Radiation Protection Dosimetry (2014), Vol. 162, No. 1–2, pp. 34 –37 Advance Access publication 23 July 2014
doi:10.1093/rpd/ncu213
SPECIFIC FEATURES OF THE INFLUENCE OF HIGH-ENERGY ELECTRON BEAMS ON THE LUMINESCENT PROPERTIES OF UNDOPED AND Nb, Fe-DOPED Al2O3 CRYSTALS V. T. Maslyuk1,*, I. G. Megela1, T. O. Okunieva1, J. M. Pekar2 and V. J. Pekar2 1 Institute of Electron Physics, Ukr. Nat. Acad. Sci., Universytetska St. 21, Uzhhorod 88000, Ukraine 2 LLC ‘Tehnocristal-Corund’, Granitna St., 5a, Uzhhorod 88000, Ukraine *Corresponding author: [email protected]
INTRODUCTION Thermal luminescent dosimetry with the use of the LiF, Li2B4O7 and a-Al2O3 crystals is being widely used in the personal dosimetry and environmental radiative monitoring(1, 2). Of not less interest is its use for the dosimetry of high-energy beams of electron and proton accelerators. In(3, 4), the influence of irradiation by the beams of accelerated hadrons and electrons on the behaviour of the dosimetric LiF crystals has been studied. Optically stimulated and thermal luminescence of the anion-defect a-Al2O3 crystals irradiated by accelerated protons has been studied in Kruzhalov et al.(5) However, in case of irradiating the thermal luminescent materials by the high-energy nuclear particles one has to take into account that, besides ionisation that results in the charge carrier capture at traps, the displaced-atomlike radiation defects may occur due to the elastic high-energy electron scattering by the lattice atoms. Of not less importance are the studies of the influence of irradiation intensity that may modify the recombination rate of the electron-hole pairs produced resulting in the variation of the material sensitivity to irradiation dose. Taking into account the aforementioned, we have studied in this work the specific features of the influence of irradiation by the high-current 10 MeV electrons accelerated at the M-30 microtron on the luminescent properties of both undoped and Nb, Fedoped Al2O3 crystals. EXPERIMENTALTECHNIQUE The a-Al2O3 single crystal specimens (both specially undoped and Nb, Fe-doped) grown by the modified Kyropoulos method(6) were investigated. The above specimens were irradiated by the gammaquanta from the 60Sp source with the irradiation dose rate at the location place of 7.35 . 1028 Gy s21.
Irradiation by the 10 MeV energy electrons accelerated at the M-30 micron was carried out in the formed field using the scattering foil and collimator conventionally used at the oncologic patient therapy. The electron fluence intensity at the specimen location place was measured by a Faraday cup used to calibrate the secondary emission transmission monitor and determine the integral irradiation dose. The fluence intensity was varied within the 2 . 109 –5 . 1010 e sm22 s21 range with the maximal accumulated fluence of 1014 e sm22. To evaluate the influence of contribution of the unavoidable gamma-quanta bremsstrahlung produced at the electron transmission through the exit window and scattering foil, the specimens under study were irradiated by the bremsstrahlung gamma-quanta at the same geometry, while electrons generating the above quanta were eliminated by adsorbing them in the 21-mm-thick aluminum layer. Here, the absence of electrons was confirmed by the measurements using the Faraday cup. Both phosphorescence at room temperature and thermal luminescence in the 25–3008S temperature range at the 0.58S s21 rate were detected by the FEU-106 photomultiplier in the photon counting mode.
RESULTS AND DISCUSSION Irradiation at the 60Sp source has demonstrated a low sensitivity to radiation influence on the luminescent properties of both undoped and Fe-doped leucosapphire crystals. The thermal luminescence curves for the Al2O3:Nb specimens irradiated by the 1.2 . 1022 –3.7 . 1021 Gy doses are shown in Figure 1. One may notice two peaks at the 160 and 2208S temperatures, respectively, and state that the fluorescence light sum of the peak at 2208S is four times larger than that at 1608S. The monotonous increase of both peaks with irradiation dose indicates the possibility of their use for the gamma-radiation dosimetry.
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at Florida International University on June 25, 2015
The influence of 10 MeV high-current electron beams accelerated by the M-30 microtron on the luminescent properties of the a-Al2O3, Al2O3:Nb and Al2O3:Fe crystals has been studied. The effect of the long-term phosphorescence at room temperature has been found that can be used to monitor electron and gamma accelerator beams.
SPECIFIC FEATURES OF THE INFLUENCE OF HIGH-ENERGY ELECTRON BEAMS
Figure 2. Kinetics of the phosphorescence decrease for the Al2O3 (1), Al2O3:Nb (2), Al2O3:Fe (3) specimens irradiated by electrons with the 1012 e cm22 fluence and the 3 . 109 e cm22 s21 intensity.
As a result of irradiating all the Al2O3 samples at the M-30 microtron, we have found the long-term phosphorescence at room temperature (with no use of additional optical and thermal excitation). The phosphorescence value and duration appeared to depend both on the radiation dose and intensity. Figure 2 shows the typical curves of the phosphorescence reduction for the Al2O3, Al2O3:Nb, Al2O3:Fe specimens irradiated by electrons with the 1012 e sm22 fluence and the 3 . 109 e sm22 s21 intensity. As seen from this figure, the maximal fluorescence value is observed for the Al2O3:Fe specimen, whereas in the undoped and Nb-doped specimens there are two areas with different fluorescence decrease rates. As our studies show, at the fluence intensity variation from 3 . 109 to 4 . 1010 f sm22 s21 the yield of the phosphorescence light sum increases approximately twice. The dependence of the fluorescence light sum yield on the irradiation intensity testifies to that the
Figure 3. Thermal luminescence curves of the Al2O3 (a), Al2O3:Nb (b), Al2O3:Fe (c) crystals irradiated by the different electron fluences.
presence of the hollow charge carrier traps with the depletion rate is less than the rate of pumping by the high-intensity electron beam. This explains the fact of earlier not observance of phosphorescence at the specimen irradiation with low intensity. The results obtained show the recombination phosphorescence mechanism(7), and, since the analysis of the kinetics of the phosphorescence decrease has shown that it does not agree satisfactorily neither with the Becquerel hyperbola nor with the exponent, one may conclude the presence of several close energetically located electron traps. 35
Downloaded from http://rpd.oxfordjournals.org/ at Florida International University on June 25, 2015
Figure 1. Thermal luminescence curves for the Al2O3:Nb specimens irradiated at the 60Co source.
V. T. MASLYUK ET AL.
The typical thermal luminescence curves for the Al2O3, Al2O3:Nb, Al2O3:Fe crystals irradiated by the different electron fluences are shown in Figure 3. As seen from figure, for the undoped crystals two peaks are observed: the main one at the 1328S temperature and a considerably less one at 2658S. The Nb-doped specimens also reveal two peaks at the 160 and 2208S temperatures, and their positions coincide with those for the specimens irradiated by the 60Sp gammaquanta. However, relation between the first and second
Figure 5. Thermal luminescence light sum yield at the peak at the 1328C temperature for the undoped Al2O3 crystal irradiated by a mixed electron and bremsstrahlung gammaquanta field (1) and the bremsstrahlung gamma-quanta (2) as a function of the fluence.
Figure 4. Thermal luminescence curves of the Al2O3 (a), Al2O3:Nb (b), Al2O3:Fe (c) crystals irradiated by the gammaquanta produced due to the bremsschtrahlung at the construction elements having electron fluences of 1011 –1014 e cm22.
36
Downloaded from http://rpd.oxfordjournals.org/ at Florida International University on June 25, 2015
peaks for the case of electron irradiation differs essentially, i.e. a tendency of the peak rise at 1608S with respect to that at 2208S is clearly observed. The Al2O3:Fe specimen demonstrates the peaks at 1508S and 1958S. Our studies have shown that due to electron irradiation up to the 1013 e sm22 fluences, all the specimens reveal the increase in the phosphorescence and thermal luminescence yields, while at the 1014 e sm22 fluences—a decreasing tendency. The possible explanation of this is the effect of the lattice radiation defects production at irradiation due to the relativistic electron elastic scattering by the crystal atoms. Probably, some of these defects types could be the centres of competing non-radiative recombination. This is confirmed by the results of irradiating the crystals under study by the bremsstrahlung gamma-quanta with no presence of electrons. The relevant results are shown in Figure 4 demonstrating the thermal luminescence curves for the Al2O3, Al2O3:Nb, Al2O3:Fe crystals irradiated by the gamma-quanta produced due to the bremsstrahlung at the construction elements with the 1011 –1014 e sm22 electron fluences. As seen from the above data, while at low doses the thermal luminescence yield in case of electron irradiation is sufficiently large than that in case of the gamma-quanta irradiation, when reaching the fluences .1013 e sm22, the opposite tendency is observed being due to the continuous increase of the thermal luminescence yield with increasing gamma-irradiation dose (see Figure 5). The results obtained testify to the above assumption. Indeed, though at irradiating crystals by the high-energy photons the radiation defects of the displaced-atom type are being produced due to their production resulting from the secondary photoelectrons, the Compton electron and from the electron–positron production processes. Their production cross section is
SPECIFIC FEATURES OF THE INFLUENCE OF HIGH-ENERGY ELECTRON BEAMS
considerably less than that in the case of electron irradiation. This, finally, results in the thermal luminescence and phosphorescence increase at the irradiation by the large gamma-quanta doses.
This work was partly supported by a research project from National Academy of Sciences, Ukraine, contract No. K –5–27/2014. REFERENCES
CONCLUSIONS
ACKNOWLEDGEMENTS Authors sincerely thank the Scientific Advisory Committee of the RAD 2014 for the invitation to present a talk and financial support of participation.
37
Downloaded from http://rpd.oxfordjournals.org/ at Florida International University on June 25, 2015
1. Weinstein, M., German, U. and Alfassi, Z. F. On neutrongamma mixed field dosymetry with LiF:Mg, Ti at radiation protection dose levels. Radiat. Prot. Dosim. 119, 314–318 (2006). 2. Akselrod, M. S., Botter-Jensen, L. and McKeever, S. W. S. Optically stimulated luminescense and its use in medical dosimetry. Radiat. Meas. 41, 78–99 (2006). 3. Bilski, P., Obryk, B. and Stuglik, Z. Behaviour of LiF:Mg,Cu,P and LiF:Mg,Ti thermoluminescent detectors for electron doses up to 1 MGy. Radiat. Meas. 45, 576– 578 (2010). 4. Obryk, B., Bilski, P. and Olko, P. Method of thermoluminescent measurement of radiation doses from micrograys up to a megagray with a single LiF:Mg,Cu,P detector. Radiat. Prot. Dosim. 144, 543–547 (2011). 5. Kruzhalov, A. V., Milman, I. I., Neshev, F. G. and Revkov, I. G. Optically stimulated luminescence of the a-Al2O3 crystals irradiated by protons. Technical Phys. Lett. 34, 809– 811 (2008). 6. Bletskan, D. I., Lukianchuk, A. P. and Pekar, Ya. M. Studies of the proper and extrinsic point defects in the sapphire substrates by the luminescent methods. Electronic materials. 3, 59–64 (2006) (in Russian). 7. Antonov-Romanovsky, V. V. Luminescent kinetics of crystallophosphores. (Moscow: Nauka) pp. 1 –317 (1966) (in Russian).
As a result of the influence on the thermal luminescent Al2O3-based materials of the high-current electron and gamma-quanta beams after the irradiation termination, a long-term phosphorescence arises at room temperature depending both on the irradiation dose and intensity. After reaching the mixed electron and gamma-quanta field fluences .1013 e sm22 the tendency to the decrease of the yield of both the phosphorescence and thermal luminescence is observed at the increasing dose explained by the production of radiation defects being the centres of non-radiative recombination. Combined measurements of the phosphorescence and thermal luminescence are promising for determining both the irradiation dose and the intensity of the high-current electron beams produced at the electron accelerators.
doi:10.1093/rpd/ncu213
SPECIFIC FEATURES OF THE INFLUENCE OF HIGH-ENERGY ELECTRON BEAMS ON THE LUMINESCENT PROPERTIES OF UNDOPED AND Nb, Fe-DOPED Al2O3 CRYSTALS V. T. Maslyuk1,*, I. G. Megela1, T. O. Okunieva1, J. M. Pekar2 and V. J. Pekar2 1 Institute of Electron Physics, Ukr. Nat. Acad. Sci., Universytetska St. 21, Uzhhorod 88000, Ukraine 2 LLC ‘Tehnocristal-Corund’, Granitna St., 5a, Uzhhorod 88000, Ukraine *Corresponding author: [email protected]
INTRODUCTION Thermal luminescent dosimetry with the use of the LiF, Li2B4O7 and a-Al2O3 crystals is being widely used in the personal dosimetry and environmental radiative monitoring(1, 2). Of not less interest is its use for the dosimetry of high-energy beams of electron and proton accelerators. In(3, 4), the influence of irradiation by the beams of accelerated hadrons and electrons on the behaviour of the dosimetric LiF crystals has been studied. Optically stimulated and thermal luminescence of the anion-defect a-Al2O3 crystals irradiated by accelerated protons has been studied in Kruzhalov et al.(5) However, in case of irradiating the thermal luminescent materials by the high-energy nuclear particles one has to take into account that, besides ionisation that results in the charge carrier capture at traps, the displaced-atomlike radiation defects may occur due to the elastic high-energy electron scattering by the lattice atoms. Of not less importance are the studies of the influence of irradiation intensity that may modify the recombination rate of the electron-hole pairs produced resulting in the variation of the material sensitivity to irradiation dose. Taking into account the aforementioned, we have studied in this work the specific features of the influence of irradiation by the high-current 10 MeV electrons accelerated at the M-30 microtron on the luminescent properties of both undoped and Nb, Fedoped Al2O3 crystals. EXPERIMENTALTECHNIQUE The a-Al2O3 single crystal specimens (both specially undoped and Nb, Fe-doped) grown by the modified Kyropoulos method(6) were investigated. The above specimens were irradiated by the gammaquanta from the 60Sp source with the irradiation dose rate at the location place of 7.35 . 1028 Gy s21.
Irradiation by the 10 MeV energy electrons accelerated at the M-30 micron was carried out in the formed field using the scattering foil and collimator conventionally used at the oncologic patient therapy. The electron fluence intensity at the specimen location place was measured by a Faraday cup used to calibrate the secondary emission transmission monitor and determine the integral irradiation dose. The fluence intensity was varied within the 2 . 109 –5 . 1010 e sm22 s21 range with the maximal accumulated fluence of 1014 e sm22. To evaluate the influence of contribution of the unavoidable gamma-quanta bremsstrahlung produced at the electron transmission through the exit window and scattering foil, the specimens under study were irradiated by the bremsstrahlung gamma-quanta at the same geometry, while electrons generating the above quanta were eliminated by adsorbing them in the 21-mm-thick aluminum layer. Here, the absence of electrons was confirmed by the measurements using the Faraday cup. Both phosphorescence at room temperature and thermal luminescence in the 25–3008S temperature range at the 0.58S s21 rate were detected by the FEU-106 photomultiplier in the photon counting mode.
RESULTS AND DISCUSSION Irradiation at the 60Sp source has demonstrated a low sensitivity to radiation influence on the luminescent properties of both undoped and Fe-doped leucosapphire crystals. The thermal luminescence curves for the Al2O3:Nb specimens irradiated by the 1.2 . 1022 –3.7 . 1021 Gy doses are shown in Figure 1. One may notice two peaks at the 160 and 2208S temperatures, respectively, and state that the fluorescence light sum of the peak at 2208S is four times larger than that at 1608S. The monotonous increase of both peaks with irradiation dose indicates the possibility of their use for the gamma-radiation dosimetry.
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at Florida International University on June 25, 2015
The influence of 10 MeV high-current electron beams accelerated by the M-30 microtron on the luminescent properties of the a-Al2O3, Al2O3:Nb and Al2O3:Fe crystals has been studied. The effect of the long-term phosphorescence at room temperature has been found that can be used to monitor electron and gamma accelerator beams.
SPECIFIC FEATURES OF THE INFLUENCE OF HIGH-ENERGY ELECTRON BEAMS
Figure 2. Kinetics of the phosphorescence decrease for the Al2O3 (1), Al2O3:Nb (2), Al2O3:Fe (3) specimens irradiated by electrons with the 1012 e cm22 fluence and the 3 . 109 e cm22 s21 intensity.
As a result of irradiating all the Al2O3 samples at the M-30 microtron, we have found the long-term phosphorescence at room temperature (with no use of additional optical and thermal excitation). The phosphorescence value and duration appeared to depend both on the radiation dose and intensity. Figure 2 shows the typical curves of the phosphorescence reduction for the Al2O3, Al2O3:Nb, Al2O3:Fe specimens irradiated by electrons with the 1012 e sm22 fluence and the 3 . 109 e sm22 s21 intensity. As seen from this figure, the maximal fluorescence value is observed for the Al2O3:Fe specimen, whereas in the undoped and Nb-doped specimens there are two areas with different fluorescence decrease rates. As our studies show, at the fluence intensity variation from 3 . 109 to 4 . 1010 f sm22 s21 the yield of the phosphorescence light sum increases approximately twice. The dependence of the fluorescence light sum yield on the irradiation intensity testifies to that the
Figure 3. Thermal luminescence curves of the Al2O3 (a), Al2O3:Nb (b), Al2O3:Fe (c) crystals irradiated by the different electron fluences.
presence of the hollow charge carrier traps with the depletion rate is less than the rate of pumping by the high-intensity electron beam. This explains the fact of earlier not observance of phosphorescence at the specimen irradiation with low intensity. The results obtained show the recombination phosphorescence mechanism(7), and, since the analysis of the kinetics of the phosphorescence decrease has shown that it does not agree satisfactorily neither with the Becquerel hyperbola nor with the exponent, one may conclude the presence of several close energetically located electron traps. 35
Downloaded from http://rpd.oxfordjournals.org/ at Florida International University on June 25, 2015
Figure 1. Thermal luminescence curves for the Al2O3:Nb specimens irradiated at the 60Co source.
V. T. MASLYUK ET AL.
The typical thermal luminescence curves for the Al2O3, Al2O3:Nb, Al2O3:Fe crystals irradiated by the different electron fluences are shown in Figure 3. As seen from figure, for the undoped crystals two peaks are observed: the main one at the 1328S temperature and a considerably less one at 2658S. The Nb-doped specimens also reveal two peaks at the 160 and 2208S temperatures, and their positions coincide with those for the specimens irradiated by the 60Sp gammaquanta. However, relation between the first and second
Figure 5. Thermal luminescence light sum yield at the peak at the 1328C temperature for the undoped Al2O3 crystal irradiated by a mixed electron and bremsstrahlung gammaquanta field (1) and the bremsstrahlung gamma-quanta (2) as a function of the fluence.
Figure 4. Thermal luminescence curves of the Al2O3 (a), Al2O3:Nb (b), Al2O3:Fe (c) crystals irradiated by the gammaquanta produced due to the bremsschtrahlung at the construction elements having electron fluences of 1011 –1014 e cm22.
36
Downloaded from http://rpd.oxfordjournals.org/ at Florida International University on June 25, 2015
peaks for the case of electron irradiation differs essentially, i.e. a tendency of the peak rise at 1608S with respect to that at 2208S is clearly observed. The Al2O3:Fe specimen demonstrates the peaks at 1508S and 1958S. Our studies have shown that due to electron irradiation up to the 1013 e sm22 fluences, all the specimens reveal the increase in the phosphorescence and thermal luminescence yields, while at the 1014 e sm22 fluences—a decreasing tendency. The possible explanation of this is the effect of the lattice radiation defects production at irradiation due to the relativistic electron elastic scattering by the crystal atoms. Probably, some of these defects types could be the centres of competing non-radiative recombination. This is confirmed by the results of irradiating the crystals under study by the bremsstrahlung gamma-quanta with no presence of electrons. The relevant results are shown in Figure 4 demonstrating the thermal luminescence curves for the Al2O3, Al2O3:Nb, Al2O3:Fe crystals irradiated by the gamma-quanta produced due to the bremsstrahlung at the construction elements with the 1011 –1014 e sm22 electron fluences. As seen from the above data, while at low doses the thermal luminescence yield in case of electron irradiation is sufficiently large than that in case of the gamma-quanta irradiation, when reaching the fluences .1013 e sm22, the opposite tendency is observed being due to the continuous increase of the thermal luminescence yield with increasing gamma-irradiation dose (see Figure 5). The results obtained testify to the above assumption. Indeed, though at irradiating crystals by the high-energy photons the radiation defects of the displaced-atom type are being produced due to their production resulting from the secondary photoelectrons, the Compton electron and from the electron–positron production processes. Their production cross section is
SPECIFIC FEATURES OF THE INFLUENCE OF HIGH-ENERGY ELECTRON BEAMS
considerably less than that in the case of electron irradiation. This, finally, results in the thermal luminescence and phosphorescence increase at the irradiation by the large gamma-quanta doses.
This work was partly supported by a research project from National Academy of Sciences, Ukraine, contract No. K –5–27/2014. REFERENCES
CONCLUSIONS
ACKNOWLEDGEMENTS Authors sincerely thank the Scientific Advisory Committee of the RAD 2014 for the invitation to present a talk and financial support of participation.
37
Downloaded from http://rpd.oxfordjournals.org/ at Florida International University on June 25, 2015
1. Weinstein, M., German, U. and Alfassi, Z. F. On neutrongamma mixed field dosymetry with LiF:Mg, Ti at radiation protection dose levels. Radiat. Prot. Dosim. 119, 314–318 (2006). 2. Akselrod, M. S., Botter-Jensen, L. and McKeever, S. W. S. Optically stimulated luminescense and its use in medical dosimetry. Radiat. Meas. 41, 78–99 (2006). 3. Bilski, P., Obryk, B. and Stuglik, Z. Behaviour of LiF:Mg,Cu,P and LiF:Mg,Ti thermoluminescent detectors for electron doses up to 1 MGy. Radiat. Meas. 45, 576– 578 (2010). 4. Obryk, B., Bilski, P. and Olko, P. Method of thermoluminescent measurement of radiation doses from micrograys up to a megagray with a single LiF:Mg,Cu,P detector. Radiat. Prot. Dosim. 144, 543–547 (2011). 5. Kruzhalov, A. V., Milman, I. I., Neshev, F. G. and Revkov, I. G. Optically stimulated luminescence of the a-Al2O3 crystals irradiated by protons. Technical Phys. Lett. 34, 809– 811 (2008). 6. Bletskan, D. I., Lukianchuk, A. P. and Pekar, Ya. M. Studies of the proper and extrinsic point defects in the sapphire substrates by the luminescent methods. Electronic materials. 3, 59–64 (2006) (in Russian). 7. Antonov-Romanovsky, V. V. Luminescent kinetics of crystallophosphores. (Moscow: Nauka) pp. 1 –317 (1966) (in Russian).
As a result of the influence on the thermal luminescent Al2O3-based materials of the high-current electron and gamma-quanta beams after the irradiation termination, a long-term phosphorescence arises at room temperature depending both on the irradiation dose and intensity. After reaching the mixed electron and gamma-quanta field fluences .1013 e sm22 the tendency to the decrease of the yield of both the phosphorescence and thermal luminescence is observed at the increasing dose explained by the production of radiation defects being the centres of non-radiative recombination. Combined measurements of the phosphorescence and thermal luminescence are promising for determining both the irradiation dose and the intensity of the high-current electron beams produced at the electron accelerators.
