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Investigations of OSL properties of LiMgPO4:Tb,B based dosimeters
Radiation Measurements xxx (2016) 1e4
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Investigations of OSL properties of LiMgPO4:Tb,B based dosimeters bel, M. Kłosowski B. Marczewska*, P. Bilski, D. Wro Institute of Nuclear Physics Polish Academy of Sciences (IFJ PAN), Radzikowskiego 152, PL-31-342, Krakow, Poland
h i g h l i g h t s A compact OSL dosimetric system was developed. Dosimetric cards are based on LiMgPO4 (LMP) material. LMP dosimeters show high sensitivity and linear dose response.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 October 2015 Received in revised form 29 January 2016 Accepted 4 February 2016 Available online xxx
Lithium magnesium phosphate LiMgPO4 (LMP) doped with Tb and B is one of new materials intended for use in optically stimulated luminescence (OSL) dosimetry. LMP doped with Tb and B luminophors were synthetized at IFJ PAN in Krakow. The investigations were carried out on self-developed dosimeters consisting of a slide with four LMP detectors and a light tight cover. LMP detectors were investigated in regard to their OSL properties using OSL reader named HELIOS adopted to the readouts of dosimetric cards. New LMP detectors showed high sensitivity to the ionizing radiation, good repeatability of OSL signal and good dose response, 25% of fading in the first two weeks after irradiation. Also, the pronounced dependence of OSL response on the energy of the measured radiation requires to apply the compensation filters. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Optically stimulated luminescence Luminophor Dosimeter OSL reader
1. Introduction Nowadays, optically stimulated luminescence is a fast developing dosimetric method due to the technical progress in stimulation technology, LED evolution, improvement of electronic and optical components. In opposite to thermoluminecence (TL), which offers a variety of luminophors, material resources for OSL method are rather poor. The number of commercially available OSL materials is basically limited only to two of them, namely Al2O3:C and BeO [Akselrod et al., 1998; Jahn et al., 2014]. This limited availability stimulates efforts for development of new OSL dosimetric materials. Some literature references indicated that lithium magnesium phosphate LiMgPO4 (LMP) is a material which can compete with such well known OSL materials like BeO or Al2O3. TL and OSL properties of LMP activated by Tb or both Tb and B were investigated by Kumar and Menon [Menon et al., 2012; Kumar et al., 2011, Singh et al., 2012]. Gai [Gai et al., 2015] has investigated OSL properties of LiMgPO4:Eu,Sm,B. LMP doped with Eu was the object
* Corresponding author. E-mail address: [email protected] (B. Marczewska).
of the study of Baran [Baran et al., 2014] and Zhang [Zhang et al., 2010]. Enciso-Maldonado [Enciso-Maldonado et al., 2015] studied lithium vacancy introduction into LMP. The research on the optimization of Tb and B concentration in regard to obtain the best OSL properties of LMP has been carried out bel et al., 2014]. Our present work also at IFJ PAN in Krakow [Wro was aimed on the developing of a compact dosimetric system consisting of the LMP in a form of thin foils, dosimeters in a form of dosimetric cards with 4 detectors inside a plastic cover and an OSL reader adapted to the measurement of each detector in covered dosimeters. We report here our latest results of the OSL readout tests, such as sensitivity, repeatability, dose and energy response, which were investigated for the developed LMP material tested in a new dosimetric system. 2. Material and methods 2.1. Material LiMgPO4:Tb,B phosphor has been prepared by solid state reaction between LiOH$H2O (Alfa Aesar 99,995%), Mg(NO3)2$6H2O (Alfa Aesar 99,999%) and NH4H2PO4 (SigmaeAldrich 99,99%) in air.
http://dx.doi.org/10.1016/j.radmeas.2016.02.004 1350-4487/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Marczewska, B., et al., Investigations of OSL properties of LiMgPO4:Tb,B based dosimeters, Radiation Measurements (2016), http://dx.doi.org/10.1016/j.radmeas.2016.02.004
2
B. Marczewska et al. / Radiation Measurements xxx (2016) 1e4
Tb4O7 (terbium oxide) and H3BO3 (boric acid) or Na2B4O7 (sodium borate) were used to introduce terbium (Tb) and boron (B) dopants into the phosphor during synthesis. Stoichiometric quantities of the starting materials were ground together in an agate mortar. The resulting mixtures were heated in an open crucible at 200 C for 1 h and next were cooled down and triturated in a mortar. Subsequently the powdered mixtures were thermally processed in following steps: 300 C for 1 h then at 600 C for the next 1 h. This process was followed by further heating at 750 C for 20 h with an intermediate regrinding after 10 h. The final products were cooled down to room temperature. In this way more than 20 different samples of LMP were manufactured. The Tb concentration was varied between 0.5 mol% and 1.2 mol %, and B concentration between 5 mol% and 10 mol%. After preliminary investigations in regard to the sensitivity and signal stability, three samples, named LMP10, LMP14 and LMP20 were selected for further studies - see Table 1. The final result of the synthesis was a white powder, which had to be turned into a solid form, in order to be mounted in dosimeters. For this purpose a technique of making fluoropolymer foils containing a powdered luminophor, originally developed for manufacturing of large area 2D TL detectors [Olko et al., 2006], was used. The foils of 0.1 mm thickness were made from a mixture of LMP powder and ETFE (ethylene-tetrafluoroethylene) polymer (1:2 ratio by mass) by hot pressing. Finally, 6 mm diameter discs were cut out of the foils. 2.2. Dosimetric cassettes and OSL measurements A new dosimetric badges were developed in frame of the present project. The slides with 4 positions for detectors and the lightproof covers were designed (Fig. 1B) and manufactured from black ABS plastic using technique of high resolution 3D printing. These covers were used during the tests, but is planned that the final version will be equipped with energy compensation filters of chosen materials (Fig. 1C). The quality of 3D printed cassettes was found satisfactory, as the tests showed that the cards and covers were mechanically stable and resistant to deformation during multiple cycles of opening and closing. The tests of light tightness were also performed and proved that there was no light leakage. The initial measurements of all new synthesized powder samples, as well as discs cut from the foils were performed using Risø DA-20 TL/OSL reader (Risø DTU, Denmark). DA-20 reader detection system is equipped with a band pass filter U-340 (Hoya) allowing to register the light with the wavelength from 300 to 400 nm (UV range). The irradiations were conducted using beta source (Sr-90/Y90) built in the DA-20 reader. For optical stimulation 28 blue LEDs (peak emission at 470 nm) were used. TL and OSL signals of powders were normalized to the mass of samples. For the readouts of the dosimeters a modified version of HELIOS-1 OSL reader [Mandowski et al., 2010; Piaskowska et al., 2013] was applied. The OSL reader (Fig. 1A), originally developed for readout of single samples, is equipped with 5 W blue diodes, photomultiplier H7360 and UG-11 filter for light detection (transmission 300e400 nm). The reader was adapted for readouts of the dosimeters by installing a specially designed mechanical system for
Table 1 The detailed composition of the selected LMP samples. Tb was always added in form of Tb4O7. Sample
Terbium
Boron
LMP10 LMP14 LMP20
0.8 mol% 0.8 mol% 0.8 mol%
5 mol% (H3BO3) 10 mol% (H3BO3) 10 mol% (Na2B4O7)
Fig. 1. A photo of HELIOS OSL reader with a dosimeter inside the measurement drawer (A) and the photos of dosimetric cassettes made of black ABS with a lightproof cover (B) and with cover with openings foreseen for filters (C).
manual transport of dosimeters into the reader, their opening inside the reader and movement of the slide to consecutive reading positions. It is planned that the final version of the system will consist of both manual and automatic readers. The readouts were proceeded with the CW-OSL stimulation mode. The signal was recorded every 0.1 s during a typical measurement period of 60 s. As a final result, the total sum of OSL was taken, after subtracting the background, which was measured for non-irradiated detectors. Irradiations of the dosimeters were done mostly with Cs-137 gamma-rays at the calibration laboratory of the IFJ PAN in w. X-rays at the Institute of Occupational Medicine in Ło dz, Krako Poland were used as well to study the dependence of the OSL response on the energy of the incident X-ray photons. 3. Results Fig. 2 presents the OSL decay curves (normalized to the mass of samples) for LMP10, LMP14 and LMP20 in comparison with Al2O3:C. OSL sensitivity of LMP powders is similar or even higher than that of Al2O3:C. It should be however noted that the applied measuring conditions (detection wavelengths) were not optimal for aluminum oxide. The LMP OSL signal was found to be unstable and to decay spontaneously after irradiation. The loss of the signal during 2 weeks after irradiation, with respect to the signal after 24 h is 25% for LMP14 and LMP20 and then stabilizes. The bleaching conditions were optimized using several discs cut from LMP10 foils and performing OSL readout in the HELIOS reader. All detectors were irradiated in uniform radiation field of gamma rays with a dose of 0.5 Gy and next bleached for 30 min under
Please cite this article in press as: Marczewska, B., et al., Investigations of OSL properties of LiMgPO4:Tb,B based dosimeters, Radiation Measurements (2016), http://dx.doi.org/10.1016/j.radmeas.2016.02.004
B. Marczewska et al. / Radiation Measurements xxx (2016) 1e4
Fig. 2. CW-OSL decay curves of LMP10, LMP14 and LMP20 samples in a form of powders, measured in Risø reader after irradiation with 1 Gy.
Fig. 3. Effect of 30 min bleaching of LMP10 foil discs under blue light of 460 nm (previously irradiated with 0.5 Gy).
various light sources: 254 nm, 302 nm, 365 nm, 460 nm 530 nm and under normal white day light. The most effective bleaching was found for blue light of 460 nm wavelength. The duration of bleaching depends on the dose. For the dose of 0.5 Gy the 30 min
3
Fig. 5. Photon energy response for LMP20 (details in the text).
bleaching is sufficient to erase the rest of the signal (Fig. 3). The tests of the measurement repeatability were performed with the group of 40 detectors mounted in 10 dosimeters, including 20 detectors of LMP-14 and 20 detectors of LMP-20. Four measurement series were carried out, each consisting of an irradiation with 10 mGy of gamma-rays, followed by a readout in the HELIOS reader and 30 min bleaching under blue light. The standard deviation of OSL signals within detectors of one type were 7.5% for LMP14 and 6.5% for LMP20. The standard deviations of the consecutive four readouts of a single detector were below 1% for LMP14 and below 2% for LMP20. This indicates very good reproducibility of the whole measuring cycle. The results of the repeatability tests were used for calculation of individual reference factors (IRFs) of all detectors, which were applied in the further measurements. The dose response measurements were proceeded for the dose range from 0.05 mGy to 1 Gy. For each dose 5 dosimeters with LMP14 detectors and 5 dosimeters with LMP20 were used. The readouts were performed immediately after irradiation. The calculation of the signal, after subtracting the background, was done using IRF computed in the previous experiments. The results are presented in Fig. 4 showing good linearity. Photon energy response measurements were performed for dosimeters made of LMP20 material. Irradiations were realized with the ISO Narrow Series X-rays (ISO 1999), with mean energy between 33 keV and 208 keV, as well as with Cs-137 gamma-rays in terms of Hp(10). The readouts of all dosimeters were done two weeks after irradiation, what should minimize any fading effects. Fig. 5 presents the relative energy response, normalized to 662 keV photons (Cs-137). The observed overresponse for low energy photons was somewhat lower than expected according to the mass energy absorption coefficients for LMP (dashed line in Fig. 5). This is probably a result of the presence of ETFE polymer. Nevertheless, it is apparent that application of filters (e.g. copper or lead) will be needed, in order to establish energy correction algorithm, in a similar manner as it is done for Al2O3 (Yukihara and McKeever, 2011). This need was already foreseen during designing the dosimeter covers, which has openings to accommodate such filters (Fig. 1C). 4. Conclusions
Fig. 4. Dose response of LMP14 and LMP20 for gamma rays (Cs-137).
A new compact dosimetric system consisting of a modified HELIOS reader and dosimeters with 4 detectors placed in light tight
Please cite this article in press as: Marczewska, B., et al., Investigations of OSL properties of LiMgPO4:Tb,B based dosimeters, Radiation Measurements (2016), http://dx.doi.org/10.1016/j.radmeas.2016.02.004
4
B. Marczewska et al. / Radiation Measurements xxx (2016) 1e4
covers was developed for laboratory applications. The tests of the system showed its simplicity and usefulness. All new LMP materials (LMP10, LMP14 and LMP20), regardless of chemical process of preparing, showed high sensitivity to the ionizing radiation, comparable to that of Al2O3:C, good reproducibility of the OSL signal, linear dose response in the range between 0.05 mGy and 1 Gy. The photon energy characteristic shows an overresponse for low energies, as expected for a compound with Zeff ¼ 11.4. Despite the high fading LMP seems to be an attractive luminophor. Further work should be devoted to improvement of time stability of OSL signal after irradiation, by the chemical or thermal treatment and selection of the compensation filters for correction of the energy dependence. Acknowledgments This work was supported by the National Centre for Research and Development (contract No PBS1/A9/4/2012). References Akselrod, M.S., Lukas, A.C., Polf, J.C., McKeever, S.W.S., 1998. Optically stimulated luminescence of Al2O3. Radiat. Meas. 29, 391e399. Baran, Mahlik S., Grinberg, M., Cai, P., Kim, S.I., Seo, H.J., 2014. Luminescence properties of different Eu sites in LiMgPO4:Eu2þ, Eu3þ. J. Phys. Condens. Matter 26, 385401. Enciso-Maldonado, L., Dyer, M.S., Jones, M.D., Li, M., Payne, J.L., Pitcher, M.J., Omir, M.K., Claridge, J.B., Blanc, F., Rosseinsky, M.J., 2015. Computational
identification and experimental realization of lithium vacancy introduction into the olivine LiMgPO4. Chem. Mater 27, 2074e2091. Gai, M., Chen, Z., Fan, Y., Yan, S., Xie, Y., Wang, J., Zhang, Y., 2015. Synthesis of LiMgPO4:Eu,Sm, B phosphors and investigation of their optically stimulated luminescence properties. Rad. Meas. 78, 48e52. ISO 4037-3, 1999. X and Gamma Reference Radiation for Calibrating Dosemeters and Doserate Meters and for Determining Their Response as a Function of Photon Energy e Part 3: Calibration of Area and Personal Dosemeters and the Measurement of Their Response as a Function of Energy and Angle of Incidence. Jahn, A., Sommer, M., Henniger, J., 2014. OSL efficiency for BeO OSL dosimeters. Rad. Meas. 71, 104e107. Kumar, M., Dhabekar, B., Menon, S.N., Chougaonkar, M.P., Mayya, Y.S., 2011. LiMgPO4:Tb, B OSL phosphor e CW and LM OSL studies. Nim. B 269, 1849e1854. Mandowski, A., Mandowska, E., Kokot, L., Bilski, P., Olko, P., Marczewska, B., 2010. _ n _ Mobilny system wykrywania zagroze radiacyjnych przy uzyciu w OSL. Elektronika 51, 136e138 (in Polish). mikrodetektoro Menon, S.N., Dhabekar, B., Alagu Raja, E., Chougaonkar, M.P., 2012. Preparation and TSL studies in Tb activated LiMgPO4 phosphor. Rad. Meas. 47, 236e240. Olko, P., Marczewska, B., Czopyk, L., Czermak, M.A., Kłosowski, M., rski, M.P.R., 2006. New 2D dosimetric technique for radiotherapy based Waligo on planar thermoluminescence detectors. Radiat. Prot. Dosim. 118 (2), 213e218. Piaskowska, A., Marczewska, B., Bilski, P., Mandowski, A., Mandowska, E., 2013. Photoluminescence measurements of LiF TL detectors. Rad. Meas. 56, 209e212. Singh, A.K., Menon, S.N., Dhabekar, B., Kadam, S., Chougaonkar, M.P., Mayya, Y.S., 2012. TLeOSL correlation studies of LiMgPO4:Tb, B dosimetric phosphor. Nim. B 274, 177e181. bel, D., Bilski, P., Marczewska, B., Kłosowski, M., 2014. TL and OSL Properties of Wro LiMgPO4:Tb, B, OMEE. Publisher IEEE. http://dx.doi.org/10.1109/OMEE.2014. 6912439. Yukihara, E.G., McKeever, S.W.S., 2011. Optically Stimulated Luminescence: Fundamentals and Applications. John Wiley&Sons, Chichester, WestSussex, UK. Zhang, S., Huang, Y., Shi, L., Seo, H.J., 2010. The luminescence characterization and structure of Eu2þ doped LiMgPO4. J. Phys. Condens. Matter 22, 235402.
Please cite this article in press as: Marczewska, B., et al., Investigations of OSL properties of LiMgPO4:Tb,B based dosimeters, Radiation Measurements (2016), http://dx.doi.org/10.1016/j.radmeas.2016.02.004
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Investigations of OSL properties of LiMgPO4:Tb,B based dosimeters bel, M. Kłosowski B. Marczewska*, P. Bilski, D. Wro Institute of Nuclear Physics Polish Academy of Sciences (IFJ PAN), Radzikowskiego 152, PL-31-342, Krakow, Poland
h i g h l i g h t s A compact OSL dosimetric system was developed. Dosimetric cards are based on LiMgPO4 (LMP) material. LMP dosimeters show high sensitivity and linear dose response.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 October 2015 Received in revised form 29 January 2016 Accepted 4 February 2016 Available online xxx
Lithium magnesium phosphate LiMgPO4 (LMP) doped with Tb and B is one of new materials intended for use in optically stimulated luminescence (OSL) dosimetry. LMP doped with Tb and B luminophors were synthetized at IFJ PAN in Krakow. The investigations were carried out on self-developed dosimeters consisting of a slide with four LMP detectors and a light tight cover. LMP detectors were investigated in regard to their OSL properties using OSL reader named HELIOS adopted to the readouts of dosimetric cards. New LMP detectors showed high sensitivity to the ionizing radiation, good repeatability of OSL signal and good dose response, 25% of fading in the first two weeks after irradiation. Also, the pronounced dependence of OSL response on the energy of the measured radiation requires to apply the compensation filters. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Optically stimulated luminescence Luminophor Dosimeter OSL reader
1. Introduction Nowadays, optically stimulated luminescence is a fast developing dosimetric method due to the technical progress in stimulation technology, LED evolution, improvement of electronic and optical components. In opposite to thermoluminecence (TL), which offers a variety of luminophors, material resources for OSL method are rather poor. The number of commercially available OSL materials is basically limited only to two of them, namely Al2O3:C and BeO [Akselrod et al., 1998; Jahn et al., 2014]. This limited availability stimulates efforts for development of new OSL dosimetric materials. Some literature references indicated that lithium magnesium phosphate LiMgPO4 (LMP) is a material which can compete with such well known OSL materials like BeO or Al2O3. TL and OSL properties of LMP activated by Tb or both Tb and B were investigated by Kumar and Menon [Menon et al., 2012; Kumar et al., 2011, Singh et al., 2012]. Gai [Gai et al., 2015] has investigated OSL properties of LiMgPO4:Eu,Sm,B. LMP doped with Eu was the object
* Corresponding author. E-mail address: [email protected] (B. Marczewska).
of the study of Baran [Baran et al., 2014] and Zhang [Zhang et al., 2010]. Enciso-Maldonado [Enciso-Maldonado et al., 2015] studied lithium vacancy introduction into LMP. The research on the optimization of Tb and B concentration in regard to obtain the best OSL properties of LMP has been carried out bel et al., 2014]. Our present work also at IFJ PAN in Krakow [Wro was aimed on the developing of a compact dosimetric system consisting of the LMP in a form of thin foils, dosimeters in a form of dosimetric cards with 4 detectors inside a plastic cover and an OSL reader adapted to the measurement of each detector in covered dosimeters. We report here our latest results of the OSL readout tests, such as sensitivity, repeatability, dose and energy response, which were investigated for the developed LMP material tested in a new dosimetric system. 2. Material and methods 2.1. Material LiMgPO4:Tb,B phosphor has been prepared by solid state reaction between LiOH$H2O (Alfa Aesar 99,995%), Mg(NO3)2$6H2O (Alfa Aesar 99,999%) and NH4H2PO4 (SigmaeAldrich 99,99%) in air.
http://dx.doi.org/10.1016/j.radmeas.2016.02.004 1350-4487/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Marczewska, B., et al., Investigations of OSL properties of LiMgPO4:Tb,B based dosimeters, Radiation Measurements (2016), http://dx.doi.org/10.1016/j.radmeas.2016.02.004
2
B. Marczewska et al. / Radiation Measurements xxx (2016) 1e4
Tb4O7 (terbium oxide) and H3BO3 (boric acid) or Na2B4O7 (sodium borate) were used to introduce terbium (Tb) and boron (B) dopants into the phosphor during synthesis. Stoichiometric quantities of the starting materials were ground together in an agate mortar. The resulting mixtures were heated in an open crucible at 200 C for 1 h and next were cooled down and triturated in a mortar. Subsequently the powdered mixtures were thermally processed in following steps: 300 C for 1 h then at 600 C for the next 1 h. This process was followed by further heating at 750 C for 20 h with an intermediate regrinding after 10 h. The final products were cooled down to room temperature. In this way more than 20 different samples of LMP were manufactured. The Tb concentration was varied between 0.5 mol% and 1.2 mol %, and B concentration between 5 mol% and 10 mol%. After preliminary investigations in regard to the sensitivity and signal stability, three samples, named LMP10, LMP14 and LMP20 were selected for further studies - see Table 1. The final result of the synthesis was a white powder, which had to be turned into a solid form, in order to be mounted in dosimeters. For this purpose a technique of making fluoropolymer foils containing a powdered luminophor, originally developed for manufacturing of large area 2D TL detectors [Olko et al., 2006], was used. The foils of 0.1 mm thickness were made from a mixture of LMP powder and ETFE (ethylene-tetrafluoroethylene) polymer (1:2 ratio by mass) by hot pressing. Finally, 6 mm diameter discs were cut out of the foils. 2.2. Dosimetric cassettes and OSL measurements A new dosimetric badges were developed in frame of the present project. The slides with 4 positions for detectors and the lightproof covers were designed (Fig. 1B) and manufactured from black ABS plastic using technique of high resolution 3D printing. These covers were used during the tests, but is planned that the final version will be equipped with energy compensation filters of chosen materials (Fig. 1C). The quality of 3D printed cassettes was found satisfactory, as the tests showed that the cards and covers were mechanically stable and resistant to deformation during multiple cycles of opening and closing. The tests of light tightness were also performed and proved that there was no light leakage. The initial measurements of all new synthesized powder samples, as well as discs cut from the foils were performed using Risø DA-20 TL/OSL reader (Risø DTU, Denmark). DA-20 reader detection system is equipped with a band pass filter U-340 (Hoya) allowing to register the light with the wavelength from 300 to 400 nm (UV range). The irradiations were conducted using beta source (Sr-90/Y90) built in the DA-20 reader. For optical stimulation 28 blue LEDs (peak emission at 470 nm) were used. TL and OSL signals of powders were normalized to the mass of samples. For the readouts of the dosimeters a modified version of HELIOS-1 OSL reader [Mandowski et al., 2010; Piaskowska et al., 2013] was applied. The OSL reader (Fig. 1A), originally developed for readout of single samples, is equipped with 5 W blue diodes, photomultiplier H7360 and UG-11 filter for light detection (transmission 300e400 nm). The reader was adapted for readouts of the dosimeters by installing a specially designed mechanical system for
Table 1 The detailed composition of the selected LMP samples. Tb was always added in form of Tb4O7. Sample
Terbium
Boron
LMP10 LMP14 LMP20
0.8 mol% 0.8 mol% 0.8 mol%
5 mol% (H3BO3) 10 mol% (H3BO3) 10 mol% (Na2B4O7)
Fig. 1. A photo of HELIOS OSL reader with a dosimeter inside the measurement drawer (A) and the photos of dosimetric cassettes made of black ABS with a lightproof cover (B) and with cover with openings foreseen for filters (C).
manual transport of dosimeters into the reader, their opening inside the reader and movement of the slide to consecutive reading positions. It is planned that the final version of the system will consist of both manual and automatic readers. The readouts were proceeded with the CW-OSL stimulation mode. The signal was recorded every 0.1 s during a typical measurement period of 60 s. As a final result, the total sum of OSL was taken, after subtracting the background, which was measured for non-irradiated detectors. Irradiations of the dosimeters were done mostly with Cs-137 gamma-rays at the calibration laboratory of the IFJ PAN in w. X-rays at the Institute of Occupational Medicine in Ło dz, Krako Poland were used as well to study the dependence of the OSL response on the energy of the incident X-ray photons. 3. Results Fig. 2 presents the OSL decay curves (normalized to the mass of samples) for LMP10, LMP14 and LMP20 in comparison with Al2O3:C. OSL sensitivity of LMP powders is similar or even higher than that of Al2O3:C. It should be however noted that the applied measuring conditions (detection wavelengths) were not optimal for aluminum oxide. The LMP OSL signal was found to be unstable and to decay spontaneously after irradiation. The loss of the signal during 2 weeks after irradiation, with respect to the signal after 24 h is 25% for LMP14 and LMP20 and then stabilizes. The bleaching conditions were optimized using several discs cut from LMP10 foils and performing OSL readout in the HELIOS reader. All detectors were irradiated in uniform radiation field of gamma rays with a dose of 0.5 Gy and next bleached for 30 min under
Please cite this article in press as: Marczewska, B., et al., Investigations of OSL properties of LiMgPO4:Tb,B based dosimeters, Radiation Measurements (2016), http://dx.doi.org/10.1016/j.radmeas.2016.02.004
B. Marczewska et al. / Radiation Measurements xxx (2016) 1e4
Fig. 2. CW-OSL decay curves of LMP10, LMP14 and LMP20 samples in a form of powders, measured in Risø reader after irradiation with 1 Gy.
Fig. 3. Effect of 30 min bleaching of LMP10 foil discs under blue light of 460 nm (previously irradiated with 0.5 Gy).
various light sources: 254 nm, 302 nm, 365 nm, 460 nm 530 nm and under normal white day light. The most effective bleaching was found for blue light of 460 nm wavelength. The duration of bleaching depends on the dose. For the dose of 0.5 Gy the 30 min
3
Fig. 5. Photon energy response for LMP20 (details in the text).
bleaching is sufficient to erase the rest of the signal (Fig. 3). The tests of the measurement repeatability were performed with the group of 40 detectors mounted in 10 dosimeters, including 20 detectors of LMP-14 and 20 detectors of LMP-20. Four measurement series were carried out, each consisting of an irradiation with 10 mGy of gamma-rays, followed by a readout in the HELIOS reader and 30 min bleaching under blue light. The standard deviation of OSL signals within detectors of one type were 7.5% for LMP14 and 6.5% for LMP20. The standard deviations of the consecutive four readouts of a single detector were below 1% for LMP14 and below 2% for LMP20. This indicates very good reproducibility of the whole measuring cycle. The results of the repeatability tests were used for calculation of individual reference factors (IRFs) of all detectors, which were applied in the further measurements. The dose response measurements were proceeded for the dose range from 0.05 mGy to 1 Gy. For each dose 5 dosimeters with LMP14 detectors and 5 dosimeters with LMP20 were used. The readouts were performed immediately after irradiation. The calculation of the signal, after subtracting the background, was done using IRF computed in the previous experiments. The results are presented in Fig. 4 showing good linearity. Photon energy response measurements were performed for dosimeters made of LMP20 material. Irradiations were realized with the ISO Narrow Series X-rays (ISO 1999), with mean energy between 33 keV and 208 keV, as well as with Cs-137 gamma-rays in terms of Hp(10). The readouts of all dosimeters were done two weeks after irradiation, what should minimize any fading effects. Fig. 5 presents the relative energy response, normalized to 662 keV photons (Cs-137). The observed overresponse for low energy photons was somewhat lower than expected according to the mass energy absorption coefficients for LMP (dashed line in Fig. 5). This is probably a result of the presence of ETFE polymer. Nevertheless, it is apparent that application of filters (e.g. copper or lead) will be needed, in order to establish energy correction algorithm, in a similar manner as it is done for Al2O3 (Yukihara and McKeever, 2011). This need was already foreseen during designing the dosimeter covers, which has openings to accommodate such filters (Fig. 1C). 4. Conclusions
Fig. 4. Dose response of LMP14 and LMP20 for gamma rays (Cs-137).
A new compact dosimetric system consisting of a modified HELIOS reader and dosimeters with 4 detectors placed in light tight
Please cite this article in press as: Marczewska, B., et al., Investigations of OSL properties of LiMgPO4:Tb,B based dosimeters, Radiation Measurements (2016), http://dx.doi.org/10.1016/j.radmeas.2016.02.004
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B. Marczewska et al. / Radiation Measurements xxx (2016) 1e4
covers was developed for laboratory applications. The tests of the system showed its simplicity and usefulness. All new LMP materials (LMP10, LMP14 and LMP20), regardless of chemical process of preparing, showed high sensitivity to the ionizing radiation, comparable to that of Al2O3:C, good reproducibility of the OSL signal, linear dose response in the range between 0.05 mGy and 1 Gy. The photon energy characteristic shows an overresponse for low energies, as expected for a compound with Zeff ¼ 11.4. Despite the high fading LMP seems to be an attractive luminophor. Further work should be devoted to improvement of time stability of OSL signal after irradiation, by the chemical or thermal treatment and selection of the compensation filters for correction of the energy dependence. Acknowledgments This work was supported by the National Centre for Research and Development (contract No PBS1/A9/4/2012). References Akselrod, M.S., Lukas, A.C., Polf, J.C., McKeever, S.W.S., 1998. Optically stimulated luminescence of Al2O3. Radiat. Meas. 29, 391e399. Baran, Mahlik S., Grinberg, M., Cai, P., Kim, S.I., Seo, H.J., 2014. Luminescence properties of different Eu sites in LiMgPO4:Eu2þ, Eu3þ. J. Phys. Condens. Matter 26, 385401. Enciso-Maldonado, L., Dyer, M.S., Jones, M.D., Li, M., Payne, J.L., Pitcher, M.J., Omir, M.K., Claridge, J.B., Blanc, F., Rosseinsky, M.J., 2015. Computational
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Please cite this article in press as: Marczewska, B., et al., Investigations of OSL properties of LiMgPO4:Tb,B based dosimeters, Radiation Measurements (2016), http://dx.doi.org/10.1016/j.radmeas.2016.02.004