Stress Mitigation of X-ray Beamline Monochromators using a Topography Test Unit

Transcription

1 128 Stress Mitigation of X-ray Beamline Monochromators using a Topography Test Unit J. Maj 1, G. Waldschmidt 1 and A. Macrander 1, I. Koshelev 2, R. Huang 2, L. Maj 3, A. Maj 4 1 Argonne National Laboratory, Argonne, IL IMCA-CAT, Center for Advanced Radiation Sources, The University of Chicago, Northeastern Ohio Universities College of Medicine, Rootstown, OH, Rosalind Franklin University of Medicine and Science, North Chicago, IL ABSTRACT Silicon and diamond monochromators (crystals), often used in the Advanced Photon Source X-ray beamlines, require a good quality surface finish and stress-free installation to ensure optimal performance. The device used to mount the crystal has been shown to be a major contributing source of stress. In this case, an adjustable mounting device is an effective method of reducing stresses and improve the rocking curve to levels much closer to ideal. Analysis by a topography test unit has been used to determine the distribution of stresses and to measure the rocking curve, as well as create CCD images of the crystal. This paper describes the process of measuring these stresses and manipulating the mounting device and crystal to create a substantially improved monochromator. INTRODUCTION In order to ensure the performance of the monochromator, its manufacturing and mounting are carefully controlled. Since the monochromators are located within a vacuum environment, it is important to make adjustments prior to installation in the beamline to avoid venting the system. At the Advanced Photon Source (APS), a Topography Test Unit (TTU) [1] diffractometer has been used as a crystal characterization tool to evaluate X-ray monochromators for various types of materials including silicon, diamond, and germanium [2]. The TTU has been shown to successfully measure the stresses on the surface of crystals [3, 4] including stresses due to the holder [5]. Based on measurements of the rocking curve, adjustments of the crystal were made to achieve a result as close to theoretical as possible. A CCD camera has been used to supplement the function of the TTU by adding the capability to visually inspect locations of residual stress in the crystal. Often the problems due to mounting strains have been partially overcome by using varying amounts of a thick paste, or eutectic material. However, this means of overcoming stresses has consequences due to the varying thickness of eutectic material along the surface of the crystal. On the other hand, with the use of the TTU and an

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 129 adjustable crystal holder, much of the stress can be eliminated while maintaining other important performance parameters. This paper describes the process of measuring the stresses and manipulating the mounting device and crystal to create a substantially improved monochromator. Beamline Crystal Sample Scintillation Detector Adjustable Screws CCD Camera TTU Monochromator Crystal sample with adjustable mounting device Figure 1. TTU diffractometer test unit with adjustable crystal holder. MONOCHROMATOR TEST SETUP Measurements were taken in the X-Ray Laboratory at the APS with a TTU unit, see Fig. 1, that utilized an 18-kW Rigaku rotating anode generator for X-ray production. This generator used either a copper or molybdenum target to supply 1.54 Å or 0.71 Å X-rays, respectively. In order to eliminate dispersion, a TTU monochromator was chosen such that it matched the Bragg reflection of the crystal under study. Furthermore, the TTU monochromator crystal was cut asymmetrically to produce a large collimated beam for topographic measurements of the sample crystal. Upon reflection from the crystal sample, the diffracted X-rays were detected either by a scintillation detector or a CCD camera. The scintillation detector was used for sample orientation and measurements of the spatially integrated rocking curve. The CCD camera with 1024 x 1024 pixels and 60 x 60 micron pixel size was used for visualization purposes of the crystal topography, although detailed analysis of local stresses was also possible using the CCD image. All movements of the instrument were motorized within a 2Θ range from 0º to 120º. The

4 130 motorized motion as well as the detector acquisitions were automated using an EPICS [6] software control platform. The crystals to be analyzed were typically placed directly on a flat metal surface. The surface was lapped and also may be covered with gallium-lithium to make better contact with the crystal for mechanical support and thermal conductivity. Stress mitigation was most easily achieved with an adjustable crystal holder where the tension in the crystal was modified using screws, see Fig. 1. The assembled crystal was then installed on the TTU as a measured sample for topographic analysis. The scintillation detector was initially aligned to the 2θ scattering angle of the tested crystal. At the same time the crystallography plane of the crystal was rotated to the Bragg angle until the scintillation detector was aligned to the maximum intensity of the diffraction, and the rocking curve was generated. The angular location of the scintillation detector as well as the crystal were then held fixed. However, the tilt angle of the crystal was adjusted and the theta scan was repeated. The tilt angle was varied in order to sufficiently characterize the crystal and determine the optimal rocking curve where minimum FWHM was achieved. After stress mitigation. Before stress mitigation Figure 2. Results before and after stress mitigation using the Topographic Test Unit and CCD camera. The X-ray intensity is shown to increase by approximately 30% after stress mitigation has been performed. The dimension of the crystal sample used in the CCD pictures is 70 mm x 75 mm.

5 131 After a comparison of the theoretical rocking curve with the curve measured by the TTU, and with the aid of the CCD camera images, the crystal holder was adjusted in order to mitigate the mounting stresses. Adjustments to the crystal sample mount were made to alleviate the stresses until no further improvement was made. As shown in Fig. 2, the X- ray intensity throughput improved from 30,000 to 38,000 photons, while the FWHM width of the rocking curve reduced from 4.0 arcsec to 3.0 arcsec. Furthermore, the intensity profile of the CCD images became more uniform. As a result, significant enhancements of the performance of the monochromator were achieved. MONOCHROMATOR PERFORMANCE ENHANCEMENTS Hundreds of topograms of crystals have been performed for the APS Users and many of these crystals have been installed in the beamline with excellent results. Monochromators have been installed in beamlines for biological, chemical, and materials research. Substantial improvements have been documented using this stress mitigation method with respect to the rocking curve, which has resulted in enhanced experimental performance. One beamline that evidenced this improved performance at the APS is dedicated to macromolecular crystallography for biological research. The IMCA beamline operates in a typical configuration with an energy range from 7.5 kev to 17.5 kev, accepting white beam of 100 µrad vertically and 1.5 mrad horizontally. The requirements for the Si (111) crystal system used as the first crystal of a Double Crystal Monochromator (DCM) are to provide an energy reproducibility better than the achievable energy resolution and to deliver a high-flux throughput. Initially, poor energy reproducibility was found with a variation of 5 ev, in addition to long-term beam position drifts. In the original mounting scheme, Ga-In eutectic caused the first Si (111) crystal to conform to the mounting surface, producing considerable mounting strains in the crystal, especially where the crystal was thin and the mounting surface was not flat. To address these issues, a crystal mount with high surface flatness and low surface roughness was fabricated from a copper substrate with nickel electroplating (Johnsen UltraVac, Burlington, Canada). Although a high-quality mount was used, the monochromator still deviated from the desired optical quality. As a result, the crystal holder was modified with clamping screws to allow the stress along the crystal to be adjusted after analysis with the TTU diffractometer. Additionally, the crystal thickness was increased from 5 mm to 12 mm, and strain relief grooves were cut to minimize the effects of clamping the crystal to the mount surface. The crystal was mounted by using a thin 10-µm layer of Ga-In eutectic. While utilizing the TTU diffractometer and the CCD camera, adjustments were made to the mounting screws of the crystal holder until the measured rocking curve width of the crystal Si (333) diffraction peak was minimized. The rocking curve delivered by the total Si crystal surface in (333) diffraction geometry decreased from 4.0 arc sec (FWHM) to 3.0 arc sec (FWHM). The stress reduction using the TTU facility, in conjunction with an

6 132 improved monochromator design, resulted in an energy reproducibility of 0.2 ev in addition to achieving improved flux (at ev standard configuration). Also, the rocking curve broadened by less that 0.5 arcsec due to an operating thermal load of ~40W which produced a typical crystal thermal bump. The improved monochromator performance in flux throughput and delivered energy reproducibility is essential for obtaining high quality crystallographic data from macromolecular protein crystals as well as for conducting successful multi-wavelength anomalous dispersion (MAD) experiments for the de novo three-dimensional atomic structure of protein. The rocking curve of the altered crystal and mount arrangement for the Si (111) monochromator has remained stable since installation over a year ago, thus the performance enhancements created by the stress mitigation appear to be long-term. CONCLUSION The Topography Test Unit at the Advanced Photon Source, in conjunction with adjustable mounting devices, has been shown to successfully reduce stresses in crystal monochromators. Performance enhancements for the APS Users have been evident in applications ranging from biological, chemical, and materials research. The technique described in this paper resulted in an improvement of approximately 30% in the FWHM of the rocking curve and in the throughput of the monochromator. As a result, substantially improved energy reproducibility and high-flux throughput were achieved during beamline operation using the modified monochromators ACKNOWLEDGMENTS The authors express their appreciation to J. Lang, P. Baldo, B. Pruit, M. Westbrook, K. Getze, B. C. Cha, B. Lazarska, and O. Makarov for their contributions via valuable discussion. This work is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. W ENG-38. REFERENCES [1] S. Krasnicki, J. A. Maj, G. Czop, Y Zhong Conventional X-ray Facilities at APS. 3-Way Optics Workshop. May 30, APS/ANL [2] J.P.F. Sellschop, SPIE, Conference on Crystals and Multilayers, San Diego, 3448 (1998) 40 [3] J. Maj, A.T. Macrander, S. F. Krasnicki, P. Fernandez, R. Erk, Etching of diamond for X-ray monochromators Rev. Sci. Instrum. 73 (2002) 1546 [4] P.B. Fernandez, T. Graber, W.K. Lee et al, Nucl. Instrum. Methods A 400 (1997) 476 [5] N. Toda, H. Sumiya, S. Satoh et al., SPIE Conference on High Heat Flux and Synchrotron Radiation Beamlines / Crystal / Bragg Optics for Synchrotron Radiation Beamlines, San Diego, 3151 (1997) 329 [6] Experimental Physics and Industrial Control System,