- Email: [email protected]
Beryllium oxide as optically stimulated luminescence dosimeter
Radiation Measurements 43 (2008) 353 – 356 www.elsevier.com/locate/radmeas
Beryllium oxide as optically stimulated luminescence dosimeter M. Sommer ∗ , A. Jahn, J. Henniger Institute of Nuclear and Particle Physics, Technische Universitaet Dresden, Radiation Physics Group, D-01062 Dresden, Germany
Abstract In the last years a dosimetric system using optically stimulated luminescence (OSL) of beryllium oxide (BeO) was developed by the radiation physics group in Dresden. Blue light LED stimulation and reading of luminescence light with an enclosed photo sensor module are performed from opposite detector sides. A software controls stimulation, records the amplified and digitized photo sensor signal and generates an unified OSL signal. With the help of calibration these OSL signal can be used to specify dose. The linearity of dose response reaches from Gy level up to a few Gy. At higher dose the OSL signal shows a saturation which can be correctly described by a saturation function up to 100 Gy. Fading of the signal is negligible. The material BeO has an excellent resistance to environmental influences. Due to the near tissue equivalence and the resulting low photon energy dependence the BeO detectors are predestinated for dosimetric use. Assembling several detectors to an area offers the ability to examine strongly inhomogeneous radiation fields. Measurements of the radial dose function of a brachytherapy seed and dose rate distributions in front of a 55 Fe source are presented. © 2007 Elsevier Ltd. All rights reserved. Keywords: Optically stimulated luminescence; OSL; Beryllium oxide; BeO; Dosimetry
1. Introduction and motivation Investigations on the luminescence properties of Beryllium oxide (BeO) were accomplished by Albrecht and Mandeville (1956). One year later the material BeO was used as TLluminophor for the first time (Moore, 1957). However the light-induced fading, measured by Tochilin et al. (1969), prevented the use of BeO as TL-material in a broad spectrum. According to the effect of light-induced fading, Rhyner and Miller (1970) suggested BeO as an optically stimulated luminescence (OSL) luminophor. But not until the work of Bulur and Göksu (1998) the OSL-properties of the material were researched in detail. Due to the high request for passive radiation detectors in medicine, personal and environmental dosimetry, the radiation physics group in Dresden investigated BeO as OSL dosimeter for a number of years (Sommer and Henniger, 2006; Sommer et al., 2007). Up to now an useful dosimetric method using the material BeO was developed which can be used to specify dose in a number of applications. According to the near tissue equivalence (Zeff = 7.14) of the material BeO, the dosimetric ∗ Corresponding author.
E-mail address: [email protected] (M. Sommer). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.11.018
system offers a wide range of appliance especially in medical fields. Beside this an dose field imaging technique arises from punctual stimulation of an area detector. 2. Material The material used for the investigation is Thermalox 995䉸 from Brush Wellmann Inc. Two main forms of detectors were studied, discs of 4 mm diameter and 0.8 mm thickness and square chips of 4.7 mm edge length and 0.5 mm thickness. The discs and chips are dry pressed and sintered, as a result they have a very good mechanical, thermal and chemical stability. Due to this and the insensitivity to humidity, even irradiations of bare detectors in water or in nutrient solution are possible. According to the intensive use of BeO in electronic industries the BeO chips can be purchased at clearly lower prices compared to standard luminophors. Furthermore the square chips offer the creation of an area detector by assembling several chips. So a complete 2D-area of an radiation field can be examined. Optical stimulation of BeO is effective in a broad band between about 420 and 550 nm with the maximum at 435 nm (Bulur and Göksu, 1998). The TL emission spectrum of BeO
354
M. Sommer et al. / Radiation Measurements 43 (2008) 353 – 356
is reported to have its mean peak at 335 nm (McKeever et al., 1995). The exact OSL emission is unknown. It is assumed to be in the same region like TL, that is why all known OSL measurements are carried out in the UV region below about 370 nm. 3. Measurement method According to Sommer et al. (2007) stimulation and reading of luminescence light are performed from opposite sides of the detector. So short distances are achieved between stimulation source, dosimeter and light sensor. The BeO chips are stimulated using intensive blue (455 nm) light emitting diodes. The small UV part of LED emission is removed by simple UV absorption foils. Luminescence light is detected by an Hamamatsu photo sensor module (PSM) H5784-03 with integrated bialkali photomultiplier, high voltage supply and preamplifier. Schott DUG11X optical filters in front of the PSM avoid light of stimulation spectrum reaching the photomultiplier. The voltage output of the sensor module is amplified in four channels of different gains and time constants. A self-developed software selects the most fitting channel and converts the OSL signal via the known gain factors into an unified value of OSL measure. The measurement of irradiated BeO detectors shows a nearly exponential decay of the voltage signal. The integral over the decay curve in a definite time interval (e.g. 0–10 s) is defined as unified OSL signal, which can be used to specify dose (see Fig. 1). According to the desire of creating a radiation imaging technique, the system was extended with a 2D axial system. So, a BeO area detector can be positioned between the stimulation element and the PSM. The stimulation light is focused on a point of 1 mm diameter by an aperture. So the punctual measurement is possible. A grid pattern of punctual measurements gives the information about the dose distribution in a radiation field. The most important limitation of its resolution is the scattering of stimulation light in the detector material. Therefore, the farther surround of the stimulation point is uncontrollably bleached.
Besides this, the OSL from surrounding contributes to the measurement. From this it follows a sensible pitch of approximately 2 mm. The new system only exemplifies for the matter of principle feasibility of tissue equivalent radiation imaging with the help of beryllium oxide. Particularly with regard to the duration of readings and the resolution of the images it is worthy of improvement. 4. Results and discussion 4.1. Explanation of the decay curve form The main part of the BeO OSL decay can be described with the single trap model combined with stimulation power dependence and light attenuation inside the detector. Single trap differential equations lead to a simple exponential decay curve form. But the decay constant and the amplitude strongly depend on stimulation power density, while the integral over the whole curve is only a function of dose (BZtter-Jensen et al., 2003): IOSL = −
dn = n · S · , dt
IOSL (t) = n0 · · S · e−t·S· = A0 (D) · S · e−S/·t . Because of the strong light attenuation in BeO (Lembo et al., 1990) the depth dependence of stimulation power inside the detectors must be added as well as weakening of the emitted OSL light: t d −S0 ·e−s ·x · −S ·x −E ·(d−x) dx IOSL(t)= A0 (D(x)) · S0 · e ·e ·e 0
with n0 and n the initial and actual concentration of trapped electrons; the photoionization cross section for trapped electrons; S the stimulation power density, S0 on the detector surface; unified decay constant; A0 the amplitude constant, depends only on the dose; x the dosimeter depth from stimulation side; D the dose, if necessary as a function of the depth; S,E the light attenuation coefficients (S,E ≈2.7 mm−1 ). Only a numerical integration of the equation is possible. It results in a stretched exponential decay which is consistent with the measured decay curves over a large range. Besides the characterized main part of the BeO decay, there is another important component. In particular for higher doses above a few 10 mGy, a slow signal photo-transferred from deep traps dominates the bleaching of the detectors (cp. Bulur, 2007). 4.2. Dosimetric properties
Fig. 1. OSL curve of BeO detector irradiated with 34 mGy.
The dosimetric properties of BeO examined in previous investigations are excellent for use in all dosimetric fields (Sommer et al., 2007). Due to the effective atomic number (Zeff = 7.14) BeO can be qualified as tissue equivalent in dosimetric practice. The absorption of dose in BeO fits the energy response of tissue well. Only a weakly underestimation of radiation doses at low energy could be measured.
M. Sommer et al. / Radiation Measurements 43 (2008) 353 – 356
355
Fig. 2. Dose characteristic of BeO chips. The linear range covers more than six orders of magnitude. Left inside: low dose growth over storage time under different environmental conditions. Right inside: response ratio of saturation doses due to 1 Gy.
Furthermore, after storage for some hours, long time fading is negligible under environmental conditions. Temperature dependence of the OSL signal could not be monitored in the normal environmental temperature range. All investigated detector forms show nearly the same dosimetric properties. Due to the influence of detector volume, thickness and surface area the absolute response distinguish between the different detectors. For the 30 mg (4.7 mm×4.7 mm× 0.5 mm) standard detector a wide range of linearity of the dose characteristic reaches from about 1 Gy up to a few Gy. For higher doses the signal saturates. This effect can be described with an saturation function which permits to specify dose up to about 100 Gy well (see Fig. 2). 4.3. Radiation imaging Using the point by point measurement the radiation field of a collimated 55 Fe source was monitored in different distances. Hundred square chips formed an area detector of 47 mm × 47 mm. It was irradiated in distances of 1 and 2.8 cm. The step width of measurement was 2.4 mm and 20 × 20 points were evaluated. A bicubic interpolation was used to convert the 400 results into an image of the dose rate distribution. The images are shown in Fig. 3a (1 cm distance) and Fig. 3b (2.8 cm distance). The dose rate distribution of the area irradiated in distance of 1 cm to the source shows a clear border between irradiated and non-irradiated area. In contrast the dose rate image of the detectors irradiated in the distance of 2.8 cm reveals a broad blurring between the two areas. Because of scattering and weakening of radiation the border smears. Furthermore the highest dose rate values in the spot of the radiation source for both distances follow the reduction of dose within the square of the distance. So, with the help of the punctual measurements, 2D-radiation fields could be monitored and basic principles in radiation physics could be controlled.
Fig. 3. (a) Dose rate distribution of an area vertical to the collimated beam of a 55 Fe radiation source, distance of 1 cm between the area and the radiation source, irradiation time 50 min. (b) Dose rate distribution of an area vertical to collimated beam of a 55 Fe radiation source, distance of 2.8 cm between the area and the radiation source, irradiation time 2 h.
Due to the stimulation of only a small point of the detector, the decrease of active detector volume leads to a lower OSL signal. Coincidently, the small points show a higher dispersal of response, which is better adjusted for stimulation of the whole detector. In addition, because of constructional conditions, the distance between detector and PSM is greater than that in normal single detector reader (12 mm instead of 8 mm). Summarizing, the lowest detection limit of the method is only in the range of a few milligray. To enhance the reading speed and the resolution a laser in combination with a focussing optic should be used. With the more intensive and focussed stimulation a lower detection limit will be reached too. 5. Application The described OSL dosimetry system was used to specify dose in a number of applications including all dose regions
356
M. Sommer et al. / Radiation Measurements 43 (2008) 353 – 356
6. Summary and future perspectives The OSL of BeO can be used to acquire dose in several applications. By measurement of OSL curve, calculating the OSL signal and with the help of a calibration the applied dose can be determined. Due to the excellent dosimetric properties, e.g. the wide range of linearity of dose characteristic and the good lowest detection limit, the described measurement method offers a wide range of application in dosimetry. Based on the near tissue equivalence and the possibility to use BeO detectors for radiation imaging techniques the shown OSL technique is predestinated for medical use. By systematic improvements of the lowest level of detection and resolution, the punctual stimulation permits detailed investigation of 2Dradiation fields. Fig. 4. Radial dose function (RDF) of 125 I brachytherapy seed.
from environmental over diagnostic and therapeutic medical doses up to high dose experiments in the range of 50 Gy (Moeckel et al., 2007). Furthermore doses in experiments with various particle radiation, e.g. and 12 C, were investigated with BeO detectors. The investigation of the radial dose function (RDF) of an 125 I brachytherapy seed shall be shown as an example for the use of the OSL measurement method with BeO. According to the requirements of the AAPM TG 43 formalism (Nath et al., 1995), the description of the radiation field of interstitial brachytherapy sources is factorized in air kerma strength, dose rate constant, geometry factor, RDF and anisotropy function. The product of the first two values gives the dose rate in water on a reference point 10 mm from the source mid point transversal to the source axis. The geometry factor represents the square distance law combined with line source corrections. Then the RDF represents radiation attenuation and scattering on the radial axis and the anisotropy function represents the angle dependence of dose for a constant distance relative to the RDF point. Usually the listed functions are described and measured in three different ways: measurements with small scintillators in water, MC calculations and solid state dosimetry in plastic phantoms, commonly with LiF microcube TL detectors. Here the RDF of an 125 I Isoseed䉸 model I25.S17 from Bebig GmbH Berlin is presented. The irradiations were performed in a Plastic Water 䉸 phantom. Thin BeO detectors with dimensions of 6.4 mm × 6.4 mm × 0.2 mm lay in a line in planes parallel to the source axis in distances of 5, 10 and 20 mm. By point stimulation a high number of points in the source transversal could be recorded for specification of the RDF (see Fig. 4). The radial dose function shows an exponential decay, which mostly illustrates the attenuation of radiation in phantom material. Therefore the decay constants matches to the mass energy absorption coefficient at the photon energy of 125 I in the used material.
Acknowledgments The authors would like to thank Dr. W. Wahl and M. Figel (GSF) for their support to this work and Dr. Y. Göksu (GSF) and Dr. A. Bos (Delft University of Technology) for important discussions of luminescence problems. In addition we want to thank M. Andreeff (University Clinic Dresden, Clinic for Nuclear Medicine) and Prof. W. Enghardt (Medical Faculty Carl Gustav Carus of TU Dresden, Forschungszentrum Dresden—Rossendorf) for their support on applications. References Albrecht, H.O., Mandeville, C.E., 1956. Storage of energy in BeO. Phys Rev. 101, 1250. BZtter-Jensen, L., McKeever, S.W.S., Wintle, A.G., 2003. Optically Stimulated Luminescence Dosimetry. Elsevier, Amsterdam. Bulur, E., 2007. Photo-transferred luminescence from BeO ceramics. Radiat., Meas., doi:10.1016/j.radmeas.2007.02.065. Bulur, E., Göksu, H.Y., 1998. OSL from BeO ceramics: new observation from an old material. Radiat. Meas. Trans 29, 639–650. Lembo, L., Pimpinella, M., Mukherjee, B., 1990. Self optical attenuation coefficient of TL glow in BeO detectors. Radiat. Prot. Dosim. 33 (1/4), 43–45. McKeever, S.W.S., Moscovitch, M., Townsend, P.D., 1995. Thermoluminescence Dosimetry Materials: Properties and Uses. Nuclear Technology Publishing, Ashford. Moeckel, D., Mueller, H., Pawelke, J., Sommer, M., Will, E., Enghardt, W., 2007. Quantification of + activity generated by hard photons by means of PET. Phys. Med. Biol. 52, 2515–2530. Moore, L.E., 1957. Thermoluminescence of sodium sulfate and lead sulfate, and miscellaneous sulfates, carbonates, and oxides. J. Phys. Chem. 61, 636–639. Nath, R., Anderson, L.L., Luxton, G., Weaver, K.A., Williamson, J.F., Meigooni, A.S., 1995. Dosimetry of interstitial brachytherapy sources: recommendations of the AAPM Radiation Therapy Committee Task Group no. 43. Med. Phys. 22 (2), 209–234. Rhyner, G.R., Miller, W.G., 1970. Radiation dosimetry by optically-stimulated luminescence of BeO. Health Phys. 18, 681–684. Sommer, M., Henniger, J., 2006. Investigation of a BeO-based optically stimulation luminescence dosimeter. Radiat. Prot. Dosim. 119, 394–397. Sommer, M., Freudenberg, R., Henniger, J., 2007. New aspects of a BeO-based optically stimulated luminescence dosimeter. Radiat. Meas., doi:10.1016/j.radmeas.2007.01.052. Tochilin, E., Goldstein, N., Miller, W.G., 1969. Beryllium oxide as a thermoluminescent dosimeter. Health Phys. 16, 1–7.
Beryllium oxide as optically stimulated luminescence dosimeter M. Sommer ∗ , A. Jahn, J. Henniger Institute of Nuclear and Particle Physics, Technische Universitaet Dresden, Radiation Physics Group, D-01062 Dresden, Germany
Abstract In the last years a dosimetric system using optically stimulated luminescence (OSL) of beryllium oxide (BeO) was developed by the radiation physics group in Dresden. Blue light LED stimulation and reading of luminescence light with an enclosed photo sensor module are performed from opposite detector sides. A software controls stimulation, records the amplified and digitized photo sensor signal and generates an unified OSL signal. With the help of calibration these OSL signal can be used to specify dose. The linearity of dose response reaches from Gy level up to a few Gy. At higher dose the OSL signal shows a saturation which can be correctly described by a saturation function up to 100 Gy. Fading of the signal is negligible. The material BeO has an excellent resistance to environmental influences. Due to the near tissue equivalence and the resulting low photon energy dependence the BeO detectors are predestinated for dosimetric use. Assembling several detectors to an area offers the ability to examine strongly inhomogeneous radiation fields. Measurements of the radial dose function of a brachytherapy seed and dose rate distributions in front of a 55 Fe source are presented. © 2007 Elsevier Ltd. All rights reserved. Keywords: Optically stimulated luminescence; OSL; Beryllium oxide; BeO; Dosimetry
1. Introduction and motivation Investigations on the luminescence properties of Beryllium oxide (BeO) were accomplished by Albrecht and Mandeville (1956). One year later the material BeO was used as TLluminophor for the first time (Moore, 1957). However the light-induced fading, measured by Tochilin et al. (1969), prevented the use of BeO as TL-material in a broad spectrum. According to the effect of light-induced fading, Rhyner and Miller (1970) suggested BeO as an optically stimulated luminescence (OSL) luminophor. But not until the work of Bulur and Göksu (1998) the OSL-properties of the material were researched in detail. Due to the high request for passive radiation detectors in medicine, personal and environmental dosimetry, the radiation physics group in Dresden investigated BeO as OSL dosimeter for a number of years (Sommer and Henniger, 2006; Sommer et al., 2007). Up to now an useful dosimetric method using the material BeO was developed which can be used to specify dose in a number of applications. According to the near tissue equivalence (Zeff = 7.14) of the material BeO, the dosimetric ∗ Corresponding author.
E-mail address: [email protected] (M. Sommer). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.11.018
system offers a wide range of appliance especially in medical fields. Beside this an dose field imaging technique arises from punctual stimulation of an area detector. 2. Material The material used for the investigation is Thermalox 995䉸 from Brush Wellmann Inc. Two main forms of detectors were studied, discs of 4 mm diameter and 0.8 mm thickness and square chips of 4.7 mm edge length and 0.5 mm thickness. The discs and chips are dry pressed and sintered, as a result they have a very good mechanical, thermal and chemical stability. Due to this and the insensitivity to humidity, even irradiations of bare detectors in water or in nutrient solution are possible. According to the intensive use of BeO in electronic industries the BeO chips can be purchased at clearly lower prices compared to standard luminophors. Furthermore the square chips offer the creation of an area detector by assembling several chips. So a complete 2D-area of an radiation field can be examined. Optical stimulation of BeO is effective in a broad band between about 420 and 550 nm with the maximum at 435 nm (Bulur and Göksu, 1998). The TL emission spectrum of BeO
354
M. Sommer et al. / Radiation Measurements 43 (2008) 353 – 356
is reported to have its mean peak at 335 nm (McKeever et al., 1995). The exact OSL emission is unknown. It is assumed to be in the same region like TL, that is why all known OSL measurements are carried out in the UV region below about 370 nm. 3. Measurement method According to Sommer et al. (2007) stimulation and reading of luminescence light are performed from opposite sides of the detector. So short distances are achieved between stimulation source, dosimeter and light sensor. The BeO chips are stimulated using intensive blue (455 nm) light emitting diodes. The small UV part of LED emission is removed by simple UV absorption foils. Luminescence light is detected by an Hamamatsu photo sensor module (PSM) H5784-03 with integrated bialkali photomultiplier, high voltage supply and preamplifier. Schott DUG11X optical filters in front of the PSM avoid light of stimulation spectrum reaching the photomultiplier. The voltage output of the sensor module is amplified in four channels of different gains and time constants. A self-developed software selects the most fitting channel and converts the OSL signal via the known gain factors into an unified value of OSL measure. The measurement of irradiated BeO detectors shows a nearly exponential decay of the voltage signal. The integral over the decay curve in a definite time interval (e.g. 0–10 s) is defined as unified OSL signal, which can be used to specify dose (see Fig. 1). According to the desire of creating a radiation imaging technique, the system was extended with a 2D axial system. So, a BeO area detector can be positioned between the stimulation element and the PSM. The stimulation light is focused on a point of 1 mm diameter by an aperture. So the punctual measurement is possible. A grid pattern of punctual measurements gives the information about the dose distribution in a radiation field. The most important limitation of its resolution is the scattering of stimulation light in the detector material. Therefore, the farther surround of the stimulation point is uncontrollably bleached.
Besides this, the OSL from surrounding contributes to the measurement. From this it follows a sensible pitch of approximately 2 mm. The new system only exemplifies for the matter of principle feasibility of tissue equivalent radiation imaging with the help of beryllium oxide. Particularly with regard to the duration of readings and the resolution of the images it is worthy of improvement. 4. Results and discussion 4.1. Explanation of the decay curve form The main part of the BeO OSL decay can be described with the single trap model combined with stimulation power dependence and light attenuation inside the detector. Single trap differential equations lead to a simple exponential decay curve form. But the decay constant and the amplitude strongly depend on stimulation power density, while the integral over the whole curve is only a function of dose (BZtter-Jensen et al., 2003): IOSL = −
dn = n · S · , dt
IOSL (t) = n0 · · S · e−t·S· = A0 (D) · S · e−S/·t . Because of the strong light attenuation in BeO (Lembo et al., 1990) the depth dependence of stimulation power inside the detectors must be added as well as weakening of the emitted OSL light: t d −S0 ·e−s ·x · −S ·x −E ·(d−x) dx IOSL(t)= A0 (D(x)) · S0 · e ·e ·e 0
with n0 and n the initial and actual concentration of trapped electrons; the photoionization cross section for trapped electrons; S the stimulation power density, S0 on the detector surface; unified decay constant; A0 the amplitude constant, depends only on the dose; x the dosimeter depth from stimulation side; D the dose, if necessary as a function of the depth; S,E the light attenuation coefficients (S,E ≈2.7 mm−1 ). Only a numerical integration of the equation is possible. It results in a stretched exponential decay which is consistent with the measured decay curves over a large range. Besides the characterized main part of the BeO decay, there is another important component. In particular for higher doses above a few 10 mGy, a slow signal photo-transferred from deep traps dominates the bleaching of the detectors (cp. Bulur, 2007). 4.2. Dosimetric properties
Fig. 1. OSL curve of BeO detector irradiated with 34 mGy.
The dosimetric properties of BeO examined in previous investigations are excellent for use in all dosimetric fields (Sommer et al., 2007). Due to the effective atomic number (Zeff = 7.14) BeO can be qualified as tissue equivalent in dosimetric practice. The absorption of dose in BeO fits the energy response of tissue well. Only a weakly underestimation of radiation doses at low energy could be measured.
M. Sommer et al. / Radiation Measurements 43 (2008) 353 – 356
355
Fig. 2. Dose characteristic of BeO chips. The linear range covers more than six orders of magnitude. Left inside: low dose growth over storage time under different environmental conditions. Right inside: response ratio of saturation doses due to 1 Gy.
Furthermore, after storage for some hours, long time fading is negligible under environmental conditions. Temperature dependence of the OSL signal could not be monitored in the normal environmental temperature range. All investigated detector forms show nearly the same dosimetric properties. Due to the influence of detector volume, thickness and surface area the absolute response distinguish between the different detectors. For the 30 mg (4.7 mm×4.7 mm× 0.5 mm) standard detector a wide range of linearity of the dose characteristic reaches from about 1 Gy up to a few Gy. For higher doses the signal saturates. This effect can be described with an saturation function which permits to specify dose up to about 100 Gy well (see Fig. 2). 4.3. Radiation imaging Using the point by point measurement the radiation field of a collimated 55 Fe source was monitored in different distances. Hundred square chips formed an area detector of 47 mm × 47 mm. It was irradiated in distances of 1 and 2.8 cm. The step width of measurement was 2.4 mm and 20 × 20 points were evaluated. A bicubic interpolation was used to convert the 400 results into an image of the dose rate distribution. The images are shown in Fig. 3a (1 cm distance) and Fig. 3b (2.8 cm distance). The dose rate distribution of the area irradiated in distance of 1 cm to the source shows a clear border between irradiated and non-irradiated area. In contrast the dose rate image of the detectors irradiated in the distance of 2.8 cm reveals a broad blurring between the two areas. Because of scattering and weakening of radiation the border smears. Furthermore the highest dose rate values in the spot of the radiation source for both distances follow the reduction of dose within the square of the distance. So, with the help of the punctual measurements, 2D-radiation fields could be monitored and basic principles in radiation physics could be controlled.
Fig. 3. (a) Dose rate distribution of an area vertical to the collimated beam of a 55 Fe radiation source, distance of 1 cm between the area and the radiation source, irradiation time 50 min. (b) Dose rate distribution of an area vertical to collimated beam of a 55 Fe radiation source, distance of 2.8 cm between the area and the radiation source, irradiation time 2 h.
Due to the stimulation of only a small point of the detector, the decrease of active detector volume leads to a lower OSL signal. Coincidently, the small points show a higher dispersal of response, which is better adjusted for stimulation of the whole detector. In addition, because of constructional conditions, the distance between detector and PSM is greater than that in normal single detector reader (12 mm instead of 8 mm). Summarizing, the lowest detection limit of the method is only in the range of a few milligray. To enhance the reading speed and the resolution a laser in combination with a focussing optic should be used. With the more intensive and focussed stimulation a lower detection limit will be reached too. 5. Application The described OSL dosimetry system was used to specify dose in a number of applications including all dose regions
356
M. Sommer et al. / Radiation Measurements 43 (2008) 353 – 356
6. Summary and future perspectives The OSL of BeO can be used to acquire dose in several applications. By measurement of OSL curve, calculating the OSL signal and with the help of a calibration the applied dose can be determined. Due to the excellent dosimetric properties, e.g. the wide range of linearity of dose characteristic and the good lowest detection limit, the described measurement method offers a wide range of application in dosimetry. Based on the near tissue equivalence and the possibility to use BeO detectors for radiation imaging techniques the shown OSL technique is predestinated for medical use. By systematic improvements of the lowest level of detection and resolution, the punctual stimulation permits detailed investigation of 2Dradiation fields. Fig. 4. Radial dose function (RDF) of 125 I brachytherapy seed.
from environmental over diagnostic and therapeutic medical doses up to high dose experiments in the range of 50 Gy (Moeckel et al., 2007). Furthermore doses in experiments with various particle radiation, e.g. and 12 C, were investigated with BeO detectors. The investigation of the radial dose function (RDF) of an 125 I brachytherapy seed shall be shown as an example for the use of the OSL measurement method with BeO. According to the requirements of the AAPM TG 43 formalism (Nath et al., 1995), the description of the radiation field of interstitial brachytherapy sources is factorized in air kerma strength, dose rate constant, geometry factor, RDF and anisotropy function. The product of the first two values gives the dose rate in water on a reference point 10 mm from the source mid point transversal to the source axis. The geometry factor represents the square distance law combined with line source corrections. Then the RDF represents radiation attenuation and scattering on the radial axis and the anisotropy function represents the angle dependence of dose for a constant distance relative to the RDF point. Usually the listed functions are described and measured in three different ways: measurements with small scintillators in water, MC calculations and solid state dosimetry in plastic phantoms, commonly with LiF microcube TL detectors. Here the RDF of an 125 I Isoseed䉸 model I25.S17 from Bebig GmbH Berlin is presented. The irradiations were performed in a Plastic Water 䉸 phantom. Thin BeO detectors with dimensions of 6.4 mm × 6.4 mm × 0.2 mm lay in a line in planes parallel to the source axis in distances of 5, 10 and 20 mm. By point stimulation a high number of points in the source transversal could be recorded for specification of the RDF (see Fig. 4). The radial dose function shows an exponential decay, which mostly illustrates the attenuation of radiation in phantom material. Therefore the decay constants matches to the mass energy absorption coefficient at the photon energy of 125 I in the used material.
Acknowledgments The authors would like to thank Dr. W. Wahl and M. Figel (GSF) for their support to this work and Dr. Y. Göksu (GSF) and Dr. A. Bos (Delft University of Technology) for important discussions of luminescence problems. In addition we want to thank M. Andreeff (University Clinic Dresden, Clinic for Nuclear Medicine) and Prof. W. Enghardt (Medical Faculty Carl Gustav Carus of TU Dresden, Forschungszentrum Dresden—Rossendorf) for their support on applications. References Albrecht, H.O., Mandeville, C.E., 1956. Storage of energy in BeO. Phys Rev. 101, 1250. BZtter-Jensen, L., McKeever, S.W.S., Wintle, A.G., 2003. Optically Stimulated Luminescence Dosimetry. Elsevier, Amsterdam. Bulur, E., 2007. Photo-transferred luminescence from BeO ceramics. Radiat., Meas., doi:10.1016/j.radmeas.2007.02.065. Bulur, E., Göksu, H.Y., 1998. OSL from BeO ceramics: new observation from an old material. Radiat. Meas. Trans 29, 639–650. Lembo, L., Pimpinella, M., Mukherjee, B., 1990. Self optical attenuation coefficient of TL glow in BeO detectors. Radiat. Prot. Dosim. 33 (1/4), 43–45. McKeever, S.W.S., Moscovitch, M., Townsend, P.D., 1995. Thermoluminescence Dosimetry Materials: Properties and Uses. Nuclear Technology Publishing, Ashford. Moeckel, D., Mueller, H., Pawelke, J., Sommer, M., Will, E., Enghardt, W., 2007. Quantification of + activity generated by hard photons by means of PET. Phys. Med. Biol. 52, 2515–2530. Moore, L.E., 1957. Thermoluminescence of sodium sulfate and lead sulfate, and miscellaneous sulfates, carbonates, and oxides. J. Phys. Chem. 61, 636–639. Nath, R., Anderson, L.L., Luxton, G., Weaver, K.A., Williamson, J.F., Meigooni, A.S., 1995. Dosimetry of interstitial brachytherapy sources: recommendations of the AAPM Radiation Therapy Committee Task Group no. 43. Med. Phys. 22 (2), 209–234. Rhyner, G.R., Miller, W.G., 1970. Radiation dosimetry by optically-stimulated luminescence of BeO. Health Phys. 18, 681–684. Sommer, M., Henniger, J., 2006. Investigation of a BeO-based optically stimulation luminescence dosimeter. Radiat. Prot. Dosim. 119, 394–397. Sommer, M., Freudenberg, R., Henniger, J., 2007. New aspects of a BeO-based optically stimulated luminescence dosimeter. Radiat. Meas., doi:10.1016/j.radmeas.2007.01.052. Tochilin, E., Goldstein, N., Miller, W.G., 1969. Beryllium oxide as a thermoluminescent dosimeter. Health Phys. 16, 1–7.