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Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Advantages and disadvantages of luminescence dosimetry Pawe1 Olko Institute of Nuclear Physics Polish Academy of Science (IFJ PAN), Krakow, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 August 2009 Received in revised form 17 December 2009 Accepted 6 January 2010
Owing to their excellent dosimetric properties, luminescence detectors of ionizing radiation are now extensively applied in individual dosimetry services. The most frequently used personal dosemeters are based on Optically Stimulated Luminescence (OSL), radiophotoluminescence (RPL) or thermoluminescence (TL). Luminescence detectors have also found several applications in clinical dosimetry, especially around new radiation modalities in radiotherapy, such as Intensity Modulated Radiotherapy (IMRT) or ion beam radiotherapy. Requirements of luminescence detectors applied in individual and clinical dosimetry and some recent developments in luminescence of detectors and techniques leading to significant improvements of the functionality and accuracy of dosimetry systems are reviewed and discussed. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: TLD OSL RPL Personal dosimetry Medical dosimetry
1. Introduction Despite the rapid development of active real-time electronic dosemeters, passive dosemeters which integrate absorbed dose over a period of time are still in demand and are frequently obligatory in radiation protection. Luminescence dosemeters are now gaining popularity in individual dosimetric services, replacing dosimetric films, which were commonly used in the past century (Olko et al., 2006). The principle of application of luminescence materials to the dosimetry of ionizing radiation relies on the relationship between the dose absorbed and the intensity of light emitted. Three groups of luminescence detectors are applied in personal dosimetry: thermoluminescence detectors (TLD), detectors based of Optically Stimulated Luminescence (OSL) and radiophotoluminescence (RPL) glasses. The major advantages of luminescence dosemeters are their high sensitivity, allowing doses as low as 1 mGy to be measured; their linear response with dose up to at least 1 Gy, their good energy response after X-ray irradiation (for materials with Zeff close to that of tissue), their reusability, and their resistance to high humidity and to high magnetic fields. The aim of this paper is to review and discuss the recent progress of luminescence detectors and techniques to meet the growing requirements of personal and medical dosimetry, excluding radiography. We wish also to indicate areas in which it will most likely be difficult to achieve further progress. One of the problems of
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luminescence dosimetry, particularly with respect to high-sensitive LiF:Mg,Cu,P and Al2O3:C, is the decrease in the response of these materials with increasing ionization density of the radiation field, which may lead to underestimation of dose after heavy charged particle irradiation. Since the dose response and ionization density (LET) response and photon energy response are interrelated, there are some basic limits in the development of a universal luminescence material able to evaluate dose over a very large dose range, simultaneously featuring flat LET/energy response over that range. 2. Requirements for luminescence dosemeters in personal dosimetry The requirements for personal dosemeters are to a large extent internationally standardized in IEC or ISO documents. For TL dosemeters these are IEC 62387-1 (2007), IEC 61066 (2006), ISO 12794 (2000)and for dosimetric films ISO 1757 (1996) standards. These standards, closely related to the type of the system, specify performance criteria, mainly for type testing of personal dosimetry systems, to harmonize tests of the system and reference radiations. Part 3 of IEC 62387, which is under preparation, will specify characteristics of other dosimetric systems, including OSL systems. The response of a personal dosimeter can be expressed as:
M ¼
N0 G rn rE;a renv
(1)
where N0 is the calibration factor, rn is the relative factor for nonlinear dose response, rE,a is the energy and angular response and renv is the relative response due to environmental exposures
P. Olko / Radiation Measurements 45 (2010) 506–511
(the influence of mechanical and electromagnetic disturbances are not taken into account here). In principle, rE,a and rn are not independent quantities for luminescence detectors because with growing ionization density the supralinearity/sublinearity of dose response decreases and the saturation dose increases (Olko et al., 2006). For routine radiation protection this effect is negligible but for doses above 1 Gy this must be accounted for. Energy and angular response are corrected by a single factor rE,a because attenuation of low-energy photons affects the angular response. Some of the requirements for TL personal dosemeters used for determining the value of HP(10) (IEC 62387-1, 2007) are listed in Table 1. The dose range starting with several mSv up to 1 Sv are easily measurable by all personal dosimetry systems based on luminescence detectors and any deviations from linearity, using a careful readout procedure, are typically no worse than a few percent without correction for dose response. Energy and angular correction rE,a, may vary up to 40% which leads to (1/1.4 ¼ 0.71 to þ 1/0.6 ¼ þ1.67) performance criteria for the response M of the personal dosemeter. The energy response of a luminescence dosemeter can be corrected using high-Z filters, typically Al, Sn or Cu, which attenuate the low-energy photon fluence and diminish the enhanced energy deposition in a high-Z luminescence detector. The remaining problem for high-Z luminophor is dosimetry of lowenergy mixed X-b fields, such as Pm-147 beta rays with an average energy of about 62 keV measured with low-energy photons. The filters cut-out the low-energy beta component which leads to underestimation of dose but a detector without such a filter produces a signal which overestimates the photon dose. Therefore, for low-energy mixed radiation fields tissue-equivalent detectors, the signal of which is roughly proportional to absorbed dose in tissue, are recommended. Among TL detectors these are lithium borate and lithium fluoride detectors. Altogether, in difficult environmental conditions, in mixed radiation fields, and over wide dose ranges, the relative difference between the true value and the readout of a personal dosemeter may exceed 100%. Current dose limits in radiation protection based on the Linear-No-Threshold hypothesis appear to overestimate the risk to the extent that such uncertainties in dosimetry are still acceptable.
3. Advantages of luminescence detectors in personal dosimetry Personal dosimetry remains the main application field of luminescence detectors. According to ESOREX data (Frasch and Petrova, 2007), in 2004 about 1 million workers in EU were routinely issued with personal dosemeters, most of them (about
Table 1 Selected requirements of personal dosemeters based on TLDs used for measurements of HP(10) and HP(0.07). Characteristics under test
Minimal measuring range
Performance requirement
Relative response due to the non-linearity Relative response due to mean photon energy and angular response (for HP (10)) Relative response due to mean beta radiation energy (for HP(10)) Relative response due to mean beta energy (for HP(0.07))
0.1 mSv < HP(10) < 1 Sv
9% to þ 11%
80 keV to 1.25 MeV and angels from 0 to 60
29% to þ67%
0.8 MeV
Indicated value maximum 10% of HP(0.07) 29% to þ67%
0.2 MeV to 0.8 MeV and angels from 0 to 60
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65%) in the medical sector. In a review of dosimetric services in Europe, performed by EURADOS over the years 2002–2004, of 91 participating services 61 used dosemeters based on thermoluminescence detectors, mainly LiF:Mg,Ti (TLD-100, TLD-700, MTS-N and MTS-7), LiF:Mg,Cu,P (GR-200, MCP-N) and lithium borate (Olko et al., 2006). After a few years experience in US, Luxel dosemeters based on Al2O3:C became also available in France, Ireland and the United Kingdom. In 2008, the French IRSN dosimetric service introduced RadioPhotoLuminescence (RPL) silver-doped glasses which replaced dosimetric films, used over several decades (IRSN, 2007). In Germany, it is planned to terminate personal dosimetry based on films by 2011. It seems unlikely that over the next decade active electronic dosemeters or other promising systems such as Direct Ion Storage dosemeters (Fuchs et al., 2008) would replace luminescence systems, mainly because of the low price, high accuracy and resistance to environmental conditions of luminescence detectors. What makes luminescence dosemeters so popular? Excellent ‘‘pro- and contra’’ arguments were exchanged in a dispute between McKeever and Moscovitch (2003) on advantages and disadvantages of Optically Stimulated Luminescence and thermoluminescence dosimetry. In Table 2 the main arguments of this dispute are listed. Over the last few years several additional features of luminescence detectors become apparent, which make them even more attractive for personal dosimetry: - The first personal neutron dosimeter based on OSL technology was developed and introduced into dosimetric services (Yukihara et al., 2008). In the albedo-type dosemeter, neutrons thermalized in the body are detected by a thin Al2O3:C detector covered with a thin layer of 6LiCO3 to register short-range products of the 6Li(n, 4He)3H reaction. Uncovered Al2O3:C shows low sensitivity to thermal neutrons and is used to determine the g-ray contribution. This type of neutron dosemeter demonstrates similar neutron energy response to classical TLD albedo dosemeters based on pairs of 6LiF/7LiF, suitable essentially for neutrons of energy below 1 MeV. - Direct measurement of energy deposited by fast neutrons in luminescence detectors is inefficient because detector interaction cross sections are quite different from those in tissue and the relative luminescence efficiency is relatively low for densely ionizing products of nuclear reactions or for recoil protons. Alternatively, the dose due to fast neutrons can be determined by counting tracks of charged recoil particles and assessing their LET. As early as in 1993, Lommler et al. (1993) developed a laser-scanned RPL glass system able to count
Table 2 Advantages of OSL and TL dosimetry – summary of debate between S. W. S. McKeever and Moscovitch (2003). Advantages of OSL dosemeters
Advantages of TL dosemeters
– – – –
– High sensitivity of new materials like LiF:Mg,Cu,P – Easy handling (no light sensitivity) – Simple TLD readout – Effective and flexible gas heating – Neutron dosimetry possible with Li-6 and Li-7 – TL glow curve used as a quality control – Individual calibration of dosemeters possible – Dose re-estimation possible (but not used) – Very flat photon energy response for LiF
– – – – – –
High luminescence efficiency Stable sensitivity High precision and accuracy Control of the luminescence emitted High speed of readout Identification of static exposure Multiple re-analyses possible Elimination of complex thermal annealing steps, Dose imaging possible Low-power consumption for portable readers
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individual recoil tracks of 5 MeV neutrons over fluences up to 107 n cm2 . Fluorescent nuclear track detectors (FNTDs) have recently been developed by Sykora et al. (2008) which allow one to image individual tracks of heavy charged particles, including their angle of incidence. The system is based on aluminium oxide crystals containing aggregate oxygen vacancies and doped with carbon and magnesium (Al2O3:C,Mg), imaged using laser scanning confocal fluorescence microscopy. The recent improvements of the system permit observation of even single electron tracks (Sykora and Akselrod, 2010). - An interesting method of increasing the detection efficiency of high-LET particles using standard LiF:Mg,Ti (TLD-100) and combined TL and OSL readout has been suggested by Oster et al. (2008). OSL is supposed to originate from F2 or F3 centres which demonstrate the properties of ‘two-hit’ centres, whereas the main dosimetric TL peak of LiF:Mg,Ti arises from a ‘one-hit’ complex defect. Therefore, OSL emission spectra consist of two broad visible bands at 640 nm and 520 nm, whereas main TL emission is observed at about 410–420 nm. For rather high doses (120–480 Gy) a 500-fold increase of relative response of b-rays to a-particles after OSL as compared to TL readouts was observed. This discovery potentially allows discrimination between low and high-ionization density radiation, important in mixed-field radiation dosimetry. However, any practical implications of these finding for dosimetry will be possible only when the effect is confirmed at much lower doses, particularly in the mGy region. - In personal dosimetry based on dosimetric films it is possible to identify cases of static exposure by the source (e.g. if a dosemeter is left by the source over a longer period) by eye recognition of the sharp edges of the filters. In 2000, Akselrod et al. (2000) presented a laser-scanned OSL system with a CCD camera, applied to read a thin layer of Al2O3 covered with a periodically punched 0.5 mm Cu filter. For static exposures with X-rays energies below 150 keV and doses exceeding a few mSv, a clearly distinguishable pattern of punched holes is visible on the CCD images. Budzanowski et al. (2006) applied a newly developed TLD reader with a CCD camera (Marczewska et al., 2006) to read LiF:Mg,Cu,P TL pellets irradiated under a 1 mm thick Pb filter, with a central hole of 1 mm diameter. The CCD image from the statically irradiated dosemeter showed a clear image of the hole, which is not visible for dynamic exposures. A quantitative identification, based on the analysis of the frequency distribution of image intensity, was proposed by Kopec et al. (2010). The method works efficiently for doses above 5 mSv and X-ray energies below 200 keV. After modification of one of the filters in the RADOS personal dosemeter, the system will be used routinely by the dosimetric service of IFJ PAN. - Nakajima and Hashizume (1969) proposed to determine the time delay between exposure and readout of a TL detector by taking into account the different decay rates of particular peaks in TL glow curve. Budzanowski et al. (1999) demonstrated that by deconvoluting the MCP-N (LiF:Mg,Cu,P) glow curve it is possible to determine the time elapsed since exposure up to about 40 days using the peak 3/peak 4 and peak 2/peak 4 ratios. Weinstein et al. (2003) draw similar conclusions, pointing out difficulties in using peak 2 in TLD-100 for this purpose. This feature is still not used in routine personal dosimetry but it could be applied in some special circumstances, e.g. to determine the time elapsed after a radiation accident. A serious disadvantage of historical TLD systems in personal dosimetry is the necessity of manual handling, annealing and readout. Several procedures are time-consuming e.g. annealing of
LiF:Mg,Ti takes 1 h 400 C and 2 h at 100 C (or sometimes 16 h at 80 C). However, in the modern TLD dosimetric systems such as Thermo, Panasonic or Dosacus, annealing, readout and data processing are fully automatic. This is especially true for the largescale, highly efficient Landauer system, with laser readout of Al2O3:C detectors, which takes only a few seconds per readout. That luminescence detectors are presently unsurpassed in personal dosimetry is due to their advantageous physical properties and to the continuous improvement of their readout techniques. 4. Requirements for luminescence dosemeters in radiotherapy Accuracy of dosimetry measurements is understood as the proximity of their expectation value of the measured quantity to the true value. Other than clinical recommendations, generally within 5%, there is no general recommendation for accuracy of dose delivery in radiotherapy but there is a continued trend to decrease the limits. Dosimetric accuracy at the level of 5%, which is believed today to represent a tolerance of deviation between the response of tumour and healthy tissue, is postulated in dosimetric protocols (TRS 398, 2000). The International Atomic Energy Agency performs regularly postal quality audits of radiotherapy beams using TLD dosemeters in the form of capsules filled with LiF:Mg,Ti powder. Results reported by hospitals are accepted which differ by less than 5% from the true value. For differences between 5% and 10% the checks are repeated but in the case of higher discrepancies additional measures are undertaken, including expert’s visits to the hospital. ICRU 24 (1976) recommended 2% but it was impossible and probably also not necessary to reach this goal. When developing new dosimetric systems, medical physicists postulate even better accuracies on the particular properties of the system, such as energy response, dose response, angular dependence etc. to assure that the combined extended uncertainty will be well bellow 5%. In Table 3 we presented a part of requirements on 2-D dosimetry systems, working in integration mode, developed within the FP7 MAESTRO project (Bucciolini et al., 2005). At the moment, no passive dosimetry system based on luminescence detectors is able to fulfil those requirements, particularly for two-dimensional (2-D) dosimetry. 5. Luminescence dosemeters in radiotherapy Historically, TLDs, particularly LiF:Mg,Ti detectors were frequently used in radiotherapy for relative dosimetry (Horowitz, 1984). Today, active dosemeters, based mainly on ionization chambers, silicon diodes, MOSFET detectors are almost exclusively used in clinics because they allow real-time readout of results, easy operation and minimum maintenance. Yet, intensive research is
Table 3 Requirement table for 2-D dosimetry systems, working in integration mode (Bucciolini et al, 2005). Parameter
Range
Requirement
Dose rate dependence: Short term precision Linearity vs. absorbed dose Background signal
0.01–4 Gy/min
<1% <0.5% <1% <0.1% of the radiation induced signal <1%
Energy dependence Angular dependence Reproducibility (different elements of 2-D matrix)
0.001–20 Gy
Photons 4–25 MV Protons 20–200 MeV
<1% <1%
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under way to introduce new dosimetric systems based on luminescence principles, not only in the traditional in-phantom measurements and mailed dosimetric service of radiotherapy beams but also for two-dimensional (2-D) TL dosimetry and promising solutions are being developed for on-line in-vivo dosimetry using OSL systems. 5.1. Postal dosimetric service of radiotherapy units Today, the most frequent routine applications of TLDs in radiotherapy are mailed dosimetric services of radiotherapy units, such as those organized by the IAEA or by MD Anderson Hospital in Houston, US. In the frame of IAEA-WHO postal dose comparison service for cobalt-60 and MV X-rays beams, LiF TL dosemeters are irradiated under specified conditions in a simplified water phantom at the participating radiotherapy centre and returned to the IAEA’s Dosimetry Laboratory for evaluation. The TL dosemeters are black polyethylene capsules of 20 mm inner length, 3 mm inner diameter, filled with approximately 155 mg of TLD powder (Izewska et al., 2002). Several corrections for dose dependence, energy dependence (for MV X-rays) and fading are applied to increase accuracy. The stability of the reader is controlled using uniformly irradiated TLD powder from a large amount of powder stored for several years to minimize the influence of fading. The accuracy of the a measurement can be better that 1% but the overall extended uncertainty of the method, including calibration is at the level of 2% (Izewska et al., 2002). Such results can be achieved only in laboratories which use strict operational procedures and have access to highly skilled staff, since many operations are performed manually and require good working experience. Therefore, there is still demand to develop systems with comparable accuracy but with simplified requirements in terms of personnel training. Yukihara et al. (2008) using a set of Al2O3:C detectors for depth dose distribution measurements in water phantom achieved relative uncertainty of a single measurement below 1.1%. A flat (G 0.5%) energy response for 6–18 MV X-rays makes this OSL system a potential candidate for a mailed dosimetry service of radiotherapy units. Also RPL glasses have been recently tested for these purposes, showing lower fading but slightly worse angular dependence for 6 MV photons, as compared to LiF:Mg,Ti (Rah et al., 2009). Field tests of the new systems should prove their usefulness in clinical conditions and their resistance to environmental factors. For rapidly increasing number of operational proton and carbon-ion radiotherapy centres no mailed dosimetry checks are yet available. One of the problems in this case is the significant ionization density (LET) dependence of the response of luminescence detectors after doses of heavy charged particles. The energy of the ions is modulated to cover the irradiated volume with a uniform dose, called Spreading Out of the Bragg Peak (SOBP), where a broad spectrum of energies is present. Relative luminescence efficiency for 11 keV/mm carbon ions vs. 60Co g-rays for Al2O3:C is about 0.80 (Yukihara et al., 2006), and 0.51 for LiF:Mg,Cu,P (Bilski et al., 2008). Heavily doped supralinear MTT-N (LiF:Mg,Ti) shows h ¼ 1.07 for 2.2 keV/mm 4He ions and h ¼ 1.02 for 11 keV/mm 12C ions, but high fading of this material makes precise postal dosimetry difficult. For scanning ion beams TLD foils could be used in external audits to test the dose level and the uniformity of the dose distribution (Olko et al., 2008). 5.2. In-phantom measurements Small size luminescence detectors are used for point-like dosimetry in dosimetric phantoms. The most frequently used Rando-Anderson anthropomorphic phantoms have up to a few hundred drilled holes, which can be filled with luminescence
509
detectors. Only for this phantom type over 3000 publications and reports are available on dosimetric measurements using TLDs. The most frequently applied TLD materials are LiF:Mg,Ti, LiF:Mg,Cu,P and Li2B4O7, (Zeff ¼ 7.4–8.2) in the form of chips, pellets and rods. For typical fractionated doses used in radiotherapy (2 Gy) corrections for energy and dose response for the relevant beams must be applied. Recently, there is growing interest in in-phantom TLD dosimetry outside the treatment field, to measure the undesirable doses in radiotherapy, mainly for estimation of risk of secondary cancers (Harrison, 2007). Usually such measurements are technically easier because the doses are in the mGy region and linearity correction, typical for the Gy region, is not necessary. TLDs in clinical environment are almost exclusively used for research purposes only because in routine clinical work manual installation and readout of large numbers of detectors is laborious and timeconsuming. Application of bare OSL or RPL detectors is more complicated in clinical conditions due to their light sensitivity.
5.3. 2-D thermoluminescence dosimetry In modern radiotherapy, two-dimensional (2-D) dosimetry with high spatial resolution is required for modalities where steep gradients of dose distributions occur, such as is the case in scanned beams of protons or carbon ions or even in various modalities using photon (MV X-ray) medical accelerator beams. The progress in the development of highly sensitive Charge Coupled Device (CCD) cameras and the decrease of their prices opened the possibilities to develop CCD-TLD readers to 2-D thermoluminescence dosimetry (Konnai et al., 2008). A two-dimensional (2-D) thermoluminescence (TL) dosimetry system consisting of LiF:Mg,Cu,P (MCP-N)- or CaSO4:Dy based TL foils and two large-area (up to 200 200 mm2) TLD-CCD readers was developed at the Institute of Nuclear Physics (IFJ PAN) (Olko et al., 2008). The main advantages of the developed 2-D TL dosimetry system are: direct (in ASCII matrix format, DICOM-adaptable) digital output of dosimetry data from a precalibrated 2-D detector which shows linearity over the dose range from 10 mGy to 10 Gy, good spatial resolution (below 1 mm), reusability of the TL foil detector, and good mechanical properties of the TL foil (flexibility and water-resistance). The system was applied for 2-D dosimetry of 6 MV radiotherapy beam, dosimetry of brachytherapy sources (106Ru, 226Ra, X-ray needle), dosimetry of 62 MeV proton beam, raster 12C radiotherapy beam and others. The system can potentially complement or compete with other existing 2-D techniques, such as X-ray or dye film dosimetry. The convenience of use and the short time required to evaluate 2-D TLD foils make them very suitable for Quality Assurance in radiotherapy, e.g. verification of treatment plans. A very promising application of luminescence techniques is in in-vivo dosimetry systems, based of a luminescent crystal (e.g. Al2O3:C) attached to an optical fibre cable, used both for transmitting the stimulation light and the emitted OSL signal. The crystal can be attached to the body or, because of the very low diameter of the cable (2 mm), introduced into body cavities. Their small size and negligible temperature dependence give some advantage to active OSL over the presently used semiconductor diodes. The OSL system can operate in active mode, measuring the prompt radioluminescence signal (RL), or in integration mode. A number of systems are available as working prototypes and were tested in clinical conditions. One of the most spectacular application of the system has recently been developed to verify dose delivery in remotely afterloaded brachytherapy, allowing for automatic online in-vivo dosimetry directly in the tumour region using thin OSL probes placed in catheters (Andersen et al., 2009). The system was clinically tested on the Varian GammaMed Plus 192Ir PDR
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afterloader. Excellent reviews of in-vivo OSL systems were published by Akselrod et al. (2007) and by Yukihara and McKeever (2008). 6. Limitations due to ionization density effects Despite the wide availability of different luminescence detectors for dosimetry, there is continued interest in the development of new dosimetric materials for the rapidly growing medical field, for dosimetry of high-LET radiation and for dosemeters which can mimic the response of biological systems. In seeking new materials some basic limitations of the luminescence dosemeters must be taken into account. The main problem of luminescence dosimetry, particularly with respect to high-sensitive LiF:Mg,Cu,P or Al2O3:C, is the decrease in the response of these materials with increasing ionization density of the radiation field, which may lead to underestimation of dose after heavy charged particle irradiation. Low relative luminescence efficiency, even below 0.1 for a-particles, is related to the relatively low saturation dose for g-ray doses, typically not exceeding a few hundred Gy. After irradiation with heavy ions or low-energy X-rays, in the microscopic spatial distribution of energy deposition events in the detector volume, one may expect regions of extremely high local dose in the close vicinity of the ion track and and/or clustering of ionization events along paths of low-energy delta-rays or photoelectrons. If the dose in these regions is high enough, local saturation of the TL signal may occur. The response of TL detectors at high g-ray dose, close to the saturation of their response, should then be closely related to the energy response of these detectors after irradiation by charged ions (high-LET radiation) and/or after low doses of photons (gamma-rays and low-energy X-rays) and some systematic description of this relationship should be possible. One solution to assure a flat dose and energy response of luminescence detectors is to combine the response of several types of detectors (LiF:Mg,Ti: MTS-N and MTT with enhanced Ti concentration) or of different peaks/decay curves (TL peaks 3 and 5 in CaF2:Tm or OSL in Al2O3:C). An interesting approach to increase the dose range of LiF:Mg,Cu,P (MCP-N) from the mGy to several hundreds of kGy (11 orders of magnitude!) was proposed by Bilski et al. (2008) who observed the appearance of high temperature peaks above 400 C. Now, MCP-N detectors are installed around the experimental area of Large Hadron Collider in CERN to measure very high radiation doses to sensitive detectors and electronics (Obryk et al., 2009). 7. Conclusions While most radiation fields can be adequately surveyed by luminescence detectors, neutron dosimetry is still an unresolved issue due to the complicated relationship between the neutron cross sections in tissue and in the TL material, compounded by the variation of relative response of luminescence detectors with the ionization density of charged heavy secondary particles. The ultimate goal of biologically relevant dosimetry using luminescence detectors would consist in developing a TLD material which could actually mimic the response of some biological systems with regard to dose and radiation quality. It would appear that the LETdependent enhancement of the biological effectiveness (RBE) can only be mimicked by detectors which exhibit supralinearity after gray doses. Here, intensive materials research, better understanding of detector characteristics and development and application of suitable theoretical models to describe the response of luminescent detectors after doses of radiation of different quality are needed to achieve success in this endeavour.
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Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Advantages and disadvantages of luminescence dosimetry Pawe1 Olko Institute of Nuclear Physics Polish Academy of Science (IFJ PAN), Krakow, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 August 2009 Received in revised form 17 December 2009 Accepted 6 January 2010
Owing to their excellent dosimetric properties, luminescence detectors of ionizing radiation are now extensively applied in individual dosimetry services. The most frequently used personal dosemeters are based on Optically Stimulated Luminescence (OSL), radiophotoluminescence (RPL) or thermoluminescence (TL). Luminescence detectors have also found several applications in clinical dosimetry, especially around new radiation modalities in radiotherapy, such as Intensity Modulated Radiotherapy (IMRT) or ion beam radiotherapy. Requirements of luminescence detectors applied in individual and clinical dosimetry and some recent developments in luminescence of detectors and techniques leading to significant improvements of the functionality and accuracy of dosimetry systems are reviewed and discussed. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: TLD OSL RPL Personal dosimetry Medical dosimetry
1. Introduction Despite the rapid development of active real-time electronic dosemeters, passive dosemeters which integrate absorbed dose over a period of time are still in demand and are frequently obligatory in radiation protection. Luminescence dosemeters are now gaining popularity in individual dosimetric services, replacing dosimetric films, which were commonly used in the past century (Olko et al., 2006). The principle of application of luminescence materials to the dosimetry of ionizing radiation relies on the relationship between the dose absorbed and the intensity of light emitted. Three groups of luminescence detectors are applied in personal dosimetry: thermoluminescence detectors (TLD), detectors based of Optically Stimulated Luminescence (OSL) and radiophotoluminescence (RPL) glasses. The major advantages of luminescence dosemeters are their high sensitivity, allowing doses as low as 1 mGy to be measured; their linear response with dose up to at least 1 Gy, their good energy response after X-ray irradiation (for materials with Zeff close to that of tissue), their reusability, and their resistance to high humidity and to high magnetic fields. The aim of this paper is to review and discuss the recent progress of luminescence detectors and techniques to meet the growing requirements of personal and medical dosimetry, excluding radiography. We wish also to indicate areas in which it will most likely be difficult to achieve further progress. One of the problems of
E-mail address: [email protected] 1350-4487/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2010.01.016
luminescence dosimetry, particularly with respect to high-sensitive LiF:Mg,Cu,P and Al2O3:C, is the decrease in the response of these materials with increasing ionization density of the radiation field, which may lead to underestimation of dose after heavy charged particle irradiation. Since the dose response and ionization density (LET) response and photon energy response are interrelated, there are some basic limits in the development of a universal luminescence material able to evaluate dose over a very large dose range, simultaneously featuring flat LET/energy response over that range. 2. Requirements for luminescence dosemeters in personal dosimetry The requirements for personal dosemeters are to a large extent internationally standardized in IEC or ISO documents. For TL dosemeters these are IEC 62387-1 (2007), IEC 61066 (2006), ISO 12794 (2000)and for dosimetric films ISO 1757 (1996) standards. These standards, closely related to the type of the system, specify performance criteria, mainly for type testing of personal dosimetry systems, to harmonize tests of the system and reference radiations. Part 3 of IEC 62387, which is under preparation, will specify characteristics of other dosimetric systems, including OSL systems. The response of a personal dosimeter can be expressed as:
M ¼
N0 G rn rE;a renv
(1)
where N0 is the calibration factor, rn is the relative factor for nonlinear dose response, rE,a is the energy and angular response and renv is the relative response due to environmental exposures
P. Olko / Radiation Measurements 45 (2010) 506–511
(the influence of mechanical and electromagnetic disturbances are not taken into account here). In principle, rE,a and rn are not independent quantities for luminescence detectors because with growing ionization density the supralinearity/sublinearity of dose response decreases and the saturation dose increases (Olko et al., 2006). For routine radiation protection this effect is negligible but for doses above 1 Gy this must be accounted for. Energy and angular response are corrected by a single factor rE,a because attenuation of low-energy photons affects the angular response. Some of the requirements for TL personal dosemeters used for determining the value of HP(10) (IEC 62387-1, 2007) are listed in Table 1. The dose range starting with several mSv up to 1 Sv are easily measurable by all personal dosimetry systems based on luminescence detectors and any deviations from linearity, using a careful readout procedure, are typically no worse than a few percent without correction for dose response. Energy and angular correction rE,a, may vary up to 40% which leads to (1/1.4 ¼ 0.71 to þ 1/0.6 ¼ þ1.67) performance criteria for the response M of the personal dosemeter. The energy response of a luminescence dosemeter can be corrected using high-Z filters, typically Al, Sn or Cu, which attenuate the low-energy photon fluence and diminish the enhanced energy deposition in a high-Z luminescence detector. The remaining problem for high-Z luminophor is dosimetry of lowenergy mixed X-b fields, such as Pm-147 beta rays with an average energy of about 62 keV measured with low-energy photons. The filters cut-out the low-energy beta component which leads to underestimation of dose but a detector without such a filter produces a signal which overestimates the photon dose. Therefore, for low-energy mixed radiation fields tissue-equivalent detectors, the signal of which is roughly proportional to absorbed dose in tissue, are recommended. Among TL detectors these are lithium borate and lithium fluoride detectors. Altogether, in difficult environmental conditions, in mixed radiation fields, and over wide dose ranges, the relative difference between the true value and the readout of a personal dosemeter may exceed 100%. Current dose limits in radiation protection based on the Linear-No-Threshold hypothesis appear to overestimate the risk to the extent that such uncertainties in dosimetry are still acceptable.
3. Advantages of luminescence detectors in personal dosimetry Personal dosimetry remains the main application field of luminescence detectors. According to ESOREX data (Frasch and Petrova, 2007), in 2004 about 1 million workers in EU were routinely issued with personal dosemeters, most of them (about
Table 1 Selected requirements of personal dosemeters based on TLDs used for measurements of HP(10) and HP(0.07). Characteristics under test
Minimal measuring range
Performance requirement
Relative response due to the non-linearity Relative response due to mean photon energy and angular response (for HP (10)) Relative response due to mean beta radiation energy (for HP(10)) Relative response due to mean beta energy (for HP(0.07))
0.1 mSv < HP(10) < 1 Sv
9% to þ 11%
80 keV to 1.25 MeV and angels from 0 to 60
29% to þ67%
0.8 MeV
Indicated value maximum 10% of HP(0.07) 29% to þ67%
0.2 MeV to 0.8 MeV and angels from 0 to 60
507
65%) in the medical sector. In a review of dosimetric services in Europe, performed by EURADOS over the years 2002–2004, of 91 participating services 61 used dosemeters based on thermoluminescence detectors, mainly LiF:Mg,Ti (TLD-100, TLD-700, MTS-N and MTS-7), LiF:Mg,Cu,P (GR-200, MCP-N) and lithium borate (Olko et al., 2006). After a few years experience in US, Luxel dosemeters based on Al2O3:C became also available in France, Ireland and the United Kingdom. In 2008, the French IRSN dosimetric service introduced RadioPhotoLuminescence (RPL) silver-doped glasses which replaced dosimetric films, used over several decades (IRSN, 2007). In Germany, it is planned to terminate personal dosimetry based on films by 2011. It seems unlikely that over the next decade active electronic dosemeters or other promising systems such as Direct Ion Storage dosemeters (Fuchs et al., 2008) would replace luminescence systems, mainly because of the low price, high accuracy and resistance to environmental conditions of luminescence detectors. What makes luminescence dosemeters so popular? Excellent ‘‘pro- and contra’’ arguments were exchanged in a dispute between McKeever and Moscovitch (2003) on advantages and disadvantages of Optically Stimulated Luminescence and thermoluminescence dosimetry. In Table 2 the main arguments of this dispute are listed. Over the last few years several additional features of luminescence detectors become apparent, which make them even more attractive for personal dosimetry: - The first personal neutron dosimeter based on OSL technology was developed and introduced into dosimetric services (Yukihara et al., 2008). In the albedo-type dosemeter, neutrons thermalized in the body are detected by a thin Al2O3:C detector covered with a thin layer of 6LiCO3 to register short-range products of the 6Li(n, 4He)3H reaction. Uncovered Al2O3:C shows low sensitivity to thermal neutrons and is used to determine the g-ray contribution. This type of neutron dosemeter demonstrates similar neutron energy response to classical TLD albedo dosemeters based on pairs of 6LiF/7LiF, suitable essentially for neutrons of energy below 1 MeV. - Direct measurement of energy deposited by fast neutrons in luminescence detectors is inefficient because detector interaction cross sections are quite different from those in tissue and the relative luminescence efficiency is relatively low for densely ionizing products of nuclear reactions or for recoil protons. Alternatively, the dose due to fast neutrons can be determined by counting tracks of charged recoil particles and assessing their LET. As early as in 1993, Lommler et al. (1993) developed a laser-scanned RPL glass system able to count
Table 2 Advantages of OSL and TL dosimetry – summary of debate between S. W. S. McKeever and Moscovitch (2003). Advantages of OSL dosemeters
Advantages of TL dosemeters
– – – –
– High sensitivity of new materials like LiF:Mg,Cu,P – Easy handling (no light sensitivity) – Simple TLD readout – Effective and flexible gas heating – Neutron dosimetry possible with Li-6 and Li-7 – TL glow curve used as a quality control – Individual calibration of dosemeters possible – Dose re-estimation possible (but not used) – Very flat photon energy response for LiF
– – – – – –
High luminescence efficiency Stable sensitivity High precision and accuracy Control of the luminescence emitted High speed of readout Identification of static exposure Multiple re-analyses possible Elimination of complex thermal annealing steps, Dose imaging possible Low-power consumption for portable readers
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individual recoil tracks of 5 MeV neutrons over fluences up to 107 n cm2 . Fluorescent nuclear track detectors (FNTDs) have recently been developed by Sykora et al. (2008) which allow one to image individual tracks of heavy charged particles, including their angle of incidence. The system is based on aluminium oxide crystals containing aggregate oxygen vacancies and doped with carbon and magnesium (Al2O3:C,Mg), imaged using laser scanning confocal fluorescence microscopy. The recent improvements of the system permit observation of even single electron tracks (Sykora and Akselrod, 2010). - An interesting method of increasing the detection efficiency of high-LET particles using standard LiF:Mg,Ti (TLD-100) and combined TL and OSL readout has been suggested by Oster et al. (2008). OSL is supposed to originate from F2 or F3 centres which demonstrate the properties of ‘two-hit’ centres, whereas the main dosimetric TL peak of LiF:Mg,Ti arises from a ‘one-hit’ complex defect. Therefore, OSL emission spectra consist of two broad visible bands at 640 nm and 520 nm, whereas main TL emission is observed at about 410–420 nm. For rather high doses (120–480 Gy) a 500-fold increase of relative response of b-rays to a-particles after OSL as compared to TL readouts was observed. This discovery potentially allows discrimination between low and high-ionization density radiation, important in mixed-field radiation dosimetry. However, any practical implications of these finding for dosimetry will be possible only when the effect is confirmed at much lower doses, particularly in the mGy region. - In personal dosimetry based on dosimetric films it is possible to identify cases of static exposure by the source (e.g. if a dosemeter is left by the source over a longer period) by eye recognition of the sharp edges of the filters. In 2000, Akselrod et al. (2000) presented a laser-scanned OSL system with a CCD camera, applied to read a thin layer of Al2O3 covered with a periodically punched 0.5 mm Cu filter. For static exposures with X-rays energies below 150 keV and doses exceeding a few mSv, a clearly distinguishable pattern of punched holes is visible on the CCD images. Budzanowski et al. (2006) applied a newly developed TLD reader with a CCD camera (Marczewska et al., 2006) to read LiF:Mg,Cu,P TL pellets irradiated under a 1 mm thick Pb filter, with a central hole of 1 mm diameter. The CCD image from the statically irradiated dosemeter showed a clear image of the hole, which is not visible for dynamic exposures. A quantitative identification, based on the analysis of the frequency distribution of image intensity, was proposed by Kopec et al. (2010). The method works efficiently for doses above 5 mSv and X-ray energies below 200 keV. After modification of one of the filters in the RADOS personal dosemeter, the system will be used routinely by the dosimetric service of IFJ PAN. - Nakajima and Hashizume (1969) proposed to determine the time delay between exposure and readout of a TL detector by taking into account the different decay rates of particular peaks in TL glow curve. Budzanowski et al. (1999) demonstrated that by deconvoluting the MCP-N (LiF:Mg,Cu,P) glow curve it is possible to determine the time elapsed since exposure up to about 40 days using the peak 3/peak 4 and peak 2/peak 4 ratios. Weinstein et al. (2003) draw similar conclusions, pointing out difficulties in using peak 2 in TLD-100 for this purpose. This feature is still not used in routine personal dosimetry but it could be applied in some special circumstances, e.g. to determine the time elapsed after a radiation accident. A serious disadvantage of historical TLD systems in personal dosimetry is the necessity of manual handling, annealing and readout. Several procedures are time-consuming e.g. annealing of
LiF:Mg,Ti takes 1 h 400 C and 2 h at 100 C (or sometimes 16 h at 80 C). However, in the modern TLD dosimetric systems such as Thermo, Panasonic or Dosacus, annealing, readout and data processing are fully automatic. This is especially true for the largescale, highly efficient Landauer system, with laser readout of Al2O3:C detectors, which takes only a few seconds per readout. That luminescence detectors are presently unsurpassed in personal dosimetry is due to their advantageous physical properties and to the continuous improvement of their readout techniques. 4. Requirements for luminescence dosemeters in radiotherapy Accuracy of dosimetry measurements is understood as the proximity of their expectation value of the measured quantity to the true value. Other than clinical recommendations, generally within 5%, there is no general recommendation for accuracy of dose delivery in radiotherapy but there is a continued trend to decrease the limits. Dosimetric accuracy at the level of 5%, which is believed today to represent a tolerance of deviation between the response of tumour and healthy tissue, is postulated in dosimetric protocols (TRS 398, 2000). The International Atomic Energy Agency performs regularly postal quality audits of radiotherapy beams using TLD dosemeters in the form of capsules filled with LiF:Mg,Ti powder. Results reported by hospitals are accepted which differ by less than 5% from the true value. For differences between 5% and 10% the checks are repeated but in the case of higher discrepancies additional measures are undertaken, including expert’s visits to the hospital. ICRU 24 (1976) recommended 2% but it was impossible and probably also not necessary to reach this goal. When developing new dosimetric systems, medical physicists postulate even better accuracies on the particular properties of the system, such as energy response, dose response, angular dependence etc. to assure that the combined extended uncertainty will be well bellow 5%. In Table 3 we presented a part of requirements on 2-D dosimetry systems, working in integration mode, developed within the FP7 MAESTRO project (Bucciolini et al., 2005). At the moment, no passive dosimetry system based on luminescence detectors is able to fulfil those requirements, particularly for two-dimensional (2-D) dosimetry. 5. Luminescence dosemeters in radiotherapy Historically, TLDs, particularly LiF:Mg,Ti detectors were frequently used in radiotherapy for relative dosimetry (Horowitz, 1984). Today, active dosemeters, based mainly on ionization chambers, silicon diodes, MOSFET detectors are almost exclusively used in clinics because they allow real-time readout of results, easy operation and minimum maintenance. Yet, intensive research is
Table 3 Requirement table for 2-D dosimetry systems, working in integration mode (Bucciolini et al, 2005). Parameter
Range
Requirement
Dose rate dependence: Short term precision Linearity vs. absorbed dose Background signal
0.01–4 Gy/min
<1% <0.5% <1% <0.1% of the radiation induced signal <1%
Energy dependence Angular dependence Reproducibility (different elements of 2-D matrix)
0.001–20 Gy
Photons 4–25 MV Protons 20–200 MeV
<1% <1%
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under way to introduce new dosimetric systems based on luminescence principles, not only in the traditional in-phantom measurements and mailed dosimetric service of radiotherapy beams but also for two-dimensional (2-D) TL dosimetry and promising solutions are being developed for on-line in-vivo dosimetry using OSL systems. 5.1. Postal dosimetric service of radiotherapy units Today, the most frequent routine applications of TLDs in radiotherapy are mailed dosimetric services of radiotherapy units, such as those organized by the IAEA or by MD Anderson Hospital in Houston, US. In the frame of IAEA-WHO postal dose comparison service for cobalt-60 and MV X-rays beams, LiF TL dosemeters are irradiated under specified conditions in a simplified water phantom at the participating radiotherapy centre and returned to the IAEA’s Dosimetry Laboratory for evaluation. The TL dosemeters are black polyethylene capsules of 20 mm inner length, 3 mm inner diameter, filled with approximately 155 mg of TLD powder (Izewska et al., 2002). Several corrections for dose dependence, energy dependence (for MV X-rays) and fading are applied to increase accuracy. The stability of the reader is controlled using uniformly irradiated TLD powder from a large amount of powder stored for several years to minimize the influence of fading. The accuracy of the a measurement can be better that 1% but the overall extended uncertainty of the method, including calibration is at the level of 2% (Izewska et al., 2002). Such results can be achieved only in laboratories which use strict operational procedures and have access to highly skilled staff, since many operations are performed manually and require good working experience. Therefore, there is still demand to develop systems with comparable accuracy but with simplified requirements in terms of personnel training. Yukihara et al. (2008) using a set of Al2O3:C detectors for depth dose distribution measurements in water phantom achieved relative uncertainty of a single measurement below 1.1%. A flat (G 0.5%) energy response for 6–18 MV X-rays makes this OSL system a potential candidate for a mailed dosimetry service of radiotherapy units. Also RPL glasses have been recently tested for these purposes, showing lower fading but slightly worse angular dependence for 6 MV photons, as compared to LiF:Mg,Ti (Rah et al., 2009). Field tests of the new systems should prove their usefulness in clinical conditions and their resistance to environmental factors. For rapidly increasing number of operational proton and carbon-ion radiotherapy centres no mailed dosimetry checks are yet available. One of the problems in this case is the significant ionization density (LET) dependence of the response of luminescence detectors after doses of heavy charged particles. The energy of the ions is modulated to cover the irradiated volume with a uniform dose, called Spreading Out of the Bragg Peak (SOBP), where a broad spectrum of energies is present. Relative luminescence efficiency for 11 keV/mm carbon ions vs. 60Co g-rays for Al2O3:C is about 0.80 (Yukihara et al., 2006), and 0.51 for LiF:Mg,Cu,P (Bilski et al., 2008). Heavily doped supralinear MTT-N (LiF:Mg,Ti) shows h ¼ 1.07 for 2.2 keV/mm 4He ions and h ¼ 1.02 for 11 keV/mm 12C ions, but high fading of this material makes precise postal dosimetry difficult. For scanning ion beams TLD foils could be used in external audits to test the dose level and the uniformity of the dose distribution (Olko et al., 2008). 5.2. In-phantom measurements Small size luminescence detectors are used for point-like dosimetry in dosimetric phantoms. The most frequently used Rando-Anderson anthropomorphic phantoms have up to a few hundred drilled holes, which can be filled with luminescence
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detectors. Only for this phantom type over 3000 publications and reports are available on dosimetric measurements using TLDs. The most frequently applied TLD materials are LiF:Mg,Ti, LiF:Mg,Cu,P and Li2B4O7, (Zeff ¼ 7.4–8.2) in the form of chips, pellets and rods. For typical fractionated doses used in radiotherapy (2 Gy) corrections for energy and dose response for the relevant beams must be applied. Recently, there is growing interest in in-phantom TLD dosimetry outside the treatment field, to measure the undesirable doses in radiotherapy, mainly for estimation of risk of secondary cancers (Harrison, 2007). Usually such measurements are technically easier because the doses are in the mGy region and linearity correction, typical for the Gy region, is not necessary. TLDs in clinical environment are almost exclusively used for research purposes only because in routine clinical work manual installation and readout of large numbers of detectors is laborious and timeconsuming. Application of bare OSL or RPL detectors is more complicated in clinical conditions due to their light sensitivity.
5.3. 2-D thermoluminescence dosimetry In modern radiotherapy, two-dimensional (2-D) dosimetry with high spatial resolution is required for modalities where steep gradients of dose distributions occur, such as is the case in scanned beams of protons or carbon ions or even in various modalities using photon (MV X-ray) medical accelerator beams. The progress in the development of highly sensitive Charge Coupled Device (CCD) cameras and the decrease of their prices opened the possibilities to develop CCD-TLD readers to 2-D thermoluminescence dosimetry (Konnai et al., 2008). A two-dimensional (2-D) thermoluminescence (TL) dosimetry system consisting of LiF:Mg,Cu,P (MCP-N)- or CaSO4:Dy based TL foils and two large-area (up to 200 200 mm2) TLD-CCD readers was developed at the Institute of Nuclear Physics (IFJ PAN) (Olko et al., 2008). The main advantages of the developed 2-D TL dosimetry system are: direct (in ASCII matrix format, DICOM-adaptable) digital output of dosimetry data from a precalibrated 2-D detector which shows linearity over the dose range from 10 mGy to 10 Gy, good spatial resolution (below 1 mm), reusability of the TL foil detector, and good mechanical properties of the TL foil (flexibility and water-resistance). The system was applied for 2-D dosimetry of 6 MV radiotherapy beam, dosimetry of brachytherapy sources (106Ru, 226Ra, X-ray needle), dosimetry of 62 MeV proton beam, raster 12C radiotherapy beam and others. The system can potentially complement or compete with other existing 2-D techniques, such as X-ray or dye film dosimetry. The convenience of use and the short time required to evaluate 2-D TLD foils make them very suitable for Quality Assurance in radiotherapy, e.g. verification of treatment plans. A very promising application of luminescence techniques is in in-vivo dosimetry systems, based of a luminescent crystal (e.g. Al2O3:C) attached to an optical fibre cable, used both for transmitting the stimulation light and the emitted OSL signal. The crystal can be attached to the body or, because of the very low diameter of the cable (2 mm), introduced into body cavities. Their small size and negligible temperature dependence give some advantage to active OSL over the presently used semiconductor diodes. The OSL system can operate in active mode, measuring the prompt radioluminescence signal (RL), or in integration mode. A number of systems are available as working prototypes and were tested in clinical conditions. One of the most spectacular application of the system has recently been developed to verify dose delivery in remotely afterloaded brachytherapy, allowing for automatic online in-vivo dosimetry directly in the tumour region using thin OSL probes placed in catheters (Andersen et al., 2009). The system was clinically tested on the Varian GammaMed Plus 192Ir PDR
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afterloader. Excellent reviews of in-vivo OSL systems were published by Akselrod et al. (2007) and by Yukihara and McKeever (2008). 6. Limitations due to ionization density effects Despite the wide availability of different luminescence detectors for dosimetry, there is continued interest in the development of new dosimetric materials for the rapidly growing medical field, for dosimetry of high-LET radiation and for dosemeters which can mimic the response of biological systems. In seeking new materials some basic limitations of the luminescence dosemeters must be taken into account. The main problem of luminescence dosimetry, particularly with respect to high-sensitive LiF:Mg,Cu,P or Al2O3:C, is the decrease in the response of these materials with increasing ionization density of the radiation field, which may lead to underestimation of dose after heavy charged particle irradiation. Low relative luminescence efficiency, even below 0.1 for a-particles, is related to the relatively low saturation dose for g-ray doses, typically not exceeding a few hundred Gy. After irradiation with heavy ions or low-energy X-rays, in the microscopic spatial distribution of energy deposition events in the detector volume, one may expect regions of extremely high local dose in the close vicinity of the ion track and and/or clustering of ionization events along paths of low-energy delta-rays or photoelectrons. If the dose in these regions is high enough, local saturation of the TL signal may occur. The response of TL detectors at high g-ray dose, close to the saturation of their response, should then be closely related to the energy response of these detectors after irradiation by charged ions (high-LET radiation) and/or after low doses of photons (gamma-rays and low-energy X-rays) and some systematic description of this relationship should be possible. One solution to assure a flat dose and energy response of luminescence detectors is to combine the response of several types of detectors (LiF:Mg,Ti: MTS-N and MTT with enhanced Ti concentration) or of different peaks/decay curves (TL peaks 3 and 5 in CaF2:Tm or OSL in Al2O3:C). An interesting approach to increase the dose range of LiF:Mg,Cu,P (MCP-N) from the mGy to several hundreds of kGy (11 orders of magnitude!) was proposed by Bilski et al. (2008) who observed the appearance of high temperature peaks above 400 C. Now, MCP-N detectors are installed around the experimental area of Large Hadron Collider in CERN to measure very high radiation doses to sensitive detectors and electronics (Obryk et al., 2009). 7. Conclusions While most radiation fields can be adequately surveyed by luminescence detectors, neutron dosimetry is still an unresolved issue due to the complicated relationship between the neutron cross sections in tissue and in the TL material, compounded by the variation of relative response of luminescence detectors with the ionization density of charged heavy secondary particles. The ultimate goal of biologically relevant dosimetry using luminescence detectors would consist in developing a TLD material which could actually mimic the response of some biological systems with regard to dose and radiation quality. It would appear that the LETdependent enhancement of the biological effectiveness (RBE) can only be mimicked by detectors which exhibit supralinearity after gray doses. Here, intensive materials research, better understanding of detector characteristics and development and application of suitable theoretical models to describe the response of luminescent detectors after doses of radiation of different quality are needed to achieve success in this endeavour.
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