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X-ray excited optical luminescence studies of rare earth activated thermoluminescent phosphors
Int. d. AppL Radiat. Isot. Vol. 35, No. 2, pp. 141-142, 1984
the emission centers for a possible inference regarding the charge state of the rare earth ion.
,~ Pergamon Press Ltd 1984. Printed in Great Britain 0020-708X/'84 53.00 + 0.00
2. Experimental
X-Ray Excited Optical Luminescence Studies of Rare Earth Activated Thermoluminescent Phosphors J. S. NAGPAL I* and M. J. K A M A T ' tDivision of Radiologicat Protection and 'Spectroscopy Division, Bhabha Atomic Research Centre, Trombay, Bombay-400 085, India (Received 14 April 1983)
XEOL spectra of CaSO4: Dy, MgF:: Dy, and CaO: Dy are typical of Dy 3. and similar to that of TL emission spectra. However, the studies fail to reveal the charge conversion state of the R E ion on irradiation.Minor differences,as regards the intensity ratio of various lines are observed depending upon the host matrix and the radiation history.
TL phosphors, used in the present study, were prepared indigenously, starting with analytical Reagent grade chemicals and 99.9% pure rare earth oxides. CaSO4 samples were prepared by the acid evaporation methodfi ) MgF2: Dy was prepared by the dry fusion method "°~ and CaO:Dy was obtained by heating coprecipitated CaCO3:DY. Samples for XEOL studies were prepared by depositing powder on a non-luminescent strip and using a minute quantity of silicone resin. XEOL measurements were made using an indigenously assembled simple apparatus. The sample was placed in the x-ray beam (W target, 40 kV, 20 mA) such that it was inclined at an angle of 45° to the x-ray beam as well as to the 0.25 m Ebert monochromator axis. The x-ray tube was turned " O N " and the XEOL spectra at RT (20°-25°C) were recorded on a strip chart recorder over the wavelength range 180-700 nm. An EMI 6256 B photomultiplier was used as the optical detector.
3. Results and Discussion
XEOL spectrum of unirradiated CaSO 4" Dy TL phosphor is shown in Fig. 1. It exhibits sharp lines, at 486 and 577 nm. The lines can be assigned to the transitions with the 4 v level of the Dy J+ ion. The 4T,,:--*6x,~, transition gives the first peak, second peak is assigned to'the 4Fg,i--'~6Hu: transition. These assigned values agree within the experimental limits 1. Introduction with TL emission peaks reported ty Nambi et al. tS~However, A variety of rare earth activated thermoluminescent (TL) the measured wavelengths are closer to those assigned by Merz and Pershan(9)in CaF::Dy. phosphors is in use for various radiation dosimetric and The phosphor CaSO, is a crystallineinsulatorhaving an other applications. Some of them are highly sensitive and have been employed for environmental monitoringY -2) energy gap of 8.2 eV. Its valence band representsthe range However, the underlying theory of thermoluminescence is of energy statesof the valence electronsthat bind the atoms not yet understood completely. TL glow curve analysis, in the solid state, with all energy levels being filled.The optical absorption measurements and photoluminescence;°) conduction band is initially empty. The absorption of and electron spin resonance studies(4"5) are useful in in- discreteamounts of energy from photo-and auger electrons vestigating the radiation induced defects, x-Ray excited formed by the primary absorption of x-rays by the host optical luminescence CXEOL) and cathodoluminescence are CaSO4 may resultin the excitationof an electron from the the two methods which allow the study of the phosphors valence band into the conduction band, leaving a positive during irradiation. Cathodoluminescence of CaSO4: Dy TL hole in the valence band. The recombination of the electron phosphor has been recently reported by Mathews and and the hole (disallowed in a defect-free host) takes place at Stoebe: e~ However, for cathodoluminescence studies the the defect (formed by substitutional impurity 2 Dy 3+ replacing 3 Ca2*). This emission results from characteristic samples have to be in vacuum. x-Ray excited optical luminescence (XEOL) in rare earth transitions in the defect structure. In the case of rare earths activated phosphors is generally attributed to the interaction. the excitation of f-f transitions gives rise to the sharp line of primary incident x-ray photons with host atom, produc- luminescent spectra because of the protection afforded by ing a large number of density of secondary excitants which the outer closed 5s2 and 5p 6 orbits. On 7-irradiation of the have a greater probability of eventually exciting the lumi- phosphor the XEOL intensities decrease continuously and nescence of the rare earth ions. On direct absorption of the for i0 s Gy (107 rad) of dose, the lines disappear; indicating primary x-rays, inner shell photoelectrons (K, L, M) are absence of Dy 3÷ ions. Even though our earlier absorption ejected from the host and impurity atoms. Alternatively, it studies0' have indicated that on irradiation of CaSO4: Dy, is presumed that the absorbed energy may be internally converted into Auger electrons which in turn interact with extranuciear electrons resulting in the production of secondary excitants having a medium energy (several eV). Finally, energy may be transferred from host ions to the impurity ions giving rise to optical luminescence: ~ i This study reports on X E O L of T L phosphors CaSO,:Dy, CaSO4:Tb. MgF,:Dy and CaO:Dy. The aim is to determine the emission spectrum and characteristicsof
* Author to w h o m dressed. A.R.L 35~2--E
SO0 Waveleflqth (nm)
all correspondence should be ad-
600
Fig. 1. XEOL spectrum of CaSO4:Dy at RT (20°-25°C). 141
142
Technical Note
1
i 200
300
400 500 Wovellln~ltll (nm)
600
700
Fig. 2. XEOL spectra (A) CaO:Dy (B) MgFz:Dy at RT (20°-25'C). Dy J~ gets reduced to Dy-", XEOL spectral studies provide no evidence for the same. XEOL spectra of unirradiated CaO:Dy and MgF.,:Dy samples, shown in Fig. 2, are similar to those of CaSO4: Dy and characteristic of Dy 3~" ion. Even though the method of preparation of samples for the present study is such that quantitative comparisons from one sample to another can not be made (since mass of the phosphor and uniform spreading of the phosphor is not controlled precisely), it is possible only to compare the relative ratio of intensities of various spectral lines in the same sample. In the CaSO4 matrix, the first peak (486 am) is more intense as compared to the second peak (577 nm) whereas in CaO and MgFe matrices, the second peak is predominant over the first one (Figs 1 and 2). In fact the 486 nm peak emission is very feeble in MgF:. The emission around 400 nm, observed in MgF::Dy, has also been observed in photoluminescence studies, ~'°~and the host matrix is presumed to be responsible for the same. Thus we see that even though the emission lines for the rare earth remain unchanged within experimental limits, when placed in different matrices; the respective probabilities for various transitions seem to be affected which needs further investigation and has not been paid due attention. XEOL spectra of irradiated CaO:Dy and MgF2:Dy again fail to bring out any evidence of Dy~+--Dy :+. Figure 3 shows XEOL spectrum of unirradiated CaO: Dy. Intensity of the second line (583 nm) is higher than that of the first (487.5nm). On 7-irradiations (2.3 x 104Gy,
200
I 300
I I 400 500 Wovelen~lth (nml
600
Fig. 4. XEOL spectrum of CaSO4:Tb at RT (20°-25°C).
7.5 x 10~ Gy), XEOL intensities decrease, but the intensity of the 583 nm line decreases by a greater extent so that for the 7.5 × l0 s Gy irradiated CaO:Dy sample the intensity of the 487.5 ram peak is higher as compared to that of 583 nm. At that stage it can only be inferred that non-radiative probability is higher for the 4v)f--.6H,3.z transition in 7-irradiated CaO: Dy. XEOL spectrum of unirradiated CaSO4:Tb is characteristic of the Tb 3+ ion (Fig. 4). On 7-irradiation, the XEOL intensities decrease.
4. C o ~ l m | o m Thus even though XEOL studies yield a fine structure of the emission spectra of rare earth doped TL samples, the same do not help determine the charge state of the rare earth ion in irradiated samples. Perhaps the percentage of the ions Dy ~+ converted to Dy 2. is very small. Very minute differences in the XEOL spectra are observed for a rare earth ion put in different matrices. Even though we have not been able to assign the reasons for the same, the effect of the host matrix does exist. Acknowledgements--Tbe authors are thankful to Mrs R. Kaimal for experimental assistance, to Dr K. (3. Vohra for constant encouragement and to P. Gangadharan for many useful discussions.
A References B
i o:
~/ ;I" , :I~
~ ', ! ./: ,| 1~ \ t :
.......................-..
/
/,
~ ........
.;
~,
r 4OO
~"
/ :~ ".
...................... " / " i ~;~'"........
........
~00
l'-. 600
Wovelength (nm)
Fig. 3. XEOL spectra of CaO:Dy at RT (20°-25°C). (A) unirradiated (B) 7-irradiated (2.3x104Gy) and (C) 7-irradiated (7.5 × 104Gy).
1. McKinlay A. F. Thermoluminescent Dosimetry, Med. Phys. Handbook 5, p. 40. (Adam Hilger, Bristol, 1981). 2. Ramanathan G., Nagpal J. S. and Gangadharan P. Nucl. lnstrum. Methods 164, 601 (1979). 3. Nagpal J. S. Int. J. AppL Radiat. Isot. 31, 333 (1980). 4. Huzimura R. and Atarashi K. Phys. Stat. Solidi a 70, 649 (1982). 5. Danby Robert J., Boas J. F., Calvert R. L. and Pilbrow J. B. J. phys. Chem. 15, 2483 0982). 6. Mathews R. J. and Stoebe T. G. Phys. Stat. Solid Chem. a 71, 55 (1982). 7. D'Silva A. P. and Fassel V. A. X-ray Excited Optical Luminescence of the Rare Earths (Eds Gschneidner K. A. Jr and Eyring L.) p. 441 (North Holland, Amsterdam, 1979). 8. Nambi K. S. V. and Bapat V. N. J. Phys. Chem. 13, 1555 (1980). 9. Merz J. L. and Pershan P. S. Phys. Rev. 162, 217 (1967). 10. Nagpal J. S., Kathuria V. K. and Bapat V. N. Int. J. AppL Radiat Isot. 32, 147 (1981).
the emission centers for a possible inference regarding the charge state of the rare earth ion.
,~ Pergamon Press Ltd 1984. Printed in Great Britain 0020-708X/'84 53.00 + 0.00
2. Experimental
X-Ray Excited Optical Luminescence Studies of Rare Earth Activated Thermoluminescent Phosphors J. S. NAGPAL I* and M. J. K A M A T ' tDivision of Radiologicat Protection and 'Spectroscopy Division, Bhabha Atomic Research Centre, Trombay, Bombay-400 085, India (Received 14 April 1983)
XEOL spectra of CaSO4: Dy, MgF:: Dy, and CaO: Dy are typical of Dy 3. and similar to that of TL emission spectra. However, the studies fail to reveal the charge conversion state of the R E ion on irradiation.Minor differences,as regards the intensity ratio of various lines are observed depending upon the host matrix and the radiation history.
TL phosphors, used in the present study, were prepared indigenously, starting with analytical Reagent grade chemicals and 99.9% pure rare earth oxides. CaSO4 samples were prepared by the acid evaporation methodfi ) MgF2: Dy was prepared by the dry fusion method "°~ and CaO:Dy was obtained by heating coprecipitated CaCO3:DY. Samples for XEOL studies were prepared by depositing powder on a non-luminescent strip and using a minute quantity of silicone resin. XEOL measurements were made using an indigenously assembled simple apparatus. The sample was placed in the x-ray beam (W target, 40 kV, 20 mA) such that it was inclined at an angle of 45° to the x-ray beam as well as to the 0.25 m Ebert monochromator axis. The x-ray tube was turned " O N " and the XEOL spectra at RT (20°-25°C) were recorded on a strip chart recorder over the wavelength range 180-700 nm. An EMI 6256 B photomultiplier was used as the optical detector.
3. Results and Discussion
XEOL spectrum of unirradiated CaSO 4" Dy TL phosphor is shown in Fig. 1. It exhibits sharp lines, at 486 and 577 nm. The lines can be assigned to the transitions with the 4 v level of the Dy J+ ion. The 4T,,:--*6x,~, transition gives the first peak, second peak is assigned to'the 4Fg,i--'~6Hu: transition. These assigned values agree within the experimental limits 1. Introduction with TL emission peaks reported ty Nambi et al. tS~However, A variety of rare earth activated thermoluminescent (TL) the measured wavelengths are closer to those assigned by Merz and Pershan(9)in CaF::Dy. phosphors is in use for various radiation dosimetric and The phosphor CaSO, is a crystallineinsulatorhaving an other applications. Some of them are highly sensitive and have been employed for environmental monitoringY -2) energy gap of 8.2 eV. Its valence band representsthe range However, the underlying theory of thermoluminescence is of energy statesof the valence electronsthat bind the atoms not yet understood completely. TL glow curve analysis, in the solid state, with all energy levels being filled.The optical absorption measurements and photoluminescence;°) conduction band is initially empty. The absorption of and electron spin resonance studies(4"5) are useful in in- discreteamounts of energy from photo-and auger electrons vestigating the radiation induced defects, x-Ray excited formed by the primary absorption of x-rays by the host optical luminescence CXEOL) and cathodoluminescence are CaSO4 may resultin the excitationof an electron from the the two methods which allow the study of the phosphors valence band into the conduction band, leaving a positive during irradiation. Cathodoluminescence of CaSO4: Dy TL hole in the valence band. The recombination of the electron phosphor has been recently reported by Mathews and and the hole (disallowed in a defect-free host) takes place at Stoebe: e~ However, for cathodoluminescence studies the the defect (formed by substitutional impurity 2 Dy 3+ replacing 3 Ca2*). This emission results from characteristic samples have to be in vacuum. x-Ray excited optical luminescence (XEOL) in rare earth transitions in the defect structure. In the case of rare earths activated phosphors is generally attributed to the interaction. the excitation of f-f transitions gives rise to the sharp line of primary incident x-ray photons with host atom, produc- luminescent spectra because of the protection afforded by ing a large number of density of secondary excitants which the outer closed 5s2 and 5p 6 orbits. On 7-irradiation of the have a greater probability of eventually exciting the lumi- phosphor the XEOL intensities decrease continuously and nescence of the rare earth ions. On direct absorption of the for i0 s Gy (107 rad) of dose, the lines disappear; indicating primary x-rays, inner shell photoelectrons (K, L, M) are absence of Dy 3÷ ions. Even though our earlier absorption ejected from the host and impurity atoms. Alternatively, it studies0' have indicated that on irradiation of CaSO4: Dy, is presumed that the absorbed energy may be internally converted into Auger electrons which in turn interact with extranuciear electrons resulting in the production of secondary excitants having a medium energy (several eV). Finally, energy may be transferred from host ions to the impurity ions giving rise to optical luminescence: ~ i This study reports on X E O L of T L phosphors CaSO,:Dy, CaSO4:Tb. MgF,:Dy and CaO:Dy. The aim is to determine the emission spectrum and characteristicsof
* Author to w h o m dressed. A.R.L 35~2--E
SO0 Waveleflqth (nm)
all correspondence should be ad-
600
Fig. 1. XEOL spectrum of CaSO4:Dy at RT (20°-25°C). 141
142
Technical Note
1
i 200
300
400 500 Wovellln~ltll (nm)
600
700
Fig. 2. XEOL spectra (A) CaO:Dy (B) MgFz:Dy at RT (20°-25'C). Dy J~ gets reduced to Dy-", XEOL spectral studies provide no evidence for the same. XEOL spectra of unirradiated CaO:Dy and MgF.,:Dy samples, shown in Fig. 2, are similar to those of CaSO4: Dy and characteristic of Dy 3~" ion. Even though the method of preparation of samples for the present study is such that quantitative comparisons from one sample to another can not be made (since mass of the phosphor and uniform spreading of the phosphor is not controlled precisely), it is possible only to compare the relative ratio of intensities of various spectral lines in the same sample. In the CaSO4 matrix, the first peak (486 am) is more intense as compared to the second peak (577 nm) whereas in CaO and MgFe matrices, the second peak is predominant over the first one (Figs 1 and 2). In fact the 486 nm peak emission is very feeble in MgF:. The emission around 400 nm, observed in MgF::Dy, has also been observed in photoluminescence studies, ~'°~and the host matrix is presumed to be responsible for the same. Thus we see that even though the emission lines for the rare earth remain unchanged within experimental limits, when placed in different matrices; the respective probabilities for various transitions seem to be affected which needs further investigation and has not been paid due attention. XEOL spectra of irradiated CaO:Dy and MgF2:Dy again fail to bring out any evidence of Dy~+--Dy :+. Figure 3 shows XEOL spectrum of unirradiated CaO: Dy. Intensity of the second line (583 nm) is higher than that of the first (487.5nm). On 7-irradiations (2.3 x 104Gy,
200
I 300
I I 400 500 Wovelen~lth (nml
600
Fig. 4. XEOL spectrum of CaSO4:Tb at RT (20°-25°C).
7.5 x 10~ Gy), XEOL intensities decrease, but the intensity of the 583 nm line decreases by a greater extent so that for the 7.5 × l0 s Gy irradiated CaO:Dy sample the intensity of the 487.5 ram peak is higher as compared to that of 583 nm. At that stage it can only be inferred that non-radiative probability is higher for the 4v)f--.6H,3.z transition in 7-irradiated CaO: Dy. XEOL spectrum of unirradiated CaSO4:Tb is characteristic of the Tb 3+ ion (Fig. 4). On 7-irradiation, the XEOL intensities decrease.
4. C o ~ l m | o m Thus even though XEOL studies yield a fine structure of the emission spectra of rare earth doped TL samples, the same do not help determine the charge state of the rare earth ion in irradiated samples. Perhaps the percentage of the ions Dy ~+ converted to Dy 2. is very small. Very minute differences in the XEOL spectra are observed for a rare earth ion put in different matrices. Even though we have not been able to assign the reasons for the same, the effect of the host matrix does exist. Acknowledgements--Tbe authors are thankful to Mrs R. Kaimal for experimental assistance, to Dr K. (3. Vohra for constant encouragement and to P. Gangadharan for many useful discussions.
A References B
i o:
~/ ;I" , :I~
~ ', ! ./: ,| 1~ \ t :
.......................-..
/
/,
~ ........
.;
~,
r 4OO
~"
/ :~ ".
...................... " / " i ~;~'"........
........
~00
l'-. 600
Wovelength (nm)
Fig. 3. XEOL spectra of CaO:Dy at RT (20°-25°C). (A) unirradiated (B) 7-irradiated (2.3x104Gy) and (C) 7-irradiated (7.5 × 104Gy).
1. McKinlay A. F. Thermoluminescent Dosimetry, Med. Phys. Handbook 5, p. 40. (Adam Hilger, Bristol, 1981). 2. Ramanathan G., Nagpal J. S. and Gangadharan P. Nucl. lnstrum. Methods 164, 601 (1979). 3. Nagpal J. S. Int. J. AppL Radiat. Isot. 31, 333 (1980). 4. Huzimura R. and Atarashi K. Phys. Stat. Solidi a 70, 649 (1982). 5. Danby Robert J., Boas J. F., Calvert R. L. and Pilbrow J. B. J. phys. Chem. 15, 2483 0982). 6. Mathews R. J. and Stoebe T. G. Phys. Stat. Solid Chem. a 71, 55 (1982). 7. D'Silva A. P. and Fassel V. A. X-ray Excited Optical Luminescence of the Rare Earths (Eds Gschneidner K. A. Jr and Eyring L.) p. 441 (North Holland, Amsterdam, 1979). 8. Nambi K. S. V. and Bapat V. N. J. Phys. Chem. 13, 1555 (1980). 9. Merz J. L. and Pershan P. S. Phys. Rev. 162, 217 (1967). 10. Nagpal J. S., Kathuria V. K. and Bapat V. N. Int. J. AppL Radiat Isot. 32, 147 (1981).