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The influence of impurities on the quality of bismuth germanate (BGO) scintillator crystals
248
Journal of Crystal Growth 69 (1984) 248 252 North-Holland, Amsterdam
THE INFLUENCE OF IMPURITIES ON THE QUALITY OF BISMUTH GERMANATE (BGO) SCINTILLATOR CRYSTALS R.G.L. BARNES Johnson Mat they Chemicals Ltd., Orchard Road, Royston, Hertfordshire, UK
Received 26 July 1984; manuscript received in final form 29 October 1984
The distribution behaviour of prevalent impurities, including Pt, Fe, Si and Ca, in BGO scintillator crystals is described. The possibility of locally high impurity concentrations in the region of major growth defects has been examined by electron beam microprobe analysis, and has been found not to be significant. Conclusions are drawn about the purity requirements for pre-reacted BGO capable of efficient conversion from melt to single crystal and providing the required scintillation performance.
I. Infroduction The large world requirement for single crystals of bismuth germanate (Bi4Ge3O12, BGO) for use as a scintillator, notably in high energy physics calorimeters, places severe demands on the quality and consistency of the crystal-growing raw material needed on a tonnage scale. In addition, the cost of such a material has to be contained at an acceptable level, and thus it is essential that realistic specifications are set with regards to purity, for example. Pre-reacted BGO of closely controlled stoichiometry has been developed as the best way of meeting these needs, with properly defined impurity limits, The incorporation of an impurity into an ionic crystal grown from the melt, without structural breakdown, is primarily determined by the compatibility of the ionic size and charge of the impurity with those of the host material. Secondary factors include ionicity (or degree of covalency), degree of site packing in the host structure, and the need for charge compensation for an aliovalent ion. The ability of a growing crystal to accommodate a potentially deleterious impurity can therefore vary quite widely, but in general structural integrity is retained in oxides as long as impurities do not exceed a few parts per million by weight (ppm). Once grown, however, single crystals possess per-
formance characteristics which may differ from one to another; these are therefore influenced by impurities at a significantly lower level than is important solely for growth. Such behaviour by doped BGO crystals has already been reported elsewhere [1], and detrimental impurities typically have to be below 1 ppm. The related purity necessary for pre-rea~tedBGO as raw material should depend on the corresponding impurity segregation coefficient and the proportion of the melt which is grown. We have examined the distribution behaviour of a range of prevalent impurities in BGO crystals grown from doped materials, also seeking to establish whether locally high concentrations occur in the region of the “veils” which are known to form fairly readily in BGO crystals. The perturbation from equilibrium growth conditions causing these veils might also cause a serious deviation from regular segregation effects, and would need to be taken into account when subsequently compiling a raw material specification.
2. Experimental Pre-reacted BGO samples doped, respectively, with Al, Ca, Fe, Mg, Pt and Si were synthesized, with the doping levels given in table 1. These were
0022-0248/84/$03.OO © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
R. G.L. Barnes
249
Influence of impurities on quality of BGO
Table I Dopant parameters utilised Dopant
Doping level
Analytical detection limit
Segregation coefficient found
Ionic radius (A) 0.50 0.47 (t) 0.99 0.74(2+) 0.64(3+) 0.66 0.80(2+) 0.65(4+) 0.38 (t)
Al
10 ppm
0.1 ppm
0.15
Ca Fe
I ppm I ppm
0.1 ppm 0.1 ppm
1.0 0.65
Mg Pt
5 ppm 100 ppm
0.1 ppm <1 ppm
0.15 0.03
Si a)
1, 10 ppm (t)
—
0.1 ppm
0.4
a)
[3]
Size mismatch 3~ with Bi
Size with mismatch Ge4~
48% 6% 3% 23% 33% 31% 17% 32% 24%
tetrahedral coordination; otherwise octahedral..
grown into single crystals by the Czochralski process by Metal Crystals Limited, of Harston, UK. Boules of approximately 25 mm diameter were pulled at 3 to 4 mm/h up to approximately 50% of a charge of 600 g. The boules were sectioned, and representative portions of the weighed sections and melt residues were analysed by emission spectrography. The total impurity contents of each growth run agreed closely with the nominal doping levels, and were used to give the initial impurity content of each melt. One section was chosen as exhibiting a distinct veil formation and containing an average of 5 ppm of Pt. This was cut through the veil, and the exposed face was polished and examined by electron beam microprobe analysis, after carbon coating, at the Johnson Matthey Research Centre, Sonning Common, UK. After line scans had failed to reveal any substantial differences across the veiled and unveiled regions, detailed elemental mapping for Bi, Ge and Pt was carried out on a 50 ~sm square section across a veil edge, counting on individual 800 nm locations and storing the data for subsequent processing.
3. Results
extremes), and to take advantage of improved detection sensitivities provided by the JMC Analytical Laboratories at Royston. The analytical data were computed in the form of a cumulative percentage of the total impurity content, which offers a number of advantages. These include: (1) Larger samples can be used than if seeking to analyse a particular spot in the crystal. (2) Replicate analyses on a given section can be carried out to give improved precision. (3) An apparently stepwise increase in impurity content down a boule, with its inherent uncertainties, is obviated. (4) It provides an immediate derivation of con-
100 .
ppm doping ~ cI~fl~
°
• .~
~ ‘~
,‘
60
/
~
20 0
The doping levels had been chosen to provide information in the concentration regions of interest without the need for extrapolation of analytical data (which can be erroneous when reaching into
//
“
•
0 0
20
40
60 80 of melt solidified
°“°
Fig. 1. Distribution of Si in doped BGO.
100
250
R.G.L. Barnes
/ Influence
of impurities on quality of BGO
60 ~.
5~
S log
0)
40
~coI
Ca
Si ~Ca
>
‘
~Mg
0
Fe
10
20
30
40
50
60
1
% of melt solidified Fig. 2. Distribution of impurities in doped BGO.
centration as a fraction of the total for the calculation of segregation coefficients (see below). The analyses in this form are plotted in fig. 1 against the fraction of the melt grown for the two different doping levels of Si. A single curve can be drawn for these, demonstrating no difference in behaviour for an order of magnitude difference in concentration levels. Corresponding curves for all six impurities studied are drawn in fig. 2, with the points omitted for clarity. A straight line with a gradient of I corresponds to a segregation coefficient of 1, since this is the line representing impurity deposition at the same proportionate rate as the host material, Curved lines below this (fig. 2) correspond to a progressive decrease in segregation coefficient. It can be seen that Ca exhibits a segregation coefficient of approximately 1, with all of the others less than 1. The expression [2]
2 0
02
0
log (1 g I Fig. 3. Impurity distribution in doped BGO crystals.
curves in fig. 2. A plot of log (C~/C0)versus log(1 g) is given in fig. 3. The intercepts of the lines at log(1 g) 0 give k, the effective segregation coefficients under the growth conditions used (which are typical of those in current industrial operations). The values of k obtained are included in table 1. The result of the microprobe analysis showed no detectable concentrations of Pt, and no significant variation in the concentration of Bi and Ge and their ratio. —
k(~ I
C~=k0C0(1—g) where C0 is the initial impurity concentration in the whole melt, C~is the impurity concentration in the solid when a fraction g has solidified, and k0 is the equilibrium segregation coefficient, has been adapted such that C~/C0 is the gradient of the
4. Discussion From the linearity observed in fig. 3, the calculated segregation coefficients are shown to be constant over the range of conditions explored. The
R. G.L. Barnes
/
Influence of impurities on quality of BGO
251
Ca will be incorporated into a growing crystal at a B
0
1
•
VS
constant rate, with the degree of rejection of the other impurities in the order Fe < Si
i
4~
vs Ole
0 75
0 50
spect to oxidation.) However, an important consequence of a low segregation coefficient is a rapid
4+ 0 25
M
0
10
20
M 30
2 +
M
3 60
increase in incorporation during a later stage of growth, perhaps to such an extent that structural breakdown will be induced. Such limits of impurity incorporation are also being investigated.
% ionic size mismatch Fig. 4. Dependence of segregation coefficient k on ionic size mismatch in BGO.
5. Conclusions
lower limit for any local concentration of an impurity is equivalent to the order of a 100 nm Pt particle in a 2 ~.tmcube of BGO, from the detection limits of the microprobe equipment. No such concentrations were found in the 3600 locations counted, which sets an upper limit of the order of 30 ppm for averaged local Pt concentrations in a 50 p~msquare section of a veil. This may be compared with the mean Pt concentration determined to be 5 ppm, and hence gross local concentrations are considered not to be associated with the presence of veils. The segregation coefficients determined therefore apply throughout the crystals grown. It is apparent from the values in table 1 that ionic charge is less important than size in determining impurity incorporation in the BGO lattice. A cornparison of the segregation coefficients with the ionic size misfit with the host cations 2~ andis presented Fe3~ are in fig. 4. Values for both Fe shown, with no apparent preference for either, and Al3 + fits well with occupation of the octahedral Bi3 + sites in preference to the tetrahedral Ge4 + sites. The range of segregation coefficients show that
A detailed investigation has been carried out into the distribution behaviour of Al, Ca, Fe, Mg, Pt and Si as impurities in Czochralski-grown BGO. Locally high concentrations associated with the presence of “veils” have been sought by electron beam microprobe analysis, but have not been found. From this and the linearity of results obtamed, segregation coefficients determined from a cumulative treatment of analytical data are held to be valid throughout the grown crystals over the range of growth fractions and impurity concentrations explored. Impurity concentrations in crystal-growing raw materials are generally targeted below the levels acceptable in the grown crystal. In seeking larger and larger melt fractions pulled, impurities with low segregation coefficients, which might be considered as a favourable factor, assume prominence; the pulled fraction necessary for economical working decides the tolerance which should be set then on impurities in the raw material. With the need to keep impurities throughout a grown crystal below, typically, I ppm [1] to ensure scintillation performance, the data presented here will allow sensible limits below that level to be set for C 0 for various working situations.
252
R.G.L. Barnes
Influence of impurities on quality of BGO
Acknowledgements
References
The author wishes to thank E.R. Emerton and A. Razzaq for assistance in the preparation of some of the experimental materials, and D. Fogg for all the analytical work. Valuable discussions with Dr. R. Sims, and helpful suggestions with the manuscript by Dr. M.J.S. Gynane, are also gratefully acknowledged.
[1] R.G.L. Barnes, R. Sims, M.D. Rousseau and M. Sproston, IEEE Trans. Nucl. Sci. NS 31(1984) 249. [2] W.G. Pfann, Zone Melting (Wiley, New York, 1958). [3] From data published in Lange’s Handbook of Chemistry, 12th ed., Ed. J.A. Dean (McGraw-Hill, 1979).
Journal of Crystal Growth 69 (1984) 248 252 North-Holland, Amsterdam
THE INFLUENCE OF IMPURITIES ON THE QUALITY OF BISMUTH GERMANATE (BGO) SCINTILLATOR CRYSTALS R.G.L. BARNES Johnson Mat they Chemicals Ltd., Orchard Road, Royston, Hertfordshire, UK
Received 26 July 1984; manuscript received in final form 29 October 1984
The distribution behaviour of prevalent impurities, including Pt, Fe, Si and Ca, in BGO scintillator crystals is described. The possibility of locally high impurity concentrations in the region of major growth defects has been examined by electron beam microprobe analysis, and has been found not to be significant. Conclusions are drawn about the purity requirements for pre-reacted BGO capable of efficient conversion from melt to single crystal and providing the required scintillation performance.
I. Infroduction The large world requirement for single crystals of bismuth germanate (Bi4Ge3O12, BGO) for use as a scintillator, notably in high energy physics calorimeters, places severe demands on the quality and consistency of the crystal-growing raw material needed on a tonnage scale. In addition, the cost of such a material has to be contained at an acceptable level, and thus it is essential that realistic specifications are set with regards to purity, for example. Pre-reacted BGO of closely controlled stoichiometry has been developed as the best way of meeting these needs, with properly defined impurity limits, The incorporation of an impurity into an ionic crystal grown from the melt, without structural breakdown, is primarily determined by the compatibility of the ionic size and charge of the impurity with those of the host material. Secondary factors include ionicity (or degree of covalency), degree of site packing in the host structure, and the need for charge compensation for an aliovalent ion. The ability of a growing crystal to accommodate a potentially deleterious impurity can therefore vary quite widely, but in general structural integrity is retained in oxides as long as impurities do not exceed a few parts per million by weight (ppm). Once grown, however, single crystals possess per-
formance characteristics which may differ from one to another; these are therefore influenced by impurities at a significantly lower level than is important solely for growth. Such behaviour by doped BGO crystals has already been reported elsewhere [1], and detrimental impurities typically have to be below 1 ppm. The related purity necessary for pre-rea~tedBGO as raw material should depend on the corresponding impurity segregation coefficient and the proportion of the melt which is grown. We have examined the distribution behaviour of a range of prevalent impurities in BGO crystals grown from doped materials, also seeking to establish whether locally high concentrations occur in the region of the “veils” which are known to form fairly readily in BGO crystals. The perturbation from equilibrium growth conditions causing these veils might also cause a serious deviation from regular segregation effects, and would need to be taken into account when subsequently compiling a raw material specification.
2. Experimental Pre-reacted BGO samples doped, respectively, with Al, Ca, Fe, Mg, Pt and Si were synthesized, with the doping levels given in table 1. These were
0022-0248/84/$03.OO © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
R. G.L. Barnes
249
Influence of impurities on quality of BGO
Table I Dopant parameters utilised Dopant
Doping level
Analytical detection limit
Segregation coefficient found
Ionic radius (A) 0.50 0.47 (t) 0.99 0.74(2+) 0.64(3+) 0.66 0.80(2+) 0.65(4+) 0.38 (t)
Al
10 ppm
0.1 ppm
0.15
Ca Fe
I ppm I ppm
0.1 ppm 0.1 ppm
1.0 0.65
Mg Pt
5 ppm 100 ppm
0.1 ppm <1 ppm
0.15 0.03
Si a)
1, 10 ppm (t)
—
0.1 ppm
0.4
a)
[3]
Size mismatch 3~ with Bi
Size with mismatch Ge4~
48% 6% 3% 23% 33% 31% 17% 32% 24%
tetrahedral coordination; otherwise octahedral..
grown into single crystals by the Czochralski process by Metal Crystals Limited, of Harston, UK. Boules of approximately 25 mm diameter were pulled at 3 to 4 mm/h up to approximately 50% of a charge of 600 g. The boules were sectioned, and representative portions of the weighed sections and melt residues were analysed by emission spectrography. The total impurity contents of each growth run agreed closely with the nominal doping levels, and were used to give the initial impurity content of each melt. One section was chosen as exhibiting a distinct veil formation and containing an average of 5 ppm of Pt. This was cut through the veil, and the exposed face was polished and examined by electron beam microprobe analysis, after carbon coating, at the Johnson Matthey Research Centre, Sonning Common, UK. After line scans had failed to reveal any substantial differences across the veiled and unveiled regions, detailed elemental mapping for Bi, Ge and Pt was carried out on a 50 ~sm square section across a veil edge, counting on individual 800 nm locations and storing the data for subsequent processing.
3. Results
extremes), and to take advantage of improved detection sensitivities provided by the JMC Analytical Laboratories at Royston. The analytical data were computed in the form of a cumulative percentage of the total impurity content, which offers a number of advantages. These include: (1) Larger samples can be used than if seeking to analyse a particular spot in the crystal. (2) Replicate analyses on a given section can be carried out to give improved precision. (3) An apparently stepwise increase in impurity content down a boule, with its inherent uncertainties, is obviated. (4) It provides an immediate derivation of con-
100 .
ppm doping ~ cI~fl~
°
• .~
~ ‘~
,‘
60
/
~
20 0
The doping levels had been chosen to provide information in the concentration regions of interest without the need for extrapolation of analytical data (which can be erroneous when reaching into
//
“
•
0 0
20
40
60 80 of melt solidified
°“°
Fig. 1. Distribution of Si in doped BGO.
100
250
R.G.L. Barnes
/ Influence
of impurities on quality of BGO
60 ~.
5~
S log
0)
40
~coI
Ca
Si ~Ca
>
‘
~Mg
0
Fe
10
20
30
40
50
60
1
% of melt solidified Fig. 2. Distribution of impurities in doped BGO.
centration as a fraction of the total for the calculation of segregation coefficients (see below). The analyses in this form are plotted in fig. 1 against the fraction of the melt grown for the two different doping levels of Si. A single curve can be drawn for these, demonstrating no difference in behaviour for an order of magnitude difference in concentration levels. Corresponding curves for all six impurities studied are drawn in fig. 2, with the points omitted for clarity. A straight line with a gradient of I corresponds to a segregation coefficient of 1, since this is the line representing impurity deposition at the same proportionate rate as the host material, Curved lines below this (fig. 2) correspond to a progressive decrease in segregation coefficient. It can be seen that Ca exhibits a segregation coefficient of approximately 1, with all of the others less than 1. The expression [2]
2 0
02
0
log (1 g I Fig. 3. Impurity distribution in doped BGO crystals.
curves in fig. 2. A plot of log (C~/C0)versus log(1 g) is given in fig. 3. The intercepts of the lines at log(1 g) 0 give k, the effective segregation coefficients under the growth conditions used (which are typical of those in current industrial operations). The values of k obtained are included in table 1. The result of the microprobe analysis showed no detectable concentrations of Pt, and no significant variation in the concentration of Bi and Ge and their ratio. —
k(~ I
C~=k0C0(1—g) where C0 is the initial impurity concentration in the whole melt, C~is the impurity concentration in the solid when a fraction g has solidified, and k0 is the equilibrium segregation coefficient, has been adapted such that C~/C0 is the gradient of the
4. Discussion From the linearity observed in fig. 3, the calculated segregation coefficients are shown to be constant over the range of conditions explored. The
R. G.L. Barnes
/
Influence of impurities on quality of BGO
251
Ca will be incorporated into a growing crystal at a B
0
1
•
VS
constant rate, with the degree of rejection of the other impurities in the order Fe < Si
i
4~
vs Ole
0 75
0 50
spect to oxidation.) However, an important consequence of a low segregation coefficient is a rapid
4+ 0 25
M
0
10
20
M 30
2 +
M
3 60
increase in incorporation during a later stage of growth, perhaps to such an extent that structural breakdown will be induced. Such limits of impurity incorporation are also being investigated.
% ionic size mismatch Fig. 4. Dependence of segregation coefficient k on ionic size mismatch in BGO.
5. Conclusions
lower limit for any local concentration of an impurity is equivalent to the order of a 100 nm Pt particle in a 2 ~.tmcube of BGO, from the detection limits of the microprobe equipment. No such concentrations were found in the 3600 locations counted, which sets an upper limit of the order of 30 ppm for averaged local Pt concentrations in a 50 p~msquare section of a veil. This may be compared with the mean Pt concentration determined to be 5 ppm, and hence gross local concentrations are considered not to be associated with the presence of veils. The segregation coefficients determined therefore apply throughout the crystals grown. It is apparent from the values in table 1 that ionic charge is less important than size in determining impurity incorporation in the BGO lattice. A cornparison of the segregation coefficients with the ionic size misfit with the host cations 2~ andis presented Fe3~ are in fig. 4. Values for both Fe shown, with no apparent preference for either, and Al3 + fits well with occupation of the octahedral Bi3 + sites in preference to the tetrahedral Ge4 + sites. The range of segregation coefficients show that
A detailed investigation has been carried out into the distribution behaviour of Al, Ca, Fe, Mg, Pt and Si as impurities in Czochralski-grown BGO. Locally high concentrations associated with the presence of “veils” have been sought by electron beam microprobe analysis, but have not been found. From this and the linearity of results obtamed, segregation coefficients determined from a cumulative treatment of analytical data are held to be valid throughout the grown crystals over the range of growth fractions and impurity concentrations explored. Impurity concentrations in crystal-growing raw materials are generally targeted below the levels acceptable in the grown crystal. In seeking larger and larger melt fractions pulled, impurities with low segregation coefficients, which might be considered as a favourable factor, assume prominence; the pulled fraction necessary for economical working decides the tolerance which should be set then on impurities in the raw material. With the need to keep impurities throughout a grown crystal below, typically, I ppm [1] to ensure scintillation performance, the data presented here will allow sensible limits below that level to be set for C 0 for various working situations.
252
R.G.L. Barnes
Influence of impurities on quality of BGO
Acknowledgements
References
The author wishes to thank E.R. Emerton and A. Razzaq for assistance in the preparation of some of the experimental materials, and D. Fogg for all the analytical work. Valuable discussions with Dr. R. Sims, and helpful suggestions with the manuscript by Dr. M.J.S. Gynane, are also gratefully acknowledged.
[1] R.G.L. Barnes, R. Sims, M.D. Rousseau and M. Sproston, IEEE Trans. Nucl. Sci. NS 31(1984) 249. [2] W.G. Pfann, Zone Melting (Wiley, New York, 1958). [3] From data published in Lange’s Handbook of Chemistry, 12th ed., Ed. J.A. Dean (McGraw-Hill, 1979).