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Development of a horizontally and vertically focused neutron monochromator using stacked elastically bent Si single crystals
Nuclear Instruments and Methods in Physics Research A 482 (2002) 799–805
Development of a horizontally and vertically focused neutron monochromator using stacked elastically bent Si single crystals H. Kimuraa,*, R. Kiyanagia, A. Kojimaa,b, Y. Nodaa, N. Minakawac, Y. Moriic, N. Takesued a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Miyagi 980-8577, Japan b Graduate school of Science and Technology, Chiba University, Yayoi, Inage, Chiba 263-8522, Japan c Advanced Science Research Center, JAERI, Tokai, Ibaraki 319-1195, Japan d Neutron Scattering Laboratory, ISSP, University of Tokyo, Tokai, Ibaraki 319-1195, Japan Received 14 April 2001; received in revised form 20 July 2001; accepted 21 July 2001
Abstract A horizontally and vertically focused monochromator has been developed for the 4-axis neutron diffractometer applied for a single crystal structure analysis. Silicon perfect single crystals are bent elastically in order to focus monochromatic neutrons horizontally. The monochromatic beam can also be focused vertically by stacking the horizontally bent crystals. Tilting motion of each stacked bent crystal is controlled independently by stepping pulse motors for optimizing and reproducing perfectly the vertical focusing at the sample position. The intensity of neutrons ( wavelength monochromatized by the new monochromator increases remarkably, and is comparable to with a 1.57 A ( wavelength. The high tunability of the doubly focusing system that of pyrolytic graphite monochromator with a 2.44 A established in the present study can be adopted easily when obtaining the shorter wavelength of neutrons. r 2002 Elsevier Science B.V. All rights reserved. PACS: 07.85.Jy Keywords: Neutron diffraction; Elastically bent crystal; Independent control; Doubly focused monochromator
1. Introduction Single crystal structure analysis using X-ray is a powerful and popular method for determining the averaged position of atoms in a unit cell. However, in the case that light atoms represented by hydrogen play an important role for physical
*Corresponding author. E-mail address: [email protected] (H. Kimura).
properties such as phase transitions, it is difficult for X-ray to determine accurately the position of light atoms. On the other hand, neutrons can determine their positions due to a high sensitivity even for light atoms, which contributes to a further understanding about the mechanism of the structural phase transitions. Although the principles of single crystal structure analysis using neutron are well established, there are few examples due to the lack of enough neutron intensity to perform the high-quality analysis.
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 7 0 8 - 9
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H. Kimura et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 799–805
To overcome the weak neutron flux, since several decades, real space focusing techniques for monochromators and analyzers have been studied and developed, which increases neutron beam flux at sample position. Vertically focusing monochromators consisting of arrays of mosaic crystals such as pyrolytic graphite (PG) were first applied [1] and have been in routine use till date. On the other hand , horizontal focusing techniques [2] were less common because of a more complicated mechanical design. In all cases of horizontal focusing monochromators, mosaic crystals (PG) were also used to compose the focusing assemblies. However, in recent years, the horizontal focusing by using elastically bent perfect silicon (Si) crystals applied for monochromators has been developed and employed in some neutron beam lines [3,4], which is due to the improvement and sizing down of the focusing device. The advantages of bent perfect crystals compared with the mosaic crystals are the deterministic nature of their ‘‘mosaicness’’ and the variation of wavelength for monochromatic beam by selecting the mirror indices of Bragg reflection. Furthermore in the case of Si crystals, their low cost and standard crystalline quality encourage their employment. Although it was believed that the neutron intensity monochromatized by the bent perfect crystals cannot compete with that of mosaic crystals, recently [4] success was achieved in increasing the intensity comparable to that of mosaic monochromators owing to the development of special horizontal bender. Furthermore, in order to get more neutron flux at the sample position, the horizontally and vertically focused monochromator has been discussed [4] and constructed by stacking the bending devices vertically, and their tilt angles are optimized to be focused on the sample position. However, it was very difficult to focus vertically because the vertical tilt angle for each device was controlled only with one pulse motor using a camshaft mechanism, which is a very popular system for the vertically focusing devices. For the mosaic crystals, reflected beam divergence in the vertical direction is broad (B31 of FWHM), which makes the focusing much easier. On the other hand, in the bent perfect crystals, much finer tuning of the tilting angles is
needed due to the sharpness of the vertical beam divergence. Thus, in the present study, we have tried to develop a new vertically focusing system which can control independently the vertical tilt angle of each stacked bending device. As for the horizontal bender, we utilized the method being developed and applied already to several diffractometers. In this paper the horizontal bender and the newly developed vertical focusing device are introduced in Section 2. The practical optimization of bending curvature for the Si crystal and assembly of the nine vertically stacked bending crystals by using neutron two-axis spectrometer are presented in Section 3. The detailed characters of the monochromatic beam are evaluated in Section 4. Theoretical approaches for the integrated reflecting power in the case of an elastically bent perfect crystal, which can be calculated by using dynamical diffraction theory, should be referred elsewhere [3–6].
2. Bending and vertical focusing device In the preliminary study, Si single crystal plates cut in parallel with (5 5 3) plane were used as a bender crystal, but the reflected intensity of monochromatic neutrons was not enough for applying to the monochromator. Therefore, Si(4 2 2) plates with a size of W100 H10 T5 mm3 were utilized in this study, with intensity being two times larger than that of (5 5 3) reflection. As shown in Fig. 1(a), the elastically bending device for one Si crystal plate is very simple [4], which consists of hard steel parts to hold crystals, piano wire to pull both edges of the crystal, and screw (denoted as Screw-o in Fig. 1(a)) inserted at the bender framework to adjust the rotating (o) angle of the crystal. By squeezing the Screw-T attached to the piano wire, the tension in the piano wire is increased and the crystal is bent homogeneously. The vertical focusing under the horizontal bend is achieved by stacking the bending devices. As mentioned in Section 1, the previous focusing system using camshaft with one stepping pulse motor has not been successful. Therefore in
H. Kimura et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 799–805
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Fig. 1. Schematic drawings of the monochromator: (a) horizontal bending device and (b) vertical tilting system of the stacked bending device.
the present study, the pulse motors are attached for all the stacked devices except for the part located at the center. The schematic drawing of the newly designed focusing system is shown in Fig. 1(b). The nine horizontally focusing devices with the eight pulse motors are stacked vertically in the present work. The whole dimensions of the monochromators are W285 H193 T100 mm3. Simplified structure of the bending device and sizing down of the pulse motor enable us to insert such a complicated mechanism into a monochromator housing.
3. Horizontal bending and assembling the stacked Si crystals In order to optimize the bending curvature of the Si crystal, peak profiles and integrated intensities of Bragg reflection as a function of bending were measured. The experiments were carried out with a triple-axis spectrometer TAS-2, which was used in two-axis mode without analyzer
in the present study, installed at the T2 thermal guide of JRR-3 M at Japan Atomic Energy Research Institute (JAERI). The wavelength of ( by Si (3 1 1) initial neutron was fixed at 1.57 A monochromator. Instead of direct measurements of the bending curvature using micrometers [4], a strain gauge was attached on the largest surface of Si plates to measure the internal strain of the crystals (see Fig. 1(a)). A strain gauge is one of the strain-measuring devices, whose internal resistance (R) increases with increase in the extension of devices due to a strain. An external strain e (DL/ L, where L is the length of the device) can be obtained from e=KSDR/R, where KS corresponds to a sensitivity which depends on the devices. In the present work, we used stainless steel (SUS304) as the strain gauge with KS = 2.1. All the results are summarized in Figs. 2 and 3. Fig. 2(a) shows the strain dependence of the (4 2 2) rocking curves. The intensity increases on increasing the internal strain e, which is due to the reduction of primary extinction effects. Note that the small peak seen on the right side of the main
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H. Kimura et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 799–805
Fig. 2. (a) Rocking curves of Si(422) Bragg reflection and (b) integrated intensity as a function of internal strain e (=DL/L) In the present study, e=300 106 (shown by bold arrows) was applied for all the crystals.
Fig. 3. Rocking curves of Si(4 2 2) Bragg reflection for the nine single crystals (Nos. 1–9) with the internal strain (e=300 106).
peak is not originated from the crystal, but is a characteristic property of thermal neutron beam collimated by T2 neutron guide tube. The integrated intensity as a function of the strain is also plotted in Fig. 2(b). Although the intensity increased with increasing e, it tended to saturate into a constant value around e =300( 10-6). Results indicate that there is almost no gain for intensity at e X 300 (shown by the arrow in Fig. 1(b)). In a preliminary experiment, the maximum strain loaded into the crystal was estimated to be e = 430 where the crystal was broken. As a result, the optimum strain, where the bending curvature should be optimized, was determined to be e =300 (70% of the broken point). For the next step, all the nine Si crystal plates with the strain of e = 300 were stacked vertically and assembled. In this stage, there are two important things. Firstly, the equal strains should be actually applied for all the crystals, which gives the same profile and intensity in diffraction measurements. Secondly, the crystal angle o satisfying the (4 2 2) Bragg condition should be identical for all the stacked crystals. As for the second requirement, we could fulfill it by using Screw-o (see Fig. 1(a)) which can move only one side of horizontal device, corresponding approximately to the o rotation. Fig. 3 shows the rocking curves for all the stacked crystals, whose line widths are about 0.061 of FWHM. The profiles for all the samples are perfectly identical even in their intensities, strongly confirming that the crystals could be bent quite systematically and reproducibly. Furthermore, it also proves that the crystal angle o was ideally tuned within the error of 0.0051. In the previous system of the vertical focusing by using one camshaft, it was a crucial fault that the offset angle for the vertical tilting between one and the other crystals cannot be changed. Therefore neutrons are not focused well when the offset angle is large enough compared with the vertical beam divergence (p0.61 of FWHM in the present thermal guide). However, on the other hand, the present focusing system can change the offset angle to zero by the independent control of tilting angle for each bending device. In fact, the identical intensities shown in Fig. 3 exhibit, that the (4 2 2) reciprocal vectors are
H. Kimura et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 799–805
perfectly concordant along not only the horizontal but also the vertical direction for all the crystal plates.
4. Characters of the doubly focused monochromatic beam The Si(4 2 2) horizontally bent monochromator with vertical stacking of the bending devices was installed in T2-2 beam port [7] on T2 thermal guide of JRR-3 M at JAERI. A schematic sketch of the T2-2 configuration is given in Fig. 4. In this beam port, as seen in Fig. 4, three fixed scattering angles for the monochromator (2yM) of 42.61, 901 and 113.21 are available. In the present study, 2yM was selected at 901 where the monochromatic beam has ( since the wavelength a wavelength of 1.57 A, distribution of neutron flux in the T2 thermal ( guide tube has a maximum around 1.5 A. The tuning of the monochromator at the T2-2 beam port was performed by the following procedures. A cadmium plate with a square hole of 20 20 mm2 was mounted on the sample position. First of all, the center plate of Si monochromator was adjusted by the tilting mechanism of the goniometer head of the monochromator. Then, the vertical tilting angles of the eight Si plates except for the center plate were tuned to maximize the intensity of monochromatic beam passed through the square hole. In order to
Fig. 4. A schematic sketch of the T2-2 configuration.
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evaluate the focusing effect for the horizontal direction as well as for the vertical one, a neutron imaging plate [8] was used, which gives us a twodimensional distribution of the monochromatic beam intensity. The beam width as a function of distance from the monochromator position (L) is shown in the upper panel of Fig. 5. The left and right vertical axes in the upper figure denote the vertical- and horizontal-beam width of FWHM, respectively. One can see clearly that the monochromatic beam indeed focuses at the sample position for both the vertical and horizontal directions. The lower panel in Fig. 5 shows the beam images for several L, indicating that the sample position has a pretty good symmetrical shape. Note that at the detector position (L = 2960 mm), the beam has two components due to the beam character of the T2 guide tube. The monochromatic beam size at the sample position was tuned to be H10 V20 mm2 in FWHM. The beam profiles in the vertical direction at L=660 mm and at L=1970 mm (sample position) are provided in Fig. 6. The eight small peaks seen in the profile at L=660 mm correspond to the superposed intensity with each Si(4 2 2) plate of the nearest neighbor. The neutron intensity at the sample position is about 6 times larger than that at L=660 mm due to the effectively vertical focusing, which has a quite good reproducibility. The beam intensity by the present Si(4 2 2) monochromator was compared with that by PG(0 0 2) monochromator. According to T2-2 configuration shown in Fig. 4, the wavelength of ( in neutron from the PG(0 0 2) was fixed at 2.44 A order to use a l/2-filter, of whose configuration has been previously used at T2-2 beam port. The ( at flux of PG(0 0 2) monochromator with 2.44 A T2-2 beam port was measured to be almost comparable with that taken at a triple-axis spectrometer in a reactor hall under the very tight beam collimation such as 100 –400 –400 . It was confirmed by using various single crystals that ( the Bragg reflections of the samples with 1.57 A have a comparable intensity with that obtained by the previous configuration. That is, the brilliance of incoming neutrons at the sample position for the present doubly focused monochromator is almost comparable with that of PG(0 0 2) a
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H. Kimura et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 799–805
Fig. 5. Upper panel: Vertical and horizontal width of monochromatic beam as a function of distance from monochromator L shown with closed and open circles, respectively. Lower panel: Neutron beam images at each L.
improved without the loss of neutron flux so that the observable reciprocal lattice space became 8 times larger than that in the pervious stage, which makes the reliability of crystal structure analysis extremely improved. It should be pointed out that at the present stage, the monochromatic beam is contaminated by l/2 neutrons due to Si(8 4 4) reflection, of whose amplitude is less than 0.5% of l contribution. However, it will be overcome in the near future by applying Si(5 5 3) or other bending crystals. Fig. 6. Vertical beam profiles at L =660 and 1970 mm. In the present study, L=1970 mm corresponds to the sample position.
5. Summary
( Fig. 7 well-established monochromator for 2.44 A. shows the comparison of beam profiles for PG(0 0 2) and Si(4 2 2) monochromator taken by ( for Imaging plates. The wavelength is 2.44 A ( for Si(4 2 2) monochromaPG(0 0 2) and 1.57 A tors. As a consequence, for small samples with the typical size of 5 5 5 mm3, it was successfully
Vertical focusing of the stacked elastically bent Si crystals was completely successful due to the independent control of each vertical tilting angle. Owing to the suitable horizontal bending, the monochromatic beam intensity increased with the gain factor 10. Furthermore, the reproducible vertical focusing using nine Si crystals made the
H. Kimura et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 799–805
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pensable for the crystal structure analysis with a high reliability. It should be emphasized that a wavelength selectivity and an easy control of mosaic spread with a low cost for Si crystal are also quite important.
Acknowledgements The authors thank Y. Ishii of JAERI for the TAS-2 experiments, N. Niimura of JAERI and H. Yoshizawa of the Tokyo University for valuable discussions. All the works were supported by Grant-in-Aid for Science Research of Japan Society for Promotion of Science (A, No. 09354004).
References
Fig. 7. (a) Horizontal and (b) vertical beam profiles at the ( sample position for PG(002) monochromator with l=2.44 A ( and for the Si(422) one with l=1.57 A.
intensity comparable to that by the PG(0 0 2) monochromator which is one of the best neutron monochromators. The doubly focusing system for monochromators developed in the present study has a great potential in the case of requiring shorter wavelength of neutrons, which is indis-
[1] T. Riste, Nucl. Instr. and Meth. 86 (1970). [2] R. Scherm, G. Dolling, R. Ritter, E. Schedler, W. Teuchert, V. Wagner, Nucl. Instr. and Meth. 143 (1977) 77. $ [3] J. Kulda, Saroun, Nucl. Instr. and Meth. A 379 (1996) 155. [4] I. Tanaka, N. Niimura, P. Mikula, J. Appl. Crystallogr. 32 (1999) 525. [5] J. Kulda, Acta Crystallogr. A 40 (1984) 120. [6] J. Kulda, P. Luk!as&, Phys. Status Solidi (B) 153 (1989) 435. [7] Y. Noda, H. Kimura, R. Kiyanagi, A. Kojima, Y. Morii, N. Minakawa, N. Takesue, J. Phys. Soc. Jpn. 70 (Suppl. A) (2001) 456. [8] N. Niimura, Y. Karasawa, I. Tanaka, J. Miyahara, K. Takashi, H. Saito, S. Koizumi, M. Hidaka, Nucl. Instr. and Meth. A 349 (1994) 521.
Development of a horizontally and vertically focused neutron monochromator using stacked elastically bent Si single crystals H. Kimuraa,*, R. Kiyanagia, A. Kojimaa,b, Y. Nodaa, N. Minakawac, Y. Moriic, N. Takesued a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Miyagi 980-8577, Japan b Graduate school of Science and Technology, Chiba University, Yayoi, Inage, Chiba 263-8522, Japan c Advanced Science Research Center, JAERI, Tokai, Ibaraki 319-1195, Japan d Neutron Scattering Laboratory, ISSP, University of Tokyo, Tokai, Ibaraki 319-1195, Japan Received 14 April 2001; received in revised form 20 July 2001; accepted 21 July 2001
Abstract A horizontally and vertically focused monochromator has been developed for the 4-axis neutron diffractometer applied for a single crystal structure analysis. Silicon perfect single crystals are bent elastically in order to focus monochromatic neutrons horizontally. The monochromatic beam can also be focused vertically by stacking the horizontally bent crystals. Tilting motion of each stacked bent crystal is controlled independently by stepping pulse motors for optimizing and reproducing perfectly the vertical focusing at the sample position. The intensity of neutrons ( wavelength monochromatized by the new monochromator increases remarkably, and is comparable to with a 1.57 A ( wavelength. The high tunability of the doubly focusing system that of pyrolytic graphite monochromator with a 2.44 A established in the present study can be adopted easily when obtaining the shorter wavelength of neutrons. r 2002 Elsevier Science B.V. All rights reserved. PACS: 07.85.Jy Keywords: Neutron diffraction; Elastically bent crystal; Independent control; Doubly focused monochromator
1. Introduction Single crystal structure analysis using X-ray is a powerful and popular method for determining the averaged position of atoms in a unit cell. However, in the case that light atoms represented by hydrogen play an important role for physical
*Corresponding author. E-mail address: [email protected] (H. Kimura).
properties such as phase transitions, it is difficult for X-ray to determine accurately the position of light atoms. On the other hand, neutrons can determine their positions due to a high sensitivity even for light atoms, which contributes to a further understanding about the mechanism of the structural phase transitions. Although the principles of single crystal structure analysis using neutron are well established, there are few examples due to the lack of enough neutron intensity to perform the high-quality analysis.
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 7 0 8 - 9
800
H. Kimura et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 799–805
To overcome the weak neutron flux, since several decades, real space focusing techniques for monochromators and analyzers have been studied and developed, which increases neutron beam flux at sample position. Vertically focusing monochromators consisting of arrays of mosaic crystals such as pyrolytic graphite (PG) were first applied [1] and have been in routine use till date. On the other hand , horizontal focusing techniques [2] were less common because of a more complicated mechanical design. In all cases of horizontal focusing monochromators, mosaic crystals (PG) were also used to compose the focusing assemblies. However, in recent years, the horizontal focusing by using elastically bent perfect silicon (Si) crystals applied for monochromators has been developed and employed in some neutron beam lines [3,4], which is due to the improvement and sizing down of the focusing device. The advantages of bent perfect crystals compared with the mosaic crystals are the deterministic nature of their ‘‘mosaicness’’ and the variation of wavelength for monochromatic beam by selecting the mirror indices of Bragg reflection. Furthermore in the case of Si crystals, their low cost and standard crystalline quality encourage their employment. Although it was believed that the neutron intensity monochromatized by the bent perfect crystals cannot compete with that of mosaic crystals, recently [4] success was achieved in increasing the intensity comparable to that of mosaic monochromators owing to the development of special horizontal bender. Furthermore, in order to get more neutron flux at the sample position, the horizontally and vertically focused monochromator has been discussed [4] and constructed by stacking the bending devices vertically, and their tilt angles are optimized to be focused on the sample position. However, it was very difficult to focus vertically because the vertical tilt angle for each device was controlled only with one pulse motor using a camshaft mechanism, which is a very popular system for the vertically focusing devices. For the mosaic crystals, reflected beam divergence in the vertical direction is broad (B31 of FWHM), which makes the focusing much easier. On the other hand, in the bent perfect crystals, much finer tuning of the tilting angles is
needed due to the sharpness of the vertical beam divergence. Thus, in the present study, we have tried to develop a new vertically focusing system which can control independently the vertical tilt angle of each stacked bending device. As for the horizontal bender, we utilized the method being developed and applied already to several diffractometers. In this paper the horizontal bender and the newly developed vertical focusing device are introduced in Section 2. The practical optimization of bending curvature for the Si crystal and assembly of the nine vertically stacked bending crystals by using neutron two-axis spectrometer are presented in Section 3. The detailed characters of the monochromatic beam are evaluated in Section 4. Theoretical approaches for the integrated reflecting power in the case of an elastically bent perfect crystal, which can be calculated by using dynamical diffraction theory, should be referred elsewhere [3–6].
2. Bending and vertical focusing device In the preliminary study, Si single crystal plates cut in parallel with (5 5 3) plane were used as a bender crystal, but the reflected intensity of monochromatic neutrons was not enough for applying to the monochromator. Therefore, Si(4 2 2) plates with a size of W100 H10 T5 mm3 were utilized in this study, with intensity being two times larger than that of (5 5 3) reflection. As shown in Fig. 1(a), the elastically bending device for one Si crystal plate is very simple [4], which consists of hard steel parts to hold crystals, piano wire to pull both edges of the crystal, and screw (denoted as Screw-o in Fig. 1(a)) inserted at the bender framework to adjust the rotating (o) angle of the crystal. By squeezing the Screw-T attached to the piano wire, the tension in the piano wire is increased and the crystal is bent homogeneously. The vertical focusing under the horizontal bend is achieved by stacking the bending devices. As mentioned in Section 1, the previous focusing system using camshaft with one stepping pulse motor has not been successful. Therefore in
H. Kimura et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 799–805
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Fig. 1. Schematic drawings of the monochromator: (a) horizontal bending device and (b) vertical tilting system of the stacked bending device.
the present study, the pulse motors are attached for all the stacked devices except for the part located at the center. The schematic drawing of the newly designed focusing system is shown in Fig. 1(b). The nine horizontally focusing devices with the eight pulse motors are stacked vertically in the present work. The whole dimensions of the monochromators are W285 H193 T100 mm3. Simplified structure of the bending device and sizing down of the pulse motor enable us to insert such a complicated mechanism into a monochromator housing.
3. Horizontal bending and assembling the stacked Si crystals In order to optimize the bending curvature of the Si crystal, peak profiles and integrated intensities of Bragg reflection as a function of bending were measured. The experiments were carried out with a triple-axis spectrometer TAS-2, which was used in two-axis mode without analyzer
in the present study, installed at the T2 thermal guide of JRR-3 M at Japan Atomic Energy Research Institute (JAERI). The wavelength of ( by Si (3 1 1) initial neutron was fixed at 1.57 A monochromator. Instead of direct measurements of the bending curvature using micrometers [4], a strain gauge was attached on the largest surface of Si plates to measure the internal strain of the crystals (see Fig. 1(a)). A strain gauge is one of the strain-measuring devices, whose internal resistance (R) increases with increase in the extension of devices due to a strain. An external strain e (DL/ L, where L is the length of the device) can be obtained from e=KSDR/R, where KS corresponds to a sensitivity which depends on the devices. In the present work, we used stainless steel (SUS304) as the strain gauge with KS = 2.1. All the results are summarized in Figs. 2 and 3. Fig. 2(a) shows the strain dependence of the (4 2 2) rocking curves. The intensity increases on increasing the internal strain e, which is due to the reduction of primary extinction effects. Note that the small peak seen on the right side of the main
802
H. Kimura et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 799–805
Fig. 2. (a) Rocking curves of Si(422) Bragg reflection and (b) integrated intensity as a function of internal strain e (=DL/L) In the present study, e=300 106 (shown by bold arrows) was applied for all the crystals.
Fig. 3. Rocking curves of Si(4 2 2) Bragg reflection for the nine single crystals (Nos. 1–9) with the internal strain (e=300 106).
peak is not originated from the crystal, but is a characteristic property of thermal neutron beam collimated by T2 neutron guide tube. The integrated intensity as a function of the strain is also plotted in Fig. 2(b). Although the intensity increased with increasing e, it tended to saturate into a constant value around e =300( 10-6). Results indicate that there is almost no gain for intensity at e X 300 (shown by the arrow in Fig. 1(b)). In a preliminary experiment, the maximum strain loaded into the crystal was estimated to be e = 430 where the crystal was broken. As a result, the optimum strain, where the bending curvature should be optimized, was determined to be e =300 (70% of the broken point). For the next step, all the nine Si crystal plates with the strain of e = 300 were stacked vertically and assembled. In this stage, there are two important things. Firstly, the equal strains should be actually applied for all the crystals, which gives the same profile and intensity in diffraction measurements. Secondly, the crystal angle o satisfying the (4 2 2) Bragg condition should be identical for all the stacked crystals. As for the second requirement, we could fulfill it by using Screw-o (see Fig. 1(a)) which can move only one side of horizontal device, corresponding approximately to the o rotation. Fig. 3 shows the rocking curves for all the stacked crystals, whose line widths are about 0.061 of FWHM. The profiles for all the samples are perfectly identical even in their intensities, strongly confirming that the crystals could be bent quite systematically and reproducibly. Furthermore, it also proves that the crystal angle o was ideally tuned within the error of 0.0051. In the previous system of the vertical focusing by using one camshaft, it was a crucial fault that the offset angle for the vertical tilting between one and the other crystals cannot be changed. Therefore neutrons are not focused well when the offset angle is large enough compared with the vertical beam divergence (p0.61 of FWHM in the present thermal guide). However, on the other hand, the present focusing system can change the offset angle to zero by the independent control of tilting angle for each bending device. In fact, the identical intensities shown in Fig. 3 exhibit, that the (4 2 2) reciprocal vectors are
H. Kimura et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 799–805
perfectly concordant along not only the horizontal but also the vertical direction for all the crystal plates.
4. Characters of the doubly focused monochromatic beam The Si(4 2 2) horizontally bent monochromator with vertical stacking of the bending devices was installed in T2-2 beam port [7] on T2 thermal guide of JRR-3 M at JAERI. A schematic sketch of the T2-2 configuration is given in Fig. 4. In this beam port, as seen in Fig. 4, three fixed scattering angles for the monochromator (2yM) of 42.61, 901 and 113.21 are available. In the present study, 2yM was selected at 901 where the monochromatic beam has ( since the wavelength a wavelength of 1.57 A, distribution of neutron flux in the T2 thermal ( guide tube has a maximum around 1.5 A. The tuning of the monochromator at the T2-2 beam port was performed by the following procedures. A cadmium plate with a square hole of 20 20 mm2 was mounted on the sample position. First of all, the center plate of Si monochromator was adjusted by the tilting mechanism of the goniometer head of the monochromator. Then, the vertical tilting angles of the eight Si plates except for the center plate were tuned to maximize the intensity of monochromatic beam passed through the square hole. In order to
Fig. 4. A schematic sketch of the T2-2 configuration.
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evaluate the focusing effect for the horizontal direction as well as for the vertical one, a neutron imaging plate [8] was used, which gives us a twodimensional distribution of the monochromatic beam intensity. The beam width as a function of distance from the monochromator position (L) is shown in the upper panel of Fig. 5. The left and right vertical axes in the upper figure denote the vertical- and horizontal-beam width of FWHM, respectively. One can see clearly that the monochromatic beam indeed focuses at the sample position for both the vertical and horizontal directions. The lower panel in Fig. 5 shows the beam images for several L, indicating that the sample position has a pretty good symmetrical shape. Note that at the detector position (L = 2960 mm), the beam has two components due to the beam character of the T2 guide tube. The monochromatic beam size at the sample position was tuned to be H10 V20 mm2 in FWHM. The beam profiles in the vertical direction at L=660 mm and at L=1970 mm (sample position) are provided in Fig. 6. The eight small peaks seen in the profile at L=660 mm correspond to the superposed intensity with each Si(4 2 2) plate of the nearest neighbor. The neutron intensity at the sample position is about 6 times larger than that at L=660 mm due to the effectively vertical focusing, which has a quite good reproducibility. The beam intensity by the present Si(4 2 2) monochromator was compared with that by PG(0 0 2) monochromator. According to T2-2 configuration shown in Fig. 4, the wavelength of ( in neutron from the PG(0 0 2) was fixed at 2.44 A order to use a l/2-filter, of whose configuration has been previously used at T2-2 beam port. The ( at flux of PG(0 0 2) monochromator with 2.44 A T2-2 beam port was measured to be almost comparable with that taken at a triple-axis spectrometer in a reactor hall under the very tight beam collimation such as 100 –400 –400 . It was confirmed by using various single crystals that ( the Bragg reflections of the samples with 1.57 A have a comparable intensity with that obtained by the previous configuration. That is, the brilliance of incoming neutrons at the sample position for the present doubly focused monochromator is almost comparable with that of PG(0 0 2) a
804
H. Kimura et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 799–805
Fig. 5. Upper panel: Vertical and horizontal width of monochromatic beam as a function of distance from monochromator L shown with closed and open circles, respectively. Lower panel: Neutron beam images at each L.
improved without the loss of neutron flux so that the observable reciprocal lattice space became 8 times larger than that in the pervious stage, which makes the reliability of crystal structure analysis extremely improved. It should be pointed out that at the present stage, the monochromatic beam is contaminated by l/2 neutrons due to Si(8 4 4) reflection, of whose amplitude is less than 0.5% of l contribution. However, it will be overcome in the near future by applying Si(5 5 3) or other bending crystals. Fig. 6. Vertical beam profiles at L =660 and 1970 mm. In the present study, L=1970 mm corresponds to the sample position.
5. Summary
( Fig. 7 well-established monochromator for 2.44 A. shows the comparison of beam profiles for PG(0 0 2) and Si(4 2 2) monochromator taken by ( for Imaging plates. The wavelength is 2.44 A ( for Si(4 2 2) monochromaPG(0 0 2) and 1.57 A tors. As a consequence, for small samples with the typical size of 5 5 5 mm3, it was successfully
Vertical focusing of the stacked elastically bent Si crystals was completely successful due to the independent control of each vertical tilting angle. Owing to the suitable horizontal bending, the monochromatic beam intensity increased with the gain factor 10. Furthermore, the reproducible vertical focusing using nine Si crystals made the
H. Kimura et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 799–805
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pensable for the crystal structure analysis with a high reliability. It should be emphasized that a wavelength selectivity and an easy control of mosaic spread with a low cost for Si crystal are also quite important.
Acknowledgements The authors thank Y. Ishii of JAERI for the TAS-2 experiments, N. Niimura of JAERI and H. Yoshizawa of the Tokyo University for valuable discussions. All the works were supported by Grant-in-Aid for Science Research of Japan Society for Promotion of Science (A, No. 09354004).
References
Fig. 7. (a) Horizontal and (b) vertical beam profiles at the ( sample position for PG(002) monochromator with l=2.44 A ( and for the Si(422) one with l=1.57 A.
intensity comparable to that by the PG(0 0 2) monochromator which is one of the best neutron monochromators. The doubly focusing system for monochromators developed in the present study has a great potential in the case of requiring shorter wavelength of neutrons, which is indis-
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