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Lyoluminescence, thermoluminescence and photodecomposition in microcrystalline powder of KCl, KBr, KI and KI:KNO3 crystals
Nuclear Instruments and Methods in Physics Research B 192 (2002) 280–290 www.elsevier.com/locate/nimb
Lyoluminescence, thermoluminescence and photodecomposition in microcrystalline powder of KCl, KBr, KI and KI:KNO3 crystals S.J. Dhoble
a,*
, P.M. Bhujbal b, N.S. Dhoble c, S.V. Moharil
b
a
Kamla Nehru College, Sakkardara Square, Nagpur 440 009, India Department of Physics, Nagpur University, Nagpur 440 010, India Sevadal Women’s Science College, Sakkardara Square, Nagpur 440 009, India b
c
Received 5 April 2001; received in revised form 25 September 2001
Abstract Particle size effect in c-radiolysis of KCl, KBr, KI and KI:KNO3 single crystal and microcrystalline powders has been studied using optical absorption, thermoluminescence and lyoluminescence (LL) techniques. Colouration characteristics in KCl, KBr microcrystalline powders are reported to be different from KI and KI:KNO3 for various particle sizes. F centre concentration decreases with decreasing particle size in KCl and KBr. While in KI and KI:KNO3 , V centre concentration increases, on the other hand F centre concentration decreases with decreasing particle size. Rate of formation of I 3 is 100 times less in KI:KNO3 as compared to KI crystal and powders. This is attributed to the higher rate of F-H recombination due to the impurity ions in KI:KNO3 . Colour centre concentration, particle size and dissolution rate, all three factors affect LL intensity together. The results presented here may be considered as of only academic interest in solid state materials. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Colour centres in alkali halides have now been studied for 50 years. It is known that the colouration produced by electrolysis is very stable [1]. In the last two decades, we have shown [2–6] that the colouration in microcrystalline powders obtained by crushing the electrolytically coloured single crystal is not stable. In case of potassium halides, all the colouration was lost within a day [7] and Deshmukh and his co-workers established a cor-
*
Corresponding author.
relation between microhardness, dislocation mobility and the stability of colouration in microcrystalline powders [2–6]. Recently a theoretical approach has been made to the stability of colour centres in microcrystalline powders of alkali halides [8]. The emission of visible glow during the dissolution of irradiated crystal, known as lyoluminescence (LL), was first reported in sodium chloride by Westermark and Grapinyieser [9]. Later on alternative mechanisms proposed for the emission by Annstrom [10] and Arnikar et al. [11] also supported the formation of hydrated electron (eaq ) as the first step during dissolution. The
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 4 7 7 - 9
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combination of eaq with the hole at the V type centre lead to the emission.
X þ e aq ! ðXaq Þ ! Xaq þ hm
where X ¼ Cl , Br , I etc. In recent years some interesting work on LL has been done. Mahapatra showed the effect of solvent temperature in the range 10–90 °C on LL yield in saccharides [12], measurement of low radiation dose by LL techniques [13] and decay times, LL in c-irradiated saccharide [14]. In 1997, Galand showed the LL in alkali halides, carbonates, phosphate and concluded that the incorporation of bivalent earth alkali and transition metal cations in KCl leads to an enhancement of LL [15] and LL sensitisation of carbohydrates and amino acids [16]. Chandra explained theoretical and experimental qualitative correlation in LL of alkali halide [17]. Colour centres have mostly been studied in single crystals, while applications such as dosimetry, of the ionizing radiation using thermoluminescence (TL) and LL more often involves measurements on powders. The application of LL to radiation dosimetry was initiated by Ettinger and co-workers [18]. It is generally believed that the mechanism of colour centres production is similar for single crystal and microcrystalline powder. Colour centres production by c-irradiation in NaCl, KCl and KBr is reported by Deshmukh and co-workers [19–22] in crystal and microcrystalline powder. While for KI, irradiation results into production of both F and V centres, fine powders of KI develop a brown colour following such c-irradiation which is characteristic of tri-iodide absorption [23]. Pode et al. [24] and Shirke et al. [25] have studied the dependence of iodine liberation on the particle size in irradiated KI. On development of LL dosimetry materials, investigators concentrated on an enhancement in LL intensity, observed in certain flouroscent [26,27], chemiluminescent solutions [28,29], dye lasers [30] and excess colour centres by electrolytic colouration [31]. In this paper we report the dependence of the nature of the radiolysis products, LL and TL on the particle size in potassium halides. Also the effect of doping of KNO3 (1 mol%) in KI is studied for various particle size measurements.
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2. Experimental Single crystals of KCl, KBr and KI were grown from melt using the Czochralski method. G.R. grade powder from Merck was used as starting material. For preparation of KI:KNO3 single crystal desired amount of KNO3 (1 mol%) was added to the melt. It was observed that the growth was possible only in dry atmosphere when the melt was clear. Once the crystals were grown, they were relatively more stable to humidity. Only prolonged exposure to a humid atmosphere (humidity less than 40%) turned them smoky. All the experiments were performed during the dry season when the relative humidity was much below 40%. Crystals of size 5 5 0:5 mm3 were cleaved from the as grown block. These were exposed to c-rays from 60 Co source. Some of the crystals were crushed and particles of the desired size were obtained by sieving. These were also exposed to c-rays. Optical spectra were measured with a CZ VSU 2P spectrophotometer with 45/0 reflectance attachment for the powders. LL was studied with the usual setup of lyoluminescent cell, photomultiplier (Hamamastu 931 B), amplifier and recorder. Either distilled water or distilled water containing luminol (7 104 mol%) was used as the solvent. In all measurements 200 mg of sample and 4 ml of solvent was used. TL glow curve was also studied by a similar setup, the modifications being that the lyoluminescent cell was replaced by a drawer containing a heater plate on which 5 mg of sample was heated each time at linear heating rate of 150 K/min.
3. Results and discussion Optical spectra of KCl, KBr and KI powders exposed to c-rays show the various bands. These were identified as the well known F and V type centres [32]. Intensity of F and V bands decrease with decreasing particle size in KCl and KBr. But for KI optical spectra are different from those of KCl and KBr, which is shown in Fig. 1. Spectra of KCl/KBr are not given as they did not contain any thing new. Fig. 1 shows the optical spectra of KI single crystal and powders exposed to c-rays in
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Fig. 1. Optical spectra of a KI single crystal and fine powders of various sizes exposed to c-rays for various particle sizes: curve 1, single crystal; curve 2, 850–1000 lm; curve 3, 710–850 lm, curve 4, 425–500 lm; curve 5, 300–355 lm; curve 6, 250– 300 lm; curve 7, 210–250 lm.
the range 400–900 nm. A prominent F band peaking at 710 nm is seen for the single crystal. For powders the Kubelka–Munk function F ðRÞ ¼ ð1 R2 Þ=2R, where R is the reflectance, is plotted as a function of wavelength. A prominent F band is again seen at around 710 nm. With decreasing particle size intensity of this band decreases. The absorption in the violet region also increases at the same time. This is the tail of the tri-iodide absorption band peaking at 354 nm [33,34]. It is thus seen that, with the decreasing particle size, the nature of the ‘radiolysis’ product changes. The F centre concentration decreases while the tri-iodide centre concentration increases. These changes are so pronounced that they can be observed ‘‘visually’’. The single crystal develops a blue colour while the powders assume a brownish tinge. The difference between the ‘‘radioloysis’’ products was observed when powders were exposed to c-rays. If the crystals are coloured and then crushed into powders, then these powders contained F centres and no predominance of the triiodide absorption was found. Even in this case, there is a difference between the behaviour of F centres in crystals and powders. F centres in single
Fig. 2. Reflectance spectra of microcrystalline powders of KI:KNO3 exposed to c-rays for various particle sizes: (1) 850– 1000 lm; (2) 500–707 lm; (3) 300–420 lm; (4) 210–250 lm; (5) 150–210 lm; (6) 63–150 lm. Also shown are the spectra of KI for particle sizes (a) 707–850 lm and (b) 425–500 lm.
crystal are stable while those in powders decay within 24 h [5]. Reflectance spectra were recorded for c-irradiated microcrystalline powder of KI:KNO3 (Fig. 2). Similar to KI, bands at 685 nm, 390 nm corresponding to formation of F centres and I 2 respectively [35] are seen. Yet another band of lesser intensity at 490 nm is observed which may be attributed to formation of I2 [36]. It appears that in KI doped with KNO3 , F centre formation is very weak compared to KI and also the band at 685 nm is observed for some coarse particles only. The absorption spectra of KI:KNO3 in solution indicate bands at 354 nm corresponding to I 3 and at 390 nm (I ) as well. Variations of intensities of 2 354 and 390 nm bands with particle size for KI and KI:KNO3 are shown in Fig. 3(a) and (b) respectively. In case of KI (Fig. 3(a)) it is observed that intensities of both bands first increase with decreasing particle size up to 100 and 350 lm respectively and then decrease. The increase in
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Fig. 3. Variation of optical density with particle size in (a) KI and (b) KI:KNO3 .
intensity is due to the ease of radiation induced decomposition with increasing surface area. However, with further decrease in particle size, V centre aggregates are produced resulting in decrease in intensity. In Fig. 3(b) it is observed that intensity of 390 nm band increases with decreasing particle size while for 354 nm it decreases up to 100 lm and then increases slightly. Surprisingly a minimum is observed at the same particle size of 100 lm in KI. KNO3 at which a maximum is observed in case of KI for 354 nm band (Fig. 3(a)). The reduction in intensity is attributed to the presence of NO 3 or its radiolytic product NO [37]. In either case inter2 action with I may take place in such a way so as 2 to decrease the rate of formation of I . With de3 creasing particle size the reaction becomes prominent thereby giving still smaller rate of formation of I 3 . This fact becomes further evident from Fig. 4, which shows variation of GðI 3 Þ with particle size for KI and KI:KNO3 . With decreasing particle size GðI 3 Þ values also increase and decrease for KI and KI:KNO3 (upto 100 lm) respectively. Further G values of KI are higher by two orders of magnitude compared to KI:KNO3 suggesting higher rate of formation of I 3 in the former case. Fig. 5 shows the rate of formation of I 2 in KI and KI:KNO3 powders at different c-exposures. It is observed that in KI (curve a) formation of I 2 increases up to 2840 C/kg, after which it decreases
Fig. 4. Variation of GðI 3 Þ values with particle size in KI and KI:KNO3 .
due to aggregation of V centres which are formed more at higher exposures. On the other hand F centre formation (curve c) occurs at a minimum exposure of 1550 C/kg1 and then its concentration increases linearly with exposure. In KI:KNO3 , both the rates of formation of F and V centres (curve b) are very weak compared to KI powders. This may arise due to doping of KI with NO 3 impurities which interrupts the [1 1 0] replacement sequence resulting into higher F-H recombination
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Fig. 5. Variations of rate of formation of I 2 species in KI and KI:KNO3 powders with of c-exposures: (a) KI, (b) KI:KNO3 and (c) F centre formation in KI.
immediately after creation. This is schematically represented in Fig. 6. Hayes and Nichls [38] have shown that more V type centres are produced with [1 0 0] symmetry axis in KCl and KBr crystals containing divalent impurities. In the present case after doping KI with KNO3 some I lattice posi tions are occupied by NO 3 . Since NO3 (ionic ra ) dius 2.19 A) is larger than I (ionic radius 2.06 A
the whole lattice may get somewhat compressed obstructing the mobility of radiation damage products. After c-irradiation NO 2 are formed or even otherwise the replacement sequence of X 2 will get interrupted resulting in increase in F-H recombination immediately after creation. Only a small concentration of stable defects like F and V type centres remains. Consistent with the observations of Atari [39], we found that addition of a small quantity of luminol increased the LL intensity by order of magnitudes. It was found that the optimum luminol concentration was 7 104 mol% and the pH of the solvent 12. Fig. 7 shows the typical LL glow curve for KBr powder exposed to c-rays with water containing luminol as a solvent. It is seen that for the particle size 1000–300 lm the band width of the glow curve is large and it decreases with the decreasing particle size. Below 45 lm a sharp glow curve was observed with respect to intensity vs time. Glow curves show that the width of the LL glow peak depends on the dissolution rate of particles in solvent and LL intensity on the particle size of crystal and F and V colour centre concentration. On other hand with decreasing
Fig. 6. Schematic representation for (a) formation of I 2 centres in normal KI crystals (b) NO3 doped KI crystal and (c) hurdles created by NO /NO in disrupting the sequence. Adapted from [38]. 3 2
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decrease in LL intensity (due to lesser F and V centres) is seen as expected. But the decrease is only around 50 times less compared to 1000–850 lm. This indicates that LL intensity also depends on the rate of dissolution of the microcrystalline powder. Thus emission of light during dissolution depends more on the rate of dissolution, than on particle size. As per Fig. 7, particle size goes on decreasing, the band width of the LL glow curve decreases. As colour centre concentration decreases, LL intensity decreased but at the same time due to decreasing particle size rate of dissolution enhanced, hence LL intensity is increased. Band width of LL glow curve depends on particle size and LL intensity depends on colour centre concentration as well as particle size. For KCl/KI/ KI:KNO3 as the LL glow curve is similar to KBr, it is not shown here. Fig. 8 shows the variation of LL intensity with particle size of c-rays exposed KBr, KCl, KI and KI:KNO3 powders with water containing luminol used as solvent. Results show the LL intensity decreases from particle size of 1000 to 425 nm due to decreases in F and V centre concentration in
Fig. 7. The LL glow curves of KBr powder with water containing luminol (7 104 mol%) as solvent exposed to 490 C/kg for various partide sizes: (a) 850–1000 lm; (b) 710–850 lm; (c) 500–710 lm; (d) 425–500 lm; (e) 355–425 lm; (f) 300–355 lm; (g) 250–300 lm; (h) 210–250 lm; (i) 120–210 lm; (j) 85–120 lm; (k) 45–85 lm; (l) below 45 lm.
particle size F and V type centre concentration also decreases in KCl and KBr crystals while for KI and KI:KNO3 only F centre concentration decreases, however, V centre concentration increases. For finer particle size (below 200 lm) of KI and KI:KNO3 crystal LL intensity decreases due to aggregation of V type centres. Therefore,
Fig. 8. Variation in LL glow peak heights with particles size of (a) KBr, (b) KCl (c) KI and (d) KI:KNO3 with water containing luminol (7 104 mol%) as solvent (c-rays exposure 560 C/kg).
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KCl and KBr crystal, while in KI and KI:KNO3 only F centre concentration decreases (V centre production rate in coarse particle size is very much smaller). Below the 425 lm up to 150 lm LL intensity increases due to faster dissolution rate of powder in solvent. However, below 150 lm LL intensity decreases in KBr and KCl due to poor radiolysis products, while in KI and KI:KNO3 , LL intensity decreases due to aggregation of V centres and F centres concentration is nil in finer particle size. These results are quite similar to the mechanism proposed by Ahnstrom [10] and Atari [28] that it is due to recombination of hydrated electron and V centre at the water–solid interface and is adequate to explain the above fact as well as higher LL intensity is observed for coarse particle size. In case of halides irradiated with c-rays then crushed for different particle size, high LL intensity is observed for coarse particle size, showing decrease with the decreasing particle size up to around 250 lm (Fig. 9). In KI and KI:KNO3 as F centre production is less as seen from optical spectra of KI and KI:KNO3 , LL intensity is found to be less by 10 orders of magnitude compared to KBr and KCl for coarse particle size in KI and 100
times less in KI:KNO3 powders. Below 325 lm F centre concentration decreases in KCl, KBr, KI and KI:KNO3 , thus LL intensity shows decrease due to less radiolysis products. From the above presented results it can be concluded that an enhancement in LL intensity depends on the colour centre concentration in microcrystalline powder. For coarse particle size, formation of radiolysis products is much higher than fine particles, however rate of dissolution is very less in the former than in the latter. The TL in KCl, KBr and KI originates in the radiative recombinations of V type (interstitial halogen) centres with F centres [40]. It is expected that the changes in colouration characteristics of powders in comparison with crystals will be reflected in the TL also. In KCl and KBr powders TL intensity decreases with decreasing particle size while differences in the colouration characteristics of single crystals and powders of KI are observed due to F and V colour centre concentration in powders. Fig. 10 shows the TL glow curves of KI single crystals and fine powders exposed to 490 C/kg. In
Fig. 9. The height of the LL peak in particles obtained by crushing c-rays irradiated single crystals with water containing luminol (7 104 mol%) as solvent (c-rays exposure 560 C/kg): (a) KBr, (b) KCl, (c) KI and (d) KI:KNO3 .
Fig. 10. The TL glow curves of KI powders c-rays exposed to 490 C/kg for various particle sizes: (a) single crystal; (b) 850– 1000 lm; (c) 710–850 lm; (d) 500–710 lm; (e) 425–500 lm; (f) 355–425 lm; (g) 300–355 lm; (h) 250–300 lm; (i) 210–250 lm.
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Fig. 12. Variation of TL glow peak heights with particle size of KI:KNO3 crystal exposed to c-rays (490 C/kg) for (a) 370 K peak and (b) 500 K peak. Error bars are not shown on curves to maintain clarity.
Fig. 11. Variation in glow peak heights with particle size of KI crystals. The heights of the 370 K (curve a) and 500 K (curve b) peaks are plotted as a function of particle size for an exposure of 490 C/kg. A similar variation in the Kubelka–Munk function F ðRÞ at the peak of the F band is plotted in curve c. The heights of the 370 K peak for 390 C/kg (curve d), 195 C/kg (curve c) and 39 C/kg (curve f) are plotted. Error bars are not shown on curves d and e to maintain clarity.
all the glow curves, two glow peaks at around 370 and 500 K can be seen. With decreasing particle size we find that the height of the 370 K peak increases up to 425–500 lm (Fig. 11, curve a) and then decreases. A more or less similar behaviour was observed for the 500 K peak (Fig. 11, curve b). In the context of Fig. 7, it is surprising to note that there is no large decrease in TL intensities with decreasing particle size. The F centre concentration shows a marked monotonic decrease (Fig. 11, curve c), while the TL initially increases with decreasing particle size and then again decreases below 400 lm. Similar results were obtained for lower exposures also (Fig. 11, curves d–f). TL of KI:KNO3 also show peaks at 370 and 500 K. Variation of TL intensity of KI:KNO3 with particle size is plotted in Fig. 12. A broad maximum is observed in the particle range 425–700 lm beyond which it shows decreasing trend (curve a). In addition TL peak at 500 K is observed for 150–210 and 65–150 lm only. For other particle size only a hump is observed.
A comparison of Figs. 11(a), (b) and 12 shows that the shape of both the curves is more or less similar. It may be noted that maximum intensity of TL peak for KI is about 5 103 (arb. units) while for KI:KNO3 it is about 8 (arb. units) only. Obviously, it means in case of KI the concentration of F and V centres is much more compared to doped KI. Maximum TL intensity is observed for particle size range 300–500 lm and the maximum concentration of I 2 corresponding to 390 nm absorption is also obtained for similar particle size range of 250–355 lm (Fig. 3(a)), a somewhat overlapping range. This can be understood as TL due to recombination of F and V centres. While the concentration of the former continuously decreases with particle size and that of later increases up to 300 lm. The maxima in TL intensity will correspond to a maxima in the product of concentration of F and V type centres. This indicates that below 300 lm, though more V type centres are produced, some of them are in aggregated state and hence a decrease in intensity of 390 nm band is observed below 300 lm. Upon dissolution the aggregates may break ultimately forming I 3 concentration corresponding to particle size of 100 lm. Thus I 3 concentration measured at 354 nm indicates increase up to 100 lm. In KI:KNO3 , 354 nm band will not be a measure of all V type centres as there will be some V type centres involving NO 3 as well. Hence Fig. 3(a) and (b) cannot be compared.
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Change in the deformation and irradiation sequence results in changes in the manifestation of deformation effects. In optical studies, it has been found that, if the crystals are coloured first and then crushed into powders then these powders show predominantly F centre colouration (blue). The crushed powders exposed to c-rays, on the other hand, show predominant tri-iodide colouration (brown). Effect of changing the crushing–– irradiation sequence on TL measurements was also studied. Fig. 13 shows the TL glow curves of KI powders obtained by crushing the irradiated (490 C/kg) single crystals. Again, we see two peaks at around 370 and 500 K. The former decreases with decreasing particle size (Fig. 14). The later decreases initially but increases again for fine powders (Fig. 15). This is understandable in view of the fact that deformations are known to favour high temperature peaks in alkali halides [41]. Thus, some colour centres will be bleached mechanically during crushing and overall TL will decrease with decreasing particle size. However, the deformations intro-
Fig. 14. The height of the 370 K peak in KI powder obtained by crushing irradiated single crystals for various exposures: curve a, 490 C/kg; curve b, 390 C/kg; curve c, 195 C/kg; curve d, 39 C/kg.
Fig. 15. The height of the 500 K peak in powders obtained by crushing irradiated single crystals for various exposures: curve a, 490 C/kg; curve b, 390 C/kg; curve c, 195 C/kg, curve d, 39 C/kg.
Fig. 13. The TL glow curves of KI powders obtained by crushing the single crystals exposed to 490 C/kg: (a) single crystal; (b) 850–1000 lm; (c) 710–850 lm; (d) 500–710 lm; (e) 425–500 lm; (f) 355–425 lm; (g) 300–355 lm; (h) 250–300 lm; (i) 210–250 lm.
duced during crushing will favour the 500 K peak. Hence the 370 K peak decreases rapidly with decreasing particle size while the 500 K peak shows an increase for finer size. Similar results were obtained for lower exposures also (Fig. 13(b)–(d)).
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Thus we find that the colouration characteristics of KI powders and single crystals are different. Although the relative heights of glow peaks change with decreasing particle size, the overall TL intensities are not correlated with the changes observed in optical spectra of single crystals and powders. Since F centres are the only electron centres in pure KI responsible for TL, it was expected that the decrease in F centre concentration would be reflected in the corresponding TL intensities.
4. Conclusions Based on the results presented here, it is found that in c-irradiated potassium halides optical absorption spectral, TL and LL measurements intensity depends on the particle size due to radiolytic products. TL and LL intensities cannot be directly correlated to the colour centre concentration. LL intensity depends on the colour centre concentration or dissolution rate but it depends on both factors simultaneously. Formation of I increases with decreasing particle size. 2 However, for KI it decreases beyond 100 lm due to V centre aggregation. It has been shown that I 2 and I 3 species are formed in KI but formation is decreased by 100 times in KI:KNO3 . This has been explained due to increased F-H recombination resulting from non-mobility of H centres because of presence of NO 3 hurdles. Such a study is helpful not only in giving information for LL dosimetry, TL dosimetry, but will also definitely add to our knowledge of defect interaction in general and solids in particular. On the other hand TL, LL, optical absorption gives information about the defect correlation, stability, ionization, recombination in microcrystalline powder.
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Lyoluminescence, thermoluminescence and photodecomposition in microcrystalline powder of KCl, KBr, KI and KI:KNO3 crystals S.J. Dhoble
a,*
, P.M. Bhujbal b, N.S. Dhoble c, S.V. Moharil
b
a
Kamla Nehru College, Sakkardara Square, Nagpur 440 009, India Department of Physics, Nagpur University, Nagpur 440 010, India Sevadal Women’s Science College, Sakkardara Square, Nagpur 440 009, India b
c
Received 5 April 2001; received in revised form 25 September 2001
Abstract Particle size effect in c-radiolysis of KCl, KBr, KI and KI:KNO3 single crystal and microcrystalline powders has been studied using optical absorption, thermoluminescence and lyoluminescence (LL) techniques. Colouration characteristics in KCl, KBr microcrystalline powders are reported to be different from KI and KI:KNO3 for various particle sizes. F centre concentration decreases with decreasing particle size in KCl and KBr. While in KI and KI:KNO3 , V centre concentration increases, on the other hand F centre concentration decreases with decreasing particle size. Rate of formation of I 3 is 100 times less in KI:KNO3 as compared to KI crystal and powders. This is attributed to the higher rate of F-H recombination due to the impurity ions in KI:KNO3 . Colour centre concentration, particle size and dissolution rate, all three factors affect LL intensity together. The results presented here may be considered as of only academic interest in solid state materials. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Colour centres in alkali halides have now been studied for 50 years. It is known that the colouration produced by electrolysis is very stable [1]. In the last two decades, we have shown [2–6] that the colouration in microcrystalline powders obtained by crushing the electrolytically coloured single crystal is not stable. In case of potassium halides, all the colouration was lost within a day [7] and Deshmukh and his co-workers established a cor-
*
Corresponding author.
relation between microhardness, dislocation mobility and the stability of colouration in microcrystalline powders [2–6]. Recently a theoretical approach has been made to the stability of colour centres in microcrystalline powders of alkali halides [8]. The emission of visible glow during the dissolution of irradiated crystal, known as lyoluminescence (LL), was first reported in sodium chloride by Westermark and Grapinyieser [9]. Later on alternative mechanisms proposed for the emission by Annstrom [10] and Arnikar et al. [11] also supported the formation of hydrated electron (eaq ) as the first step during dissolution. The
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 4 7 7 - 9
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combination of eaq with the hole at the V type centre lead to the emission.
X þ e aq ! ðXaq Þ ! Xaq þ hm
where X ¼ Cl , Br , I etc. In recent years some interesting work on LL has been done. Mahapatra showed the effect of solvent temperature in the range 10–90 °C on LL yield in saccharides [12], measurement of low radiation dose by LL techniques [13] and decay times, LL in c-irradiated saccharide [14]. In 1997, Galand showed the LL in alkali halides, carbonates, phosphate and concluded that the incorporation of bivalent earth alkali and transition metal cations in KCl leads to an enhancement of LL [15] and LL sensitisation of carbohydrates and amino acids [16]. Chandra explained theoretical and experimental qualitative correlation in LL of alkali halide [17]. Colour centres have mostly been studied in single crystals, while applications such as dosimetry, of the ionizing radiation using thermoluminescence (TL) and LL more often involves measurements on powders. The application of LL to radiation dosimetry was initiated by Ettinger and co-workers [18]. It is generally believed that the mechanism of colour centres production is similar for single crystal and microcrystalline powder. Colour centres production by c-irradiation in NaCl, KCl and KBr is reported by Deshmukh and co-workers [19–22] in crystal and microcrystalline powder. While for KI, irradiation results into production of both F and V centres, fine powders of KI develop a brown colour following such c-irradiation which is characteristic of tri-iodide absorption [23]. Pode et al. [24] and Shirke et al. [25] have studied the dependence of iodine liberation on the particle size in irradiated KI. On development of LL dosimetry materials, investigators concentrated on an enhancement in LL intensity, observed in certain flouroscent [26,27], chemiluminescent solutions [28,29], dye lasers [30] and excess colour centres by electrolytic colouration [31]. In this paper we report the dependence of the nature of the radiolysis products, LL and TL on the particle size in potassium halides. Also the effect of doping of KNO3 (1 mol%) in KI is studied for various particle size measurements.
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2. Experimental Single crystals of KCl, KBr and KI were grown from melt using the Czochralski method. G.R. grade powder from Merck was used as starting material. For preparation of KI:KNO3 single crystal desired amount of KNO3 (1 mol%) was added to the melt. It was observed that the growth was possible only in dry atmosphere when the melt was clear. Once the crystals were grown, they were relatively more stable to humidity. Only prolonged exposure to a humid atmosphere (humidity less than 40%) turned them smoky. All the experiments were performed during the dry season when the relative humidity was much below 40%. Crystals of size 5 5 0:5 mm3 were cleaved from the as grown block. These were exposed to c-rays from 60 Co source. Some of the crystals were crushed and particles of the desired size were obtained by sieving. These were also exposed to c-rays. Optical spectra were measured with a CZ VSU 2P spectrophotometer with 45/0 reflectance attachment for the powders. LL was studied with the usual setup of lyoluminescent cell, photomultiplier (Hamamastu 931 B), amplifier and recorder. Either distilled water or distilled water containing luminol (7 104 mol%) was used as the solvent. In all measurements 200 mg of sample and 4 ml of solvent was used. TL glow curve was also studied by a similar setup, the modifications being that the lyoluminescent cell was replaced by a drawer containing a heater plate on which 5 mg of sample was heated each time at linear heating rate of 150 K/min.
3. Results and discussion Optical spectra of KCl, KBr and KI powders exposed to c-rays show the various bands. These were identified as the well known F and V type centres [32]. Intensity of F and V bands decrease with decreasing particle size in KCl and KBr. But for KI optical spectra are different from those of KCl and KBr, which is shown in Fig. 1. Spectra of KCl/KBr are not given as they did not contain any thing new. Fig. 1 shows the optical spectra of KI single crystal and powders exposed to c-rays in
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Fig. 1. Optical spectra of a KI single crystal and fine powders of various sizes exposed to c-rays for various particle sizes: curve 1, single crystal; curve 2, 850–1000 lm; curve 3, 710–850 lm, curve 4, 425–500 lm; curve 5, 300–355 lm; curve 6, 250– 300 lm; curve 7, 210–250 lm.
the range 400–900 nm. A prominent F band peaking at 710 nm is seen for the single crystal. For powders the Kubelka–Munk function F ðRÞ ¼ ð1 R2 Þ=2R, where R is the reflectance, is plotted as a function of wavelength. A prominent F band is again seen at around 710 nm. With decreasing particle size intensity of this band decreases. The absorption in the violet region also increases at the same time. This is the tail of the tri-iodide absorption band peaking at 354 nm [33,34]. It is thus seen that, with the decreasing particle size, the nature of the ‘radiolysis’ product changes. The F centre concentration decreases while the tri-iodide centre concentration increases. These changes are so pronounced that they can be observed ‘‘visually’’. The single crystal develops a blue colour while the powders assume a brownish tinge. The difference between the ‘‘radioloysis’’ products was observed when powders were exposed to c-rays. If the crystals are coloured and then crushed into powders, then these powders contained F centres and no predominance of the triiodide absorption was found. Even in this case, there is a difference between the behaviour of F centres in crystals and powders. F centres in single
Fig. 2. Reflectance spectra of microcrystalline powders of KI:KNO3 exposed to c-rays for various particle sizes: (1) 850– 1000 lm; (2) 500–707 lm; (3) 300–420 lm; (4) 210–250 lm; (5) 150–210 lm; (6) 63–150 lm. Also shown are the spectra of KI for particle sizes (a) 707–850 lm and (b) 425–500 lm.
crystal are stable while those in powders decay within 24 h [5]. Reflectance spectra were recorded for c-irradiated microcrystalline powder of KI:KNO3 (Fig. 2). Similar to KI, bands at 685 nm, 390 nm corresponding to formation of F centres and I 2 respectively [35] are seen. Yet another band of lesser intensity at 490 nm is observed which may be attributed to formation of I2 [36]. It appears that in KI doped with KNO3 , F centre formation is very weak compared to KI and also the band at 685 nm is observed for some coarse particles only. The absorption spectra of KI:KNO3 in solution indicate bands at 354 nm corresponding to I 3 and at 390 nm (I ) as well. Variations of intensities of 2 354 and 390 nm bands with particle size for KI and KI:KNO3 are shown in Fig. 3(a) and (b) respectively. In case of KI (Fig. 3(a)) it is observed that intensities of both bands first increase with decreasing particle size up to 100 and 350 lm respectively and then decrease. The increase in
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Fig. 3. Variation of optical density with particle size in (a) KI and (b) KI:KNO3 .
intensity is due to the ease of radiation induced decomposition with increasing surface area. However, with further decrease in particle size, V centre aggregates are produced resulting in decrease in intensity. In Fig. 3(b) it is observed that intensity of 390 nm band increases with decreasing particle size while for 354 nm it decreases up to 100 lm and then increases slightly. Surprisingly a minimum is observed at the same particle size of 100 lm in KI. KNO3 at which a maximum is observed in case of KI for 354 nm band (Fig. 3(a)). The reduction in intensity is attributed to the presence of NO 3 or its radiolytic product NO [37]. In either case inter2 action with I may take place in such a way so as 2 to decrease the rate of formation of I . With de3 creasing particle size the reaction becomes prominent thereby giving still smaller rate of formation of I 3 . This fact becomes further evident from Fig. 4, which shows variation of GðI 3 Þ with particle size for KI and KI:KNO3 . With decreasing particle size GðI 3 Þ values also increase and decrease for KI and KI:KNO3 (upto 100 lm) respectively. Further G values of KI are higher by two orders of magnitude compared to KI:KNO3 suggesting higher rate of formation of I 3 in the former case. Fig. 5 shows the rate of formation of I 2 in KI and KI:KNO3 powders at different c-exposures. It is observed that in KI (curve a) formation of I 2 increases up to 2840 C/kg, after which it decreases
Fig. 4. Variation of GðI 3 Þ values with particle size in KI and KI:KNO3 .
due to aggregation of V centres which are formed more at higher exposures. On the other hand F centre formation (curve c) occurs at a minimum exposure of 1550 C/kg1 and then its concentration increases linearly with exposure. In KI:KNO3 , both the rates of formation of F and V centres (curve b) are very weak compared to KI powders. This may arise due to doping of KI with NO 3 impurities which interrupts the [1 1 0] replacement sequence resulting into higher F-H recombination
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Fig. 5. Variations of rate of formation of I 2 species in KI and KI:KNO3 powders with of c-exposures: (a) KI, (b) KI:KNO3 and (c) F centre formation in KI.
immediately after creation. This is schematically represented in Fig. 6. Hayes and Nichls [38] have shown that more V type centres are produced with [1 0 0] symmetry axis in KCl and KBr crystals containing divalent impurities. In the present case after doping KI with KNO3 some I lattice posi tions are occupied by NO 3 . Since NO3 (ionic ra ) dius 2.19 A) is larger than I (ionic radius 2.06 A
the whole lattice may get somewhat compressed obstructing the mobility of radiation damage products. After c-irradiation NO 2 are formed or even otherwise the replacement sequence of X 2 will get interrupted resulting in increase in F-H recombination immediately after creation. Only a small concentration of stable defects like F and V type centres remains. Consistent with the observations of Atari [39], we found that addition of a small quantity of luminol increased the LL intensity by order of magnitudes. It was found that the optimum luminol concentration was 7 104 mol% and the pH of the solvent 12. Fig. 7 shows the typical LL glow curve for KBr powder exposed to c-rays with water containing luminol as a solvent. It is seen that for the particle size 1000–300 lm the band width of the glow curve is large and it decreases with the decreasing particle size. Below 45 lm a sharp glow curve was observed with respect to intensity vs time. Glow curves show that the width of the LL glow peak depends on the dissolution rate of particles in solvent and LL intensity on the particle size of crystal and F and V colour centre concentration. On other hand with decreasing
Fig. 6. Schematic representation for (a) formation of I 2 centres in normal KI crystals (b) NO3 doped KI crystal and (c) hurdles created by NO /NO in disrupting the sequence. Adapted from [38]. 3 2
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decrease in LL intensity (due to lesser F and V centres) is seen as expected. But the decrease is only around 50 times less compared to 1000–850 lm. This indicates that LL intensity also depends on the rate of dissolution of the microcrystalline powder. Thus emission of light during dissolution depends more on the rate of dissolution, than on particle size. As per Fig. 7, particle size goes on decreasing, the band width of the LL glow curve decreases. As colour centre concentration decreases, LL intensity decreased but at the same time due to decreasing particle size rate of dissolution enhanced, hence LL intensity is increased. Band width of LL glow curve depends on particle size and LL intensity depends on colour centre concentration as well as particle size. For KCl/KI/ KI:KNO3 as the LL glow curve is similar to KBr, it is not shown here. Fig. 8 shows the variation of LL intensity with particle size of c-rays exposed KBr, KCl, KI and KI:KNO3 powders with water containing luminol used as solvent. Results show the LL intensity decreases from particle size of 1000 to 425 nm due to decreases in F and V centre concentration in
Fig. 7. The LL glow curves of KBr powder with water containing luminol (7 104 mol%) as solvent exposed to 490 C/kg for various partide sizes: (a) 850–1000 lm; (b) 710–850 lm; (c) 500–710 lm; (d) 425–500 lm; (e) 355–425 lm; (f) 300–355 lm; (g) 250–300 lm; (h) 210–250 lm; (i) 120–210 lm; (j) 85–120 lm; (k) 45–85 lm; (l) below 45 lm.
particle size F and V type centre concentration also decreases in KCl and KBr crystals while for KI and KI:KNO3 only F centre concentration decreases, however, V centre concentration increases. For finer particle size (below 200 lm) of KI and KI:KNO3 crystal LL intensity decreases due to aggregation of V type centres. Therefore,
Fig. 8. Variation in LL glow peak heights with particles size of (a) KBr, (b) KCl (c) KI and (d) KI:KNO3 with water containing luminol (7 104 mol%) as solvent (c-rays exposure 560 C/kg).
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KCl and KBr crystal, while in KI and KI:KNO3 only F centre concentration decreases (V centre production rate in coarse particle size is very much smaller). Below the 425 lm up to 150 lm LL intensity increases due to faster dissolution rate of powder in solvent. However, below 150 lm LL intensity decreases in KBr and KCl due to poor radiolysis products, while in KI and KI:KNO3 , LL intensity decreases due to aggregation of V centres and F centres concentration is nil in finer particle size. These results are quite similar to the mechanism proposed by Ahnstrom [10] and Atari [28] that it is due to recombination of hydrated electron and V centre at the water–solid interface and is adequate to explain the above fact as well as higher LL intensity is observed for coarse particle size. In case of halides irradiated with c-rays then crushed for different particle size, high LL intensity is observed for coarse particle size, showing decrease with the decreasing particle size up to around 250 lm (Fig. 9). In KI and KI:KNO3 as F centre production is less as seen from optical spectra of KI and KI:KNO3 , LL intensity is found to be less by 10 orders of magnitude compared to KBr and KCl for coarse particle size in KI and 100
times less in KI:KNO3 powders. Below 325 lm F centre concentration decreases in KCl, KBr, KI and KI:KNO3 , thus LL intensity shows decrease due to less radiolysis products. From the above presented results it can be concluded that an enhancement in LL intensity depends on the colour centre concentration in microcrystalline powder. For coarse particle size, formation of radiolysis products is much higher than fine particles, however rate of dissolution is very less in the former than in the latter. The TL in KCl, KBr and KI originates in the radiative recombinations of V type (interstitial halogen) centres with F centres [40]. It is expected that the changes in colouration characteristics of powders in comparison with crystals will be reflected in the TL also. In KCl and KBr powders TL intensity decreases with decreasing particle size while differences in the colouration characteristics of single crystals and powders of KI are observed due to F and V colour centre concentration in powders. Fig. 10 shows the TL glow curves of KI single crystals and fine powders exposed to 490 C/kg. In
Fig. 9. The height of the LL peak in particles obtained by crushing c-rays irradiated single crystals with water containing luminol (7 104 mol%) as solvent (c-rays exposure 560 C/kg): (a) KBr, (b) KCl, (c) KI and (d) KI:KNO3 .
Fig. 10. The TL glow curves of KI powders c-rays exposed to 490 C/kg for various particle sizes: (a) single crystal; (b) 850– 1000 lm; (c) 710–850 lm; (d) 500–710 lm; (e) 425–500 lm; (f) 355–425 lm; (g) 300–355 lm; (h) 250–300 lm; (i) 210–250 lm.
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Fig. 12. Variation of TL glow peak heights with particle size of KI:KNO3 crystal exposed to c-rays (490 C/kg) for (a) 370 K peak and (b) 500 K peak. Error bars are not shown on curves to maintain clarity.
Fig. 11. Variation in glow peak heights with particle size of KI crystals. The heights of the 370 K (curve a) and 500 K (curve b) peaks are plotted as a function of particle size for an exposure of 490 C/kg. A similar variation in the Kubelka–Munk function F ðRÞ at the peak of the F band is plotted in curve c. The heights of the 370 K peak for 390 C/kg (curve d), 195 C/kg (curve c) and 39 C/kg (curve f) are plotted. Error bars are not shown on curves d and e to maintain clarity.
all the glow curves, two glow peaks at around 370 and 500 K can be seen. With decreasing particle size we find that the height of the 370 K peak increases up to 425–500 lm (Fig. 11, curve a) and then decreases. A more or less similar behaviour was observed for the 500 K peak (Fig. 11, curve b). In the context of Fig. 7, it is surprising to note that there is no large decrease in TL intensities with decreasing particle size. The F centre concentration shows a marked monotonic decrease (Fig. 11, curve c), while the TL initially increases with decreasing particle size and then again decreases below 400 lm. Similar results were obtained for lower exposures also (Fig. 11, curves d–f). TL of KI:KNO3 also show peaks at 370 and 500 K. Variation of TL intensity of KI:KNO3 with particle size is plotted in Fig. 12. A broad maximum is observed in the particle range 425–700 lm beyond which it shows decreasing trend (curve a). In addition TL peak at 500 K is observed for 150–210 and 65–150 lm only. For other particle size only a hump is observed.
A comparison of Figs. 11(a), (b) and 12 shows that the shape of both the curves is more or less similar. It may be noted that maximum intensity of TL peak for KI is about 5 103 (arb. units) while for KI:KNO3 it is about 8 (arb. units) only. Obviously, it means in case of KI the concentration of F and V centres is much more compared to doped KI. Maximum TL intensity is observed for particle size range 300–500 lm and the maximum concentration of I 2 corresponding to 390 nm absorption is also obtained for similar particle size range of 250–355 lm (Fig. 3(a)), a somewhat overlapping range. This can be understood as TL due to recombination of F and V centres. While the concentration of the former continuously decreases with particle size and that of later increases up to 300 lm. The maxima in TL intensity will correspond to a maxima in the product of concentration of F and V type centres. This indicates that below 300 lm, though more V type centres are produced, some of them are in aggregated state and hence a decrease in intensity of 390 nm band is observed below 300 lm. Upon dissolution the aggregates may break ultimately forming I 3 concentration corresponding to particle size of 100 lm. Thus I 3 concentration measured at 354 nm indicates increase up to 100 lm. In KI:KNO3 , 354 nm band will not be a measure of all V type centres as there will be some V type centres involving NO 3 as well. Hence Fig. 3(a) and (b) cannot be compared.
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Change in the deformation and irradiation sequence results in changes in the manifestation of deformation effects. In optical studies, it has been found that, if the crystals are coloured first and then crushed into powders then these powders show predominantly F centre colouration (blue). The crushed powders exposed to c-rays, on the other hand, show predominant tri-iodide colouration (brown). Effect of changing the crushing–– irradiation sequence on TL measurements was also studied. Fig. 13 shows the TL glow curves of KI powders obtained by crushing the irradiated (490 C/kg) single crystals. Again, we see two peaks at around 370 and 500 K. The former decreases with decreasing particle size (Fig. 14). The later decreases initially but increases again for fine powders (Fig. 15). This is understandable in view of the fact that deformations are known to favour high temperature peaks in alkali halides [41]. Thus, some colour centres will be bleached mechanically during crushing and overall TL will decrease with decreasing particle size. However, the deformations intro-
Fig. 14. The height of the 370 K peak in KI powder obtained by crushing irradiated single crystals for various exposures: curve a, 490 C/kg; curve b, 390 C/kg; curve c, 195 C/kg; curve d, 39 C/kg.
Fig. 15. The height of the 500 K peak in powders obtained by crushing irradiated single crystals for various exposures: curve a, 490 C/kg; curve b, 390 C/kg; curve c, 195 C/kg, curve d, 39 C/kg.
Fig. 13. The TL glow curves of KI powders obtained by crushing the single crystals exposed to 490 C/kg: (a) single crystal; (b) 850–1000 lm; (c) 710–850 lm; (d) 500–710 lm; (e) 425–500 lm; (f) 355–425 lm; (g) 300–355 lm; (h) 250–300 lm; (i) 210–250 lm.
duced during crushing will favour the 500 K peak. Hence the 370 K peak decreases rapidly with decreasing particle size while the 500 K peak shows an increase for finer size. Similar results were obtained for lower exposures also (Fig. 13(b)–(d)).
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Thus we find that the colouration characteristics of KI powders and single crystals are different. Although the relative heights of glow peaks change with decreasing particle size, the overall TL intensities are not correlated with the changes observed in optical spectra of single crystals and powders. Since F centres are the only electron centres in pure KI responsible for TL, it was expected that the decrease in F centre concentration would be reflected in the corresponding TL intensities.
4. Conclusions Based on the results presented here, it is found that in c-irradiated potassium halides optical absorption spectral, TL and LL measurements intensity depends on the particle size due to radiolytic products. TL and LL intensities cannot be directly correlated to the colour centre concentration. LL intensity depends on the colour centre concentration or dissolution rate but it depends on both factors simultaneously. Formation of I increases with decreasing particle size. 2 However, for KI it decreases beyond 100 lm due to V centre aggregation. It has been shown that I 2 and I 3 species are formed in KI but formation is decreased by 100 times in KI:KNO3 . This has been explained due to increased F-H recombination resulting from non-mobility of H centres because of presence of NO 3 hurdles. Such a study is helpful not only in giving information for LL dosimetry, TL dosimetry, but will also definitely add to our knowledge of defect interaction in general and solids in particular. On the other hand TL, LL, optical absorption gives information about the defect correlation, stability, ionization, recombination in microcrystalline powder.
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