Characterization by γ-ray diffractometry of the mosaic structure of Bi4Ge3O12, Bi12GeO20 and BaF2 crystals

Nuclear Instruments and Methods in Physics Research North-Holland

A302 (1991) 455-459

455

Characterization by -y-ray diffractometry of the mosaic structure of Bi 4 Ge 3 012 , Bi l2 Ge020 and BaF2 crystals F. Allegretti a, R. Caciuffo h.l, F. de Notaristefani `, F. Ferroni G . Majni h.2, M . Mattioli d arid D. Rinaldi n,z

O. Francescangeli

Dipartimento di Chimica-Fisica ed Elettrochimica, Untversità di Milano, Milano, Itah Dipartimento di Scienze dei Materiah e della Terra, Umversttà di Ancona, Via Brecce Branche, 1-60131, Aricona, Italy Dipartimento di Fivica, Università di Roma La Sapienza, Roma, Italy `~ Is4ttuto Nazionale di Fivtca Nucleare, Sezione di Ronia, Roma, Italv t'

Received

21

November

1990

-y-ray diffraction measurements have been performed to characterize the degree of perfection of several crystals of BGO and barium fluoride . In particular the mosaic structure and homogeneity of samples grown with different techniques have been determined 1 . Introduction High density scintillating monocrystals have recently become popular in high energy physics. Eleven thousand bismuth orthogermanate Bt 4Ge 30i2 (BGO) crystals [1] have been used to build the L3 [2] electromagnetic calorimeter [3] at LEP and the use of barium fluoride (BaF,) [4,5] crystals has been proposed for many experiments to be performed in a near future [6]. The unique property of the BGO lies in its very short emission radiation length (X(, = 1 .13 cm), allowing the construction of very compact calorimeters . Barium fluoride is a high-density inorganic scintillator with three emission spectra to the range 195-310 rim, with decay time constants varying between 0.87 and 600 ns [5,7]. Because of its high resistance to radiation damage and its fast light emission, BaF2 is particularly interesting in view of the construction of high precision electromagnetic calorimeters [8-10] for the next generation of hadron colliders. BGO is also used in medical research as detector in electron-positron tomography . Another important stoichiometric compound in the phase diagram of the system B120,-GeO2 is Bi,,Geo20 , whose great interest is due to the large number of discrete electron trapping levels . Because of this . Bi i2GeO2O is very important for the development of electron optical devices [I I] . For all these purposes, it is absolutely necessary to i INFN, Sezione di Roma, Italy. z INFN, Sezione di Bologna, Italy. 0168-9002/91/$03 .50 9~~ 1991 -

have optically transparent defect-free crystals cut out from a single crystalline bulk . From an industrial point of view, the know-how is not yet consolidated and the production of huge quantities of high quality crystals requires the performance of quality controls . In this article we suggest a quick and reliable method based on -y-ray diffraction to check the degree of monocrystallinity of the samples. This method can be used to supplement other standard acceptance tests of quality. We describe the experimental technique in section 2, experimental results and discussion of the data are presented to section 3.

2. Experimental technique y-ray diffractometry is a powerful technique to perform detailed investigations of the mosaic structure and lattice imperfections in single crystal samples [12-17] and it is now routinely used to test the quality of crystals for neutron or X-ray monochromators. A spectrometer for y-ray diffraction consists of a y-source 198 (usually a thin foil of Au or i37Cs), a variable-aperture lead collimator, a high-angular precision goniometer at the sample position and a Nal(TI) scintillator counter. A schematic view of the experimental setup installed at the Institut Laue-Langevin and used in this work is shown in fig. 1 . A highly monochromatic beam is obtained without the need of monochromators due to the intrinsic linewidth of the source. In the case of i9"Au, the wavelength of the y-ray beam is X = 0.0302 A and

Elsevier Science Publishers B V (North-Holland)

456

F Allegretti et al. / Characterization of mosaic structures by -y-ray diffractometry atoms lie on the (12a) and (48e) special positions,

CESIUM SOURCE

respectively . The lattice parameter at room temperature is ao = 10 .527 Â. Some samples were grown at the

Physical Chemistry Department of the University of Milan by a modified Bridgman method [18,19], capable

of giving large defect-free ingots . Parallelepipeds of dimension 10 X 20 X 60 mm were sawn from the grown boules with the longest dimension parallel to the [100] or

[111] crystallographic direction.

Other specimens

grown with the Czochralski technique and provided by

Crismatec SA, Grenoble, were also investigated in order to reveal possible correlations between crystal quality

and growth technique. The second

compound

studied was the bismuth

germanium oxide Bi j2Ge020 , an n-type photorefractive material crystallizing in the body centred cubic system with space group 123 [20] . The lattice parameter at room

temperature is ao = 10 .145 Â. Its complex crystallographic structure originates a great number of discrete electron trapping levels, making this material very attractive because of its potential use in electron-optical

devices. The samples were prepared using the Czochralski method by Crismatec SA, in the form of paralleleFig. 1 . Schematic view of the experimental setup for -y-ray diffraction installed at the Institut Laue -Langevm, Grenoble .

the wavelength resolution is of the order of 10 -6 while 137 for the Cs source a wavelength X = 0 .01876 k is obtained with a wavelength resolution of the order of 10 -

5

A rocking curve for a given Bragg reflection is obtained by rotating the sample around a vertical axis and recording the diffracted intensity as a function of the rocking angle w with a detector positioned at twice the Bragg angle. Angular divergences smaller than - 30"

pipeds oriented with the longest dimension parallel to the [100] crystallographic axis.

Finally, we have studied some specimens of the fluorite type BaF2 compounds (Fm3m space group, ao = 6.2 A), also provided by Crismatec and grown by the

same method .

The - y-ray rocking curves r(w) were measured in

symmetrical Laue geometry in different volume ele-

ments of the samples with an w step of 3.6". The reflectivity distribution is given by [17] : r(w)= Ptt(w)-P. PT

(in the case of the gold source) or 90" (for the cesium

where PH(w) is the Bragg reflected intensity, PB is the background intensity measured at both sides of the

proportional to the mosaic distribution function [16] . Because of the small Bragg angles involved, broad-

attenuated by the sample, out of the reflecting position . If the measured rocking curve has a full width at half

source) are easily obtained . This usually eliminates the need of deconvolution and the rocking curve is directly

ening of the rocking curve due to lattice strains are not important and only lattice tilt effects are observed .

Moreover, the small Bragg angles make it easier to

perform measurements in the Laue geometry. This is possible because the absorption of y-ray radiation in this energy range is quite small. The low absorption also allows the study of thick crystals or encapsulated samples.

137

Measurements have been performed using the Cs spectrometer of the Institut Laue-Langevin (Grenoble,

France) on different samples of Bi 4Ge 3O12 , Bi 12 GeO2o and BaF2. Bismuth orthogermanate Bi 4Ge 3O12 is a cubic crystal belonging to the 143d space group with four

formula units per unit cell . The Bi atoms occupy the

(16c) position (Wyckoff notation), while the Ge and 0

Bragg peak and PT is the intensity of the incident beam

maximum (FWHM) much larger than the instrument resolution, the crystal is assumed to have a mosaic

structure and the probability function W(w) which describes the angular orientation of the mosaic blocks in the crystal is calculated according to [17] : 1 In ( 1 _ 2r(wj ) W(w,) = 1 Lin l1w (1-2r(wn)) n

(2)

were the sum over n is extended to all experimental points . On the other hand, if the FWHM of the measured

rocking curve is comparable to or smaller than the

457

F Allegretti et al / Characterization of mosaic structures by y-ray diffractometry instrument resolution, the sample could be considered to be almost perfect crystal. In that case the measured rocking curve is given by the convolution of the instrument resolution function with the intrinsic reflectivity [21] . 3. Results and discussion

3

Fig. 2 shows the rocking curves obtained for the [400] reflection of a Bi4Ge3012 sample, grown by the Bridgman technique. Measurements were performed on different volume elements of the sample separated by a distance of 2 cm (the dimensions of the entrance slit, defining the beam cross section, were 10 x 1 mm) . The shape of the rocking curve appears to be quite the same along the crystal showing that the sample is highly homogeneous. Moreover the value of the FWHM, which is of about 90" corresponding to the instrumental resolution, indicates good crystal quality. However, a second feature is visible in the diffraction pattern separated from the main peak by about 143" and characterized by a larger FWHM, of about 100" . This indicates that the sample contains two regions, having an almost perfect structure, the crystallographic planes of which form an angle of 143" . The data have been fitted by two Gaussian lineshapes, the sum of which is represented by the solid curve in fig. 2. The ratio of the integrals of the two Gaussians, which is 6.6, represents an estimate of the volume proportion occupied by the two regions. As the incident beam is scanned along the sample, an increase of the position of the maximum of the rocking curve towards smaller angles is observed . This fact suggests that the sample is cylindrically bent with a curvature radius, estimated from the observed shift, of the order of 50 m.

3

ROCKING

ANGLE x 10 -

ti)(deg)

Fig. 2 Rocking curves obtained for different positions of a Bi 4Ge 3012 sample grown by the Bridgman method and ori ented along the [400] Bragg reflection . The solid lines are the best fit to the experimental data using Gaussian lineshapes.

w U

o

cD z Y O O

0 6 .5

70

75

ROCKING

80 ANGLE

x 10 - ~

85

90

(0 (deg

I

95

Fig. 3. Rocking curves obtained for the [400] Bragg reflection of a B1 4Ge 3012 sample grown with the Bridgman method. The three curves a), b) and c) refer to different volume elements separated by a distance of 2 cm in the direction of the longest axis of the sample . The solid lines are the best fit to the experimental data using Gaussian lineshapes.

Fig. 3 shows the rocking curves obtained from the [400] Bragg reflection of a second Bi 4Ge 3012 sample grown with the same technique. In this case the experimental data may be fitted to two Gaussians lineshapes of FWHM -- 150" with a very small angular separation (8w - 110") as shown in a), b) and c) for three different volume elements . Also this sample is highly homogeneous along its volume . A further example of rocking curves obtained for bismuth orthogermanate crystals grown by the Bridgman method is shown in fig. 4. In this case, the [1111 Bragg reflection was used . The results suggest that the crystal quality of the sample changes over its volume . For some volume elements, the data may in fact be fitted by two Gaussians while in other positions three Gaussian lineshapes are needed . The angular distribution of the mosaic blocks and their angular separation is not constant over the volume . The FWHM range from 187" to 374", while the angular separation between the blocks is of about 450" . Figs . 5 and 6 show the rocking curves obtained for two different volume elements of two Bi 4Ge3012 samples grown with a different technique, namely the

458

F Allegretti et al / Charactenzation of mosaic structures hy -y-rar diffractometri

U

o

U z U O

ROCKING

ANGLE

(0 Ideg]

2 0

x 10 ,

Fig 4. Rocking curves obtained for different positions of a bismuth orthogermanate sample grown with the Bridgman method and oriented along the [111] Bragg reflection The solid lines are the best fit to the experimental data using Gaussian lineshapes

2 4

2 8

ROCKING

3 2

ANGLE x

3 6

4 0

to (deg )

10 -,

Fig 6. Rocking curves obtained at different positions of a B1 4Ge 1 012 sample grown with the Czochralski method and oriented along the [330] direction The two curves refer to different volume elements and the solid lines are the best fit to the experimental data using Gaussian Ilneshapes .

Czochralski method, by Crismatec SA, Grenoble . Fig. 5 refers to a sample oriented along the [400] direction

while fig. 6 refers to the [330] reflection of a different

crystal. In these cases the data may be fitted to a single Gaussian function and the crystals appear to be homogeneous and almost perfect along all their volume (the FWHM is equal to the instrument resolution)

Fig. 7 shows the results obtained for the [400] reflec-

tion of a Bi 12 GeOZo crystal for three different volume

elements . At least three peaks are visible, with FWHM ranging from 85" to 243" . The sample is then com-

posed by two main perfect blocks and one mosaic. region with an overall angular spread of about 406" . The ratio of the larger to the smaller peak area is about

2.7 . Scans over different volume elements reveal that the sample quality is not very homogeneous.

Finally, fig. 8 shows the r(w) distribution measured

for a BaF2 sample using the [220] Bragg reflection . The

3

C Z Y U O

70

74

78

ROCKING

ANGLE x

10 -,

82

86

(A (deg )

Fig 5 Rocking curves obtained at different positions of a B1 4Ge,0,, sample grown with the Czochralski method and oriented along the [400] direction The two curves refer to different volume elements and the solid lines are the best fit to the experimental data using Gaussian lmeshapes.

ROCKING

ANGLE x 10'

0)

(deg)

Fig. 7. Rocking curves obtained at different positions of a B1 12Ge020 sample oriented along the [400] Bragg reflection The solid lines are the best fit to the experimental data by Gausslan lineshapes

459

F Allegretti et al / Characterization of mosaic structures by y-ray dtffractometry Acknowledgements

10 w

It is a pleasure to thank Dr . A. Magerl of the Institut Laue-Langevm who gave us the possibility to use the

8 6

U

4

Z Y U 0

2

-y-ray diffractometer and Drs. A. Boeuf and B. Hamelin for their valuable help during the experiment. Thanks also go to Prof . De Martin of the Istttuto di Strut_y 0

i

0 4

iL 0 8

ROCKING

1 2 ANGLE

x

10 -,

y

i

1 6

tunstica of the University of Milan for his collaboration

I

to the orientation of some crystals .

2 0

The work was supported by INFN (Istttuto Naz-

0.l (deg I

ionale

Fig. 8. Rocking curve measured for a BaF. sample oriented along the [220] Bragg reflection The solid lines are the best fit to the experimental data measured rocking curve has a FWHM of - 131" and remains almost unchanged over the crystal. This result

shows that large BaF2 blocks of suitable quality can be easily grown.

4. Conclusions The data presented in the previous section are obtained by y-ray diffraction measurements performed to characterize the crystalline degree of perfection of several samples (Bi 4Ge30, 2, Bi12Ge020 , BaF2 ) grown with

different

Czochralski) .

techniques

(Bridgman-modified

and

The monocrystallinity and the homogeneity of the

samples investigated are essential for the construction

of very compact calorimeters (Bi4Ge 10, 2 , BaF2) or for electron-optical devices (Bii,Ge020 ).

The results suggest that the Bridgman modified method could be quite attractive for the growth of BGO crystals to be used in the realization of electromagnetic calorimeters for high-energy physics experiments. -y-ray diffraction could be used as a fast method to select

specimens having the same crystal quality and therefore to avoid inhomogeneities in the calorimeter response. The volume proportion occupied by different mosaic

blocks in the sample may be estimated from the area of the different subpeaks .

The study of the effect of radiation damage on the

crystalline quality of these materials is in progress .

di

Fisica

Nucleare)

RICERCHE, Milano.

and

by

ENICHEM

References [1] R Nusche, J. Appl . Phys 36B (1965) 2358 . [21 L3 Collaboration. Letter of Intent (1982) . [3] B. Adeva et al ., Nucl . Instr and Meth . A289 (1990) 35 [41 M.R . Farubhi and C F. Swineheart, IEEE Trans. Nucl Sci. NS-18 (1971) 200. [5] M. Laval et al ., Nucl . Instr. and Meth . 206 (1983) 169. [6] L3 Collaboration, Expression of interest to the Superconducting Super Collider Laboratory (1990) . [7l P Schotanus, C.W E. Van Eqk, R W. Hollander and J Pgpelink, Nucl Instr . and Meth. A259 (1987) 586. [81 S. Malewski and D Anderson, Nucl . Instr. and Meth A241 (1985) 76 [91 S Malewski et al ., Nucl Instr. and Meth . A260 (1987) 373 [10] D.F . Anderson et al , Nucl Instr. and Meth . 228 (1984) 33 [ll] M. Peltier and F. Micheron, J Appl Phys . 48 (1977) 3683 [12] A Freund and J R. Schneider, J. Crys . Growth 13/14 (1972) 247 [13] H. Maier-Leibnitz, J.R Schneider, Proc Adv. Inst . on the Experimental Aspects of X-ray and Neutron Diffraction, Aarhus, Denmark (1972) . [14] A. Freund, Dissertation Technische Universitât Muenchen, Germany (1973) unpublished . [15] J.R. Schneider, J Appl . Cryst 7 (1974) 541 . [16] J R. Schneider, J. Appl . Cryst. 7 (1974) 547 [17] J.R. Schneider, J. Appl . Cryst. 2 (1976) 394. [18] F. Allegretti, R. Riva . B. Borgia, F. de Notaristefam and S Pizzim, J. Cryst. Growth 94 (1989) 373. [19] F. Allegretti and S Pizzini, Nucl . Instr. and Meth . A279 (1989) 402. [20] N . Benlelloun, M Tapiero and J P Zielenger, J. Appl Phys . 64 (1988) 4013 [21] W H. Zachariasen, Theory of X-Ray Diffraction in Crystals (Wiley, New York, 1945)