- Email: [email protected]
On luminescence properties of CsI crystals scavenged by Mg2+
Materials Letters 65 (2011) 2416–2418
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
On luminescence properties of CsI crystals scavenged by Mg 2+ V.L. Cherginets a,⁎, T.P. Rebrova b, Yu.N. Datsko b, V.F. Goncharenko b, N.N. Kosinov b, V.Yu. Pedash b a b
Kharkov Karazin National University, Svobody Sq., 4 Kharkov, 61077, Ukraine Institute for Scintillation Materials, National Academy of Sciences of Ukraine, Lenin Avenue, 60 Kharkov, 61001, Ukraine
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 23 November 2010 Accepted 28 April 2011 Available online 5 May 2011
Features of radioluminescence spectra and scintillation light decay curves of CsI crystals grown from the melt preliminarily scavenged by different amounts of Mg2+ (MgCl2) are reported. The scintillation light of CsI usually contains 3 components with the decay time constants equal to 7 ns (I), 30 ns (II) and 2 μs (III). The addition of Mg 2+ amount equivalent to the total concentration of oxide ions in molten CsI results in complete destruction of carbonate and sulfate ions. This is confirmed by the disappearance of 370–550 nm band in the radioluminescence spectrum of CsI crystal; thereat the fraction of component III in CsI scintillation light decreases. The excessive in relation to O2− concentration amounts of Mg2+ lead to the decrease of O2− equilibrium molality in CsI melt from 10 − 3 to 10 −10 mol kg− 1. That causes a considerable increase of fraction of component I as compared with component II from 0.63:0.35 to 0.88:0.07. The fraction of the slow component III approaches the minimum at the equivalent ratio of initial molalities of Mg 2+ and O2−. © 2011 Elsevier B.V. All rights reserved.
Keywords: Optical materials Melt scavenging Radioluminescence Scintillation light decay
1. Introduction
2. Experimental
Single crystals based on alkali metal halides are widely used for the detection of ionizing irradiation in various scientific and engineering applications. Their quality is dependent on the purity of the growth melts. The presence of oxoanions results in decrease of their radiation strength and reduction of the light yield [1]. Therefore, the problem of removal of these admixtures from the melts is urgent enough. Owing to relatively low light yield (0.06 with respect to NaI:Tl) and efficiently high radiation strength, undoped CsI is an excellent detecting material for the monitoring of power fluxes of high-energy particles. However, the presence of CO32 − in CsI crystals makes them hygroscopic. The absorption of moisture is accompanied with the formation of hydrocarbonate and hydroxide ions in the bulk of the crystal:
CsI (Aldrich, 99.999%) with the total molality of O 2− in the melt equal to 5∙10 − 4 mol kg − 1 was used. The molality of O 2− was determined by the titration procedure [2]. Anhydrous MgCl2 was prepared by drying MgCl2·6H2O + 2NH4Cl mixture (both components were of reagent quality) in vacuum at careful elevation of the temperature (30–40 °C/h) to 500 °C followed by melting of MgCl2. Mg 2+-doped CsI single crystals were grown by the Bridgman– Stockbarger method; MgCl2 concentration in CsI melt varied from 5∙10 − 4 to 10 −2 mol kg − 1. The crystal growth was performed in sealed ampoules in vacuum. The melt containing MgCl2 was kept for 12 h to provide complete running of chemical reactions. The temperature at the diaphragm was 630 °C and the bottom zone was without heating. The rate of ampoule pulling down was 7 mm/h. The detectors with a diameter of 12 mm and height of 20 mm were cut from the ingot. The radioluminescence spectra were obtained under 59.6 keV 241Am excitation using MDR-23 monochromator additionally equipped by PMT-100 and PMT-86 photo multiplier tubes. The signal obtained from the PMT is registered after preliminary amplification. The decay curves were obtained by such a manner. The studied sample was placed directly on photocathode of PMT XP 9822QB without optical contact. Other surfaces of the sample were coated by 3 layers of TETRATEC film. The measurements of the pulse shape were performed using the natural background irradiation without additional γ-ray sources for some hours. Thereat the events of enough for the registration intensity by LeCroy WaveSurfer 422 oscilloscope took place sometimes per minute. The summary pulse was considered as the scintillation light of the studied crystal.
2−
CO3
−
−
+ H2 O = HCO3 + OH :
ð1Þ
The formed ions undergo radiolysis that worsens the working parameters of CsI (transparency) and reduces their life time. The purpose of the present work is to study the effect of Mg 2+ ions on the luminescence parameters of CsI crystals.
⁎ Corresponding author. E-mail address: [email protected] (V.L. Cherginets). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.04.097
V.L. Cherginets et al. / Materials Letters 65 (2011) 2416–2418
2417
3. Results and discussion Introduction of cations in the melts is an effective way for destruction of different oxoanions, e.g., carbonate and sulfate ions undergo breaking down: 2−
+ Mg
2−
+ Mg
CO3 SO4
2+
= MgO↓ + CO2 ↑
ð2Þ
2+
= MgO↓ + SO3 ↑:
ð3Þ
These reactions result in the formation of volatile acidic gas and insoluble metal oxide. Completeness of the oxide deposition from melts is dependent on its solubility product: MgO↓ = Mg
2+
2−
+O
; Ks;MgO = mMg2 + mO2−
ð4Þ
where mMg2 + and mO2 − are molalities of Mg 2+ and O 2− in the saturated solution, respectively. MgO possesses Ks,MgO value of about of 10 −12 mol 2 kg − 2 that provides its extremely low solubility in molten salts. Therefore, Mg 2+ should be one of the most effective scavengers of the growth melts. The possibility of cation application for the scavenging before the crystal growth was discussed earlier [3], but there is no information about its practical realization. This way of purification allows O 2− concentration to vary in melts in a very wide range. From the known values of the initial molality of O 2− (5∙10 −4 mol kg − 1) and Mg 2+ in the melt one can estimate the equilibrium molality of O 2− according to the following equation true for the excess of Mg 2+, i.e., m0Mg2+ N m0O2 −: mO2 = 10
−12
0
0
= mMg2 + −mO2− :
ð5Þ
The equivalent addition of Mg 2+ permits to decrease mO2 − down to 10 −6 mol kg − 1 (here pO ≡ − log mO 2 − = 6), and at m0Mg2+ = 0.01 mol kg − 1 mO2 − ≈ 10 − 10 mol kg − 1 (pO = 10). The addition of Mg 2+ to the melt causes precipitation of MgO. As for densities, at 900 K (the melting point of CsI) the density of MgO is approximately equal to 3.5 g cm − 3 that is somewhat greater than the density of molten CsI (3.1 g cm − 3). Near the melting point CsI crystals possess the density equal to 4.4 g cm − 3. Therefore, we never observe the precipitate at the bottom of the growth ampoule since it is displaced by the crystallization front and formed ribs around of the crystallized ingot. During the crystallization the excessive (as compared with O 2−) amount of Mg 2+ is displaced upwards. The top part of the ingot becomes turbid due to crystallization of the components of CsI–MgCl2 reciprocal system. However, CsI crystals doped with 5∙10 − 4 and 10 −3 mol kg − 1 of Mg 2+ are completely transparent owing to negligible excess of Mg 2+. These features make the cation scavenging very appropriate for crystal growth by Czochralski or Kyropoulos since MgO precipitates on the bottom of the growth vessel and the excess of Mg 2+ will be displaced from the growing crystal. The radioluminescence spectrum of pure CsI (Fig. 1) contains two pronounced bands with maxima located in 300–310 nm and 400–450 nm ranges [4]. The first band is the superposition of two components of the scintillation light caused by exciton luminescence with decay time constants equal to 7 (component I) and 30 ns (component II).The second band is due to vacancies and its decay time constant oscillates within 2–3 μs (slow component III). In commercial CsI the slow component is caused by carbonate and sulfate admixtures. The radioluminescence spectrum of commercial CsI without Mg 2+ doping (sample 1) is characterized by two bands with the maxima located at 307 and ~ 450 nm, respectively. Sample 2 contains a small excess of Mg 2+ as compared with O 2−. Its radioluminescence spectrum is characterized by the absence of the band in 370–500 mm range with a maximum at ~ 450 nm that
Fig. 1. Radioluminescence spectra of undoped CsI (1) and CsI with the addition of Mg2+ (mol kg− 1): 2 — 10− 3, 3 — 10− 2. The maxima of the main bands are designated by arrows with the corresponding wavelength (nm).
confirms complete destruction of the above-said oxoanions. The maximum of the first band is shifted to 305 nm. A negligible excess of Mg 2+ in sample 2 protects the growing crystal from the action of oxygen-containing components of the atmosphere. Subsequent increase of Mg 2+ concentration (sample 3) leads to the appearance of the band with a maximum at 420 nm. This is caused by incorporation of two-charged Mg 2+ ion into CsI crystal lattice followed by the formation of cation vacancies. As for the exciton band, its maximum is shifted to 304 nm. The excess of Mg 2+ makes CsI appreciably hygroscopic. The sequential shift of the maximum of the first band from 307 to 304 nm may be explained by the increase of the fraction of component I as compared with that of component II at decreasing O 2− concentration. To perform the quantitative estimation of the mentioned components in the scintillation light, the data on the time dependence of the scintillation light decay were obtained. The scintillation pulse decay curve for CsI presented in Fig. 2 is described by the following equation: −t = 7 −t = 30 −t = 2000 I = I0 ⋅ AI e + AII e + AIII e
ð6Þ
where I is the scintillation light intensity at the time t (ns); I0, the initial intensity of the scintillation light, AI, AII and AIII are initial fractions of components I, II and III, respectively. These parameters for all the samples are presented in Table 1. More pictorial presentation for the dependence of decay constants vs. pO may be got from Fig. 2b. The decrease of O 2− concentration in the melt results in the increase of AI as compared with AII that makes CsI faster. As for the dependence of AIII vs. pO, one can see that it is dependent on the degree of CsI lattice deformation. Indeed, the crystals grown from the commercial CsI are characterized by AIII values near 0.02 due to the presence of carbonate and sulfate ions in the growth melt. At the equivalent Mg 2+ addition the fraction of AIII component considerably diminishes (down to 0.007), however it does not disappear completely. The most possible reason consists in the presence of admixture cations (Ca 2+, Ba 2+, Na +) in the growth melt that leads to the arise of cation vacancies [5]. Subsequent increase of Mg 2+ concentration leads to the appearance of component III, and AIII increases to 0.05 at pO = 10. Undoubtedly, in the latter case the slow component III arises again owing to Mg 2+ action on the CsI lattice. The microsecond component caused by anion admixtures is slower than that caused by cation doping (3 μs against 2 μs, respectively).
2418
V.L. Cherginets et al. / Materials Letters 65 (2011) 2416–2418 Table 1 Parameters of the scintillation light decay in CsI with different concentrations of Mg2+ and O2−. Sample
0 mMg 2+
mO 2−
pO
AI
AII
AIII
1 1a 2 2a 3
0 (pure) 5∙10− 4 1∙10− 3 5∙10− 3 1∙10− 2
5∙10− 4 1∙10− 6 1∙10− 9 2∙10− 10 1∙10− 10
3.3 6.0 9.0 9.7 10.0
0.63 0.74 0.75 0.86 0.88
0.35 0.25 0.24 0.10 0.07
0.02 0.01 0.007 0.04 0.05
(3 μs) (3 μs) (2 μs) (2 μs) (2 μs)
is equivalent to O 2− concentration. Thereat, the ratio of the fractions of components I and II (approximately 3:1) is observed at O 2− concentration corresponding to the congruent MgO solubility in CsI. The addition of 5∙10− 3 mol kg− 1 of MgCl2 to CsI allows to obtain a material with the highest fraction of AI N 0.85. For commercial CsI crystals AI oscillates within 0.6–0.7 and obtained under special conditions CsI is characterized by AI ~ 0.75 [1]. Further modification of undoped CsI is possible by searching for cations with solubility in halide melts lower in comparison with that of MgO. 4. Conclusions Treatment of CsI melt with Mg 2+ causes considerable reduction of concentration of oxygen-containing admixtures and leads to disappearing bands caused by these admixtures from radioluminescence spectrum of CsI crystals and essential reduction (down to 0.007) of fraction of the microsecond component of the scintillation light. Addition of excessive (in respect to total concentration of O 2−) amounts of Mg 2+ to CsI results in an increase of fraction of the fastest for this material component of the scintillation light (decay time constant is 7 ns) to 0.85–0.88. References Fig. 2. Features of scintillation light decay of CsI single crystals scavenged by Mg2+: a) the curve of scintillation light decay for samples of CsI containing 10− 3 mol kg− 1 of Mg2+ (1, gray) and fractions of 7 ns (2, dashed line), 30 ns (2, dotted line) and 2 μs (3, solid line) components. b) The dependence of initial fractions of components of scintillation light of CsI (A) on pO values in CsI growth melt: AI (quadrates), 2 — AII (rings), 3 — AIII (triangles).
From Fig. 2b it follows that Mg 2+-modified CsI single crystals are characterized by better functional properties when the amount of Mg 2+
[1] Kubota S, Sakuragi S, Hashimoto S, Ruan J. A new scintillation material: pure CsI with 10 ns decay time. Nucl Instr Meth 1988;A268:275–7. [2] Rebrova TP, Cherginets VL, Ponomarenko TV. A study of Lux–Flood acid–base reactions in KBr melt at 800 °C. Rus J Phys Chem A 2009;83:1879–82. [3] Cherginets Oxoacidity VL. Reactions of oxocompounds in ionic solvents. Amsterdam: Elsevier; 2005. [4] Balamurugan N, Arulchakkaravarthi A, Selvakumar S, Lenin M, Kumar Rakesh, Muralithar S, Sivaji K, Ramasamy P. Growth and characterization of undoped and thallium doped cesium iodide single crystals. J Cryst Growth 2006;286:294–9. [5] Pushak AS, Myagkota SV, Stryganyuk GB, Voloshinovskii AS, Pashuk IS. Luminescent properties of CsI–Me2+ (Me = Ca, Sr, Ba) single crystals. Abstr. 10th Int.Conf.Inorg. Scintillators and their Applications, Jeju, Korea; 2009. p. 91.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
On luminescence properties of CsI crystals scavenged by Mg 2+ V.L. Cherginets a,⁎, T.P. Rebrova b, Yu.N. Datsko b, V.F. Goncharenko b, N.N. Kosinov b, V.Yu. Pedash b a b
Kharkov Karazin National University, Svobody Sq., 4 Kharkov, 61077, Ukraine Institute for Scintillation Materials, National Academy of Sciences of Ukraine, Lenin Avenue, 60 Kharkov, 61001, Ukraine
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 23 November 2010 Accepted 28 April 2011 Available online 5 May 2011
Features of radioluminescence spectra and scintillation light decay curves of CsI crystals grown from the melt preliminarily scavenged by different amounts of Mg2+ (MgCl2) are reported. The scintillation light of CsI usually contains 3 components with the decay time constants equal to 7 ns (I), 30 ns (II) and 2 μs (III). The addition of Mg 2+ amount equivalent to the total concentration of oxide ions in molten CsI results in complete destruction of carbonate and sulfate ions. This is confirmed by the disappearance of 370–550 nm band in the radioluminescence spectrum of CsI crystal; thereat the fraction of component III in CsI scintillation light decreases. The excessive in relation to O2− concentration amounts of Mg2+ lead to the decrease of O2− equilibrium molality in CsI melt from 10 − 3 to 10 −10 mol kg− 1. That causes a considerable increase of fraction of component I as compared with component II from 0.63:0.35 to 0.88:0.07. The fraction of the slow component III approaches the minimum at the equivalent ratio of initial molalities of Mg 2+ and O2−. © 2011 Elsevier B.V. All rights reserved.
Keywords: Optical materials Melt scavenging Radioluminescence Scintillation light decay
1. Introduction
2. Experimental
Single crystals based on alkali metal halides are widely used for the detection of ionizing irradiation in various scientific and engineering applications. Their quality is dependent on the purity of the growth melts. The presence of oxoanions results in decrease of their radiation strength and reduction of the light yield [1]. Therefore, the problem of removal of these admixtures from the melts is urgent enough. Owing to relatively low light yield (0.06 with respect to NaI:Tl) and efficiently high radiation strength, undoped CsI is an excellent detecting material for the monitoring of power fluxes of high-energy particles. However, the presence of CO32 − in CsI crystals makes them hygroscopic. The absorption of moisture is accompanied with the formation of hydrocarbonate and hydroxide ions in the bulk of the crystal:
CsI (Aldrich, 99.999%) with the total molality of O 2− in the melt equal to 5∙10 − 4 mol kg − 1 was used. The molality of O 2− was determined by the titration procedure [2]. Anhydrous MgCl2 was prepared by drying MgCl2·6H2O + 2NH4Cl mixture (both components were of reagent quality) in vacuum at careful elevation of the temperature (30–40 °C/h) to 500 °C followed by melting of MgCl2. Mg 2+-doped CsI single crystals were grown by the Bridgman– Stockbarger method; MgCl2 concentration in CsI melt varied from 5∙10 − 4 to 10 −2 mol kg − 1. The crystal growth was performed in sealed ampoules in vacuum. The melt containing MgCl2 was kept for 12 h to provide complete running of chemical reactions. The temperature at the diaphragm was 630 °C and the bottom zone was without heating. The rate of ampoule pulling down was 7 mm/h. The detectors with a diameter of 12 mm and height of 20 mm were cut from the ingot. The radioluminescence spectra were obtained under 59.6 keV 241Am excitation using MDR-23 monochromator additionally equipped by PMT-100 and PMT-86 photo multiplier tubes. The signal obtained from the PMT is registered after preliminary amplification. The decay curves were obtained by such a manner. The studied sample was placed directly on photocathode of PMT XP 9822QB without optical contact. Other surfaces of the sample were coated by 3 layers of TETRATEC film. The measurements of the pulse shape were performed using the natural background irradiation without additional γ-ray sources for some hours. Thereat the events of enough for the registration intensity by LeCroy WaveSurfer 422 oscilloscope took place sometimes per minute. The summary pulse was considered as the scintillation light of the studied crystal.
2−
CO3
−
−
+ H2 O = HCO3 + OH :
ð1Þ
The formed ions undergo radiolysis that worsens the working parameters of CsI (transparency) and reduces their life time. The purpose of the present work is to study the effect of Mg 2+ ions on the luminescence parameters of CsI crystals.
⁎ Corresponding author. E-mail address: [email protected] (V.L. Cherginets). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.04.097
V.L. Cherginets et al. / Materials Letters 65 (2011) 2416–2418
2417
3. Results and discussion Introduction of cations in the melts is an effective way for destruction of different oxoanions, e.g., carbonate and sulfate ions undergo breaking down: 2−
+ Mg
2−
+ Mg
CO3 SO4
2+
= MgO↓ + CO2 ↑
ð2Þ
2+
= MgO↓ + SO3 ↑:
ð3Þ
These reactions result in the formation of volatile acidic gas and insoluble metal oxide. Completeness of the oxide deposition from melts is dependent on its solubility product: MgO↓ = Mg
2+
2−
+O
; Ks;MgO = mMg2 + mO2−
ð4Þ
where mMg2 + and mO2 − are molalities of Mg 2+ and O 2− in the saturated solution, respectively. MgO possesses Ks,MgO value of about of 10 −12 mol 2 kg − 2 that provides its extremely low solubility in molten salts. Therefore, Mg 2+ should be one of the most effective scavengers of the growth melts. The possibility of cation application for the scavenging before the crystal growth was discussed earlier [3], but there is no information about its practical realization. This way of purification allows O 2− concentration to vary in melts in a very wide range. From the known values of the initial molality of O 2− (5∙10 −4 mol kg − 1) and Mg 2+ in the melt one can estimate the equilibrium molality of O 2− according to the following equation true for the excess of Mg 2+, i.e., m0Mg2+ N m0O2 −: mO2 = 10
−12
0
0
= mMg2 + −mO2− :
ð5Þ
The equivalent addition of Mg 2+ permits to decrease mO2 − down to 10 −6 mol kg − 1 (here pO ≡ − log mO 2 − = 6), and at m0Mg2+ = 0.01 mol kg − 1 mO2 − ≈ 10 − 10 mol kg − 1 (pO = 10). The addition of Mg 2+ to the melt causes precipitation of MgO. As for densities, at 900 K (the melting point of CsI) the density of MgO is approximately equal to 3.5 g cm − 3 that is somewhat greater than the density of molten CsI (3.1 g cm − 3). Near the melting point CsI crystals possess the density equal to 4.4 g cm − 3. Therefore, we never observe the precipitate at the bottom of the growth ampoule since it is displaced by the crystallization front and formed ribs around of the crystallized ingot. During the crystallization the excessive (as compared with O 2−) amount of Mg 2+ is displaced upwards. The top part of the ingot becomes turbid due to crystallization of the components of CsI–MgCl2 reciprocal system. However, CsI crystals doped with 5∙10 − 4 and 10 −3 mol kg − 1 of Mg 2+ are completely transparent owing to negligible excess of Mg 2+. These features make the cation scavenging very appropriate for crystal growth by Czochralski or Kyropoulos since MgO precipitates on the bottom of the growth vessel and the excess of Mg 2+ will be displaced from the growing crystal. The radioluminescence spectrum of pure CsI (Fig. 1) contains two pronounced bands with maxima located in 300–310 nm and 400–450 nm ranges [4]. The first band is the superposition of two components of the scintillation light caused by exciton luminescence with decay time constants equal to 7 (component I) and 30 ns (component II).The second band is due to vacancies and its decay time constant oscillates within 2–3 μs (slow component III). In commercial CsI the slow component is caused by carbonate and sulfate admixtures. The radioluminescence spectrum of commercial CsI without Mg 2+ doping (sample 1) is characterized by two bands with the maxima located at 307 and ~ 450 nm, respectively. Sample 2 contains a small excess of Mg 2+ as compared with O 2−. Its radioluminescence spectrum is characterized by the absence of the band in 370–500 mm range with a maximum at ~ 450 nm that
Fig. 1. Radioluminescence spectra of undoped CsI (1) and CsI with the addition of Mg2+ (mol kg− 1): 2 — 10− 3, 3 — 10− 2. The maxima of the main bands are designated by arrows with the corresponding wavelength (nm).
confirms complete destruction of the above-said oxoanions. The maximum of the first band is shifted to 305 nm. A negligible excess of Mg 2+ in sample 2 protects the growing crystal from the action of oxygen-containing components of the atmosphere. Subsequent increase of Mg 2+ concentration (sample 3) leads to the appearance of the band with a maximum at 420 nm. This is caused by incorporation of two-charged Mg 2+ ion into CsI crystal lattice followed by the formation of cation vacancies. As for the exciton band, its maximum is shifted to 304 nm. The excess of Mg 2+ makes CsI appreciably hygroscopic. The sequential shift of the maximum of the first band from 307 to 304 nm may be explained by the increase of the fraction of component I as compared with that of component II at decreasing O 2− concentration. To perform the quantitative estimation of the mentioned components in the scintillation light, the data on the time dependence of the scintillation light decay were obtained. The scintillation pulse decay curve for CsI presented in Fig. 2 is described by the following equation: −t = 7 −t = 30 −t = 2000 I = I0 ⋅ AI e + AII e + AIII e
ð6Þ
where I is the scintillation light intensity at the time t (ns); I0, the initial intensity of the scintillation light, AI, AII and AIII are initial fractions of components I, II and III, respectively. These parameters for all the samples are presented in Table 1. More pictorial presentation for the dependence of decay constants vs. pO may be got from Fig. 2b. The decrease of O 2− concentration in the melt results in the increase of AI as compared with AII that makes CsI faster. As for the dependence of AIII vs. pO, one can see that it is dependent on the degree of CsI lattice deformation. Indeed, the crystals grown from the commercial CsI are characterized by AIII values near 0.02 due to the presence of carbonate and sulfate ions in the growth melt. At the equivalent Mg 2+ addition the fraction of AIII component considerably diminishes (down to 0.007), however it does not disappear completely. The most possible reason consists in the presence of admixture cations (Ca 2+, Ba 2+, Na +) in the growth melt that leads to the arise of cation vacancies [5]. Subsequent increase of Mg 2+ concentration leads to the appearance of component III, and AIII increases to 0.05 at pO = 10. Undoubtedly, in the latter case the slow component III arises again owing to Mg 2+ action on the CsI lattice. The microsecond component caused by anion admixtures is slower than that caused by cation doping (3 μs against 2 μs, respectively).
2418
V.L. Cherginets et al. / Materials Letters 65 (2011) 2416–2418 Table 1 Parameters of the scintillation light decay in CsI with different concentrations of Mg2+ and O2−. Sample
0 mMg 2+
mO 2−
pO
AI
AII
AIII
1 1a 2 2a 3
0 (pure) 5∙10− 4 1∙10− 3 5∙10− 3 1∙10− 2
5∙10− 4 1∙10− 6 1∙10− 9 2∙10− 10 1∙10− 10
3.3 6.0 9.0 9.7 10.0
0.63 0.74 0.75 0.86 0.88
0.35 0.25 0.24 0.10 0.07
0.02 0.01 0.007 0.04 0.05
(3 μs) (3 μs) (2 μs) (2 μs) (2 μs)
is equivalent to O 2− concentration. Thereat, the ratio of the fractions of components I and II (approximately 3:1) is observed at O 2− concentration corresponding to the congruent MgO solubility in CsI. The addition of 5∙10− 3 mol kg− 1 of MgCl2 to CsI allows to obtain a material with the highest fraction of AI N 0.85. For commercial CsI crystals AI oscillates within 0.6–0.7 and obtained under special conditions CsI is characterized by AI ~ 0.75 [1]. Further modification of undoped CsI is possible by searching for cations with solubility in halide melts lower in comparison with that of MgO. 4. Conclusions Treatment of CsI melt with Mg 2+ causes considerable reduction of concentration of oxygen-containing admixtures and leads to disappearing bands caused by these admixtures from radioluminescence spectrum of CsI crystals and essential reduction (down to 0.007) of fraction of the microsecond component of the scintillation light. Addition of excessive (in respect to total concentration of O 2−) amounts of Mg 2+ to CsI results in an increase of fraction of the fastest for this material component of the scintillation light (decay time constant is 7 ns) to 0.85–0.88. References Fig. 2. Features of scintillation light decay of CsI single crystals scavenged by Mg2+: a) the curve of scintillation light decay for samples of CsI containing 10− 3 mol kg− 1 of Mg2+ (1, gray) and fractions of 7 ns (2, dashed line), 30 ns (2, dotted line) and 2 μs (3, solid line) components. b) The dependence of initial fractions of components of scintillation light of CsI (A) on pO values in CsI growth melt: AI (quadrates), 2 — AII (rings), 3 — AIII (triangles).
From Fig. 2b it follows that Mg 2+-modified CsI single crystals are characterized by better functional properties when the amount of Mg 2+
[1] Kubota S, Sakuragi S, Hashimoto S, Ruan J. A new scintillation material: pure CsI with 10 ns decay time. Nucl Instr Meth 1988;A268:275–7. [2] Rebrova TP, Cherginets VL, Ponomarenko TV. A study of Lux–Flood acid–base reactions in KBr melt at 800 °C. Rus J Phys Chem A 2009;83:1879–82. [3] Cherginets Oxoacidity VL. Reactions of oxocompounds in ionic solvents. Amsterdam: Elsevier; 2005. [4] Balamurugan N, Arulchakkaravarthi A, Selvakumar S, Lenin M, Kumar Rakesh, Muralithar S, Sivaji K, Ramasamy P. Growth and characterization of undoped and thallium doped cesium iodide single crystals. J Cryst Growth 2006;286:294–9. [5] Pushak AS, Myagkota SV, Stryganyuk GB, Voloshinovskii AS, Pashuk IS. Luminescent properties of CsI–Me2+ (Me = Ca, Sr, Ba) single crystals. Abstr. 10th Int.Conf.Inorg. Scintillators and their Applications, Jeju, Korea; 2009. p. 91.