Radiation Protection Dosimetry Advance Access published December 30, 2013 Radiation Protection Dosimetry (2013), pp. 1–7
doi:10.1093/rpd/nct352
CHARACTERISATION OF OSL AND OSLN DROPLETS FOR DOSIMETRY L. F. Nascimento1,2,*, E. D’Agostino1, A. C. S. Vaniqui1, C. Saldarriaga1, F. Vanhavere1 and Y. De Deene2,3 1 SCK†CEN Belgian Nuclear Research Centre, Boeretang 200, Mol, Belgium 2 Radiotherapy Department, Gent University, De Pintelaan, 185, Gent 9000, Belgium 3 Institute of Medical Physics, School of Physics, University of Sydney, Sydney, Australia
In spite of considerable progress in neutron dosimetry, there is no dosemeter that is capable of measuring neutron doses independently of the neutron spectrum with good accuracy. Carbon-doped aluminium oxide (Al2O3:C) is a sensitive material for ionising radiation (beta-ray, X ray and electron) and has been used for applications in personal and medical dosimetry as an optically stimulated luminescence (OSL) dosemeter. Al2O3:C has a low sensitivity to neutron radiation; this prevents its application to neutron fields, representing a disadvantage of Al2O3:C-OSL when compared with LiF, which is used as a thermoluminescent detector. Recently an improvement for neutron dosimetry (Passmore and Kirr. Neutron response characterisation of an OSL neutron dosemeter. Radiat. Prot. Dosim. 2011;144:155 –60) uses Al2O3:C coated with 6Li2CO3 (OSLN),which gives the highsensitive response as known for Al2O3:C with the advantage of being also sensitive to thermal neutrons. In this article, the authors compare small-size detectors (droplets) of Al2O3:C (OSL) and of Al2O3:C1 6Li2CO3 (OSLN) and discuss the advantages and drawbacks of both materials, regarding size vs. response.
INTRODUCTION Man-made neutron fields are found in the controlled areas of power plants, research reactors and of particle accelerators for medical and industrial applications as well as for basic research in nuclear, high-energy particle and condensed matter physics. Several approaches have been tested to increase the neutron sensitivity of Al2O3:C. Options evolved to embed or coat Al2O3:C grains with materials containing neutron converters such as 6Li, 10B or Gd (157Gd or 155Gd)(1), which have a high neutron-capture cross section. The secondary radiation created by the neutron-capture reaction can then deposit energy in the Al2O3:C, giving rise to an optically stimulated luminescence (OSL) signal. With this approach, the readout method remains the same and the emission wavelength is unchanged. The neutron-sensitive OSL material used in this study [Al2O3:Cþ 6Li2CO3 (OSLN) droplet] was obtained by coating Al2O3:C with 6Li2CO3 material in the form of powder (grain size ,35 mm). This offers the possibility of preparing small detectors with high spatial resolution (due to the high sensitivity of Al2O3:C), particularly important in steep dose gradients, which are common, for example, in modern radiotherapy techniques. Furthermore, the neutron sensitivity of the Al2O3:C and neutron converter powder mixture is inversely proportional to the Al2O3:C grain size(2). OSLN droplets have advantages when compared with other detectors such as LiF Micro cube(3) and Gafchromic films(4). The OSL
readout is simpler and faster than for thermoluminescence (TL)(5) and for films. For TL, problems can occur if proper attention is not paid to the thermal treatments of the detectors. In addition to this, TL readout is delayed, and the need to re-set the dosemeter before the next measurement is also a disadvantage, when comparing with OSL. Besides, Al2O3:C is known for its high sensitivity to ionising radiation, which makes it possible to be prepared in smaller sizes compared with the LiF microcubes. Gafchromic films are used for dose-mapping, but the readout and analysis of the data are far more time-consuming than those for OSL. In this study, the authors investigated and compared the dosimetric response of Al2O3:C (OSL) and Al2O3:Cþ 6Li2CO3 (OSLN) optically stimulated luminescence droplets. In the following sections, response of the OSL and OSLN droplets is presented regarding irradiations with fast neutrons (252Cf ), thermal neutrons and beta-beams.
MATERIALS AND METHODS OSL (Al2O3:C) and OSLN (Al2O3:Cþ 6Li2CO3) droplets (Figure 1) were exposed to different radiation beams. Al2O3:C and Al2O3:Cþ 6Li2CO3 thin powder was provided by Landauer, Inc. Small-size droplets were prepared by mixing the powder with a home-made polymer, using a protocol described in a previous work(6). Before each irradiation, the droplets were
# The Author 2013. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at National Dong Hwa University Library on April 10, 2014
*Corresponding author: [email protected]
L. F. NASCIMENTO ET AL.
bleached using a blue light for 1000 s at full LED power (50 mW cm22). The purpose of this bleaching process is to empty all electron-hole dosimetric pair traps(2). OSL droplets are disc-shaped samples with a diameter of 2 mm and thickness of 0.5 mm (Figure 1), with a concentration of 10 mg ml21 (total powder mass in volume of polymer); OSLN droplets are disc-shaped samples with a diameter of 2 mm and thickness of 0.5 mm and were prepared with different concentrations of OSLN material: OSLN2—2 mg ml21 and OSLN5—5 mg ml21 (total powder mass divided by the volume of the home-made polymer). All the droplets have a volume of 1 ml. The OSL measurements and beta-irradiations (90Sr/90Y) were performed using a Risø TL/OSL reader system, model TL/OSL-DA-20(7), equipped with a blue stimulation light source (470 nm), a bialkali EMI 9235QA photomultiplier tube and Hoya U-340 filters to block the blue light. The OSL measurements use the following protocol: (1) OSL signal with LED with 90 % of power for 600 s (45 mW cm22). (2) Calculation of the background (B) signal: average of the last 20-s signal. (3) Total integrated OSL (TOSL): average of the 600-s signal minus the background signal. 2P600 TOSL ¼ 4
t¼1
St
600
3 5 B:
ð1Þ
RESULTS AND DISCUSSION Beta-irradiation
(4) Peak OSL (POSL): average of the first 2-s signal minus the background signal. 2P2 POSL ¼ 4
t¼1
2
St
3 5 B:
ð2Þ
A linearity test was performed with LuxelTM pellets and OSL and OSLN5 droplets with beta-irradiations from the 90Sr/90Y source from the Risø reader (dose in air). The results obtained with TOSL and POSL can be seen in Figures 2 and 3, respectively, and the dose was calculated towards the reference from the 90 Sr/90Y source.
Page 2 of 7
Downloaded from http://rpd.oxfordjournals.org/ at National Dong Hwa University Library on April 10, 2014
Figure 1. Pictures (a) a OSL droplet and (b) a OSLN droplet.
For comparison, both POSL and TOSL are used to calculate dose. Samples were irradiated with thermal neutrons from the BR1 reactor at SCK†CEN. This is a 700kW air-cooled experimental reactor with graphite as moderator and natural uranium fuel. The samples were placed, free in air, in front of one of the beam tubes at the side of the reactor, which gives a unidirectional beam of thermal neutrons coming out of the reactor. The neutron flux is determined by the activation of 198Au. The beam is very well thermalised, with a cadmium ratio of 7.9(8). The total dose rate (dH*(10)/dt) delivered to the samples was 5.7 mSv min21. OSL, OSLN5, OSLN2 droplets and LuxelTM (9) pellets were irradiated with a 90Sr/90Y beta-source integrated in the Risø TL/OSL reader (126+8 mGy s21, dose in air), used to deliver reference doses to the samples. The sensitivity of the samples was also measured with a radionuclide neutron source, 252Cf. The average neutron energy from 252Cf was 2.1 MeV(10). Prompt gamma-rays from the 252Cf spontaneous fission had an average energy of 0.87 MeV. The samples were irradiated free in air, with a dose rate of 59.87 mSv h21 (10.0 cm from the source) from direct neutrons. Contribution of photons was estimated to be 3.6 %(11). The results from irradiation with neutron beams are presented in gamma-equivalent. Gamma-equivalent was calculated by normalising the OSL signal measured from a neutron irradiation (OSLn) by the OSL signal measured from a known given dose of 630 mGy with the beta-source from the Risø reader (OSLbeta, dose in air). The choice of gamma-equivalent, instead of neutron dose, was due to the lack of neutron calibration for OSLN detectors, because this is dependent on neutron energy distribution and detector response(1). To compare the different detectors (LuxelTM pellets and the droplets), the results were normalised, so the OSL response does not depend on sample mass or intrinsic sensitivity. The error bars presented in this work are the standard deviation of the average from results obtained with the detectors.
DROPLETS, Al2O3:C þ 6Li2CO3, Al2O3:C
Neutron test: 252Cf
Figure 3. Dose calculated using the POSL from LuxelTM pellets and OSL and OSLN5 droplets irradiated with beta-source from the Risø reader.
The Al2O3:C LuxelTM pellets and the OSL and OSLN5 droplets presented a linear dose response for doses up to 6 Gy (TOSL), and above that dose, they presented a supralinearity (more prominent for the OSL droplets) and saturation for OSLN. When looking for the POSL: LuxelTM pellets and OSL droplets are linear up to 6 Gy and start to deviate from linearity (LuxelTM pellets present a higher supralinearity than droplets); for OSLN, the results show a linear behaviour for doses up to 23 Gy. The supralinearity can be explained by the competition of recombination centres, which can cause the OSL to produce more signal than expected during readout (6). Deep traps compete for the capture of charges in the forbidden band at low doses and cause less OSL signal coming from dosimetric traps, and as the dose increases, the deep traps get saturated and more charges are trapped by the dosimetric centres. Saturation at even higher doses is, consequently, explained by the saturation of the main traps, where information about the absorbed dose can no longer be assessed.
Figures 4 and 5 are the results from the average of TOSL and POSL, respectively, after irradiating ten samples of the LuxelTM pellets and the OSL, OSLN5 and OSLN2 droplets with 252Cf. Each data point was obtained by varying the 252Cf irradiation time, with a dose rate of 58.4 mSv h21 (direct neutrons) at the position of the detectors. Doses ranged from 20 to 3800 mSv, free in air. Results are given in gamma-equivalent. Although the results from OSLN detectors increase with time of irradiation (and consequently, with dose), it was expected to detect a small signal from 252 Cf, because this is not a source of thermal neutrons(10), and OSLN droplets are weakly sensitive to fast and intermediate neutrons. Standard deviation for the OSLN detectors is within 20 %. The gamma-equivalent calculated from TOSL and POSL from OSLN2 and OSLN5 droplets has, on average, the double in signal contribution when compared with the OSL droplets and the LuxelTM pellets (Figures 4 and 5). This indicates that at least half of the signal is actually coming from gamma, not neutrons, and that the response to 252Cf is poor, but can be distinguished if properly calibrated. The results of the OSLN materials for the 252Cf irradiation with 90 mSv are three times higher than those of the OSL materials, but it has also a high standard deviation. For the range of doses studied in this work, there is no apparent difference between using TOSL and POSL. The intensity of TOSL and POSL is almost same, with a slightly higher signal for POSL. The signal observed on the LuxelTM pellets and the OSL droplets is due to the contribution of photons from the nuclear reactions in the 252Cf source and also in the form of secondary photons due to
Page 3 of 7
Downloaded from http://rpd.oxfordjournals.org/ at National Dong Hwa University Library on April 10, 2014
Figure 2. Dose calculated using the TOSL from LuxelTM pellets and OSL and OSLN5 droplets irradiated with beta-source from the Risø reader.
OSLN5 droplets (5 mg ml21) present a saturation for TOSL of .6 Gy due to lower concentration of dosimetric material, and consequently dosimetric traps, when compared with the OSL droplets (10 mg ml21) and the LuxelTM pellets. The OSL decay is composed of two main emissions: a fast portion (7 ns, UV emission from Fþ centre) centred at 330 nm, and an intense broad emission band (35 nm, at room temperature) centred at 420 nm from F centres (12). The fast portion of the OSL decay, which is measured from POSL, is from shallow intermediate electron traps and is known to show enhanced supralinearity with dose. The intensity of the UV emission is higher at high ionisation densities (high-LET radiation or high doses of low-LET radiation) than that at low ionisation densities (low doses of low-LET radiation). Because of that, the POSL for OSLN5 droplets, for doses of .6 Gy, does not present the same saturation as for TOSL.
L. F. NASCIMENTO ET AL.
Figure 5. Gamma-equivalent from POSL calculated from ten samples of the LuxelTM pellets and OSL, OSLN2 and OSLN5 droplets irradiated with 252Cf.
interactions of the neutrons with matter in the irradiation facility. Neutron test: BR1 The dose response to thermal neutrons is determined by exposing ten detectors of each type (LuxelTM pellets, OSL, OSLN2 and OSLN5 droplets) to doses ranging from 100 to 1900 mSv, free in air.
This was achieved by varying the irradiation time, with a dose rate of 5.7 mSv min21 at the position of the detectors. The calculated gamma-equivalent values for TOSL and POSL are presented in Figures 6 and 7, respectively. The dose response of the OSLN droplets increases with dose for the range studied, with no indication of signal saturation. Standard deviations for the OSLN detectors are within 15 %.
Page 4 of 7
Downloaded from http://rpd.oxfordjournals.org/ at National Dong Hwa University Library on April 10, 2014
Figure 4. Gamma-equivalent from TOSL calculated from ten samples of the LuxelTM pellets and OSL, OSLN2 and OSLN5 droplets irradiated with 252Cf.
DROPLETS, Al2O3:C þ 6Li2CO3, Al2O3:C
Figure 7. Gamma-equivalent calculated using the POSL from LuxelTM pellets and OSL, OSLN2 and OSLN5 droplets irradiated with thermal neutrons. The dashed line (gamma-linearity) represents the linear curve expected when considering the thermal neutron and beta-response equivalent. The inset shows the results obtained with the LuxelTM pellets and the OSL droplets compared with the expected (striped line) response calculated from the reference gamma-contribution from BR1.
Page 5 of 7
Downloaded from http://rpd.oxfordjournals.org/ at National Dong Hwa University Library on April 10, 2014
Figure 6. Gamma-equivalent calculated using the TOSL from LuxelTM pellets and OSL, OSLN2 and OSLN5 droplets irradiated with thermal neutrons. The dashed line (gamma-linearity) represents the linear curve expected when considering the thermal neutron and beta-response equivalent. The inset shows the results obtained with the LuxelTM pellets and the OSL droplets compared with the expected (striped line) response calculated from the reference gamma-contribution from BR1.
L. F. NASCIMENTO ET AL.
The normalised gamma-equivalent calculated with TOSL and POSL for OSLN2 and OSLN5 droplets look similar, with a slightly higher signal for OSLN5 droplets for doses of .1200 mSv. Figure 8 presents the absolute OSL (TOSL and POSL) for the OSLN5 and OSLN2 droplets irradiated with thermal neutrons solely from BR1 (OSLBR1). As expected, the signal from the OSLN5 droplets is, in average, 2.5 higher than the signal for the OSLN2 droplets. Photon contribution was calculated from the LuxelTM pellets and the OSL droplets comparing the irradiation in BR1 (OSLBR1) with an irradiation with the beta-source from the Risø reader (OSLbeta). The inset in Figures 6 and 7 presents the results obtained with the LuxelTM pellets and OSL droplets compared with the expected curve calculated from the BR1 reference gamma-contribution dose rate of 0.08 mSv min21. Comparing the results for the LuxelTM pellets and the OSL droplets, one can see a similar response with given dose. These points show a linear behaviour similar to the expected curve. Standard deviations for the LuxelTM pellets are within 15 % and for the OSL droplets 12 %.
CONCLUSION In this project, the authors characterise the dosimetric response of Al2O3:Cþ 6Li2CO3 (called OSLN). In
particular, they compared the OSL results from OSLN droplets with two different concentrations (2 and 5 mg ml21) with the OSL droplets composed of Al2O3:C and the commercially available material, LuxelTM pellets. As expected, the authors noticed the poor response of OSLN to fast and intermediate neutrons, which is one of the limitations of this approach. However, its strong response to thermal neutrons, for small-size detectors, with no need of use of big quantity of material, is an advantage. For instance, OSLN can be a potential solution for thermal neutron fluence measurements in boron neutron capture therapy (BNCT)(13), where OSLN can be distributed as a 2D matrix to map the dose distribution. Another possible application, which benefits from the small size of the detectors, is to assess peripheral doses (neutron contribution) in radiotherapy, by distributing the samples in a Randow phantom. Besides, OSL is a simple technique that also gives the opportunity of multiple readouts. The linearity tests with beta-source showed that the OSLN material has sensitivity as high as the OSL droplets and LuxelTM pellets, with better results for POSL when compared with TOSL. Discrimination between thermal neutrons and gamma/beta-particles can be easily assessed by combining OSL and OSLN droplets.
Page 6 of 7
Downloaded from http://rpd.oxfordjournals.org/ at National Dong Hwa University Library on April 10, 2014
Figure 8. TOSL (OSLBR1) from the OSLN5 and OSLN2 droplets irradiated with thermal neutrons in BR1 reactor. The inset shows the POSL (OSLBR1) from the OSLN5 and OSLN2 droplets irradiated with thermal neutrons.
DROPLETS, Al2O3:C þ 6Li2CO3, Al2O3:C
ACKNOWLEDGEMENTS The authors acknowledge Mark Akselrod, PhD (Landauer, USA) for providing the Al2O3:C OSL and Al2O3:Cþ 6Li2CO3 OSLN materials and Lars Lindvold, PhD and Claus Andersen, PhD (Risø, Denmark) for providing the polymer.
REFERENCES
Page 7 of 7
Downloaded from http://rpd.oxfordjournals.org/ at National Dong Hwa University Library on April 10, 2014
1. Mittani, J. C. R., Da Silva, A. A. R., Vanhavere, F., Akselrod, M. S. and Yukihara, E. G. Investigation of neutron converters for production of optically stimulated luminescence (OSL) neutron dosimeters using Al2O3:C. Nucl. Inst. Methods Phys. Res. B 260(2), 663– 671 (2007). 2. Yukihara, G. E. and McKeever, S. Optically Stimulated Luminescence: Fundamentals and Applications, first edn. Wiley (2011). 3. Bassinet, C., Robbes, I., Barbier, L., Baumann, M., Kernisant, B. and Trompier, F. Characterization of 7 LiF: Mg, Ti TLD micro-cubes. Radiat. Meas. 45(3), 646–648 (2010). 4. Devic, S., Seuntjens, J., Sham, E., Podgorsak, E. B., Schmidtlein, C. R., Kirov, A. S. and Soares, C. G. Precise radiochromic film dosimetry using a flat-bed document scanner. Med. Phys. 32, 2245 (2005). 5. McKeever, S. W. S. and Moscovitch, M. Topics under debate-On the advantages and disadvantages of optically stimulated luminescence dosimetry and thermoluminescence dosimetry. Radiat. Prot. Dosim. 104(3), 263– 270 (2003).
6. Nascimento, L. F., Saldarriaga, C. V., Vanhavere, F., D’Agostino, E., Defraene, G. and De Deene, Y. Characterization of OSL Al2O3:C droplets for medical dosimetry. Radiat. Meas. 56, 200–204 (2013). 7. Boitter-Jensen, L., Bulur, E., Duller, G. and Murray, A. Advances in luminescence instrument system. Radiat. Meas. 32, 523– 528 (2000). 8. Wagemans, J., Vittiglio, G., Malambu, E. and Abderrahim, H. A. The BR1 reactor: a versatile irradiation facility for fundamental research and industrial applications. In Advancements in Nuclear Instrumentation Measurement Methods and their Applications (ANIMMA), 2009 First International Conference on ( pp. 1– 4). IEEE (2009). 9. McKeever, S. W. S., Blair, M. W., Bulur, E., Gaza, R., Gaza, R., Kalchgruber, R., Klein, D. M. and Yukihara, E. G. Recent advances in dosimetry using the optically stimulated luminescence of Al2O3: C. Radiat. Prot. Dosim. 109(4), 269 –276 (2004). 10. Mihailescu, L. C., Borella, A., Cauwels, V., Vaniqui De Santana, A. C. and Vanhavere, F. Dosimetry calibrations with neutron sources of 252Cf and Am-Be at LNK laboratory of SCK†CEN. In: Proceeding of NEUDOS 12, Radiation Protection Dosimetry (2013). 11. Hoedlmoser, H., Boschung, M., Meier, K., Stadtmann, H., Hranitzky, C., Figel, M. and Mayer, S. Photon contributions from the 252Cf and 241Am-Be neutron sources at the PSI calibration laboratory. Radiat. Meas. 47(8), 567–570 (2012). 12. Yukihara, E. G. and McKeever, S. W. S. Spectroscopy and optically stimulated luminescence of AlO: C using timeresolved measurements. J. Appl. Phys. 100, 083512 (2006). 13. Beddoe, A. H. Boron neutron capture therapy. Br. J. Radiol. 70(835), 665–667 (1997).
doi:10.1093/rpd/nct352
CHARACTERISATION OF OSL AND OSLN DROPLETS FOR DOSIMETRY L. F. Nascimento1,2,*, E. D’Agostino1, A. C. S. Vaniqui1, C. Saldarriaga1, F. Vanhavere1 and Y. De Deene2,3 1 SCK†CEN Belgian Nuclear Research Centre, Boeretang 200, Mol, Belgium 2 Radiotherapy Department, Gent University, De Pintelaan, 185, Gent 9000, Belgium 3 Institute of Medical Physics, School of Physics, University of Sydney, Sydney, Australia
In spite of considerable progress in neutron dosimetry, there is no dosemeter that is capable of measuring neutron doses independently of the neutron spectrum with good accuracy. Carbon-doped aluminium oxide (Al2O3:C) is a sensitive material for ionising radiation (beta-ray, X ray and electron) and has been used for applications in personal and medical dosimetry as an optically stimulated luminescence (OSL) dosemeter. Al2O3:C has a low sensitivity to neutron radiation; this prevents its application to neutron fields, representing a disadvantage of Al2O3:C-OSL when compared with LiF, which is used as a thermoluminescent detector. Recently an improvement for neutron dosimetry (Passmore and Kirr. Neutron response characterisation of an OSL neutron dosemeter. Radiat. Prot. Dosim. 2011;144:155 –60) uses Al2O3:C coated with 6Li2CO3 (OSLN),which gives the highsensitive response as known for Al2O3:C with the advantage of being also sensitive to thermal neutrons. In this article, the authors compare small-size detectors (droplets) of Al2O3:C (OSL) and of Al2O3:C1 6Li2CO3 (OSLN) and discuss the advantages and drawbacks of both materials, regarding size vs. response.
INTRODUCTION Man-made neutron fields are found in the controlled areas of power plants, research reactors and of particle accelerators for medical and industrial applications as well as for basic research in nuclear, high-energy particle and condensed matter physics. Several approaches have been tested to increase the neutron sensitivity of Al2O3:C. Options evolved to embed or coat Al2O3:C grains with materials containing neutron converters such as 6Li, 10B or Gd (157Gd or 155Gd)(1), which have a high neutron-capture cross section. The secondary radiation created by the neutron-capture reaction can then deposit energy in the Al2O3:C, giving rise to an optically stimulated luminescence (OSL) signal. With this approach, the readout method remains the same and the emission wavelength is unchanged. The neutron-sensitive OSL material used in this study [Al2O3:Cþ 6Li2CO3 (OSLN) droplet] was obtained by coating Al2O3:C with 6Li2CO3 material in the form of powder (grain size ,35 mm). This offers the possibility of preparing small detectors with high spatial resolution (due to the high sensitivity of Al2O3:C), particularly important in steep dose gradients, which are common, for example, in modern radiotherapy techniques. Furthermore, the neutron sensitivity of the Al2O3:C and neutron converter powder mixture is inversely proportional to the Al2O3:C grain size(2). OSLN droplets have advantages when compared with other detectors such as LiF Micro cube(3) and Gafchromic films(4). The OSL
readout is simpler and faster than for thermoluminescence (TL)(5) and for films. For TL, problems can occur if proper attention is not paid to the thermal treatments of the detectors. In addition to this, TL readout is delayed, and the need to re-set the dosemeter before the next measurement is also a disadvantage, when comparing with OSL. Besides, Al2O3:C is known for its high sensitivity to ionising radiation, which makes it possible to be prepared in smaller sizes compared with the LiF microcubes. Gafchromic films are used for dose-mapping, but the readout and analysis of the data are far more time-consuming than those for OSL. In this study, the authors investigated and compared the dosimetric response of Al2O3:C (OSL) and Al2O3:Cþ 6Li2CO3 (OSLN) optically stimulated luminescence droplets. In the following sections, response of the OSL and OSLN droplets is presented regarding irradiations with fast neutrons (252Cf ), thermal neutrons and beta-beams.
MATERIALS AND METHODS OSL (Al2O3:C) and OSLN (Al2O3:Cþ 6Li2CO3) droplets (Figure 1) were exposed to different radiation beams. Al2O3:C and Al2O3:Cþ 6Li2CO3 thin powder was provided by Landauer, Inc. Small-size droplets were prepared by mixing the powder with a home-made polymer, using a protocol described in a previous work(6). Before each irradiation, the droplets were
# The Author 2013. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at National Dong Hwa University Library on April 10, 2014
*Corresponding author: [email protected]
L. F. NASCIMENTO ET AL.
bleached using a blue light for 1000 s at full LED power (50 mW cm22). The purpose of this bleaching process is to empty all electron-hole dosimetric pair traps(2). OSL droplets are disc-shaped samples with a diameter of 2 mm and thickness of 0.5 mm (Figure 1), with a concentration of 10 mg ml21 (total powder mass in volume of polymer); OSLN droplets are disc-shaped samples with a diameter of 2 mm and thickness of 0.5 mm and were prepared with different concentrations of OSLN material: OSLN2—2 mg ml21 and OSLN5—5 mg ml21 (total powder mass divided by the volume of the home-made polymer). All the droplets have a volume of 1 ml. The OSL measurements and beta-irradiations (90Sr/90Y) were performed using a Risø TL/OSL reader system, model TL/OSL-DA-20(7), equipped with a blue stimulation light source (470 nm), a bialkali EMI 9235QA photomultiplier tube and Hoya U-340 filters to block the blue light. The OSL measurements use the following protocol: (1) OSL signal with LED with 90 % of power for 600 s (45 mW cm22). (2) Calculation of the background (B) signal: average of the last 20-s signal. (3) Total integrated OSL (TOSL): average of the 600-s signal minus the background signal. 2P600 TOSL ¼ 4
t¼1
St
600
3 5 B:
ð1Þ
RESULTS AND DISCUSSION Beta-irradiation
(4) Peak OSL (POSL): average of the first 2-s signal minus the background signal. 2P2 POSL ¼ 4
t¼1
2
St
3 5 B:
ð2Þ
A linearity test was performed with LuxelTM pellets and OSL and OSLN5 droplets with beta-irradiations from the 90Sr/90Y source from the Risø reader (dose in air). The results obtained with TOSL and POSL can be seen in Figures 2 and 3, respectively, and the dose was calculated towards the reference from the 90 Sr/90Y source.
Page 2 of 7
Downloaded from http://rpd.oxfordjournals.org/ at National Dong Hwa University Library on April 10, 2014
Figure 1. Pictures (a) a OSL droplet and (b) a OSLN droplet.
For comparison, both POSL and TOSL are used to calculate dose. Samples were irradiated with thermal neutrons from the BR1 reactor at SCK†CEN. This is a 700kW air-cooled experimental reactor with graphite as moderator and natural uranium fuel. The samples were placed, free in air, in front of one of the beam tubes at the side of the reactor, which gives a unidirectional beam of thermal neutrons coming out of the reactor. The neutron flux is determined by the activation of 198Au. The beam is very well thermalised, with a cadmium ratio of 7.9(8). The total dose rate (dH*(10)/dt) delivered to the samples was 5.7 mSv min21. OSL, OSLN5, OSLN2 droplets and LuxelTM (9) pellets were irradiated with a 90Sr/90Y beta-source integrated in the Risø TL/OSL reader (126+8 mGy s21, dose in air), used to deliver reference doses to the samples. The sensitivity of the samples was also measured with a radionuclide neutron source, 252Cf. The average neutron energy from 252Cf was 2.1 MeV(10). Prompt gamma-rays from the 252Cf spontaneous fission had an average energy of 0.87 MeV. The samples were irradiated free in air, with a dose rate of 59.87 mSv h21 (10.0 cm from the source) from direct neutrons. Contribution of photons was estimated to be 3.6 %(11). The results from irradiation with neutron beams are presented in gamma-equivalent. Gamma-equivalent was calculated by normalising the OSL signal measured from a neutron irradiation (OSLn) by the OSL signal measured from a known given dose of 630 mGy with the beta-source from the Risø reader (OSLbeta, dose in air). The choice of gamma-equivalent, instead of neutron dose, was due to the lack of neutron calibration for OSLN detectors, because this is dependent on neutron energy distribution and detector response(1). To compare the different detectors (LuxelTM pellets and the droplets), the results were normalised, so the OSL response does not depend on sample mass or intrinsic sensitivity. The error bars presented in this work are the standard deviation of the average from results obtained with the detectors.
DROPLETS, Al2O3:C þ 6Li2CO3, Al2O3:C
Neutron test: 252Cf
Figure 3. Dose calculated using the POSL from LuxelTM pellets and OSL and OSLN5 droplets irradiated with beta-source from the Risø reader.
The Al2O3:C LuxelTM pellets and the OSL and OSLN5 droplets presented a linear dose response for doses up to 6 Gy (TOSL), and above that dose, they presented a supralinearity (more prominent for the OSL droplets) and saturation for OSLN. When looking for the POSL: LuxelTM pellets and OSL droplets are linear up to 6 Gy and start to deviate from linearity (LuxelTM pellets present a higher supralinearity than droplets); for OSLN, the results show a linear behaviour for doses up to 23 Gy. The supralinearity can be explained by the competition of recombination centres, which can cause the OSL to produce more signal than expected during readout (6). Deep traps compete for the capture of charges in the forbidden band at low doses and cause less OSL signal coming from dosimetric traps, and as the dose increases, the deep traps get saturated and more charges are trapped by the dosimetric centres. Saturation at even higher doses is, consequently, explained by the saturation of the main traps, where information about the absorbed dose can no longer be assessed.
Figures 4 and 5 are the results from the average of TOSL and POSL, respectively, after irradiating ten samples of the LuxelTM pellets and the OSL, OSLN5 and OSLN2 droplets with 252Cf. Each data point was obtained by varying the 252Cf irradiation time, with a dose rate of 58.4 mSv h21 (direct neutrons) at the position of the detectors. Doses ranged from 20 to 3800 mSv, free in air. Results are given in gamma-equivalent. Although the results from OSLN detectors increase with time of irradiation (and consequently, with dose), it was expected to detect a small signal from 252 Cf, because this is not a source of thermal neutrons(10), and OSLN droplets are weakly sensitive to fast and intermediate neutrons. Standard deviation for the OSLN detectors is within 20 %. The gamma-equivalent calculated from TOSL and POSL from OSLN2 and OSLN5 droplets has, on average, the double in signal contribution when compared with the OSL droplets and the LuxelTM pellets (Figures 4 and 5). This indicates that at least half of the signal is actually coming from gamma, not neutrons, and that the response to 252Cf is poor, but can be distinguished if properly calibrated. The results of the OSLN materials for the 252Cf irradiation with 90 mSv are three times higher than those of the OSL materials, but it has also a high standard deviation. For the range of doses studied in this work, there is no apparent difference between using TOSL and POSL. The intensity of TOSL and POSL is almost same, with a slightly higher signal for POSL. The signal observed on the LuxelTM pellets and the OSL droplets is due to the contribution of photons from the nuclear reactions in the 252Cf source and also in the form of secondary photons due to
Page 3 of 7
Downloaded from http://rpd.oxfordjournals.org/ at National Dong Hwa University Library on April 10, 2014
Figure 2. Dose calculated using the TOSL from LuxelTM pellets and OSL and OSLN5 droplets irradiated with beta-source from the Risø reader.
OSLN5 droplets (5 mg ml21) present a saturation for TOSL of .6 Gy due to lower concentration of dosimetric material, and consequently dosimetric traps, when compared with the OSL droplets (10 mg ml21) and the LuxelTM pellets. The OSL decay is composed of two main emissions: a fast portion (7 ns, UV emission from Fþ centre) centred at 330 nm, and an intense broad emission band (35 nm, at room temperature) centred at 420 nm from F centres (12). The fast portion of the OSL decay, which is measured from POSL, is from shallow intermediate electron traps and is known to show enhanced supralinearity with dose. The intensity of the UV emission is higher at high ionisation densities (high-LET radiation or high doses of low-LET radiation) than that at low ionisation densities (low doses of low-LET radiation). Because of that, the POSL for OSLN5 droplets, for doses of .6 Gy, does not present the same saturation as for TOSL.
L. F. NASCIMENTO ET AL.
Figure 5. Gamma-equivalent from POSL calculated from ten samples of the LuxelTM pellets and OSL, OSLN2 and OSLN5 droplets irradiated with 252Cf.
interactions of the neutrons with matter in the irradiation facility. Neutron test: BR1 The dose response to thermal neutrons is determined by exposing ten detectors of each type (LuxelTM pellets, OSL, OSLN2 and OSLN5 droplets) to doses ranging from 100 to 1900 mSv, free in air.
This was achieved by varying the irradiation time, with a dose rate of 5.7 mSv min21 at the position of the detectors. The calculated gamma-equivalent values for TOSL and POSL are presented in Figures 6 and 7, respectively. The dose response of the OSLN droplets increases with dose for the range studied, with no indication of signal saturation. Standard deviations for the OSLN detectors are within 15 %.
Page 4 of 7
Downloaded from http://rpd.oxfordjournals.org/ at National Dong Hwa University Library on April 10, 2014
Figure 4. Gamma-equivalent from TOSL calculated from ten samples of the LuxelTM pellets and OSL, OSLN2 and OSLN5 droplets irradiated with 252Cf.
DROPLETS, Al2O3:C þ 6Li2CO3, Al2O3:C
Figure 7. Gamma-equivalent calculated using the POSL from LuxelTM pellets and OSL, OSLN2 and OSLN5 droplets irradiated with thermal neutrons. The dashed line (gamma-linearity) represents the linear curve expected when considering the thermal neutron and beta-response equivalent. The inset shows the results obtained with the LuxelTM pellets and the OSL droplets compared with the expected (striped line) response calculated from the reference gamma-contribution from BR1.
Page 5 of 7
Downloaded from http://rpd.oxfordjournals.org/ at National Dong Hwa University Library on April 10, 2014
Figure 6. Gamma-equivalent calculated using the TOSL from LuxelTM pellets and OSL, OSLN2 and OSLN5 droplets irradiated with thermal neutrons. The dashed line (gamma-linearity) represents the linear curve expected when considering the thermal neutron and beta-response equivalent. The inset shows the results obtained with the LuxelTM pellets and the OSL droplets compared with the expected (striped line) response calculated from the reference gamma-contribution from BR1.
L. F. NASCIMENTO ET AL.
The normalised gamma-equivalent calculated with TOSL and POSL for OSLN2 and OSLN5 droplets look similar, with a slightly higher signal for OSLN5 droplets for doses of .1200 mSv. Figure 8 presents the absolute OSL (TOSL and POSL) for the OSLN5 and OSLN2 droplets irradiated with thermal neutrons solely from BR1 (OSLBR1). As expected, the signal from the OSLN5 droplets is, in average, 2.5 higher than the signal for the OSLN2 droplets. Photon contribution was calculated from the LuxelTM pellets and the OSL droplets comparing the irradiation in BR1 (OSLBR1) with an irradiation with the beta-source from the Risø reader (OSLbeta). The inset in Figures 6 and 7 presents the results obtained with the LuxelTM pellets and OSL droplets compared with the expected curve calculated from the BR1 reference gamma-contribution dose rate of 0.08 mSv min21. Comparing the results for the LuxelTM pellets and the OSL droplets, one can see a similar response with given dose. These points show a linear behaviour similar to the expected curve. Standard deviations for the LuxelTM pellets are within 15 % and for the OSL droplets 12 %.
CONCLUSION In this project, the authors characterise the dosimetric response of Al2O3:Cþ 6Li2CO3 (called OSLN). In
particular, they compared the OSL results from OSLN droplets with two different concentrations (2 and 5 mg ml21) with the OSL droplets composed of Al2O3:C and the commercially available material, LuxelTM pellets. As expected, the authors noticed the poor response of OSLN to fast and intermediate neutrons, which is one of the limitations of this approach. However, its strong response to thermal neutrons, for small-size detectors, with no need of use of big quantity of material, is an advantage. For instance, OSLN can be a potential solution for thermal neutron fluence measurements in boron neutron capture therapy (BNCT)(13), where OSLN can be distributed as a 2D matrix to map the dose distribution. Another possible application, which benefits from the small size of the detectors, is to assess peripheral doses (neutron contribution) in radiotherapy, by distributing the samples in a Randow phantom. Besides, OSL is a simple technique that also gives the opportunity of multiple readouts. The linearity tests with beta-source showed that the OSLN material has sensitivity as high as the OSL droplets and LuxelTM pellets, with better results for POSL when compared with TOSL. Discrimination between thermal neutrons and gamma/beta-particles can be easily assessed by combining OSL and OSLN droplets.
Page 6 of 7
Downloaded from http://rpd.oxfordjournals.org/ at National Dong Hwa University Library on April 10, 2014
Figure 8. TOSL (OSLBR1) from the OSLN5 and OSLN2 droplets irradiated with thermal neutrons in BR1 reactor. The inset shows the POSL (OSLBR1) from the OSLN5 and OSLN2 droplets irradiated with thermal neutrons.
DROPLETS, Al2O3:C þ 6Li2CO3, Al2O3:C
ACKNOWLEDGEMENTS The authors acknowledge Mark Akselrod, PhD (Landauer, USA) for providing the Al2O3:C OSL and Al2O3:Cþ 6Li2CO3 OSLN materials and Lars Lindvold, PhD and Claus Andersen, PhD (Risø, Denmark) for providing the polymer.
REFERENCES
Page 7 of 7
Downloaded from http://rpd.oxfordjournals.org/ at National Dong Hwa University Library on April 10, 2014
1. Mittani, J. C. R., Da Silva, A. A. R., Vanhavere, F., Akselrod, M. S. and Yukihara, E. G. Investigation of neutron converters for production of optically stimulated luminescence (OSL) neutron dosimeters using Al2O3:C. Nucl. Inst. Methods Phys. Res. B 260(2), 663– 671 (2007). 2. Yukihara, G. E. and McKeever, S. Optically Stimulated Luminescence: Fundamentals and Applications, first edn. Wiley (2011). 3. Bassinet, C., Robbes, I., Barbier, L., Baumann, M., Kernisant, B. and Trompier, F. Characterization of 7 LiF: Mg, Ti TLD micro-cubes. Radiat. Meas. 45(3), 646–648 (2010). 4. Devic, S., Seuntjens, J., Sham, E., Podgorsak, E. B., Schmidtlein, C. R., Kirov, A. S. and Soares, C. G. Precise radiochromic film dosimetry using a flat-bed document scanner. Med. Phys. 32, 2245 (2005). 5. McKeever, S. W. S. and Moscovitch, M. Topics under debate-On the advantages and disadvantages of optically stimulated luminescence dosimetry and thermoluminescence dosimetry. Radiat. Prot. Dosim. 104(3), 263– 270 (2003).
6. Nascimento, L. F., Saldarriaga, C. V., Vanhavere, F., D’Agostino, E., Defraene, G. and De Deene, Y. Characterization of OSL Al2O3:C droplets for medical dosimetry. Radiat. Meas. 56, 200–204 (2013). 7. Boitter-Jensen, L., Bulur, E., Duller, G. and Murray, A. Advances in luminescence instrument system. Radiat. Meas. 32, 523– 528 (2000). 8. Wagemans, J., Vittiglio, G., Malambu, E. and Abderrahim, H. A. The BR1 reactor: a versatile irradiation facility for fundamental research and industrial applications. In Advancements in Nuclear Instrumentation Measurement Methods and their Applications (ANIMMA), 2009 First International Conference on ( pp. 1– 4). IEEE (2009). 9. McKeever, S. W. S., Blair, M. W., Bulur, E., Gaza, R., Gaza, R., Kalchgruber, R., Klein, D. M. and Yukihara, E. G. Recent advances in dosimetry using the optically stimulated luminescence of Al2O3: C. Radiat. Prot. Dosim. 109(4), 269 –276 (2004). 10. Mihailescu, L. C., Borella, A., Cauwels, V., Vaniqui De Santana, A. C. and Vanhavere, F. Dosimetry calibrations with neutron sources of 252Cf and Am-Be at LNK laboratory of SCK†CEN. In: Proceeding of NEUDOS 12, Radiation Protection Dosimetry (2013). 11. Hoedlmoser, H., Boschung, M., Meier, K., Stadtmann, H., Hranitzky, C., Figel, M. and Mayer, S. Photon contributions from the 252Cf and 241Am-Be neutron sources at the PSI calibration laboratory. Radiat. Meas. 47(8), 567–570 (2012). 12. Yukihara, E. G. and McKeever, S. W. S. Spectroscopy and optically stimulated luminescence of AlO: C using timeresolved measurements. J. Appl. Phys. 100, 083512 (2006). 13. Beddoe, A. H. Boron neutron capture therapy. Br. J. Radiol. 70(835), 665–667 (1997).
