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Applicability of the mobile interstitial model for thermoluminescence in LiF TLD-100 at medium and high dose levels
Nuclear Inst~ments Norm-Holland
and Methods in Physics Research B 83 (1993) 196-204 Beam tntwaotionr with Materials P Atoms
Applicability of the mobile interstitial model for thermoluminescence in LiF TLD-100 at medium and high dose levels A.R. Laksh~anan
*
~@partm~t of P~ys~s, ~nive~i~
ofWuppertal,42119 Wuppertal,Germany
Received 23 April 1993
Optical absorption (OA) and thermoluminescence (TL) studies in LiF TLD-100 single crystals at high gamma-ray dose levels tend to support the validity of the mobile interstitial model in explaining the dependence of the glow curve shape with gamma-ray dose. Especially the disappearance of the dosimetric peak (peak no. V) at about 22YC and other low temperature peaks above 25°C at high gamma-ray dose levels ( > 10s Gy) is accompanied by the disappearance of the respective GA bands at 310 and 380 nm. At the highest dose studied, LiF TLD-100 exhibits a TL peak at about 35O’C (peak no. VIII) whose annealing step coincides with that of the 250 nm OA band which confirms the composite nature of this band. Dose levels greater than 7.2X lo3 Gy result in the growth of F-aggregate centres with their characteristic OA bands at 448 nm (M centre) and at 378 nm (R, centre). However, detailed studies on the OA spectra of TLD-100 indicate that the mobile interstitial model in its present form does not explain satisfactorily the supralineari~ in the response of different ‘IL peaks seen in the medium dose range (IO-lo3 Gy). OA results tend to support the competing nonlumine~nt centre model at least for peak V which envisages a part of the charge released during TL readout to be trapped by these competitors, and indicate that more than one TL process operate simultaneous~ in this material.
1. IBtroduction
The thermoluminescence 0I.J and the complex glow curve structure of LiF TLD-100 as a function of gamma-ray dose have been well studied. For instance, Sagastiielza and Alvarez Rivas [l] report that in the dose range from 1 Gy to 9.2 x 106 Gy, the intensity of each TL glow peak first increases with dose but for high enough doses, the glow peak intensity decreases and finally the glow peak vanishes. Low temperature glow peaks go through this sequence at lower doses than the glow peaks occurring at high temperatures. Similar results were reported by Chandra et al. [2] as well. A mobile interstitial model was proposed [l] to explain the dose dependence of the glow curve in this material. However, similar studies on changes in the optical absorption (OA) spectra of TLD-100 with gamma-ray dose have not been carried out so far. A direct confirmation of this model can be obtained from such studies, since the conversion of one trap to another at high doses should result in the disappearance of the OA band corresponding to the trap which is getting converted.
’ On leave from Safety Research and Health Physics Programme, Indira Gandhi Centre For Atomic Research, Kalpakkam 603102, India. 01~-583X/93/$~.~
Further, the supralinearity (increased TLJGy at higher gamma-ray dose levels) of different TL glow peaks in TLD-100 was attributed by Sagastibelza and Alvarez Rivas [l] to the same phenomenon, i.e. trap conversion during irradiation. Essentially their mobile interstitial model envisages an interstitial allegation process during i~adiation and a supralinear growth of trapped interstitials ~~esponding to a particular trap in the supralinear dose region, However, all the OA bands (notably 380, 310 and 250 nmf observed after gamma irradiation increase either linearly or sublinearly with gamma dose in this dose region, which prompted several investigators to suggest that the supralinearity in TLD-100 arises in the luminescence rather than in the trapping process. Since earlier studies [3] on irradiation at 77 K have shown that the absence of F centres does not influence the supralinear&y of peak V (22O“C), it is proper to test the validity of this hypothesis on vacancy centre related TL peaks, namely peak VII (3OO’Q or peak X (SBO°C). Recently, ~kshmanan [4-61 has correlated peak VII in TL,D-100 with an OA band at or very close to 250 nm and identi~ed it with &-type vacancy centres. In TLD-100, the interstitial aggregates do not give any OA band but any increase in recombination efficiency
in the supralinear dose region can be found out by studying the change in the intensity of the OA band giving rise to the vacancy centre i.e. peak VII recombi-
0 1993 - Elsevier Science Publishers B.V. All rights reserved
A.R. Lakshmanan
/ Thermoluminescence
197
in LiF
nation centre. Recently we foIlowed a similar procedure successfully to confirm the increased production efficiency of peak VII centres due to increased interstitial migration efficiency at elevated irradiation temperatures 151 as well as the increased efficiency of peak VII trap fo~ation at high LET [6]. Both these phenomena involve a change in trapping efficiency during irradiation and hence could be easily confined. But the phenomenon of supralinea~~ in TLD-300 is much more complex as it involves changes in the Iuminescence efficiency during the readout process.
2. oatmeals
and method
LiF: Mg, Ti @LID-100) single crystals (6 X 6 X 2 mm3) as well as LiF: Ti and UF: undoped single crystals (7.7 x 7.7 x 2 mm31 were used after a pre-irradiation anneal at 400°C for 1 h. Irradiations were carried out at RT in a @%Jogamma-ray cell with a dose rate of 39 Gy min - *. Details of TL and GA measurements and the instruments used have been already described 143. TL glow curves were recorded at a linear heating rate of 2”C!s-‘. UV/thermal anneal at 300°C was carried out by keeping the gamma-ray irradiated crystal on the heater planchet at 300°C and simultaneously irradiating it with the ultraviolet (mairdy 254 nm> light from the Pen-ray lamp described earlier 141.
Fig. 1. OA spectra of LiF TLD-100 single crystals at medium gamma-ray dose levels: 1 - 3.9X 10’ Gy, 2 - 3+9X JO* Gy, 3 1.17X 103 Gy and 4 - 3.2X lo3 Gy. The spectra numbers 2, 3 and 4 have been reduced (+I by a factor of 2, 10 and 20, respectively.
gamma doses matches well with the gradual disappearance of TL peaks I to V at high gamma doses [1,2f and is in agreement with the prediction of the mobile interstitial model El].
3. OA spectra at the high dose tevels Figs. 1 and 2 show the OA spectra of TLD-100 at different gamma-ray dose levels. In the dose range 39-3.2 X IO3 Gy, the major GA bands occur at 250, 310 and 380 nm but the intensity of the 310 and 380 nm bands with respect to that of the 250 nm baud decreases slowly from a value of about 0.5 to 0.27 with increasing dose. However, at higher dose levels, i.e. > 7,2 X IO3 Gy, the intensities of the 310 and 380 nm bands show a further decreasing trend and disappear ultimately. The 250 nm band appears to show a saturation trend > 7.2 x lo3 Gy, but its exact intensity could not be recorded for dose levels > 5.64 X IO4 Gy due to the saturation of the instrument i.e. spectrophotometer used beyond the optical density (absorbance) level of 4. The production of F-aggregate centres, namely an M centre which consists of two F centres situated together with its OA at 448 nm and an Rz centre which consists of three F centres situated together with its OA at 378 nm [7] is seen for dose levels > 7.2 x lo3 Gy. The growth of F-aggregate centres is accompanied by a decline in the growth of F centres. The gradua1 disappearance of GA bands at 310 and 380 nm at high
200
300
h hml
tco
Fig. 2. OA spectra of LiF TLD-100 single crystals at high gamma-ray dose levels: 5 - 7.2 x lo3 Gy, 6 - 5.64 x Z04 Gy and 7 - 2.83 X 10’ Gy. The spectrum number 5 has been ma~ni~ed ( x 1 by a factor of 2.
A.R Lakrhmanan / Thermoluminescencein LiF
198
200
300
boo )j
500
2w
400
300
500
X(nml
lnm)
Fig. 3. (a) OA spectra of LiF TLD-100 (solid line), LiF: Ti (dashed line) and LiF (undoped) (long-short dashed line) at the dose levei of 7.2~ lo3 Gy. (b) OA spectra of LiF TLD-100 (solid line), LiF:Ti (dashed line) and LiF (undoped) (long-short dashed line) at the dose levei of 2.83x lo5 Gy.
The application of M-centre OA in LiF (undoped) for high level g~a-ray dosimetry is well known [S,9], but the changes in the OA spectra of TLD-100 at high dose levels have not been so well recorded. The OA spectra of LiF (undoped), LiF : Ti and TLD-100 at the dose levels of 7.2 x lo3 and 2.83 x lo5 Gy are compared in figs. 3a and 3b. In LiF (undoped) and LiF : Ti, apart from the M-centre OA at 440 nm the presence of two F-aggregate centres is seen (fig. 3b) more clearly, i.e. R, (378 nm) and R, (320 nm) centres. Both are F, centres [7]. Further, the OA spectra of LiF (undoped) and LiF : Ti are marked by the absence of Mg-related OA bands at 310 and 380 nm (fig. 3a). Another significant difference between the three spectra ,(fig. 3b) is the OA between 200 and 300 nm. The TLD-100 OA spectra in this wavelength region are much broader than those of LiF (undoped) and LiF: Ti, which confirms the presence of other OA bands very close to the F band at 250 nm in TLD-100, as proposed earlier [4]. Figs. 4 and 5 show the growth of F (250 nm) and M (440 nm) centres in LiF (undoped~ and LiF : Ti, respectively, with dose at high dose levels. A comparison of figs. l-5 shows that the growth of F-aggregate centres, especially the M centre, with dose is more intense for
“1
Fig. 4. Response of 2.50 nm (upper curve) and 440 nm (lower curve) OA bands in LiF (undoped) as a function of gamma-ray dose.
A.R. Lak~~anan
250 nm. The 425°C readout erases peak VIII and removes most of the OA at 250 nm. The correlation of peak VIII with an OA band close to 250 nm is in agreement with our earlier results [4] which indicated that the 2.50 nm OA band in TLD-100 is a composite band. After the 425°C readout, the Z3 (225 nm>, F (250 nm> and an unidentified band at 285 nm remain, Most of these bands vanish on 499°C readout.
5. Changes in OA spectra during TL readout at medium dose levels Figs. 8-11 indicate the changes in different OA bands during the readout of different TL peaks at four dose levels. During the readout of peaks II and III, the 380 nm band is destroyed but the 310 nm band grows while the 250 nm band falls in intensity. Simiiarly during the readout of peak V, the 310 nm band is destroyed, accompanied by a fall in 250 nm band intensity while a new OA band around 235 nm is apparently formed. The relative intensity of the 235 c
I
3xw3
1oa
UP Gamma
Dosr
cxlo=
tCyt
%
Fig. 5. Response of 2.50nm (upper curve) and 440 nm (lower
curve) OA bands in LiF:Ti as a function of gamma-ray dose.
LiF (undoped), followed by LiF:Ti. Its growth is in TLD-100, probably due to the presence of Mg. explains the application of M-centre OA in LiF doped) for high level gamma-ray dosimetry rather the OA in LiF TLD-100 [8,9].
least This (unthan
4. OA band corresponding to peak VIII Figs. 6 and 7 show the OA spectra and TL glow curve of TLD-100 at the highest dose level studied (2.83 x lo5 Gy). In both cases, the changes seen after different readout temperatures, i.e. merely heating the crystal at 2”Cs-’ to the m~mum temperature of 180, 250, 350, 425 and 499”C, are shown. The TL glow peaks which are characteristic of TLD-100 at dose levels < lo4 Gy, namely glow peak numbers I to V as well as their characteristic OA bands at 310 and 380 nm are absent at this dose level. The glow peak number VIII (350°C) is the most prominent one in fig. 7, while peak number VII is seen with feeble intensity. These TL data match well with those reported earlier j&21. While the 250°C readout erases most of the M band at 448 nm, no prominent TL glow peak corresponding to this band could be observed. The 350°C readout erases the weak peak no. VII as well as Rz and M centres and reduces considerably the OA above
-lLoo 300 200
500 hhd
Fig. 6. OA spectrum (solid line) of LiF TLD-100 at the highest dose levei studied ~2.83X105 Gy). The spectra after the 180°C readout (dashed fine), 250°C readout (01, 350°C readout (o), 425°C readout (long-short-short dashed line) and after the 499“C readout (A) are also shown. In all these and subsequent cases, the crystal was merely heated linearly to the maximum temperature indicated at the rate of Z”Cs-‘. Absorbance values above the level of 4 are unreliable in the spe~rophotometer used. A high degree of oscillation of the plotter is noticed at such high absorbance values. In any case, the appearance of F-aggregate centres i.e. M centre (448 nm) and R, centre (378 urn) and the disappearance of Mg-related OA bands at 310 and 380 nm is seen. While no prominent TL peak could be correlated with either the M or R2 centre, the peak VIII could be correlated with an annealing step of an OA band near 2.50nm.
A.R. LalcFhmanan / Thermoluminescence
200
Temperature i*C)
Fig, 7, TL glow curves of LiF TLD-100 #resending to the OA spectra &own in fig. 6. At this dose level (2.83 X IO5 Gy), peak VIII (350°C) is the most prominent TL peak while peak numbers I to V are absent.
band with respect to that of the 310 nm band increases with dose and appears to reach its maximum value around 3.2 X 103 Gy, which incidentally coincides with the saturation dose of peak V. This indicates that the dest~ct~on of peak V traps not only results in TT.,, i.e. in re~mb~ation of charge carriers, but also in the creation of new traps whose efficiency changes with the dose. This confirms a key component in the competing
in LiF
Fig. 9. OA spectra of LiF TLD-100 at the dose level of 3.9~ lo2 Gy immediately after i~adiation (solid line), after 180°C readout (dashed line), 2WC readout (dot-dashed line), 350°C readout (long-short dashed line) and 425°C readout (long-short-short dashed line).
nonluminescent centre model. But the possibility that during the subsequent, i.e. 35O”C, readout the 235 nm band itself is converted to another unidentified centre which acts as the competitor to the lumines~nt centre is strong, since the readout of peak VII removes most
h Inml Fig. 8. OA spectra of LiF TLD-100 at the dose level of 3.9 x 10’ Gy immediately after irradiation (solid line), after 180% readout (dashed line), 250°C readout (dot-dashed line), 350°C readout (01 and 425°C readout Uong-short-short dashed line). While the 180°C readout removes TL peaks II and III (140°C and 180°C peaks repectively), the 250°C readout removes peak V (22ffC peak), the 350°C readout removes peak VII (3OO“Cpeak) and the 425°C readout removes peak VIII (350°C peak).
0
200
300
LCO A fnrnl
Fig. 10. OA spectra of LiF TLD-100 at the dose level of 1.2x lo3 Gy immediately after the irradiation (solid line), after 180°C readout (dashed line), 250°C readout (dot-dashed line), 350°C readout (0) and 425°C readout (long-short-short dashed line).
201
-3 <.,,,,I
,
5x10 f:
3x10’
102
,
,,,*,.I
3x10’ Gomma Fhy
-200
300
Loo
500
A hm) Fig. 11. OA spectra of LiF TLD-100 at the dose level of 3.2X lo3 Gy immediately after irradiation (solid line), after 180°C readout (dashed line), 250°C readout (dot-dashed line), 350°C readout (0) and 425°C readout (long-short-short dashed Iine). The growth of an OA band around 235 nm due to the readout of peak V (i.e. 250°C readout) is very prominent at this dose level. This indicates that the destruction of peak V traps not only results in the recombination of charge carriers, i.e. the production of TL, but also results in the creation of new traps, which is dose dependent. This confirms a key component in the competing nonluminescent centre model.
,
L
3x103
103
Dose IGyl
Fig. 12. The changes (i.e. reduction) in the intensities of: the 380 nm (A I OA band due to the readout of TL peaks II and III, the 310 nm (01 OA band due to peak V readout, the 250 nm (A) OA band due to peak VII readout and the intensity of the 225 nm (2s) band as observed after peak VII readout (0) as a function of gamma-ray dose at medium dose ievels in LiF TLD-100. The lines of linearity (dashed) are also shown.
interstitial model in its present form (or other models which invoke the supralinear creation of traps with dose), although qualitatively the OA band corresponding to high temperature TL peaks shows a tendency towards linear or supralinear behaviour rather than
of the OA at 235 and 250 nm leaving the 225 nm band
residual.
6. Intensities of OA bands versus dose at medium dose levels The changes in the intensities of 380, 310 and 250 nm GA bands due respectively to the readouts of TL peaks II and III, V and VII and the intensity of the 225 nm band as observed after the peak VII readout are plotted in fig. 12 as a function of dose. The TL intensities of peaks II (14O”C), III (18O”C), V and VII are plotted in fig. 13 as a function of dose. The 380 nm band shows sublinearity from the beginning, i.e. from 38.5 Gy. The 310 nm OA band shows linearity up to 3.85 X 10’ Gy and sublinearity afterwards. The 250 nm band is more or less linear up to 2.77 x lo3 Gy. The 2W nm band shows a slightly supralinear behaviour in contrast to other OA bands. But its supralinearity is much less than that reported by Takeuchi et al. [lo] in LiF : Mg and sets at right the controversy about the nature of the growth of Z, centres in TLD-100. In other words, the behaviour of OA bands does not support quantitatively the mobile
, 3x10’
1
102
I
3x102
.,,,,I
I
4
103
,
3x103
Gamma F&y Dose tGy)
Fig. 13. The TL intensities of peak II (*I, peak III (01, peak V (A > and peak VII (v I as a function of gamma-ray dose at medium dose levels in LiF TLD-100. The lines of linearity (dashed) are also shown.
sublinearity. While the 380 nm band is sublinear, the TL peaks II and III are supralinear and the 250 nm band is more or less linear whereas the TL peak VII is highly supralinear. Previous studies [3,12] have shown that the TL peak X is much more supralinear than that of the 225 nm OA band observed in fig. 12. Thus the trap conversion process envisaged by the mobile interstitial model though present at these dose levels does not explain satisfactorily the supralinearity of various peaks in TLD-100.
0.071 0.06
-
7. Charge mobility during Tt readout Figs. 8-11 indicate an interesting finding, namely the changes in different OA bands during the TL readout, This is a clear indication of charge/interstitial migration/aggregation as well as recombination with vacancies such as F centres during the TL readouts. The only difference between this model and that proposed in the original mobile interstitial migration model [l] is that the interstitial/charge migration is envisaged to take place here during readout instead of during i~adiation at the interme~ate dose levels (lo-lo3 Gy) where the TL peaks exhibit supralinea~~. This phenomenon is also consistent with the competing nonluminescent centre model proposed by several researchers [ll-131. But the individual identification of the competitors for different TL peaks is extremely complex, as can be seen from the complex changes in the OA spectra in figs. 8-11. Since the sensitized material (i.e. material after lo3 Gy + 300 to 350°C annealing) exhibits linearity in the response of different TL peaks, it is tempting to assume that the 225 nm centres are actually the competitors to luminescent centres since all the OA bands appear to converge ultimately into this band during the sensit~ation process. But the fact that irradiation at 77 K reduces drastically the 225 nm centres or peak X without affecting the supralinearity of peak V proves that the 225 nm centres are not the competitors that we are looking for [3]. Further, following irradiation at RT, peak X (which is correlated with the 225 nm band) itself exhibits pronounced supralinearity. This indicates that the TL mechanism of vacancy centre related traps (such as peaks VII, VIII and X) could be different from other traps (such as peaks I to V). However many experimental results indicate that the thermal activation energy of these ~mpet~tors and that of 225 nm centres are very close to each other.
8. Competitors to luminescent
centres
Further, a simultaneous 254 nm photo bleach at 300°C (UV/thermal anneal) removes peak X as well as
200
300
166
h
606
(nm)
Fig. 14. OA spectra of virgin (1) and sensitized (2) LiF TLD-100 single crystals after a test dose of lo* Gy. The sensitization treatment (2) involved a gamma-ray dose of lo3 Gy and a subsequent W/thermal annealing treatment at 280°C for 2h. This treatment resulted in the sensitization of peak V by a factor of 2 whereas peak VII and the 250 nm OA band were reduced by 25%. (3): (I)+ 400°C readout, i.e. after the removal of peaks V and VII. (4): (2)+ 400°C readout. The 225 nm is produced more or less with the same intensity in both cases, i.e. in spectra (3) and (4), which sets at rest the speculation that the 225 nm centres are the competitors to luminescent centres.
the OA band at 225 nm, but the sensitization
as well as the linearity of peak V in the sensitized LiF TLD-100 material persist [12]. This discards the possibility that the 225 nm centres are the competitors. Even if we assume that during the above W/the~al anneal the 225 nm centres are not destroyed but are merely emptied or converted to other centres, after a test dose and subsequent readout of peak VII, there should be a substantial change in their population (an increase in the former case and a decrease in the latter case) as compared to the freshly irradiated material. Figs. 14 and 15 present the results of this study. Fig. 14 shows the OA spectra after a dose of lo2 Gy to the virgin sample (freshly annealed) as well as after a test dose of lo2 Gy to the sensitized sample (lo3 Gy + W/thermal anneal at 280°C for 2 h). Fig. 15 shows the TL glow curves corresponding to the OA spectra in fig. 14. In the sensitized material, peak V is sensitized by a factor of 2 whereas peak VII and 250 nm OA are reduced by 25%. No appreciable change in 225 nm OA is seen. The fact that after the peak VII readout, the 225 nm band is produced with the same intensity in virgin as well as in sensitized samples proves that the centres giving rise to the 225 nm band are not the competitors to luminescent centres. These competitors
AX. Lakshmanan / Thermoluminescencein LiF
Fig. 15. TL &xv curves of virgin (solid line) and sensitized TLD-100 (dashed line) corresponding to the samples shown in the OA spectra of fig. 14.
apparently
do not give rise to any OA at least above
200 rim.
9. TL process of vacancy relnrted traps As discussed above the TL process of vacancy centre related traps should be discussed separately. Their TL process is well explained by the mobile interstitial model [Il. Their TL response with gamma-ray dose is marked with a high degree of supra~ineari~. In fact even at Low doses the Iinearity is scarcely noticed. This fact as well as the increased supralineari~ of peak X as compared to that of peak VII are a pointer to the readout temperature dependent interstitial mobiIity and their recombination with vacancies created by other secondary electron tracks, i.e. track interaction effects, as pointed out earlier 131. Fig. 14 indicates that the presence of partial sensitization of peak X in the UV/thermaI sensitized material observed earlier [33 should also be attributed to increased luminescence efficiency rather than to increased trapping efficiency 1141. This result disagrees with any possibility of trap conversion of 225 nm centres during the W/thermal anneal but indicates the partial influence of competitors to luminescent centres on the TL response of peak
203
X. However, the absence of sensitization of peak VII in this material following the WV/thermal anneal (fig. 15) is a result which is difficult to explain by any model proposed so far in its present form, as pointed out by us earlier [3,123. The fact that the OA of the 250 nm band in this material is slightly less than that in the virgin material (fig. 14) matches we11with the reduced TL sensitivity of peak VII (fig. 1.5). Lakshmanan et al. 131 predicted such a result from their statement “. . . that following the UV/thermal sensitization process, the peak 7 trap formation efficiency is reduced possibly because the 225 nm to peak 7 centre conversion process is inhibited perhaps as a result of reorientation/ realignment of peak 7 trap centres . . . “. Any treatment which results in the sensitization of peak VII is accompanied by an enhancement in the OA at 250 nm, as pointed out earlier [.$I. Thus it is clear that more than one TL process is operating simultaneously in TLD-100, some manifest more than the rest at certain dose IeveIs. When the elimination of competing noniuminescent centres is dominant at medium dose levels, tke supraiinearity of peak V results. When the trap aggregation process is dominant at high dose Ievels, the decrease in the intensity of TL of some peaks and their OA bands and their eventual disappearance result at high dose levels. For vacancy centre related traps, the temperature dependent mobiiity of interstitials and possible track interaction effects are dominant at Iow and medium dose levels where supralinearity in their TL is observed. However, at high dose levels the interstitial aggregation process causes the disappearance of these TL peaks as well, Thus the task of interpreting the experimental results in TLD-100 is extremely difficuh and should be pursued with caution.
10.Conclusions il The mobile i~terstitiai model in its present form explains well the changes in the shape of the TL glow curve and OA spectra at high dose levels. ii) The optica) absorption band corresponding to peak VIII observed at high dose levels apparently occurs close to 2% nm, thereby unfixing the composite nature of this band. Its utifity for high level gamma-ray dosimetry has been pointed out earlier IZ87. iii) SimiIar to LiF 6mdoped) or LiF: Ti, LiF TLD100 also exhibits the growth of F-aggregate centres for dose levels > 7.2 X 103 Gy while the F centres apparently show a decline in their growth. However, the OA spectra of TLD-100 from 200 to 300 nm are much broader than those of LiF fundoped) and LiF:Ti as a resdt of the presence of other OA bands cIose to the F band at 250 nm, as proposed earlier [4].
204
A.R. Lakshmanan
/ Thermoluminescence
iv) Detailed OA data indicate that the mobile interstitial model in its present form does not explain satisfactorily the supralinea~~ or sensitization of different TL peaks in TLD-100 observed at medium dose levels. In other words conversion and aggregation of traps as envisaged in this model is not the dominant mode at these dose levels. v) OA data indicate the migration of charge and the creation/~lling up of other traps during the readout of TL peaks especialiy peak V in TLD-100. This is consistent with the competing nonluminescent centre model proposed earlier. vi) Though tempting, the 225 nm centres could not be correlated with these competitors due to a variety of reasons, one being the observed production of the 225 nm OA band with the same intensity in virgin as well as in sensitized TLD-100 after a UV/thermal anneal. The reduced sensitivity of peak VII TL and the 250 nm OA band in the above sensitized material agree with our previous predictions [3,4]. vii) The 225 nm OA band in TLD-100 shows very little supralineari~ as compared to peak X TL response. This does not support the models which invoke the supralinear creation of 225 nm centres with gamma-ray dose. Other models such as the track interaction model and the competing nonluminescent centre model are invoked to explain these results.
Acknowledgements
The author is grateful to Dr. Wilfried Hoffmann and Sri. L.V. Krishnan, Head, SR and HPP, IGCAR,
in LiF
for helpful discussions and encouragement. is also supported by AvH Foundation.
This work
[l] F. Sagastibeiza and J.L. Alvarez Riias, J. Phys. C: Solid State Phys. 14 (1981) 1873. [2] B. Chandra, A.R. Lakshmanan and R.C. Bhatt, Int. J. Appl. Radiat. Isot. 33 (1982) 679. (31 A.R. Lakshmanan, B. Chandra, R.C. Bhatt, W. Hoffmann and R. Spatlek, J. Phys. D: Appl. Phys. 18 (1985) 1673. [4] A.R. Lakshmanan, Nucl. Instr. and Meth. B 82 (1993) 557. [5] A.R. Lakshmanan and U. Madhusoodanan, Phys. Status Solidi a 139 (1) in press. [6] A.R. Lakshmanan, Phys. Med. Biol. (submitted). [7) J.H. Schulman and W.D. Compton, Color Centres in Sohds (Pergamon, New York, 1962). [S] W.J. Vaugham and L.O. Miller, Health Phys. 18 (19701 578. [9] A. Waibel, Y. Goksu and D.F. Regulla, Radiat. Prot. Dosim. (in press). [lo] N. Takeuchi, K. Inabe and S. Nakamura, Phys. Status Solidi a 29 (1975) 11. [ll] J. ~mmerm~, J. Phys. C: Solid State Phys. 4 (1971) 3277. [12] A.R. Lakshmanan, B. Chandra and R.C. Bhatt, J. Phys. D: Appl. Phys. 15 (1982) 1501. [13] E.F. Mische and S.W.S. McKeever, Radiat. Prot. Dosim. 29 (1989) 159. [14] V.K. Jain, Phys. Status Soiidi a 60 (1980) 351.
and Methods in Physics Research B 83 (1993) 196-204 Beam tntwaotionr with Materials P Atoms
Applicability of the mobile interstitial model for thermoluminescence in LiF TLD-100 at medium and high dose levels A.R. Laksh~anan
*
~@partm~t of P~ys~s, ~nive~i~
ofWuppertal,42119 Wuppertal,Germany
Received 23 April 1993
Optical absorption (OA) and thermoluminescence (TL) studies in LiF TLD-100 single crystals at high gamma-ray dose levels tend to support the validity of the mobile interstitial model in explaining the dependence of the glow curve shape with gamma-ray dose. Especially the disappearance of the dosimetric peak (peak no. V) at about 22YC and other low temperature peaks above 25°C at high gamma-ray dose levels ( > 10s Gy) is accompanied by the disappearance of the respective GA bands at 310 and 380 nm. At the highest dose studied, LiF TLD-100 exhibits a TL peak at about 35O’C (peak no. VIII) whose annealing step coincides with that of the 250 nm OA band which confirms the composite nature of this band. Dose levels greater than 7.2X lo3 Gy result in the growth of F-aggregate centres with their characteristic OA bands at 448 nm (M centre) and at 378 nm (R, centre). However, detailed studies on the OA spectra of TLD-100 indicate that the mobile interstitial model in its present form does not explain satisfactorily the supralineari~ in the response of different ‘IL peaks seen in the medium dose range (IO-lo3 Gy). OA results tend to support the competing nonlumine~nt centre model at least for peak V which envisages a part of the charge released during TL readout to be trapped by these competitors, and indicate that more than one TL process operate simultaneous~ in this material.
1. IBtroduction
The thermoluminescence 0I.J and the complex glow curve structure of LiF TLD-100 as a function of gamma-ray dose have been well studied. For instance, Sagastiielza and Alvarez Rivas [l] report that in the dose range from 1 Gy to 9.2 x 106 Gy, the intensity of each TL glow peak first increases with dose but for high enough doses, the glow peak intensity decreases and finally the glow peak vanishes. Low temperature glow peaks go through this sequence at lower doses than the glow peaks occurring at high temperatures. Similar results were reported by Chandra et al. [2] as well. A mobile interstitial model was proposed [l] to explain the dose dependence of the glow curve in this material. However, similar studies on changes in the optical absorption (OA) spectra of TLD-100 with gamma-ray dose have not been carried out so far. A direct confirmation of this model can be obtained from such studies, since the conversion of one trap to another at high doses should result in the disappearance of the OA band corresponding to the trap which is getting converted.
’ On leave from Safety Research and Health Physics Programme, Indira Gandhi Centre For Atomic Research, Kalpakkam 603102, India. 01~-583X/93/$~.~
Further, the supralinearity (increased TLJGy at higher gamma-ray dose levels) of different TL glow peaks in TLD-100 was attributed by Sagastibelza and Alvarez Rivas [l] to the same phenomenon, i.e. trap conversion during irradiation. Essentially their mobile interstitial model envisages an interstitial allegation process during i~adiation and a supralinear growth of trapped interstitials ~~esponding to a particular trap in the supralinear dose region, However, all the OA bands (notably 380, 310 and 250 nmf observed after gamma irradiation increase either linearly or sublinearly with gamma dose in this dose region, which prompted several investigators to suggest that the supralinearity in TLD-100 arises in the luminescence rather than in the trapping process. Since earlier studies [3] on irradiation at 77 K have shown that the absence of F centres does not influence the supralinear&y of peak V (22O“C), it is proper to test the validity of this hypothesis on vacancy centre related TL peaks, namely peak VII (3OO’Q or peak X (SBO°C). Recently, ~kshmanan [4-61 has correlated peak VII in TL,D-100 with an OA band at or very close to 250 nm and identi~ed it with &-type vacancy centres. In TLD-100, the interstitial aggregates do not give any OA band but any increase in recombination efficiency
in the supralinear dose region can be found out by studying the change in the intensity of the OA band giving rise to the vacancy centre i.e. peak VII recombi-
0 1993 - Elsevier Science Publishers B.V. All rights reserved
A.R. Lakshmanan
/ Thermoluminescence
197
in LiF
nation centre. Recently we foIlowed a similar procedure successfully to confirm the increased production efficiency of peak VII centres due to increased interstitial migration efficiency at elevated irradiation temperatures 151 as well as the increased efficiency of peak VII trap fo~ation at high LET [6]. Both these phenomena involve a change in trapping efficiency during irradiation and hence could be easily confined. But the phenomenon of supralinea~~ in TLD-300 is much more complex as it involves changes in the Iuminescence efficiency during the readout process.
2. oatmeals
and method
LiF: Mg, Ti @LID-100) single crystals (6 X 6 X 2 mm3) as well as LiF: Ti and UF: undoped single crystals (7.7 x 7.7 x 2 mm31 were used after a pre-irradiation anneal at 400°C for 1 h. Irradiations were carried out at RT in a @%Jogamma-ray cell with a dose rate of 39 Gy min - *. Details of TL and GA measurements and the instruments used have been already described 143. TL glow curves were recorded at a linear heating rate of 2”C!s-‘. UV/thermal anneal at 300°C was carried out by keeping the gamma-ray irradiated crystal on the heater planchet at 300°C and simultaneously irradiating it with the ultraviolet (mairdy 254 nm> light from the Pen-ray lamp described earlier 141.
Fig. 1. OA spectra of LiF TLD-100 single crystals at medium gamma-ray dose levels: 1 - 3.9X 10’ Gy, 2 - 3+9X JO* Gy, 3 1.17X 103 Gy and 4 - 3.2X lo3 Gy. The spectra numbers 2, 3 and 4 have been reduced (+I by a factor of 2, 10 and 20, respectively.
gamma doses matches well with the gradual disappearance of TL peaks I to V at high gamma doses [1,2f and is in agreement with the prediction of the mobile interstitial model El].
3. OA spectra at the high dose tevels Figs. 1 and 2 show the OA spectra of TLD-100 at different gamma-ray dose levels. In the dose range 39-3.2 X IO3 Gy, the major GA bands occur at 250, 310 and 380 nm but the intensity of the 310 and 380 nm bands with respect to that of the 250 nm baud decreases slowly from a value of about 0.5 to 0.27 with increasing dose. However, at higher dose levels, i.e. > 7,2 X IO3 Gy, the intensities of the 310 and 380 nm bands show a further decreasing trend and disappear ultimately. The 250 nm band appears to show a saturation trend > 7.2 x lo3 Gy, but its exact intensity could not be recorded for dose levels > 5.64 X IO4 Gy due to the saturation of the instrument i.e. spectrophotometer used beyond the optical density (absorbance) level of 4. The production of F-aggregate centres, namely an M centre which consists of two F centres situated together with its OA at 448 nm and an Rz centre which consists of three F centres situated together with its OA at 378 nm [7] is seen for dose levels > 7.2 x lo3 Gy. The growth of F-aggregate centres is accompanied by a decline in the growth of F centres. The gradua1 disappearance of GA bands at 310 and 380 nm at high
200
300
h hml
tco
Fig. 2. OA spectra of LiF TLD-100 single crystals at high gamma-ray dose levels: 5 - 7.2 x lo3 Gy, 6 - 5.64 x Z04 Gy and 7 - 2.83 X 10’ Gy. The spectrum number 5 has been ma~ni~ed ( x 1 by a factor of 2.
A.R Lakrhmanan / Thermoluminescencein LiF
198
200
300
boo )j
500
2w
400
300
500
X(nml
lnm)
Fig. 3. (a) OA spectra of LiF TLD-100 (solid line), LiF: Ti (dashed line) and LiF (undoped) (long-short dashed line) at the dose levei of 7.2~ lo3 Gy. (b) OA spectra of LiF TLD-100 (solid line), LiF:Ti (dashed line) and LiF (undoped) (long-short dashed line) at the dose levei of 2.83x lo5 Gy.
The application of M-centre OA in LiF (undoped) for high level g~a-ray dosimetry is well known [S,9], but the changes in the OA spectra of TLD-100 at high dose levels have not been so well recorded. The OA spectra of LiF (undoped), LiF : Ti and TLD-100 at the dose levels of 7.2 x lo3 and 2.83 x lo5 Gy are compared in figs. 3a and 3b. In LiF (undoped) and LiF : Ti, apart from the M-centre OA at 440 nm the presence of two F-aggregate centres is seen (fig. 3b) more clearly, i.e. R, (378 nm) and R, (320 nm) centres. Both are F, centres [7]. Further, the OA spectra of LiF (undoped) and LiF : Ti are marked by the absence of Mg-related OA bands at 310 and 380 nm (fig. 3a). Another significant difference between the three spectra ,(fig. 3b) is the OA between 200 and 300 nm. The TLD-100 OA spectra in this wavelength region are much broader than those of LiF (undoped) and LiF: Ti, which confirms the presence of other OA bands very close to the F band at 250 nm in TLD-100, as proposed earlier [4]. Figs. 4 and 5 show the growth of F (250 nm) and M (440 nm) centres in LiF (undoped~ and LiF : Ti, respectively, with dose at high dose levels. A comparison of figs. l-5 shows that the growth of F-aggregate centres, especially the M centre, with dose is more intense for
“1
Fig. 4. Response of 2.50 nm (upper curve) and 440 nm (lower curve) OA bands in LiF (undoped) as a function of gamma-ray dose.
A.R. Lak~~anan
250 nm. The 425°C readout erases peak VIII and removes most of the OA at 250 nm. The correlation of peak VIII with an OA band close to 250 nm is in agreement with our earlier results [4] which indicated that the 2.50 nm OA band in TLD-100 is a composite band. After the 425°C readout, the Z3 (225 nm>, F (250 nm> and an unidentified band at 285 nm remain, Most of these bands vanish on 499°C readout.
5. Changes in OA spectra during TL readout at medium dose levels Figs. 8-11 indicate the changes in different OA bands during the readout of different TL peaks at four dose levels. During the readout of peaks II and III, the 380 nm band is destroyed but the 310 nm band grows while the 250 nm band falls in intensity. Simiiarly during the readout of peak V, the 310 nm band is destroyed, accompanied by a fall in 250 nm band intensity while a new OA band around 235 nm is apparently formed. The relative intensity of the 235 c
I
3xw3
1oa
UP Gamma
Dosr
cxlo=
tCyt
%
Fig. 5. Response of 2.50nm (upper curve) and 440 nm (lower
curve) OA bands in LiF:Ti as a function of gamma-ray dose.
LiF (undoped), followed by LiF:Ti. Its growth is in TLD-100, probably due to the presence of Mg. explains the application of M-centre OA in LiF doped) for high level gamma-ray dosimetry rather the OA in LiF TLD-100 [8,9].
least This (unthan
4. OA band corresponding to peak VIII Figs. 6 and 7 show the OA spectra and TL glow curve of TLD-100 at the highest dose level studied (2.83 x lo5 Gy). In both cases, the changes seen after different readout temperatures, i.e. merely heating the crystal at 2”Cs-’ to the m~mum temperature of 180, 250, 350, 425 and 499”C, are shown. The TL glow peaks which are characteristic of TLD-100 at dose levels < lo4 Gy, namely glow peak numbers I to V as well as their characteristic OA bands at 310 and 380 nm are absent at this dose level. The glow peak number VIII (350°C) is the most prominent one in fig. 7, while peak number VII is seen with feeble intensity. These TL data match well with those reported earlier j&21. While the 250°C readout erases most of the M band at 448 nm, no prominent TL glow peak corresponding to this band could be observed. The 350°C readout erases the weak peak no. VII as well as Rz and M centres and reduces considerably the OA above
-lLoo 300 200
500 hhd
Fig. 6. OA spectrum (solid line) of LiF TLD-100 at the highest dose levei studied ~2.83X105 Gy). The spectra after the 180°C readout (dashed fine), 250°C readout (01, 350°C readout (o), 425°C readout (long-short-short dashed line) and after the 499“C readout (A) are also shown. In all these and subsequent cases, the crystal was merely heated linearly to the maximum temperature indicated at the rate of Z”Cs-‘. Absorbance values above the level of 4 are unreliable in the spe~rophotometer used. A high degree of oscillation of the plotter is noticed at such high absorbance values. In any case, the appearance of F-aggregate centres i.e. M centre (448 nm) and R, centre (378 urn) and the disappearance of Mg-related OA bands at 310 and 380 nm is seen. While no prominent TL peak could be correlated with either the M or R2 centre, the peak VIII could be correlated with an annealing step of an OA band near 2.50nm.
A.R. LalcFhmanan / Thermoluminescence
200
Temperature i*C)
Fig, 7, TL glow curves of LiF TLD-100 #resending to the OA spectra &own in fig. 6. At this dose level (2.83 X IO5 Gy), peak VIII (350°C) is the most prominent TL peak while peak numbers I to V are absent.
band with respect to that of the 310 nm band increases with dose and appears to reach its maximum value around 3.2 X 103 Gy, which incidentally coincides with the saturation dose of peak V. This indicates that the dest~ct~on of peak V traps not only results in TT.,, i.e. in re~mb~ation of charge carriers, but also in the creation of new traps whose efficiency changes with the dose. This confirms a key component in the competing
in LiF
Fig. 9. OA spectra of LiF TLD-100 at the dose level of 3.9~ lo2 Gy immediately after i~adiation (solid line), after 180°C readout (dashed line), 2WC readout (dot-dashed line), 350°C readout (long-short dashed line) and 425°C readout (long-short-short dashed line).
nonluminescent centre model. But the possibility that during the subsequent, i.e. 35O”C, readout the 235 nm band itself is converted to another unidentified centre which acts as the competitor to the lumines~nt centre is strong, since the readout of peak VII removes most
h Inml Fig. 8. OA spectra of LiF TLD-100 at the dose level of 3.9 x 10’ Gy immediately after irradiation (solid line), after 180% readout (dashed line), 250°C readout (dot-dashed line), 350°C readout (01 and 425°C readout Uong-short-short dashed line). While the 180°C readout removes TL peaks II and III (140°C and 180°C peaks repectively), the 250°C readout removes peak V (22ffC peak), the 350°C readout removes peak VII (3OO“Cpeak) and the 425°C readout removes peak VIII (350°C peak).
0
200
300
LCO A fnrnl
Fig. 10. OA spectra of LiF TLD-100 at the dose level of 1.2x lo3 Gy immediately after the irradiation (solid line), after 180°C readout (dashed line), 250°C readout (dot-dashed line), 350°C readout (0) and 425°C readout (long-short-short dashed line).
201
-3 <.,,,,I
,
5x10 f:
3x10’
102
,
,,,*,.I
3x10’ Gomma Fhy
-200
300
Loo
500
A hm) Fig. 11. OA spectra of LiF TLD-100 at the dose level of 3.2X lo3 Gy immediately after irradiation (solid line), after 180°C readout (dashed line), 250°C readout (dot-dashed line), 350°C readout (0) and 425°C readout (long-short-short dashed Iine). The growth of an OA band around 235 nm due to the readout of peak V (i.e. 250°C readout) is very prominent at this dose level. This indicates that the destruction of peak V traps not only results in the recombination of charge carriers, i.e. the production of TL, but also results in the creation of new traps, which is dose dependent. This confirms a key component in the competing nonluminescent centre model.
,
L
3x103
103
Dose IGyl
Fig. 12. The changes (i.e. reduction) in the intensities of: the 380 nm (A I OA band due to the readout of TL peaks II and III, the 310 nm (01 OA band due to peak V readout, the 250 nm (A) OA band due to peak VII readout and the intensity of the 225 nm (2s) band as observed after peak VII readout (0) as a function of gamma-ray dose at medium dose ievels in LiF TLD-100. The lines of linearity (dashed) are also shown.
interstitial model in its present form (or other models which invoke the supralinear creation of traps with dose), although qualitatively the OA band corresponding to high temperature TL peaks shows a tendency towards linear or supralinear behaviour rather than
of the OA at 235 and 250 nm leaving the 225 nm band
residual.
6. Intensities of OA bands versus dose at medium dose levels The changes in the intensities of 380, 310 and 250 nm GA bands due respectively to the readouts of TL peaks II and III, V and VII and the intensity of the 225 nm band as observed after the peak VII readout are plotted in fig. 12 as a function of dose. The TL intensities of peaks II (14O”C), III (18O”C), V and VII are plotted in fig. 13 as a function of dose. The 380 nm band shows sublinearity from the beginning, i.e. from 38.5 Gy. The 310 nm OA band shows linearity up to 3.85 X 10’ Gy and sublinearity afterwards. The 250 nm band is more or less linear up to 2.77 x lo3 Gy. The 2W nm band shows a slightly supralinear behaviour in contrast to other OA bands. But its supralinearity is much less than that reported by Takeuchi et al. [lo] in LiF : Mg and sets at right the controversy about the nature of the growth of Z, centres in TLD-100. In other words, the behaviour of OA bands does not support quantitatively the mobile
, 3x10’
1
102
I
3x102
.,,,,I
I
4
103
,
3x103
Gamma F&y Dose tGy)
Fig. 13. The TL intensities of peak II (*I, peak III (01, peak V (A > and peak VII (v I as a function of gamma-ray dose at medium dose levels in LiF TLD-100. The lines of linearity (dashed) are also shown.
sublinearity. While the 380 nm band is sublinear, the TL peaks II and III are supralinear and the 250 nm band is more or less linear whereas the TL peak VII is highly supralinear. Previous studies [3,12] have shown that the TL peak X is much more supralinear than that of the 225 nm OA band observed in fig. 12. Thus the trap conversion process envisaged by the mobile interstitial model though present at these dose levels does not explain satisfactorily the supralinearity of various peaks in TLD-100.
0.071 0.06
-
7. Charge mobility during Tt readout Figs. 8-11 indicate an interesting finding, namely the changes in different OA bands during the TL readout, This is a clear indication of charge/interstitial migration/aggregation as well as recombination with vacancies such as F centres during the TL readouts. The only difference between this model and that proposed in the original mobile interstitial migration model [l] is that the interstitial/charge migration is envisaged to take place here during readout instead of during i~adiation at the interme~ate dose levels (lo-lo3 Gy) where the TL peaks exhibit supralinea~~. This phenomenon is also consistent with the competing nonluminescent centre model proposed by several researchers [ll-131. But the individual identification of the competitors for different TL peaks is extremely complex, as can be seen from the complex changes in the OA spectra in figs. 8-11. Since the sensitized material (i.e. material after lo3 Gy + 300 to 350°C annealing) exhibits linearity in the response of different TL peaks, it is tempting to assume that the 225 nm centres are actually the competitors to luminescent centres since all the OA bands appear to converge ultimately into this band during the sensit~ation process. But the fact that irradiation at 77 K reduces drastically the 225 nm centres or peak X without affecting the supralinearity of peak V proves that the 225 nm centres are not the competitors that we are looking for [3]. Further, following irradiation at RT, peak X (which is correlated with the 225 nm band) itself exhibits pronounced supralinearity. This indicates that the TL mechanism of vacancy centre related traps (such as peaks VII, VIII and X) could be different from other traps (such as peaks I to V). However many experimental results indicate that the thermal activation energy of these ~mpet~tors and that of 225 nm centres are very close to each other.
8. Competitors to luminescent
centres
Further, a simultaneous 254 nm photo bleach at 300°C (UV/thermal anneal) removes peak X as well as
200
300
166
h
606
(nm)
Fig. 14. OA spectra of virgin (1) and sensitized (2) LiF TLD-100 single crystals after a test dose of lo* Gy. The sensitization treatment (2) involved a gamma-ray dose of lo3 Gy and a subsequent W/thermal annealing treatment at 280°C for 2h. This treatment resulted in the sensitization of peak V by a factor of 2 whereas peak VII and the 250 nm OA band were reduced by 25%. (3): (I)+ 400°C readout, i.e. after the removal of peaks V and VII. (4): (2)+ 400°C readout. The 225 nm is produced more or less with the same intensity in both cases, i.e. in spectra (3) and (4), which sets at rest the speculation that the 225 nm centres are the competitors to luminescent centres.
the OA band at 225 nm, but the sensitization
as well as the linearity of peak V in the sensitized LiF TLD-100 material persist [12]. This discards the possibility that the 225 nm centres are the competitors. Even if we assume that during the above W/the~al anneal the 225 nm centres are not destroyed but are merely emptied or converted to other centres, after a test dose and subsequent readout of peak VII, there should be a substantial change in their population (an increase in the former case and a decrease in the latter case) as compared to the freshly irradiated material. Figs. 14 and 15 present the results of this study. Fig. 14 shows the OA spectra after a dose of lo2 Gy to the virgin sample (freshly annealed) as well as after a test dose of lo2 Gy to the sensitized sample (lo3 Gy + W/thermal anneal at 280°C for 2 h). Fig. 15 shows the TL glow curves corresponding to the OA spectra in fig. 14. In the sensitized material, peak V is sensitized by a factor of 2 whereas peak VII and 250 nm OA are reduced by 25%. No appreciable change in 225 nm OA is seen. The fact that after the peak VII readout, the 225 nm band is produced with the same intensity in virgin as well as in sensitized samples proves that the centres giving rise to the 225 nm band are not the competitors to luminescent centres. These competitors
AX. Lakshmanan / Thermoluminescencein LiF
Fig. 15. TL &xv curves of virgin (solid line) and sensitized TLD-100 (dashed line) corresponding to the samples shown in the OA spectra of fig. 14.
apparently
do not give rise to any OA at least above
200 rim.
9. TL process of vacancy relnrted traps As discussed above the TL process of vacancy centre related traps should be discussed separately. Their TL process is well explained by the mobile interstitial model [Il. Their TL response with gamma-ray dose is marked with a high degree of supra~ineari~. In fact even at Low doses the Iinearity is scarcely noticed. This fact as well as the increased supralineari~ of peak X as compared to that of peak VII are a pointer to the readout temperature dependent interstitial mobiIity and their recombination with vacancies created by other secondary electron tracks, i.e. track interaction effects, as pointed out earlier 131. Fig. 14 indicates that the presence of partial sensitization of peak X in the UV/thermaI sensitized material observed earlier [33 should also be attributed to increased luminescence efficiency rather than to increased trapping efficiency 1141. This result disagrees with any possibility of trap conversion of 225 nm centres during the W/thermal anneal but indicates the partial influence of competitors to luminescent centres on the TL response of peak
203
X. However, the absence of sensitization of peak VII in this material following the WV/thermal anneal (fig. 15) is a result which is difficult to explain by any model proposed so far in its present form, as pointed out by us earlier [3,123. The fact that the OA of the 250 nm band in this material is slightly less than that in the virgin material (fig. 14) matches we11with the reduced TL sensitivity of peak VII (fig. 1.5). Lakshmanan et al. 131 predicted such a result from their statement “. . . that following the UV/thermal sensitization process, the peak 7 trap formation efficiency is reduced possibly because the 225 nm to peak 7 centre conversion process is inhibited perhaps as a result of reorientation/ realignment of peak 7 trap centres . . . “. Any treatment which results in the sensitization of peak VII is accompanied by an enhancement in the OA at 250 nm, as pointed out earlier [.$I. Thus it is clear that more than one TL process is operating simultaneously in TLD-100, some manifest more than the rest at certain dose IeveIs. When the elimination of competing noniuminescent centres is dominant at medium dose levels, tke supraiinearity of peak V results. When the trap aggregation process is dominant at high dose Ievels, the decrease in the intensity of TL of some peaks and their OA bands and their eventual disappearance result at high dose levels. For vacancy centre related traps, the temperature dependent mobiiity of interstitials and possible track interaction effects are dominant at Iow and medium dose levels where supralinearity in their TL is observed. However, at high dose levels the interstitial aggregation process causes the disappearance of these TL peaks as well, Thus the task of interpreting the experimental results in TLD-100 is extremely difficuh and should be pursued with caution.
10.Conclusions il The mobile i~terstitiai model in its present form explains well the changes in the shape of the TL glow curve and OA spectra at high dose levels. ii) The optica) absorption band corresponding to peak VIII observed at high dose levels apparently occurs close to 2% nm, thereby unfixing the composite nature of this band. Its utifity for high level gamma-ray dosimetry has been pointed out earlier IZ87. iii) SimiIar to LiF 6mdoped) or LiF: Ti, LiF TLD100 also exhibits the growth of F-aggregate centres for dose levels > 7.2 X 103 Gy while the F centres apparently show a decline in their growth. However, the OA spectra of TLD-100 from 200 to 300 nm are much broader than those of LiF fundoped) and LiF:Ti as a resdt of the presence of other OA bands cIose to the F band at 250 nm, as proposed earlier [4].
204
A.R. Lakshmanan
/ Thermoluminescence
iv) Detailed OA data indicate that the mobile interstitial model in its present form does not explain satisfactorily the supralinea~~ or sensitization of different TL peaks in TLD-100 observed at medium dose levels. In other words conversion and aggregation of traps as envisaged in this model is not the dominant mode at these dose levels. v) OA data indicate the migration of charge and the creation/~lling up of other traps during the readout of TL peaks especialiy peak V in TLD-100. This is consistent with the competing nonluminescent centre model proposed earlier. vi) Though tempting, the 225 nm centres could not be correlated with these competitors due to a variety of reasons, one being the observed production of the 225 nm OA band with the same intensity in virgin as well as in sensitized TLD-100 after a UV/thermal anneal. The reduced sensitivity of peak VII TL and the 250 nm OA band in the above sensitized material agree with our previous predictions [3,4]. vii) The 225 nm OA band in TLD-100 shows very little supralineari~ as compared to peak X TL response. This does not support the models which invoke the supralinear creation of 225 nm centres with gamma-ray dose. Other models such as the track interaction model and the competing nonluminescent centre model are invoked to explain these results.
Acknowledgements
The author is grateful to Dr. Wilfried Hoffmann and Sri. L.V. Krishnan, Head, SR and HPP, IGCAR,
in LiF
for helpful discussions and encouragement. is also supported by AvH Foundation.
This work
[l] F. Sagastibeiza and J.L. Alvarez Riias, J. Phys. C: Solid State Phys. 14 (1981) 1873. [2] B. Chandra, A.R. Lakshmanan and R.C. Bhatt, Int. J. Appl. Radiat. Isot. 33 (1982) 679. (31 A.R. Lakshmanan, B. Chandra, R.C. Bhatt, W. Hoffmann and R. Spatlek, J. Phys. D: Appl. Phys. 18 (1985) 1673. [4] A.R. Lakshmanan, Nucl. Instr. and Meth. B 82 (1993) 557. [5] A.R. Lakshmanan and U. Madhusoodanan, Phys. Status Solidi a 139 (1) in press. [6] A.R. Lakshmanan, Phys. Med. Biol. (submitted). [7) J.H. Schulman and W.D. Compton, Color Centres in Sohds (Pergamon, New York, 1962). [S] W.J. Vaugham and L.O. Miller, Health Phys. 18 (19701 578. [9] A. Waibel, Y. Goksu and D.F. Regulla, Radiat. Prot. Dosim. (in press). [lo] N. Takeuchi, K. Inabe and S. Nakamura, Phys. Status Solidi a 29 (1975) 11. [ll] J. ~mmerm~, J. Phys. C: Solid State Phys. 4 (1971) 3277. [12] A.R. Lakshmanan, B. Chandra and R.C. Bhatt, J. Phys. D: Appl. Phys. 15 (1982) 1501. [13] E.F. Mische and S.W.S. McKeever, Radiat. Prot. Dosim. 29 (1989) 159. [14] V.K. Jain, Phys. Status Soiidi a 60 (1980) 351.