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Role of dopants in LiF TLD materials
Radiation Measurements 43 (2008) 303 – 308 www.elsevier.com/locate/radmeas
Invited paper
Role of dopants in LiF TLD materials J.I. Lee a,∗ , J.L. Kim a , A.S. Pradhan a , B.H. Kim a , K.S. Chung b , H.S. Choe b a Health Physics Department, Korea Atomic Energy Research Institute, P.O. Box 105 Yuseong, Daejeon 305-600, Republic of Korea b Gyeongsang National University, Jinju 660-701, Republic of Korea
Abstract Thermoluminescence (TL) in LiF was found to depend on the use of proper combination of the dopant ions and the optimised preparation procedure. LiF doped with Mg,Cu,Si exhibited high TL sensitivity (55 times that of TLD-100 LiF:Mg,Ti) and insignificant higher temperature glow peak leading to negligible residual TL signal. The occurrence of the dosimetric peak in LiF was found to be a strong function of Mg dopant. The peak temperature and the TL emission spectra of dosimetric glow peak in all samples having Mg, Cu were always found to be the same irrespective of the third dopant which influenced only the intensity of TL emission spectrum. The emission band peaking at 385 nm was assigned to Cu. © 2007 Elsevier Ltd. All rights reserved. Keywords: Mg,Cu,Si doped LiF; Thermoluminescence; Role of dopants; TL emission spectrum
1. Introduction Role of dopants in LiF is well recognised in enhancing TL sensitivity and changing the structure of glow curves for obtaining a near-tissue equivalent thermoluminescent dosimeter (TLD). For the past more than 50 years, efforts have continued in arriving at the variety of dopants but noticeable success had been for the combination of dopants such as Mg,Ti, Mg,Cu,P; Mg,Cu,NaSi and more recently for Mg,Cu,Si. Decision on the choice of the dopants appears to have been mainly based on the intrusions derived from the retrospective chemical analyses, behaviour of dopants in other materials, matching of ionic radii of the dopant ion with one of the host lattice ion, chemical valence consideration for the charge compensations, characteristic emissions, availability in a form appropriate for the process of doping, etc. However, in spite of a large number of publications on LiF:Mg,Cu,P appearing in the past several years, there is still no unanimity on the role of dopants. McKeever (1991), based on the results of unchanged thermoluminescence (TL) emission spectrum of LiF:Mg,Cu,P doped with either monovalent or divalent copper salts, concluded that ∗ Corresponding author.
E-mail address: [email protected] (J.I. Lee). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.10.040
Cu does not play any role in the TL emission process and phosphorus is the main activator responsible for TL emission. The glow curve structure was inferred to be dictated by the presence of Mg (as also observed by many others: e.g. Shoushan, 1988; Bilski et al., 1996, 1997, 1998; Lee et al., 2005). Bilski et al. (1996) found that Mg to be the essential dopant as very small changes of Mg concentrations affected both the glow curve structure and the TL sensitivity. Mg-related defects were related to the thermal susceptibility and a reversible reaction between the main dosimetric peak and the higher temperature peak was observed by Bilski et al. (1997) in LiF:Mg,Cu,P. Bilski et al. (1998) found that Mg is responsible not only for the formation of the trapping centres but also influences the dose response and LET dependence. In line with the results of McKeever (1991), Bilski et al. (1996, 1997) concluded that a threshold amount of P is needed to obtain high TL sensitivity whereas Cu does not take part in the formation of the traps and the luminescent centres but helps only in the incorporation of larger Mg content and therefore suggested that P is responsible for luminescent centers and may also be involved in peak 4 related traps. These findings were similar to those of Bos et al. (1996) who found the TL emission spectra to change with the concentration of Cu and concluded that Cu is not directly responsible either to emission centers or to traps but may hamper the higher order of clustering of Mg related defects resulting
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J.I. Lee et al. / Radiation Measurements 43 (2008) 303 – 308
in an increase in the production traps. P was shown to play a dual role of being involved in both the trapping as well as in the TL luminescence processes. A different dual role of P was proposed by Shinde et al. (2001) in that P does not act as luminescent center but acts as a hole trap and helps in higher incorporation of Cu ions into the lattice of LiF. This was based on the observation that by increasing concentration of phosphorus both the TL sensitivity and the intensity Cu2+ ESR signal increased considerably. The increase in Cu2+ was speculated to also increase in the incorporation of Cu+ which was considered to act as luminescent center. It is still not clear whether the proposition of Bilski (2002) of Cu helping in the incorporation of larger Mg content is correct or the proposition of Shinde et al. (2001) of P helping in the incorporation of larger amount of Cu ions is correct or both are correct or incorrect. More recently, Gunduroa and Moharil (2007) identified a new defect center, (PO4 )2− radical, through the ESR technique and TL measurements. They related the TL sensitivity to this center and proposed that the filled traps are related to the clusters of interstitial fluorine atoms and PO4− hole centers, whereas magnesium-related defects such as Mg2+ vacancy dipoles and their clusters along with F center act as a recombination center and the energy of recombination is transferred to Cu+ ion which acts as a luminescent center. Patil and Moharil (1995) earlier also concluded Cu+ to act as a luminescent center but that was based on their observation of the similarity between the photoluminescence spectra of LiF:Cu and LiF:Mg,Cu,P doped with copper salt along with a flux. In contrast, Mandowska et al. (2002) observed that the TL emission spectra for the same amount of Mg and Cu are not the same when P is replaced by Ti (380 nm band not observed for Ti). Also, in the absence of Cu, the spectra for the same amount of concentration of P changes with the concentration of Mg. This again raised a doubt on the luminescence center in LiF:Mg,Cu,P. Recent studies of spectral analysis of TL emission of NaCl and LiF by Davidson et al. (2007) demonstrated the emission to be from self-trapped excitons in these alkali halides and this emission could be enhanced and altered by the impurities due to impurity–vacancy dipoles. Chen and Stoebe (1998), through their study of extended X-ray absorption fine structure (EXAFS) where only the virgin samples were shown to contain Cu+ as a substitution to Li+ ion and not the irradiated samples or those subjected to annealing at higher temperatures, concluded that Cu+ is converted to Cu2+ on irradiation and the Cu+ to Cu2+ conversion takes place. Cu+ related defects were noted to act as a luminescence centers, especially for 380 nm TL emissions. The conversion of Cu+ to Cu2+ was inferred to be irreversible, which is responsible for the loss of TL sensitivity of LiF:Mg,Cu,P on annealing at temperatures above 240 ◦ C. However, Mathur et al. (2002) and more recently Kurt et al. (2006) argued that the isolated Cu+ ion cannot be the recombination center and ruled out the conversion of isolated ions of Cu+ to Cu2+ in the lattice of LiF because a temperature of 573 K cannot cause thermal ionisation of the isolated Cu+ to Cu2+ . They pointed out that Cu+ cannot
substitute Li+ ion and LiF cannot be doped with Cu+ even though the substitution does not need a charge compensator. This conclusion was drawn from the consideration that the ion ˚ of Cu+ is much larger than Li+ (0.68 A) ˚ and size (0.96 A) therefore the substitution would cause unacceptable distortion of the lattice. They, however, proposed that Cu and P ions could be incorporated in the precipitated MgF2 phase where two Mg2+ ions would be substituted by a Cu+ ion and a P3+ ion. This substitution would maintain the charge neutrality and should not distort the lattice severely as the ionic radii of Cu+ ˚ respectively, whereas the ionic and P3+ are 0.96 and 0.44 A, 2+ ˚ radii of a Mg ion is 0.66 A, so the sum of the Cu+ and P3+ ion radii match with the radii of two Mg2+ ions. Therefore, P3+ can interact with Cu+ in the form of donor–acceptor pairs (DAPs) and would give rise to donor–acceptor luminescence center where P3+ is a donor and Cu+ is an acceptor. This rational facilitated the Cu+ to Cu2+ conversion proposed by Chen and Stoebe. In the absence of P3+ ion, Cu was supposed ˚ substituting to enter the lattice as Cu2+ (ionic radii 0.72 A) 2+ for Mg . A study of Yang et al. (2005) on TL of differently doped LiF concluded that the reduction in TL sensitivity on annealing above 250 ◦ C is associated with precipitated phase of Mg and a secondary phase (complexes of Mg and other impurities). This precipitation was ascribed to the presence of higher concentrations of Mg. The X-ray diffraction (XRD) and ESR studies of Sun et al. (1994) have shown that not only the precipitation but also different crystalline phases are produced in LiF:Mg,Cu,P during the preparation process. They observed three phases: viz. polycrystalline LiF, Li4 P2 O7 and an unidentified new polycrystalline material. This unidentified phase was attributed to be responsible for TL in LiF:Mg,Cu,P and its light blue colour. Their ESR results showed no change in the Cu2+ valence on irradiation to gamma rays. They suggested that although Cu is an important impurity but the TL process is performed through O and P and an electron is captured at P, therefore, P should be the activator in LiF:Mg,Cu,P. Gunduroa and Moharil (2007) insisted that the solubility of Mg impurity in LiF is very low and therefore the fate excess Mg (up to 2000 ppm) in LiF was questioned. Shen et al. (2002) demonstrated that on storage for 30 days at 98% RH, orthophosphate is produced which changes the colour from bright blue to brown and reduces the TL sensitivity of LiF:Mg,Cu,P. It is evident that the crystalline phases, precipitates and clusters of impurities–vacancies do play a very important role in the TL process in LiF. These and the results of our own studies of more than 10 years on differently doped LiF, especially LiF:Mg,Cu,NaSi, indicated that Na could be responsible for the lack of thermal stability of LiF TLD (in line with the results of Tang, 2003). This convinced us that Na and P need to be avoided as dopant and fresh attempts are necessary for improving the TL dosimetric characteristics of TLDs based on LiF. Consequently we developed LiF:Mg,Cu,Si which enabled us to over come the drawbacks of LiF:Mg,Cu,P. It is evident that the crystalline phase, precipitates and clusters of impurities–vacancies do play a very important role in the TL process in LiF.
J.I. Lee et al. / Radiation Measurements 43 (2008) 303 – 308
LiF doped with a variety of dopants were prepared. Most of the equipment, including 137 Cs gamma rays sources, a Studsvik 90 Sr–90 Y beta-ray reference irradiator, ovens, set-up for recording the emission spectra and TLD reader and the procedures adopted are described in our earlier publications (Lee et al., 2005, 2006a, b, 2007). For comparison, commercially available LiF:Mg,Cu,P GR-200 and LiF:Mg,Cu,Na,Si TLD discs were procured. The results of the dopant concentration presented in this work are those used for the preparation of the final samples of LiF:Mg,Cu,Si (Lee et al., 2006a, 2007). For impurity analysis and the estimation of incorporation of dopant ions in the prepared samples, inductive coupled plasma mass spectroscopy (ICP-MS) was used. XRD analysis was carried out to ascertain the phases present in the final sample. Two methods of preparations were adopted: viz. (1) melting method and (2) granulation method, which differed only by the fact that in the former case the mixture of compounds of dopants and LiF was melted and in the latter case it was sintered below the melting point. After each step of thermal treatment (whether melting, sintering or other thermal treatment), the samples were subjected to fast cooling. The TL glow curves were recorded at a heating rate of 10 ◦ C s−1 . 3. Results and discussion TL in LiF was found to depend on the proper combination of the dopant ions and the optimised preparation procedure. For the preparation of Mg,Cu,Si doped LiF, the melting method resulted in better TL sensitivity whereas for the other dopant combinations, the granulation method gave better results. In all cases, Mg was found to be the most dominant dopant. The enhanced concentration of Mg (0.45 mol%) and reduced concentration of Cu (0.025 mol%) along with Si (0.9 mol%) gave the best results by using the melting method. LiF doped with these concentrations of Mg, Cu and Si exhibited high TL sensitivity (55 times that of LiF:Mg,Ti TLD-100). These concentrations were therefore used as the optimised values for the other studies in this work. Table 1 shows the results of analysis of impurities in the final sample obtained by using ICP-MS. It can be seen that the proportions of the dopants intended and finally present are different for different dopants. Only a little amount of Cu found its place in the final sample whereas most of the Mg was retained. The results of XRD analysis of the final sample of LiF:Mg,Cu,Si showed the presence LiF, MgF2 , SiO2 and an unknown phase. This was an indication of the formation of several phases. The glow curve structure of LiF:Mg,Cu,Si was found to be similar to that of LiF:Mg,Cu,P except for the much reduced relative intensity of the higher temperature glow peak responsible for the residual TL signal. The reduction in the higher temperature tail of the dosimetric glow peak was achieved by the use of a dual-step of thermal treatment at 300 ◦ C for 10 min followed by 260 ◦ C for 10 min as a final step of the preparation procedure (Lee et al., 2006a, 2007). Thermal treatment at 300 ◦ C (the first step of dual-step thermal
Table 1 Concentration of dopants used in the preparation and those finally remained (assessed) in the final sample of LiF:Mg,Cu,Si after the preparation Dopant
Used in the preparation (mol%)
Assessed in the final sample (mol%)
Mg Cu Si
0.45 0.025 0.9
0.34 0.0045 0.33
50
2
40 TL Intensity (arb. unit)
2. Material and methods
305
30 1 20
10
0 50
100
150
200
250
300
Temperature (°C) Fig. 1. Glow curves of LiF:Mg,Cu,Si sintered discs irradiated to 5.5 mGy from 90 Sr–90 Y beta-ray and subjected to 1–300 ◦ C for 10 min (first step of the dual-step thermal treatment) and 2–260 ◦ C for 10 min (second step of the dual-step thermal treatment) following the 300 ◦ C treatment.
treatment) resulted in the shift of the main peak to higher temperatures (265 ◦ C) and the treatment at 260 ◦ C (the second step of dual-step thermal treatment) brought back the dosimetric peak to 238 ◦ C with increased sensitivity and resulted in minimizing the higher temperature tail of the dosimetric glow peak (Fig. 1). The shifted peak (265 ◦ C) in the 300 ◦ C treated LiF:Mg,Cu,Si was considered to be the same as that of the higher temperature peak in LiF:Mg,Cu,P (Lee et al., 2007). It may be noted that in LiF:Mg,Cu,P, the emission spectrum (355 nm band) of the higher temperature glow peak and the dosimetric peak (385 nm band) were found to be different (Lee et al., 2006b; Kim et al., 2007). Therefore, it became important to study the TL emission spectrum of this shifted (265 ◦ C) peak (Fig. 1) in 300 ◦ C treated LiF:Mg,Cu,Si. Fig. 2 shows that the emission spectrum of the shifted (265 ◦ C) peak in 300 ◦ C treated sample and the regenerated dosimetric peak (238 ◦ C) in LiF:Mg,Cu,Si are exactly the same. It may also be noted that a thermal treatment at temperature above 250 ◦ C in LiF:Mg,Cu,P resulted in a shift in the main glow peak temperature by about 30–50 ◦ C and a corresponding shift in the emission spectrum from 380 to 350 nm (McKeever, 1991). Thus, from the considerations of the TL emission, the effect of the thermal treatment at 300 ◦ C (Lee et al., 2007) in LiF:Mg,Cu,Si (where the emission spectra of the shifted (265 ◦ C) peak in 300 ◦ C treated sample and the
306
J.I. Lee et al. / Radiation Measurements 43 (2008) 303 – 308 Table 2 Results of analysis of glow curve structure and TL sensitivity of the main peak (Fig. 3) and TL emission spectra (Fig. 4) of LiF doped with different combinations of the dopants (Mg: 0.45 mol%; Cu: 0.025 mol% and Si: 0.9 mol%)
0.15 385
0.10
Thermal treatment
TL intensity (arb. unit)
300 °C 0.05 440
355
0.00 0.10 385
Thermal treatment 300 °C / 260° C
0.05 440
355
0.00 300
400 450 500 Wavelength (nm)
550
Normalized TL intensity
TL emission peak and the deconvoluted bands (nm) of the main TL emission
LiF:Mg LiF:Cu LiF:Si LiF:Cu,Si LiF:Mg,Si LiF:Mg,Cu LiF:Mg,Cu,Si LiF:Mg,Cu,P LiF:Mg,Cu,Na,Si
Undefined (feeble) 175 (0.016) 150 (0.027) 150 (0.028) 238 (0.010) 238 (0.170) 238 (1.000) 238 (0.900) 238 (0.950)
– 434 nm 364 nm 362 nm 400 nm 384 nm 384 nm 368 nm 370 nm
1
4 5 6
100
150
200
250
(355, (355, (355, (355, (355,
399, 385, 385, 385, 385,
442) 440) 440) 440) 440)
TL sensitivity is shown in the parentheses in Column 2.
600
LiF:Cu LiF:Si LiF:Cu,Si LiF:Mg,Cu LiF:Mg,Si LiF:Mg,Cu,Si
32
50
Main TL peak temperature (◦ C) and relative TL sensitivitya
a Relative
350
Fig. 2. TL emission spectrum of LiF:Mg,Cu,Si treated at 300 ◦ C for 10 min (upper figure) and at 260 ◦ C for 10 min (second step of the dual-step thermal treatment) following the 300 ◦ C treatment (lower figure).
1 2 3 4 5 6
Differently doped LiF
300
Temperature (°C) Fig. 3. TL glow curves of 0.8 mm thick TLD discs of LiF doped with different dopants (dopant concentration values as that in the optimised LiF:Mg,Cu,Si TLD) at heating rate of 10 ◦ C s−1 after irradiation to 5.5 mGy from 90 Sr–90 Y beta-rays (TL intensities not to be compared, see Table 2).
dosimetric peak (238 ◦ C) are exactly the same) cannot draw a similarity with LiF:Mg,Cu,P (where the emission spectra shifts). The Cu+ to Cu2+ conversion suggested by Chen and Stoebe (1998) in LiF:Mg,Cu,P where the conversion of Cu+ to Cu2+ is ascribed to heating to higher temperatures which shifts the dosimetric glow peak to higher temperatures is not valid in LiF:Mg,Cu,Si. The peak shift in LiF:Mg,Cu,Si due to 300 ◦ C heat treatment during the dual-step thermal treatment appears to be due to the change in trap distribution not affecting the luminescent centre. Fig. 3 shows the TL glow curves of 0.8 mm thick TLD discs of LiF doped with different dopants. Table 2 shows the detailed
analysis of TL glow curve structure, TL sensitivity and TL emission spectra of LiF doped with different combinations of the dopants. It can be inferred that, without Mg there is no 238 ◦ C main/dosimetric peak and Mg alone (without a codopant) gives a very feeble TL with several peaks ranging from 120 to 230 ◦ C. Si and Cu,Si doped LiF exhibit TL emission peaks around 362 nm (Fig. 4) and the glow peak at 175 ◦ C (there is no 238 ◦ C peak in both the cases). In the absence of Mg, the addition of Cu does not influence the TL emission peak and the glow peak temperature. Si and Mg,Si doped LiF exhibit TL emission peaks at 362 and 400 nm and the TL glow peaks at 150 and 238 ◦ C, respectively. Thus Mg influences both the glow curve structure and the TL emission spectra. Cu and Mg,Cu doped LiF exhibit emission peaks at 434 and 385 nm and TL glow peaks at 175 and 238 ◦ C, respectively. Thus Mg again influences not only the glow peak but also the emission spectra (in line with the findings of Mandowska et al., 2002). However, the emission band peaking at 385 nm can be observed only in the presence of both the Cu and Mg. The XRD analysis supports the presence of several phases including MgF2 and unknown phases (McKeever et al., 1993; Sun et al., 1994; Yang et al., 2005; Mathur et al., 2002; Kurt et al., 2006) needed for understanding the TL process. Table 2 shows that the main TL glow peak and the main TL emission band of Mg,Cu and Mg,Cu,Si doped LiF and also of Mg,Cu,Na,Si and Mg,Cu,P doped LiF with concentrations of dopants different than LiF:Mg,Cu,Si are the same. Therefore, Si, P or Na,Si does not affect the emission spectra which remains the same as that of Mg,Cu doped LiF. This reaffirms that in the system of LiF:Mg,Cu,X, the characteristics of the TL emission band are not influenced by the third dopants (X = P or Si or Na,Si) and P may not be acting as luminescent center in LiF:Mg,Cu,P (McKeever, 1991; Bos et al., 1996; Bilski, 2002). However, each of the three dopants (Mg, Cu and Si) appears to play a crucial role in the presence of each other: viz. Mg for the position of the dosimetric (238 ◦ C) peak, Cu for the emission band peaking at 385 nm and Si for enhancing the TL sensitivity.
J.I. Lee et al. / Radiation Measurements 43 (2008) 303 – 308
434
LiF:Cu
362
LiF:Cu,Si
TL Intensity
LiF:Si
355
LiF:Mg,Si
LiF:Mg,Cu 440
355
385 355
350
References
442
385
300
sensitivity and not the TL emission spectrum or the peak temperature of the main dosimetric peak. In LiF:Mg,Cu,Si each of the three dopants (Mg, Cu and Si) appear to play a crucial role in the presence of each other: viz. Mg for the glow curve structure, Cu for the emission band peaking at 385 nm and Si for enhancing the TL sensitivity. There appears a strong possibility of further improvement in the dosimetric properties of LiF based TLDs by changing this third dopant and using an appropriate preparation procedure.
364
399
LiF:Mg,Cu,Si 440
400 450 500 Wavelength (nm)
307
550
600
Fig. 4. TL emission spectrum of the LiF doped with different dopants recorded after irradiation to 800 Gy.
4. Conclusions An improved LiF based TLD (LiF:Mg,Cu,Si) has been developed by optimising the dopant combinations and concentrations and also the preparation procedure. A dual-step thermal treatment at 300 ◦ C for 10 min followed by 260 ◦ C for 10 min as a final step of preparation procedure ensured stabilising the glow peak structure and minimising the residual TL. The thermal treatment at 300 ◦ C (the first step of dual-step thermal treatment) shifted the glow peak to higher temperature (265 ◦ C) but the emission spectra of the shifted peak and the dosimetric peak in LiF:Mg,Cu,Si was the same unlike LiF:Mg,Cu,P. The hypothesis of Cu+ to Cu2+ conversion due to heating to temperatures above 240 ◦ C is in doubt in LiF:Mg,Cu,Si. The dosimetric peak in LiF is a strong function of Mg. The dosimetric peak cannot be observed without Mg and the emission band peaking at 385 nm can be observed only in the presence of Cu. This supports the proposition that Cu in combination with Mg in the defect complexes is responsible for the luminescence in the LiF:Mg,Cu,X system and the presence of the third dopants (X = P or Si or Na,Si) only influences the TL
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Invited paper
Role of dopants in LiF TLD materials J.I. Lee a,∗ , J.L. Kim a , A.S. Pradhan a , B.H. Kim a , K.S. Chung b , H.S. Choe b a Health Physics Department, Korea Atomic Energy Research Institute, P.O. Box 105 Yuseong, Daejeon 305-600, Republic of Korea b Gyeongsang National University, Jinju 660-701, Republic of Korea
Abstract Thermoluminescence (TL) in LiF was found to depend on the use of proper combination of the dopant ions and the optimised preparation procedure. LiF doped with Mg,Cu,Si exhibited high TL sensitivity (55 times that of TLD-100 LiF:Mg,Ti) and insignificant higher temperature glow peak leading to negligible residual TL signal. The occurrence of the dosimetric peak in LiF was found to be a strong function of Mg dopant. The peak temperature and the TL emission spectra of dosimetric glow peak in all samples having Mg, Cu were always found to be the same irrespective of the third dopant which influenced only the intensity of TL emission spectrum. The emission band peaking at 385 nm was assigned to Cu. © 2007 Elsevier Ltd. All rights reserved. Keywords: Mg,Cu,Si doped LiF; Thermoluminescence; Role of dopants; TL emission spectrum
1. Introduction Role of dopants in LiF is well recognised in enhancing TL sensitivity and changing the structure of glow curves for obtaining a near-tissue equivalent thermoluminescent dosimeter (TLD). For the past more than 50 years, efforts have continued in arriving at the variety of dopants but noticeable success had been for the combination of dopants such as Mg,Ti, Mg,Cu,P; Mg,Cu,NaSi and more recently for Mg,Cu,Si. Decision on the choice of the dopants appears to have been mainly based on the intrusions derived from the retrospective chemical analyses, behaviour of dopants in other materials, matching of ionic radii of the dopant ion with one of the host lattice ion, chemical valence consideration for the charge compensations, characteristic emissions, availability in a form appropriate for the process of doping, etc. However, in spite of a large number of publications on LiF:Mg,Cu,P appearing in the past several years, there is still no unanimity on the role of dopants. McKeever (1991), based on the results of unchanged thermoluminescence (TL) emission spectrum of LiF:Mg,Cu,P doped with either monovalent or divalent copper salts, concluded that ∗ Corresponding author.
E-mail address: [email protected] (J.I. Lee). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.10.040
Cu does not play any role in the TL emission process and phosphorus is the main activator responsible for TL emission. The glow curve structure was inferred to be dictated by the presence of Mg (as also observed by many others: e.g. Shoushan, 1988; Bilski et al., 1996, 1997, 1998; Lee et al., 2005). Bilski et al. (1996) found that Mg to be the essential dopant as very small changes of Mg concentrations affected both the glow curve structure and the TL sensitivity. Mg-related defects were related to the thermal susceptibility and a reversible reaction between the main dosimetric peak and the higher temperature peak was observed by Bilski et al. (1997) in LiF:Mg,Cu,P. Bilski et al. (1998) found that Mg is responsible not only for the formation of the trapping centres but also influences the dose response and LET dependence. In line with the results of McKeever (1991), Bilski et al. (1996, 1997) concluded that a threshold amount of P is needed to obtain high TL sensitivity whereas Cu does not take part in the formation of the traps and the luminescent centres but helps only in the incorporation of larger Mg content and therefore suggested that P is responsible for luminescent centers and may also be involved in peak 4 related traps. These findings were similar to those of Bos et al. (1996) who found the TL emission spectra to change with the concentration of Cu and concluded that Cu is not directly responsible either to emission centers or to traps but may hamper the higher order of clustering of Mg related defects resulting
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in an increase in the production traps. P was shown to play a dual role of being involved in both the trapping as well as in the TL luminescence processes. A different dual role of P was proposed by Shinde et al. (2001) in that P does not act as luminescent center but acts as a hole trap and helps in higher incorporation of Cu ions into the lattice of LiF. This was based on the observation that by increasing concentration of phosphorus both the TL sensitivity and the intensity Cu2+ ESR signal increased considerably. The increase in Cu2+ was speculated to also increase in the incorporation of Cu+ which was considered to act as luminescent center. It is still not clear whether the proposition of Bilski (2002) of Cu helping in the incorporation of larger Mg content is correct or the proposition of Shinde et al. (2001) of P helping in the incorporation of larger amount of Cu ions is correct or both are correct or incorrect. More recently, Gunduroa and Moharil (2007) identified a new defect center, (PO4 )2− radical, through the ESR technique and TL measurements. They related the TL sensitivity to this center and proposed that the filled traps are related to the clusters of interstitial fluorine atoms and PO4− hole centers, whereas magnesium-related defects such as Mg2+ vacancy dipoles and their clusters along with F center act as a recombination center and the energy of recombination is transferred to Cu+ ion which acts as a luminescent center. Patil and Moharil (1995) earlier also concluded Cu+ to act as a luminescent center but that was based on their observation of the similarity between the photoluminescence spectra of LiF:Cu and LiF:Mg,Cu,P doped with copper salt along with a flux. In contrast, Mandowska et al. (2002) observed that the TL emission spectra for the same amount of Mg and Cu are not the same when P is replaced by Ti (380 nm band not observed for Ti). Also, in the absence of Cu, the spectra for the same amount of concentration of P changes with the concentration of Mg. This again raised a doubt on the luminescence center in LiF:Mg,Cu,P. Recent studies of spectral analysis of TL emission of NaCl and LiF by Davidson et al. (2007) demonstrated the emission to be from self-trapped excitons in these alkali halides and this emission could be enhanced and altered by the impurities due to impurity–vacancy dipoles. Chen and Stoebe (1998), through their study of extended X-ray absorption fine structure (EXAFS) where only the virgin samples were shown to contain Cu+ as a substitution to Li+ ion and not the irradiated samples or those subjected to annealing at higher temperatures, concluded that Cu+ is converted to Cu2+ on irradiation and the Cu+ to Cu2+ conversion takes place. Cu+ related defects were noted to act as a luminescence centers, especially for 380 nm TL emissions. The conversion of Cu+ to Cu2+ was inferred to be irreversible, which is responsible for the loss of TL sensitivity of LiF:Mg,Cu,P on annealing at temperatures above 240 ◦ C. However, Mathur et al. (2002) and more recently Kurt et al. (2006) argued that the isolated Cu+ ion cannot be the recombination center and ruled out the conversion of isolated ions of Cu+ to Cu2+ in the lattice of LiF because a temperature of 573 K cannot cause thermal ionisation of the isolated Cu+ to Cu2+ . They pointed out that Cu+ cannot
substitute Li+ ion and LiF cannot be doped with Cu+ even though the substitution does not need a charge compensator. This conclusion was drawn from the consideration that the ion ˚ of Cu+ is much larger than Li+ (0.68 A) ˚ and size (0.96 A) therefore the substitution would cause unacceptable distortion of the lattice. They, however, proposed that Cu and P ions could be incorporated in the precipitated MgF2 phase where two Mg2+ ions would be substituted by a Cu+ ion and a P3+ ion. This substitution would maintain the charge neutrality and should not distort the lattice severely as the ionic radii of Cu+ ˚ respectively, whereas the ionic and P3+ are 0.96 and 0.44 A, 2+ ˚ radii of a Mg ion is 0.66 A, so the sum of the Cu+ and P3+ ion radii match with the radii of two Mg2+ ions. Therefore, P3+ can interact with Cu+ in the form of donor–acceptor pairs (DAPs) and would give rise to donor–acceptor luminescence center where P3+ is a donor and Cu+ is an acceptor. This rational facilitated the Cu+ to Cu2+ conversion proposed by Chen and Stoebe. In the absence of P3+ ion, Cu was supposed ˚ substituting to enter the lattice as Cu2+ (ionic radii 0.72 A) 2+ for Mg . A study of Yang et al. (2005) on TL of differently doped LiF concluded that the reduction in TL sensitivity on annealing above 250 ◦ C is associated with precipitated phase of Mg and a secondary phase (complexes of Mg and other impurities). This precipitation was ascribed to the presence of higher concentrations of Mg. The X-ray diffraction (XRD) and ESR studies of Sun et al. (1994) have shown that not only the precipitation but also different crystalline phases are produced in LiF:Mg,Cu,P during the preparation process. They observed three phases: viz. polycrystalline LiF, Li4 P2 O7 and an unidentified new polycrystalline material. This unidentified phase was attributed to be responsible for TL in LiF:Mg,Cu,P and its light blue colour. Their ESR results showed no change in the Cu2+ valence on irradiation to gamma rays. They suggested that although Cu is an important impurity but the TL process is performed through O and P and an electron is captured at P, therefore, P should be the activator in LiF:Mg,Cu,P. Gunduroa and Moharil (2007) insisted that the solubility of Mg impurity in LiF is very low and therefore the fate excess Mg (up to 2000 ppm) in LiF was questioned. Shen et al. (2002) demonstrated that on storage for 30 days at 98% RH, orthophosphate is produced which changes the colour from bright blue to brown and reduces the TL sensitivity of LiF:Mg,Cu,P. It is evident that the crystalline phases, precipitates and clusters of impurities–vacancies do play a very important role in the TL process in LiF. These and the results of our own studies of more than 10 years on differently doped LiF, especially LiF:Mg,Cu,NaSi, indicated that Na could be responsible for the lack of thermal stability of LiF TLD (in line with the results of Tang, 2003). This convinced us that Na and P need to be avoided as dopant and fresh attempts are necessary for improving the TL dosimetric characteristics of TLDs based on LiF. Consequently we developed LiF:Mg,Cu,Si which enabled us to over come the drawbacks of LiF:Mg,Cu,P. It is evident that the crystalline phase, precipitates and clusters of impurities–vacancies do play a very important role in the TL process in LiF.
J.I. Lee et al. / Radiation Measurements 43 (2008) 303 – 308
LiF doped with a variety of dopants were prepared. Most of the equipment, including 137 Cs gamma rays sources, a Studsvik 90 Sr–90 Y beta-ray reference irradiator, ovens, set-up for recording the emission spectra and TLD reader and the procedures adopted are described in our earlier publications (Lee et al., 2005, 2006a, b, 2007). For comparison, commercially available LiF:Mg,Cu,P GR-200 and LiF:Mg,Cu,Na,Si TLD discs were procured. The results of the dopant concentration presented in this work are those used for the preparation of the final samples of LiF:Mg,Cu,Si (Lee et al., 2006a, 2007). For impurity analysis and the estimation of incorporation of dopant ions in the prepared samples, inductive coupled plasma mass spectroscopy (ICP-MS) was used. XRD analysis was carried out to ascertain the phases present in the final sample. Two methods of preparations were adopted: viz. (1) melting method and (2) granulation method, which differed only by the fact that in the former case the mixture of compounds of dopants and LiF was melted and in the latter case it was sintered below the melting point. After each step of thermal treatment (whether melting, sintering or other thermal treatment), the samples were subjected to fast cooling. The TL glow curves were recorded at a heating rate of 10 ◦ C s−1 . 3. Results and discussion TL in LiF was found to depend on the proper combination of the dopant ions and the optimised preparation procedure. For the preparation of Mg,Cu,Si doped LiF, the melting method resulted in better TL sensitivity whereas for the other dopant combinations, the granulation method gave better results. In all cases, Mg was found to be the most dominant dopant. The enhanced concentration of Mg (0.45 mol%) and reduced concentration of Cu (0.025 mol%) along with Si (0.9 mol%) gave the best results by using the melting method. LiF doped with these concentrations of Mg, Cu and Si exhibited high TL sensitivity (55 times that of LiF:Mg,Ti TLD-100). These concentrations were therefore used as the optimised values for the other studies in this work. Table 1 shows the results of analysis of impurities in the final sample obtained by using ICP-MS. It can be seen that the proportions of the dopants intended and finally present are different for different dopants. Only a little amount of Cu found its place in the final sample whereas most of the Mg was retained. The results of XRD analysis of the final sample of LiF:Mg,Cu,Si showed the presence LiF, MgF2 , SiO2 and an unknown phase. This was an indication of the formation of several phases. The glow curve structure of LiF:Mg,Cu,Si was found to be similar to that of LiF:Mg,Cu,P except for the much reduced relative intensity of the higher temperature glow peak responsible for the residual TL signal. The reduction in the higher temperature tail of the dosimetric glow peak was achieved by the use of a dual-step of thermal treatment at 300 ◦ C for 10 min followed by 260 ◦ C for 10 min as a final step of the preparation procedure (Lee et al., 2006a, 2007). Thermal treatment at 300 ◦ C (the first step of dual-step thermal
Table 1 Concentration of dopants used in the preparation and those finally remained (assessed) in the final sample of LiF:Mg,Cu,Si after the preparation Dopant
Used in the preparation (mol%)
Assessed in the final sample (mol%)
Mg Cu Si
0.45 0.025 0.9
0.34 0.0045 0.33
50
2
40 TL Intensity (arb. unit)
2. Material and methods
305
30 1 20
10
0 50
100
150
200
250
300
Temperature (°C) Fig. 1. Glow curves of LiF:Mg,Cu,Si sintered discs irradiated to 5.5 mGy from 90 Sr–90 Y beta-ray and subjected to 1–300 ◦ C for 10 min (first step of the dual-step thermal treatment) and 2–260 ◦ C for 10 min (second step of the dual-step thermal treatment) following the 300 ◦ C treatment.
treatment) resulted in the shift of the main peak to higher temperatures (265 ◦ C) and the treatment at 260 ◦ C (the second step of dual-step thermal treatment) brought back the dosimetric peak to 238 ◦ C with increased sensitivity and resulted in minimizing the higher temperature tail of the dosimetric glow peak (Fig. 1). The shifted peak (265 ◦ C) in the 300 ◦ C treated LiF:Mg,Cu,Si was considered to be the same as that of the higher temperature peak in LiF:Mg,Cu,P (Lee et al., 2007). It may be noted that in LiF:Mg,Cu,P, the emission spectrum (355 nm band) of the higher temperature glow peak and the dosimetric peak (385 nm band) were found to be different (Lee et al., 2006b; Kim et al., 2007). Therefore, it became important to study the TL emission spectrum of this shifted (265 ◦ C) peak (Fig. 1) in 300 ◦ C treated LiF:Mg,Cu,Si. Fig. 2 shows that the emission spectrum of the shifted (265 ◦ C) peak in 300 ◦ C treated sample and the regenerated dosimetric peak (238 ◦ C) in LiF:Mg,Cu,Si are exactly the same. It may also be noted that a thermal treatment at temperature above 250 ◦ C in LiF:Mg,Cu,P resulted in a shift in the main glow peak temperature by about 30–50 ◦ C and a corresponding shift in the emission spectrum from 380 to 350 nm (McKeever, 1991). Thus, from the considerations of the TL emission, the effect of the thermal treatment at 300 ◦ C (Lee et al., 2007) in LiF:Mg,Cu,Si (where the emission spectra of the shifted (265 ◦ C) peak in 300 ◦ C treated sample and the
306
J.I. Lee et al. / Radiation Measurements 43 (2008) 303 – 308 Table 2 Results of analysis of glow curve structure and TL sensitivity of the main peak (Fig. 3) and TL emission spectra (Fig. 4) of LiF doped with different combinations of the dopants (Mg: 0.45 mol%; Cu: 0.025 mol% and Si: 0.9 mol%)
0.15 385
0.10
Thermal treatment
TL intensity (arb. unit)
300 °C 0.05 440
355
0.00 0.10 385
Thermal treatment 300 °C / 260° C
0.05 440
355
0.00 300
400 450 500 Wavelength (nm)
550
Normalized TL intensity
TL emission peak and the deconvoluted bands (nm) of the main TL emission
LiF:Mg LiF:Cu LiF:Si LiF:Cu,Si LiF:Mg,Si LiF:Mg,Cu LiF:Mg,Cu,Si LiF:Mg,Cu,P LiF:Mg,Cu,Na,Si
Undefined (feeble) 175 (0.016) 150 (0.027) 150 (0.028) 238 (0.010) 238 (0.170) 238 (1.000) 238 (0.900) 238 (0.950)
– 434 nm 364 nm 362 nm 400 nm 384 nm 384 nm 368 nm 370 nm
1
4 5 6
100
150
200
250
(355, (355, (355, (355, (355,
399, 385, 385, 385, 385,
442) 440) 440) 440) 440)
TL sensitivity is shown in the parentheses in Column 2.
600
LiF:Cu LiF:Si LiF:Cu,Si LiF:Mg,Cu LiF:Mg,Si LiF:Mg,Cu,Si
32
50
Main TL peak temperature (◦ C) and relative TL sensitivitya
a Relative
350
Fig. 2. TL emission spectrum of LiF:Mg,Cu,Si treated at 300 ◦ C for 10 min (upper figure) and at 260 ◦ C for 10 min (second step of the dual-step thermal treatment) following the 300 ◦ C treatment (lower figure).
1 2 3 4 5 6
Differently doped LiF
300
Temperature (°C) Fig. 3. TL glow curves of 0.8 mm thick TLD discs of LiF doped with different dopants (dopant concentration values as that in the optimised LiF:Mg,Cu,Si TLD) at heating rate of 10 ◦ C s−1 after irradiation to 5.5 mGy from 90 Sr–90 Y beta-rays (TL intensities not to be compared, see Table 2).
dosimetric peak (238 ◦ C) are exactly the same) cannot draw a similarity with LiF:Mg,Cu,P (where the emission spectra shifts). The Cu+ to Cu2+ conversion suggested by Chen and Stoebe (1998) in LiF:Mg,Cu,P where the conversion of Cu+ to Cu2+ is ascribed to heating to higher temperatures which shifts the dosimetric glow peak to higher temperatures is not valid in LiF:Mg,Cu,Si. The peak shift in LiF:Mg,Cu,Si due to 300 ◦ C heat treatment during the dual-step thermal treatment appears to be due to the change in trap distribution not affecting the luminescent centre. Fig. 3 shows the TL glow curves of 0.8 mm thick TLD discs of LiF doped with different dopants. Table 2 shows the detailed
analysis of TL glow curve structure, TL sensitivity and TL emission spectra of LiF doped with different combinations of the dopants. It can be inferred that, without Mg there is no 238 ◦ C main/dosimetric peak and Mg alone (without a codopant) gives a very feeble TL with several peaks ranging from 120 to 230 ◦ C. Si and Cu,Si doped LiF exhibit TL emission peaks around 362 nm (Fig. 4) and the glow peak at 175 ◦ C (there is no 238 ◦ C peak in both the cases). In the absence of Mg, the addition of Cu does not influence the TL emission peak and the glow peak temperature. Si and Mg,Si doped LiF exhibit TL emission peaks at 362 and 400 nm and the TL glow peaks at 150 and 238 ◦ C, respectively. Thus Mg influences both the glow curve structure and the TL emission spectra. Cu and Mg,Cu doped LiF exhibit emission peaks at 434 and 385 nm and TL glow peaks at 175 and 238 ◦ C, respectively. Thus Mg again influences not only the glow peak but also the emission spectra (in line with the findings of Mandowska et al., 2002). However, the emission band peaking at 385 nm can be observed only in the presence of both the Cu and Mg. The XRD analysis supports the presence of several phases including MgF2 and unknown phases (McKeever et al., 1993; Sun et al., 1994; Yang et al., 2005; Mathur et al., 2002; Kurt et al., 2006) needed for understanding the TL process. Table 2 shows that the main TL glow peak and the main TL emission band of Mg,Cu and Mg,Cu,Si doped LiF and also of Mg,Cu,Na,Si and Mg,Cu,P doped LiF with concentrations of dopants different than LiF:Mg,Cu,Si are the same. Therefore, Si, P or Na,Si does not affect the emission spectra which remains the same as that of Mg,Cu doped LiF. This reaffirms that in the system of LiF:Mg,Cu,X, the characteristics of the TL emission band are not influenced by the third dopants (X = P or Si or Na,Si) and P may not be acting as luminescent center in LiF:Mg,Cu,P (McKeever, 1991; Bos et al., 1996; Bilski, 2002). However, each of the three dopants (Mg, Cu and Si) appears to play a crucial role in the presence of each other: viz. Mg for the position of the dosimetric (238 ◦ C) peak, Cu for the emission band peaking at 385 nm and Si for enhancing the TL sensitivity.
J.I. Lee et al. / Radiation Measurements 43 (2008) 303 – 308
434
LiF:Cu
362
LiF:Cu,Si
TL Intensity
LiF:Si
355
LiF:Mg,Si
LiF:Mg,Cu 440
355
385 355
350
References
442
385
300
sensitivity and not the TL emission spectrum or the peak temperature of the main dosimetric peak. In LiF:Mg,Cu,Si each of the three dopants (Mg, Cu and Si) appear to play a crucial role in the presence of each other: viz. Mg for the glow curve structure, Cu for the emission band peaking at 385 nm and Si for enhancing the TL sensitivity. There appears a strong possibility of further improvement in the dosimetric properties of LiF based TLDs by changing this third dopant and using an appropriate preparation procedure.
364
399
LiF:Mg,Cu,Si 440
400 450 500 Wavelength (nm)
307
550
600
Fig. 4. TL emission spectrum of the LiF doped with different dopants recorded after irradiation to 800 Gy.
4. Conclusions An improved LiF based TLD (LiF:Mg,Cu,Si) has been developed by optimising the dopant combinations and concentrations and also the preparation procedure. A dual-step thermal treatment at 300 ◦ C for 10 min followed by 260 ◦ C for 10 min as a final step of preparation procedure ensured stabilising the glow peak structure and minimising the residual TL. The thermal treatment at 300 ◦ C (the first step of dual-step thermal treatment) shifted the glow peak to higher temperature (265 ◦ C) but the emission spectra of the shifted peak and the dosimetric peak in LiF:Mg,Cu,Si was the same unlike LiF:Mg,Cu,P. The hypothesis of Cu+ to Cu2+ conversion due to heating to temperatures above 240 ◦ C is in doubt in LiF:Mg,Cu,Si. The dosimetric peak in LiF is a strong function of Mg. The dosimetric peak cannot be observed without Mg and the emission band peaking at 385 nm can be observed only in the presence of Cu. This supports the proposition that Cu in combination with Mg in the defect complexes is responsible for the luminescence in the LiF:Mg,Cu,X system and the presence of the third dopants (X = P or Si or Na,Si) only influences the TL
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