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Elastically bent perfect Ge crystal analyser
Physica B 276}278 (2000) 73}74
Elastically bent perfect Ge crystal analyser J. Kulda *, P. Mikula, J. Saroun Institut Laue-Langevin, BP 156, 38042 Grenoble Cedex 9, France Nuclear Physics Institute, ASCR, 250 68 Rez, Czech Republic
Abstract We report on tests of a horizontally focusing germanium analyser, developed for thermal and hot neutron three-axis spectrometers at the ILL. The present version uses a slightly asymmetric 311 re#ection (d "1.706 As ) from a pack of FIJ elastically bent perfect Ge crystals, whose bending radius can be varied continuously to optimise the horizontal focusing. The performance of the new analyser compares favourably with that of a Cu 200 (d "1.807 As ) device of the same FIJ dimensions. 2000 Elsevier Science B.V. All rights reserved. Keywords: Neutron instruments; Monochromators; Triple-axis spectrometers
Focusing devices using elastically bent perfect silicon crystals are current accessories of neutron instruments, providing a more e$cient variant to combinations of mosaic crystals with Soller collimators [1]. Elastically bent perfect crystals have no mosaic structure in the usual sense and their angular re#ection pro"le is rectangular with a width given just by geometrical factors } bending radius, thickness and length of the irradiated part of the crystal. The peak re#ectivity r"1!exp(!Q R/cos h) depends, however, on the materials properties via the kinematical re#ectivity Q "Fj/(v sin 2h). When using Si 111 at a wavelength above, say, 1.5 As the peak re#ectivity is close to unity (saturation). At higher neutron energies, when re#ections of higher Miller indices have to be used in order to achieve su$cient resolution, the re#ectivity will depend more strongly on the scattering length density and transmission properties of the crystal material [2,3]. In such cases other crystal materials than silicon, despite their higher cost, have to be sought. The closest choice is germanium with an almost twice bigger scattering length, while having the same crystal structure and
* Corresponding author. Tel.: #33-4-7620-7256; fax: #334-7648-3906. E-mail address: [email protected] (J. Kulda)
a similar lattice parameter, being available in the form of large perfect single crystals. The bending device of the present Ge analyser is similar to those used previously for the silicon crystals [1]. The bending radius can be adjusted in the range of R"2.5}25 m via a remotely controlled stepping motor. Contrary to the silicon analysers, only two packs of "ve Ge plates of dimensions 200;40;1.7 mm are used and no vertical focusing is employed. The main surface of the plates is cut parallel to the (2 1 1) crystallographic planes so that either the symmetric 422 re#ection (d "1.15 As ) FIJ or the slightly asymmetric 311 re#ection (d "1.705 As ) FIJ can be used. While the former one is about equivalent to the Cu 222 we shall pay attention to the latter one which provides the advantage of a forbidden second-order re#ection and hence of a low j/2 contamination. The tests reported in this paper were carried out on the IN1 hot neutron three axis spectrometer at ILL Grenoble. We have used a Cu 200 monochromator with a 20 Soller collimator before the vanadium sample (cylinder H11;25 mm), the remaining collimator positions resting open to allow horizontal focusing. Among the two possible asymmetric geometries for the Ge 311 re#ection the one with a larger angle of incidence was chosen, because it favours monochromatic focusing. The solid angle seen by the sample is maximised in this case and the intrinsic `mosaica due to the Bragg angle variation along the incident beam path in the crystals is
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 3 4 0 - X
74
J. Kulda et al. / Physica B 276}278 (2000) 73}74
Fig. 1. Vanadium scattering intensity measured at k"5.6 As \ with the optimally focused Ge 311 analyser (䢇) as compared to the intensities obtained with the Cu 200 analyser, #at (*) and vertically focused (䉭).
a comparison of the performance of the tested Ge 311 analyser to that of the standard Cu 200 analyser. The somewhat higher intensity provided by the vertically focusing Cu device is partly due to its larger size, 15;10 cm (w;h) as compared to the active size of 12;8 cm for the Ge device. The principal advantage of the Ge analyser is the absence of the second-order contamination, important at the hot source beams even at k+5 As \. Although for diamond structure crystals the second order of the odd hkl re#ections is forbidden by symmetry, non-zero intensity is usually observed for them because of anharmonicity of the thermal motion and, above all, due to multiple Bragg re#ection (Umweganregung) processes. It is therefore important to test the real ratio of j/2 suppression. The procedure is less straightforward than with "lters. Fig. 2 displays vanadium scan data obtained with the normal Ge 311 setup at k "4.8 As \ with those for G k "9.6 As \ and Ge 622, normalised to the same moniG tor count. After correcting for resolution e!ects one arrives at a second-order suppression ratio better than 1/300, di$cult to achieve with absorption "lters in this wavelength range. The authors are thankful to B. Roessli and A. Ivanov for their support during the test experiments on IN1. This work was supported by the European TMR network PECNO (ERB-MRX-CT96-0057).
Fig. 2. First (*) and second (䢇) order scattering intensities for the Ge 311 analyser, the second-order contamination is less than 0.3%.
minimum. The curvature of the Ge analyser was optimised to provide a combination of high peak intensity and low-energy width in vanadium scans. Fig. 1 displays a typical vanadium scan data at k"5.6 As \ providing
References [1] J. Kulda, J. Saroun, Nucl. Instr. and Meth. A 379 (1996) 155. [2] A. Freund, Nucl. Instr. and Meth. 213 (1983) 495. [3] J. Kulda, V. Wagner, P. Mikula, J. Saroun, Nucl. Instr. and Meth. A 338 (1994) 60.
Elastically bent perfect Ge crystal analyser J. Kulda *, P. Mikula, J. Saroun Institut Laue-Langevin, BP 156, 38042 Grenoble Cedex 9, France Nuclear Physics Institute, ASCR, 250 68 Rez, Czech Republic
Abstract We report on tests of a horizontally focusing germanium analyser, developed for thermal and hot neutron three-axis spectrometers at the ILL. The present version uses a slightly asymmetric 311 re#ection (d "1.706 As ) from a pack of FIJ elastically bent perfect Ge crystals, whose bending radius can be varied continuously to optimise the horizontal focusing. The performance of the new analyser compares favourably with that of a Cu 200 (d "1.807 As ) device of the same FIJ dimensions. 2000 Elsevier Science B.V. All rights reserved. Keywords: Neutron instruments; Monochromators; Triple-axis spectrometers
Focusing devices using elastically bent perfect silicon crystals are current accessories of neutron instruments, providing a more e$cient variant to combinations of mosaic crystals with Soller collimators [1]. Elastically bent perfect crystals have no mosaic structure in the usual sense and their angular re#ection pro"le is rectangular with a width given just by geometrical factors } bending radius, thickness and length of the irradiated part of the crystal. The peak re#ectivity r"1!exp(!Q R/cos h) depends, however, on the materials properties via the kinematical re#ectivity Q "Fj/(v sin 2h). When using Si 111 at a wavelength above, say, 1.5 As the peak re#ectivity is close to unity (saturation). At higher neutron energies, when re#ections of higher Miller indices have to be used in order to achieve su$cient resolution, the re#ectivity will depend more strongly on the scattering length density and transmission properties of the crystal material [2,3]. In such cases other crystal materials than silicon, despite their higher cost, have to be sought. The closest choice is germanium with an almost twice bigger scattering length, while having the same crystal structure and
* Corresponding author. Tel.: #33-4-7620-7256; fax: #334-7648-3906. E-mail address: [email protected] (J. Kulda)
a similar lattice parameter, being available in the form of large perfect single crystals. The bending device of the present Ge analyser is similar to those used previously for the silicon crystals [1]. The bending radius can be adjusted in the range of R"2.5}25 m via a remotely controlled stepping motor. Contrary to the silicon analysers, only two packs of "ve Ge plates of dimensions 200;40;1.7 mm are used and no vertical focusing is employed. The main surface of the plates is cut parallel to the (2 1 1) crystallographic planes so that either the symmetric 422 re#ection (d "1.15 As ) FIJ or the slightly asymmetric 311 re#ection (d "1.705 As ) FIJ can be used. While the former one is about equivalent to the Cu 222 we shall pay attention to the latter one which provides the advantage of a forbidden second-order re#ection and hence of a low j/2 contamination. The tests reported in this paper were carried out on the IN1 hot neutron three axis spectrometer at ILL Grenoble. We have used a Cu 200 monochromator with a 20 Soller collimator before the vanadium sample (cylinder H11;25 mm), the remaining collimator positions resting open to allow horizontal focusing. Among the two possible asymmetric geometries for the Ge 311 re#ection the one with a larger angle of incidence was chosen, because it favours monochromatic focusing. The solid angle seen by the sample is maximised in this case and the intrinsic `mosaica due to the Bragg angle variation along the incident beam path in the crystals is
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 3 4 0 - X
74
J. Kulda et al. / Physica B 276}278 (2000) 73}74
Fig. 1. Vanadium scattering intensity measured at k"5.6 As \ with the optimally focused Ge 311 analyser (䢇) as compared to the intensities obtained with the Cu 200 analyser, #at (*) and vertically focused (䉭).
a comparison of the performance of the tested Ge 311 analyser to that of the standard Cu 200 analyser. The somewhat higher intensity provided by the vertically focusing Cu device is partly due to its larger size, 15;10 cm (w;h) as compared to the active size of 12;8 cm for the Ge device. The principal advantage of the Ge analyser is the absence of the second-order contamination, important at the hot source beams even at k+5 As \. Although for diamond structure crystals the second order of the odd hkl re#ections is forbidden by symmetry, non-zero intensity is usually observed for them because of anharmonicity of the thermal motion and, above all, due to multiple Bragg re#ection (Umweganregung) processes. It is therefore important to test the real ratio of j/2 suppression. The procedure is less straightforward than with "lters. Fig. 2 displays vanadium scan data obtained with the normal Ge 311 setup at k "4.8 As \ with those for G k "9.6 As \ and Ge 622, normalised to the same moniG tor count. After correcting for resolution e!ects one arrives at a second-order suppression ratio better than 1/300, di$cult to achieve with absorption "lters in this wavelength range. The authors are thankful to B. Roessli and A. Ivanov for their support during the test experiments on IN1. This work was supported by the European TMR network PECNO (ERB-MRX-CT96-0057).
Fig. 2. First (*) and second (䢇) order scattering intensities for the Ge 311 analyser, the second-order contamination is less than 0.3%.
minimum. The curvature of the Ge analyser was optimised to provide a combination of high peak intensity and low-energy width in vanadium scans. Fig. 1 displays a typical vanadium scan data at k"5.6 As \ providing
References [1] J. Kulda, J. Saroun, Nucl. Instr. and Meth. A 379 (1996) 155. [2] A. Freund, Nucl. Instr. and Meth. 213 (1983) 495. [3] J. Kulda, V. Wagner, P. Mikula, J. Saroun, Nucl. Instr. and Meth. A 338 (1994) 60.