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LiF:Mg,Ti (MTT) TL Detectors optimised for high-LET radiation dosimetry
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Radiation Measurements 38 (2004) 427 – 430 www.elsevier.com/locate/radmeas
LiF:Mg,Ti (MTT) TL Detectors optimised for high-LET radiation dosimetry P. Bilskia;∗ , M. Budzanowskia , P. Olkoa , E. Mandowskab a Health
Physics Lab, Institute of Nuclear Physics, Radzikowskiego 152, Krakow PL 31-342, Poland of Physics Pedagogical University, Armii Krajowej 13/15, Czestochowa ; 42-200, Poland
b Institute
Received 31 October 2003; received in revised form 31 October 2003; accepted 7 December 2003
Abstract The properties of LiF:Mg,Ti (distributed as, e.g., TLD-100 or MTS-N), the most frequently used thermoluminescent detector, have been optimised for measurements of sparsely ionising radiation (gamma rays), typically encountered in radiation protection or clinical dosimetry. However, these detectors need also to be applied in conditions of mixed-7eld dosimetry with a high-LET component, such as those encountered in heavy ion beams or in space. At the Institute of Nuclear Physics in Krak;ow a new type of LiF:Mg,Ti detector (named MTT) has been recently developed through modi7cation of its dopant composition. This composition is intended to increase the detection e
1. Introduction For over four decades thermoluminescent detectors (TLD) have been successfully used in radiation dosimetry. Among diBerent types of TLDs undoubtedly the most popular ones are those based on LiF:Mg,Ti (TLD-100, MTS-N, etc.). While applications of these TLDs cover various radiation 7elds, the standard LiF:Mg,Ti detectors (as probably any other common TL materials) have been optimised for detection of sparsely ionising radiation (gamma rays), typically encountered in radiation protection or in
∗ Corresponding author. Tel.: +4812-6628414; fax: +4812-6628066. E-mail address: [email protected] (P. Bilski).
c 2003 Elsevier Ltd. All rights reserved. 1350-4487/$ - see front matter doi:10.1016/j.radmeas.2003.12.013
clinical dosimetry. If TL detectors are to be used in radiation 7elds with a densely ionising component, e.g. in beams of accelerated protons or heavier particles or in space, the dependence of their sensitivity on ionisation density becomes an important issue. This dependence is a common feature of all thermoluminescent materials (or at least of all TL materials of practical importance). LiF:Mg,Ti detectors are no exception, and with increasing LET of radiation their response decreases signi7cantly. At the Institute of Nuclear Physics in Krak;ow (INP) a new version of LiF:Mg,Ti (named MTT, to distinguish it from the standard MTS—type LiF:Mg,Ti) has recently been developed through a modi7cation of the dopant concentration, following the results of a systematic study of the inKuence of variation of activator composition on the properties of LiF:Mg,Ti (Bilski et al., 1999). The main goal of these
428
P. Bilski et al. / Radiation Measurements 38 (2004) 427 – 430
modi7cations was to increase the relative TL e
The general method of detector preparation is similar to that routinely used at the INP for manufacturing large amounts of LiF:Mg,Ti (MTS). Activators are introduced into LiF by heating the material at a temperature somewhat below the melting point of LiF. The main diBerence between MTS and MTT is in the activator concentration: for MTT CMg = 50 ppm and CTi = 120 ppm, i.e. about three times less of Mg and about 10 times more of Ti content, compared with the standard MTS-N. In order to activate LiF with higher amounts of Ti the temperature at which activation is carried out had to be optimised (800◦ C, instead the routine 600◦ C). The powdered material obtained after activation is then turned into solid pellets with the cold pressing and sintering technology developed and routinely used at the INP. All TL detectors used in this work were in the form of pellets of diameter of 4:5 mm and thickness 0:6 mm. No optimisation of the annealing procedure was performed and the standard LiF:Mg,Ti annealing cycle was used, namely 400◦ C for 1 h followed by 100◦ C for 2 h and a pre-readout heating at 100◦ C for 10 min. Readout was performed using a RA’94 manual reader, equipped with an ohmic heating system. A linear ramp at a rate of 5◦ C=s or 10◦ C=s was applied. The integral of the main peak was taken as a measure of the TL signal.
0 50
100
150
200
250
300
Temperature, oC
Fig. 1. Glow-curves of MTT and standard MTS detectors, normalised at peak height maxima.
measured values fitted peaks sum of fitted peaks
100
TL signal, relative units
2. Materials and methods
MTT-7 MTS-7
1000
80
60
40
20
0
3. Results and discussion 3.1. Basic properties In Fig. 1 the glow-curve of MTT detectors is compared with that of standard MTS. Measurements were performed at 5◦ C=s heating rate. Both detectors were irradiated with 100 mGy of Cs-137 gamma-rays. The main diBerence between the shape of the two glow-curves concerns the so-called peak 5 in LiF:Mg,Ti. In standard LiF:Mg,Ti this peak, located at ca. 225◦ C, is the main peak in the glow-curve. In MTT the height of this peak is greatly reduced and peak 4 at 200◦ C becomes the highest peak. Another diBerence is in the high-temperature peak region, which in MTT is more intense than that in the standard MTS glow-curve. At the low-temperature part of the glow-curve a diBerence may also be observed—the signal originating from peak 3 (at ca 160◦ C) is much stronger in MTT and in spite of the 100◦ C treatment it still contributes a signi7cant part of the total signal. This may be considered to be disadvantageous, because a strong low-temperature signal may indicate higher fading. A solution to this issue is likely to be found through optimising
300
350
400
450
500
550
600
650
Wavelength, nm
Fig. 2. Emission spectrum of the MTT material, measured after a gamma-ray dose of 8 Gy. Broken lines represent 7tted Gaussian peaks and solid line corresponds to the sum of these peaks.
the pre-readout heating conditions, e.g. by increasing the temperature (perhaps up to 150◦ C) and/or time. The gamma-ray sensitivity of MTT is much lower than that of MTS, by a factor of about 4 for the main peak height and by a factor of about 2.5 for the integrated signal. This makes MTT not too well suited for low-dose applications, however miligray doses can still be readily measured. The emission spectrum of the MTT material was measured at the Institute of Physics (Czestochowa) ; using a spectrometer equipped with a liquid nitrogen-cooled CCD camera, which is described elsewhere (Mandowska et al., 2002). The measured spectrum is presented in Fig. 2. It can be seen that the emission may be 7tted with two Gaussian bands, with maxima at 423 and 556 nm. These values are almost identical with the results measured in the same experimental conditions for standard MTS: 424 and 562 nm.
P. Bilski et al. / Radiation Measurements 38 (2004) 427 – 430 He
429 Fe
Ne
1.40
Relative response
H
1.0
Ne
0.8
Si
Fe
Response ratio MTT/MTS
C
1.35
Si
1.30 1.25 1.20
C
1.15 1.10 1.05
He
H
0.6
1.00 1
10
100
LET H2O, keV/µm
Fig. 3. Response of MTT detectors to ion beams per unit dose in water, normalised to their response after Cs-137 gamma-rays. The relative uncertainty of the data points (not marked on the graph for clarity) is between 3% and 6%.
3.2. High LET response The response of TLDs after doses of high-LET radiation was measured using ion beams. Most exposures were performed at the HIMAC accelerator in Chiba, Japan. These irradiations included helium (144 MeV=amu), carbon (386 MeV=amu), silicon (490 MeV=amu), neon (368 MeV=amu) and iron (418 MeV=amu) ions. Additionally, TLDs were exposed to proton beam (150 MeV) at the phasotron in Dubna, Russia. Dosimetry data for ion beams were supplied by the groups operating their respective accelerators. Results are presented in Fig. 3 in the form of response (i.e. the TL signal per unit dose) to ions, related to the response after irradiation with Cs-137 gamma-rays (all gamma exposures were performed at the INP irradiation facility). DiBerent data points for one ion type represent diBerent irradiations performed at various dose levels (Chiba: 10 – 100 mGy, Dubna: 100 –500 mGy) and in the case of exposures in Chiba—during two separate runs (in the years 2002 and 2003). It can be seen that up to 10 keV=m the response is more or less Kat. In fact, MTT even shows a small (up to 10%) over-response for He and C ions. However, one cannot be certain whether this over-response should be treated as signi7cant, taking into account the uncertainties of ion beam dosimetry (probably at least 5%), as well as the uncertainty of gamma calibration (2–3%). Fig. 4 illustrates the increase of the relative TL response of MTT with ionisation density, referred to MTS. As this graph presents the ratio of MTT and MTS responses, these results are not biased with the above-discussed uncertainties of absolute dosimetry (detectors of each type were always exposed together). It can be seen the MTT/MTS ratio increases up to LET = 30 keV=m, at which this ratio
1
10
100
LET H2O, keV/µm
Fig. 4. Ratio of MTS to MTT responses (for gamma-rays this ratio is equal to 1, by de7nition). Error bars represent the standard deviation of the results after diBerent exposures.
seemingly reaches a plateau at a level of 1.30 –1.35. An even higher ratio, of about 2.0, was obtained when TLDs were exposed to alpha particles from an Am-241 source. In this measurement accurate dose and energy data were not available. One can estimate that the initial alpha-particle energy at the TLD surface did not exceed 5 MeV and that these particles were completely stopped within the detector, corresponding to an average value of LET in LiF close to 300 keV=m. Both MTS and MTT detectors were exposed in the same conditions, so while the dosimetry data may not be speci7ed accurately enough, the value of the response ratio MTT/MTS is quite reliable. The presented results of LET characteristics suggest the most apparent application for MTT. It seems that the difference in the response of MTS and MTT detectors may be exploited for estimating LET corrections for both detectors, when exposed in an unknown 7eld with a high-LET component. This approach may be further extended by additionally using another type of LiF-based TLDs, namely LiF:Mg,Cu,P (denoted MCP). These detectors show a strongly decreased response to densely ionizing radiation, when compared with standard LiF:Mg,Ti (Bilski et al., 1994). Simultaneous use of all three types of TLDs (MTS, MCP and MTT), should provide three independent sources of information about the ionisation density of the radiation 7eld. This may also be supplemented by exploiting the ratio of high temperature and main peaks for MTT and MTS (Vana et al., 1996). To be reliable, the above-proposed method requires a number of further calibrations performed in various monoenergetic and mixed high-LET 7elds—this work is under way. Nevertheless, the method will be implemented in the near future for measuring cosmic radiation doses within the “Matroshka” experiment, whereby a specially constructed human phantom will be exposed in free space (outside the International Space Station) for 1 year. The phantom will incorporate a few thousand measuring positions in order to
430
P. Bilski et al. / Radiation Measurements 38 (2004) 427 – 430 1.35
Relative response
1.30
enhanced relative e
MTT protons MTS protons
1.25 1.20 1.15 1.10 1.05 1.00
Acknowledgements
0.95 0.1
1
10
Dose, Gy
Fig. 5. Dose response of MTT and MTS detectors after doses of 150 MeV protons. All values are normalised to the response of the respective detectors after 0:2 Gy of gamma-rays.
determine radiation doses to particular organs. It is planned to place the discussed three TLDs at about a half of these locations.
The authors are deeply grateful to the Chiba group for the opportunity of exposing TLDs to the NIRS-HIMAC ion beams in Chiba within the ICCHIBAN research project, with special thanks to Dr. Y. Uchihori. The authors would also like to thank Dr. A. Molokanov for performing proton irradiations at the Dubna Phasotron. This work is partly supported by a research project from the Polish State Committee of Scienti7c Research (KBN) over the years 2003–2005 (No. 4T10C03824).
3.3. Dose response
References
The dose-response of MTT after high doses was measured over the dose range between 0.1 and 10 Gy using the Phasotron proton beam in Dubna. Results normalised to the response after 0:2 Gy of gamma-rays, are presented in Fig. 5. The MTT material shows more prominent supralinearity than MTS, which was an expected result, as high-LET e
Bilski, P., Olko, P., Burgkhardt, B., Piesch, E., Waligorski, M.P.R., 1994. Thermoluminescence e
4. Conclusions A new type of a LiF:Mg,Ti-based TL detector (MTT) has been developed at the INP in Krak;ow, featuring
Radiation Measurements 38 (2004) 427 – 430 www.elsevier.com/locate/radmeas
LiF:Mg,Ti (MTT) TL Detectors optimised for high-LET radiation dosimetry P. Bilskia;∗ , M. Budzanowskia , P. Olkoa , E. Mandowskab a Health
Physics Lab, Institute of Nuclear Physics, Radzikowskiego 152, Krakow PL 31-342, Poland of Physics Pedagogical University, Armii Krajowej 13/15, Czestochowa ; 42-200, Poland
b Institute
Received 31 October 2003; received in revised form 31 October 2003; accepted 7 December 2003
Abstract The properties of LiF:Mg,Ti (distributed as, e.g., TLD-100 or MTS-N), the most frequently used thermoluminescent detector, have been optimised for measurements of sparsely ionising radiation (gamma rays), typically encountered in radiation protection or clinical dosimetry. However, these detectors need also to be applied in conditions of mixed-7eld dosimetry with a high-LET component, such as those encountered in heavy ion beams or in space. At the Institute of Nuclear Physics in Krak;ow a new type of LiF:Mg,Ti detector (named MTT) has been recently developed through modi7cation of its dopant composition. This composition is intended to increase the detection e
1. Introduction For over four decades thermoluminescent detectors (TLD) have been successfully used in radiation dosimetry. Among diBerent types of TLDs undoubtedly the most popular ones are those based on LiF:Mg,Ti (TLD-100, MTS-N, etc.). While applications of these TLDs cover various radiation 7elds, the standard LiF:Mg,Ti detectors (as probably any other common TL materials) have been optimised for detection of sparsely ionising radiation (gamma rays), typically encountered in radiation protection or in
∗ Corresponding author. Tel.: +4812-6628414; fax: +4812-6628066. E-mail address: [email protected] (P. Bilski).
c 2003 Elsevier Ltd. All rights reserved. 1350-4487/$ - see front matter doi:10.1016/j.radmeas.2003.12.013
clinical dosimetry. If TL detectors are to be used in radiation 7elds with a densely ionising component, e.g. in beams of accelerated protons or heavier particles or in space, the dependence of their sensitivity on ionisation density becomes an important issue. This dependence is a common feature of all thermoluminescent materials (or at least of all TL materials of practical importance). LiF:Mg,Ti detectors are no exception, and with increasing LET of radiation their response decreases signi7cantly. At the Institute of Nuclear Physics in Krak;ow (INP) a new version of LiF:Mg,Ti (named MTT, to distinguish it from the standard MTS—type LiF:Mg,Ti) has recently been developed through a modi7cation of the dopant concentration, following the results of a systematic study of the inKuence of variation of activator composition on the properties of LiF:Mg,Ti (Bilski et al., 1999). The main goal of these
428
P. Bilski et al. / Radiation Measurements 38 (2004) 427 – 430
modi7cations was to increase the relative TL e
The general method of detector preparation is similar to that routinely used at the INP for manufacturing large amounts of LiF:Mg,Ti (MTS). Activators are introduced into LiF by heating the material at a temperature somewhat below the melting point of LiF. The main diBerence between MTS and MTT is in the activator concentration: for MTT CMg = 50 ppm and CTi = 120 ppm, i.e. about three times less of Mg and about 10 times more of Ti content, compared with the standard MTS-N. In order to activate LiF with higher amounts of Ti the temperature at which activation is carried out had to be optimised (800◦ C, instead the routine 600◦ C). The powdered material obtained after activation is then turned into solid pellets with the cold pressing and sintering technology developed and routinely used at the INP. All TL detectors used in this work were in the form of pellets of diameter of 4:5 mm and thickness 0:6 mm. No optimisation of the annealing procedure was performed and the standard LiF:Mg,Ti annealing cycle was used, namely 400◦ C for 1 h followed by 100◦ C for 2 h and a pre-readout heating at 100◦ C for 10 min. Readout was performed using a RA’94 manual reader, equipped with an ohmic heating system. A linear ramp at a rate of 5◦ C=s or 10◦ C=s was applied. The integral of the main peak was taken as a measure of the TL signal.
0 50
100
150
200
250
300
Temperature, oC
Fig. 1. Glow-curves of MTT and standard MTS detectors, normalised at peak height maxima.
measured values fitted peaks sum of fitted peaks
100
TL signal, relative units
2. Materials and methods
MTT-7 MTS-7
1000
80
60
40
20
0
3. Results and discussion 3.1. Basic properties In Fig. 1 the glow-curve of MTT detectors is compared with that of standard MTS. Measurements were performed at 5◦ C=s heating rate. Both detectors were irradiated with 100 mGy of Cs-137 gamma-rays. The main diBerence between the shape of the two glow-curves concerns the so-called peak 5 in LiF:Mg,Ti. In standard LiF:Mg,Ti this peak, located at ca. 225◦ C, is the main peak in the glow-curve. In MTT the height of this peak is greatly reduced and peak 4 at 200◦ C becomes the highest peak. Another diBerence is in the high-temperature peak region, which in MTT is more intense than that in the standard MTS glow-curve. At the low-temperature part of the glow-curve a diBerence may also be observed—the signal originating from peak 3 (at ca 160◦ C) is much stronger in MTT and in spite of the 100◦ C treatment it still contributes a signi7cant part of the total signal. This may be considered to be disadvantageous, because a strong low-temperature signal may indicate higher fading. A solution to this issue is likely to be found through optimising
300
350
400
450
500
550
600
650
Wavelength, nm
Fig. 2. Emission spectrum of the MTT material, measured after a gamma-ray dose of 8 Gy. Broken lines represent 7tted Gaussian peaks and solid line corresponds to the sum of these peaks.
the pre-readout heating conditions, e.g. by increasing the temperature (perhaps up to 150◦ C) and/or time. The gamma-ray sensitivity of MTT is much lower than that of MTS, by a factor of about 4 for the main peak height and by a factor of about 2.5 for the integrated signal. This makes MTT not too well suited for low-dose applications, however miligray doses can still be readily measured. The emission spectrum of the MTT material was measured at the Institute of Physics (Czestochowa) ; using a spectrometer equipped with a liquid nitrogen-cooled CCD camera, which is described elsewhere (Mandowska et al., 2002). The measured spectrum is presented in Fig. 2. It can be seen that the emission may be 7tted with two Gaussian bands, with maxima at 423 and 556 nm. These values are almost identical with the results measured in the same experimental conditions for standard MTS: 424 and 562 nm.
P. Bilski et al. / Radiation Measurements 38 (2004) 427 – 430 He
429 Fe
Ne
1.40
Relative response
H
1.0
Ne
0.8
Si
Fe
Response ratio MTT/MTS
C
1.35
Si
1.30 1.25 1.20
C
1.15 1.10 1.05
He
H
0.6
1.00 1
10
100
LET H2O, keV/µm
Fig. 3. Response of MTT detectors to ion beams per unit dose in water, normalised to their response after Cs-137 gamma-rays. The relative uncertainty of the data points (not marked on the graph for clarity) is between 3% and 6%.
3.2. High LET response The response of TLDs after doses of high-LET radiation was measured using ion beams. Most exposures were performed at the HIMAC accelerator in Chiba, Japan. These irradiations included helium (144 MeV=amu), carbon (386 MeV=amu), silicon (490 MeV=amu), neon (368 MeV=amu) and iron (418 MeV=amu) ions. Additionally, TLDs were exposed to proton beam (150 MeV) at the phasotron in Dubna, Russia. Dosimetry data for ion beams were supplied by the groups operating their respective accelerators. Results are presented in Fig. 3 in the form of response (i.e. the TL signal per unit dose) to ions, related to the response after irradiation with Cs-137 gamma-rays (all gamma exposures were performed at the INP irradiation facility). DiBerent data points for one ion type represent diBerent irradiations performed at various dose levels (Chiba: 10 – 100 mGy, Dubna: 100 –500 mGy) and in the case of exposures in Chiba—during two separate runs (in the years 2002 and 2003). It can be seen that up to 10 keV=m the response is more or less Kat. In fact, MTT even shows a small (up to 10%) over-response for He and C ions. However, one cannot be certain whether this over-response should be treated as signi7cant, taking into account the uncertainties of ion beam dosimetry (probably at least 5%), as well as the uncertainty of gamma calibration (2–3%). Fig. 4 illustrates the increase of the relative TL response of MTT with ionisation density, referred to MTS. As this graph presents the ratio of MTT and MTS responses, these results are not biased with the above-discussed uncertainties of absolute dosimetry (detectors of each type were always exposed together). It can be seen the MTT/MTS ratio increases up to LET = 30 keV=m, at which this ratio
1
10
100
LET H2O, keV/µm
Fig. 4. Ratio of MTS to MTT responses (for gamma-rays this ratio is equal to 1, by de7nition). Error bars represent the standard deviation of the results after diBerent exposures.
seemingly reaches a plateau at a level of 1.30 –1.35. An even higher ratio, of about 2.0, was obtained when TLDs were exposed to alpha particles from an Am-241 source. In this measurement accurate dose and energy data were not available. One can estimate that the initial alpha-particle energy at the TLD surface did not exceed 5 MeV and that these particles were completely stopped within the detector, corresponding to an average value of LET in LiF close to 300 keV=m. Both MTS and MTT detectors were exposed in the same conditions, so while the dosimetry data may not be speci7ed accurately enough, the value of the response ratio MTT/MTS is quite reliable. The presented results of LET characteristics suggest the most apparent application for MTT. It seems that the difference in the response of MTS and MTT detectors may be exploited for estimating LET corrections for both detectors, when exposed in an unknown 7eld with a high-LET component. This approach may be further extended by additionally using another type of LiF-based TLDs, namely LiF:Mg,Cu,P (denoted MCP). These detectors show a strongly decreased response to densely ionizing radiation, when compared with standard LiF:Mg,Ti (Bilski et al., 1994). Simultaneous use of all three types of TLDs (MTS, MCP and MTT), should provide three independent sources of information about the ionisation density of the radiation 7eld. This may also be supplemented by exploiting the ratio of high temperature and main peaks for MTT and MTS (Vana et al., 1996). To be reliable, the above-proposed method requires a number of further calibrations performed in various monoenergetic and mixed high-LET 7elds—this work is under way. Nevertheless, the method will be implemented in the near future for measuring cosmic radiation doses within the “Matroshka” experiment, whereby a specially constructed human phantom will be exposed in free space (outside the International Space Station) for 1 year. The phantom will incorporate a few thousand measuring positions in order to
430
P. Bilski et al. / Radiation Measurements 38 (2004) 427 – 430 1.35
Relative response
1.30
enhanced relative e
MTT protons MTS protons
1.25 1.20 1.15 1.10 1.05 1.00
Acknowledgements
0.95 0.1
1
10
Dose, Gy
Fig. 5. Dose response of MTT and MTS detectors after doses of 150 MeV protons. All values are normalised to the response of the respective detectors after 0:2 Gy of gamma-rays.
determine radiation doses to particular organs. It is planned to place the discussed three TLDs at about a half of these locations.
The authors are deeply grateful to the Chiba group for the opportunity of exposing TLDs to the NIRS-HIMAC ion beams in Chiba within the ICCHIBAN research project, with special thanks to Dr. Y. Uchihori. The authors would also like to thank Dr. A. Molokanov for performing proton irradiations at the Dubna Phasotron. This work is partly supported by a research project from the Polish State Committee of Scienti7c Research (KBN) over the years 2003–2005 (No. 4T10C03824).
3.3. Dose response
References
The dose-response of MTT after high doses was measured over the dose range between 0.1 and 10 Gy using the Phasotron proton beam in Dubna. Results normalised to the response after 0:2 Gy of gamma-rays, are presented in Fig. 5. The MTT material shows more prominent supralinearity than MTS, which was an expected result, as high-LET e
Bilski, P., Olko, P., Burgkhardt, B., Piesch, E., Waligorski, M.P.R., 1994. Thermoluminescence e
4. Conclusions A new type of a LiF:Mg,Ti-based TL detector (MTT) has been developed at the INP in Krak;ow, featuring