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An X-ray backscattering instrument with very high energy resolution
450
Nuclear Instruments and Methods in Physics Research A246 (1986) 450-451 North-Holland, Amsterdam
Section III. Research instrumentation: (a) Photon spectrometers AN X-RAY BACKSCATI'ERING INSTRUMENT B. D O R N E R
*, E. B U R K E L
WITH VERY HIGH ENERGY RESOLUTION
a n d J. P E I S L
Sektion Physik der Ludwig-Maximilians-Universitiit Mfinchen, MYmchen, FRG
Very high energy resolution of X-rays can be achieved by extreme back reflection (Bragg angle close to 90 °) from perfect single crystals. We report about the progress obtained in building an instrument for inelastic scattering. In a test experiment at HASYLAB we achieved a line width AE =11 meV of the elastic scattered radiation at an energy E =13.8 keV. The synchrotron radiation intensity from a bending magnet at DORIS II is not yet sufficient to observe inelastically scattered radiation.
It is well k n o w n that the energy dispersion A O / A E a n d the lattice dispersion AO/Aa are proportional to tan 0, 0 being the Bragg angle. If 0 approaches 90 ° the resolution becomes very high and is limited by the b e a m divergence a n d the reflection width of the crystal only. Previous backscattering experiments with neutrons, Xrays and s y n c h r o t r o n radiation are referred to in a previous p a p e r by D o r n e r [1]. We have started the realization of an i n s t r u m e n t for high resolution inelastic scattering of s y n c h r o t r o n radiation [2] and report here a b o u t recent results of test experiments at H A S Y L A B .
a n d the calculated A E (for 0 = 90 °) is 4.9 meV. In spite of the high intensity of synchrotron radiation, we need spherically curved crystals as m o n o c h r o m a t o r s a n d analysers to increase the intensity at the expense of solid angle. The general layout of the i n s t r u m e n t is shown in fig. 1. The distances foreseen for a final version are L = 40 m, l M = 13.3 m, H = 8 mm, d~ < 7 cm. l A : 3 m and d D < 6.5 mm. These dimensions are chosen such that each c o n t r i b u t i o n to the resolution z ~ E / E does not exceed 5 × 10 v, we are aiming for an overall resolution A E - - 1 0 meV. The curved crystals have to be grooved to leave the reflecting parts unstrained.
2. The inelastic scattering instrument
3. Results of a test experiment
Detailed considerations for an inelastic scattering i n s t r u m e n t with very high resolution have been given at the preceding conference [2]. Therefore, we summarize the principal ideas only a n d describe the p l a n n e d setup. The resolution A k o b t a i n e d by reflection from a perfect single crystal has two contributions
D u r i n g our last b e a m time (October 1984) on the R 6 n t g e n test position at H A S Y L A B we used a flying setup in order to get first results before the final design a n d construction of the full instrument. The equipment, such as curved single crystals, their temperature control, v a c u u m tubes etc., h a d been p r e p a r e d a n d tested at a rotating a n o d e X-ray generator in Munich. The available space at the R~Sntgen test position was very limited. Therefore, l M could only be 4 m while L was 40 m. As a consequence H could only be 1 m m instead of the available height of the b e a m of a b o u t 8 mm. For the same reason the useful horizontal width of the white b e a m was 18 m m out of 54 m m available. These restrictions reduced the intensity so m u c h that we could not aim for a real inelastic experiment. Therefore, a pyrolytic graphite crystal in (004) reflection served as a sample. The other dimensions were d s = 6 cm; l A = 2 m a n d d D = 9 mm. d D was larger t h a n allowed for a 1 m m wide beam. Therefore a d i a p h r a g m of 0.5 m m was used. The spherically curved analyser and its temperature control worked properly, while the triangular mono-
1. Introduction
Ak/k = A,/~ + cot(0)-a0,
(1)
where k is the wave vector of the radiation, ~- the reciprocal lattice vector corresponding to the reflection, 0 the Bragg angle a n d A0 the divergence of the beam. In the limit 0 ~ 90 ° the resolution is determined b y a~which is the reciprocal of the p e n e t r a t i o n depth. The energy resolution A E is proportional to 1 / k for X-rays. This leads to the conclusion that for X-rays a high energy E has to be reflected from a high order reflection to obtain a small AE. For the p l a n n e d i n s t r u m e n t we have chosen the (777) reflection from a perfect Si crystal, where E is 13.8 keV
* Institut Laue-Langevin, Grenoble, France. 0 1 6 8 - 9 0 0 2 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
B. Dorner et al. / X-rc~v backscattering instrument with high energy resolution
"~ ~ d e t e c t o r 2 (~ " ~ i h ~ , ~ rnple -' ~
source
451
monochromotor I}, E
I
Fig. 1. Schematic layout of the instrument for inelastic X-ray scattering with an energy resolution of A E = 10 meV. Real dimensions are given in the text.
1.
E = 13.8 keY 150
100
Si (777)
0,31 K
I
306
11 meV
l
I
L
307 TAnalyser [ K]
308
Fig. 2. Resonance of monochromator and analyser in high resolution backscattering geometry. The temperature of the monochromator was sufficiently constant.
c h r o m a t o r crystal, curved only horizontally, showed too m u c h strain because the grooves were not deep enough. Therefore we used finally a flat silicon crystal as m o n o chromator. The heat load from the white b e a m on the m o n o c h r o m a t o r created a temperature increase of several degrees. But the t e m p e r a t u r e became stationary after a few minutes and it could be regulated by the control unit. In the future we plan to use a p r e m o n o c h r o m a t o r to remove the heat load from the m o n o c h r o mator.
To scan the energy resolution we varied the lattice p a r a m e t e r of the analyser with respect to that of the m o n o c h r o m a t o r by varying the temperature of the •analyser. The best scan of the resonance between m o n o c h r o m a t o r a n d analyser is given in fig. 2. The measured width of 0.31 K corresponds to A E = 11 meV using an expansion coefficient of 2.6 × 10 6 K-~ as quoted by Wacker-Chemitronic. We explain the asymmetry of the curve by reflections from the b o t t o m of the grooves in the analyser, where the lattice c o n s t a n t is a bit enlarged by strain. These parts then reflect at a lower temperature. The result shows that the geometrical setup (angles a n d diaphragms) provides the desired resolution and that the curved analyser operates properly. In the development of curved crystals we benefitted from cooperation with M. Hini, Siemens AG, Erlangen, Dr. Tischer, Siemens AG, M~nchen, H. Paul, Cristaltec, G r e n o b l e a n d H. Vosskilhler, Halle-Optische Ger~ite, Berlin. The project is supported by the Bundesministerium for Forschung und Technologie.
References [1] B. Dorner, Proc. Workshop on High-energy excitations in condensed matter, Los Alamos LA-10227-C (1984) p. 188. [2] B. Dorner and J. Peisl, Nucl. Instr. and Meth. 208 (1983) 587.
III(a). PHOTON SPECTROMETERS
Nuclear Instruments and Methods in Physics Research A246 (1986) 450-451 North-Holland, Amsterdam
Section III. Research instrumentation: (a) Photon spectrometers AN X-RAY BACKSCATI'ERING INSTRUMENT B. D O R N E R
*, E. B U R K E L
WITH VERY HIGH ENERGY RESOLUTION
a n d J. P E I S L
Sektion Physik der Ludwig-Maximilians-Universitiit Mfinchen, MYmchen, FRG
Very high energy resolution of X-rays can be achieved by extreme back reflection (Bragg angle close to 90 °) from perfect single crystals. We report about the progress obtained in building an instrument for inelastic scattering. In a test experiment at HASYLAB we achieved a line width AE =11 meV of the elastic scattered radiation at an energy E =13.8 keV. The synchrotron radiation intensity from a bending magnet at DORIS II is not yet sufficient to observe inelastically scattered radiation.
It is well k n o w n that the energy dispersion A O / A E a n d the lattice dispersion AO/Aa are proportional to tan 0, 0 being the Bragg angle. If 0 approaches 90 ° the resolution becomes very high and is limited by the b e a m divergence a n d the reflection width of the crystal only. Previous backscattering experiments with neutrons, Xrays and s y n c h r o t r o n radiation are referred to in a previous p a p e r by D o r n e r [1]. We have started the realization of an i n s t r u m e n t for high resolution inelastic scattering of s y n c h r o t r o n radiation [2] and report here a b o u t recent results of test experiments at H A S Y L A B .
a n d the calculated A E (for 0 = 90 °) is 4.9 meV. In spite of the high intensity of synchrotron radiation, we need spherically curved crystals as m o n o c h r o m a t o r s a n d analysers to increase the intensity at the expense of solid angle. The general layout of the i n s t r u m e n t is shown in fig. 1. The distances foreseen for a final version are L = 40 m, l M = 13.3 m, H = 8 mm, d~ < 7 cm. l A : 3 m and d D < 6.5 mm. These dimensions are chosen such that each c o n t r i b u t i o n to the resolution z ~ E / E does not exceed 5 × 10 v, we are aiming for an overall resolution A E - - 1 0 meV. The curved crystals have to be grooved to leave the reflecting parts unstrained.
2. The inelastic scattering instrument
3. Results of a test experiment
Detailed considerations for an inelastic scattering i n s t r u m e n t with very high resolution have been given at the preceding conference [2]. Therefore, we summarize the principal ideas only a n d describe the p l a n n e d setup. The resolution A k o b t a i n e d by reflection from a perfect single crystal has two contributions
D u r i n g our last b e a m time (October 1984) on the R 6 n t g e n test position at H A S Y L A B we used a flying setup in order to get first results before the final design a n d construction of the full instrument. The equipment, such as curved single crystals, their temperature control, v a c u u m tubes etc., h a d been p r e p a r e d a n d tested at a rotating a n o d e X-ray generator in Munich. The available space at the R~Sntgen test position was very limited. Therefore, l M could only be 4 m while L was 40 m. As a consequence H could only be 1 m m instead of the available height of the b e a m of a b o u t 8 mm. For the same reason the useful horizontal width of the white b e a m was 18 m m out of 54 m m available. These restrictions reduced the intensity so m u c h that we could not aim for a real inelastic experiment. Therefore, a pyrolytic graphite crystal in (004) reflection served as a sample. The other dimensions were d s = 6 cm; l A = 2 m a n d d D = 9 mm. d D was larger t h a n allowed for a 1 m m wide beam. Therefore a d i a p h r a g m of 0.5 m m was used. The spherically curved analyser and its temperature control worked properly, while the triangular mono-
1. Introduction
Ak/k = A,/~ + cot(0)-a0,
(1)
where k is the wave vector of the radiation, ~- the reciprocal lattice vector corresponding to the reflection, 0 the Bragg angle a n d A0 the divergence of the beam. In the limit 0 ~ 90 ° the resolution is determined b y a~which is the reciprocal of the p e n e t r a t i o n depth. The energy resolution A E is proportional to 1 / k for X-rays. This leads to the conclusion that for X-rays a high energy E has to be reflected from a high order reflection to obtain a small AE. For the p l a n n e d i n s t r u m e n t we have chosen the (777) reflection from a perfect Si crystal, where E is 13.8 keV
* Institut Laue-Langevin, Grenoble, France. 0 1 6 8 - 9 0 0 2 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
B. Dorner et al. / X-rc~v backscattering instrument with high energy resolution
"~ ~ d e t e c t o r 2 (~ " ~ i h ~ , ~ rnple -' ~
source
451
monochromotor I}, E
I
Fig. 1. Schematic layout of the instrument for inelastic X-ray scattering with an energy resolution of A E = 10 meV. Real dimensions are given in the text.
1.
E = 13.8 keY 150
100
Si (777)
0,31 K
I
306
11 meV
l
I
L
307 TAnalyser [ K]
308
Fig. 2. Resonance of monochromator and analyser in high resolution backscattering geometry. The temperature of the monochromator was sufficiently constant.
c h r o m a t o r crystal, curved only horizontally, showed too m u c h strain because the grooves were not deep enough. Therefore we used finally a flat silicon crystal as m o n o chromator. The heat load from the white b e a m on the m o n o c h r o m a t o r created a temperature increase of several degrees. But the t e m p e r a t u r e became stationary after a few minutes and it could be regulated by the control unit. In the future we plan to use a p r e m o n o c h r o m a t o r to remove the heat load from the m o n o c h r o mator.
To scan the energy resolution we varied the lattice p a r a m e t e r of the analyser with respect to that of the m o n o c h r o m a t o r by varying the temperature of the •analyser. The best scan of the resonance between m o n o c h r o m a t o r a n d analyser is given in fig. 2. The measured width of 0.31 K corresponds to A E = 11 meV using an expansion coefficient of 2.6 × 10 6 K-~ as quoted by Wacker-Chemitronic. We explain the asymmetry of the curve by reflections from the b o t t o m of the grooves in the analyser, where the lattice c o n s t a n t is a bit enlarged by strain. These parts then reflect at a lower temperature. The result shows that the geometrical setup (angles a n d diaphragms) provides the desired resolution and that the curved analyser operates properly. In the development of curved crystals we benefitted from cooperation with M. Hini, Siemens AG, Erlangen, Dr. Tischer, Siemens AG, M~nchen, H. Paul, Cristaltec, G r e n o b l e a n d H. Vosskilhler, Halle-Optische Ger~ite, Berlin. The project is supported by the Bundesministerium for Forschung und Technologie.
References [1] B. Dorner, Proc. Workshop on High-energy excitations in condensed matter, Los Alamos LA-10227-C (1984) p. 188. [2] B. Dorner and J. Peisl, Nucl. Instr. and Meth. 208 (1983) 587.
III(a). PHOTON SPECTROMETERS