CHARACTERIZATION OF X-RAY DIFFRACTION SYSTEM WITH A MICROFOCUS X-RAY SOURCE AND A POLYCAPILLARY

Transcription

1 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol CHARACTERIZATION OF X-RAY DIFFRACTION SYSTEM WITH A MICROFOCUS X-RAY SOURCE AND A POLYCAPILLARY OPTIC Mikhail Gubarev * 2, Ewa Ciszak2 3, Igor Ponomarev4, Walter Gibsod, Marshall Joy2 National Research Council 2 NASA/Marshall Space Flight Center, Huntsville, AL 35812, USA 3 Universities Space Research Association, 4950 Corporate Drive, Huntsville, AL 35805, USA 4X-ray Optical Systems, Inc., 30 Corporate Circle, Albany, NY 12203, USA Center for X-ray Optics, State University of New York at Albany, Albany, NY 12222, USA ABSTRACT We present here an optimized microfocus x-ray source and polycapillary optic system designed for diffractionof small protein crystals. The x-ray beam is formed by a 5.5mm input focal length capillary collimator coupled with a 40 micron x-ray source operating at 46Watts. Measurements of the x-ray flux, the divergence and the spectral characteristics of the beam are presented. This optimized system provides a seven fold greater flux than our recently reported configuration [M. Gubarev, et al., J. of Applied Crystallography (2000) 33, We now make a comparison with a SkWatts rotating anode generator (Rigaku) coupled with confocal multilayer focusing mirrors (Osmic, CMF12-38Cu6). The microfocus x-ray source and polycapillary collimator system delivers 60% of the x-ray flux from the rotating anode system. Additional ways to improve our microfocus x-ray system, and thus increase the x-ray flux will be discussed. INTRODUCTION High power rotating anode x-ray generators are standard x-ray sources for laboratory diffraction systems. To increase the x-ray flux delivered onto sample crystal, the rotating anode generators are coupled with x-ray optics, such as total reflection mirrors [l], graded multilayer monochromators [2] or polycapillary optics [3]. A polycapillary optics can collect the emitted x- rays over a larger solid angle compared to other types of optics 141, and, hence, utilize the greatest portion of x-ray energy onto the crystal. However, in order to most efficiently subtend a

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 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol large solid angle emitted from the x-ray source, a polycapillary optic has to be placed as close as possible to the anode spot, and the x-ray generator has to have a small anode spot. In that respect, high power rotating anode x-ray generators can be substituted with low-power microfocus x-ray sources. We have built a prototype microfocus x-ray source and polycapillary optic system for single crystal diffractometry from macromolecular crystals. The schematic drawing of the x-ray system for macromolecular crystallography is shown in Figure 5.5 mm, 25mm, 30 mm I I 1 I I I / *, E I --- ~~ I- X-Ray Polycapillary Ni Pinhole Crystal Source optic filter Sample Figure 1. Schematic representation of the microfocus x-ray system. X-RAY SYSTEM In our experiments we used an UltraBrite microfocus x-ray source with a copper anode manufactured by Oxford Instruments, Inc. The microfocus x-ray generator was previously characterized [J]. An operating voltage of 47 kv was found to be optimal for maximizing x-ray output and minimizing the anode spot diameter. The anode spot diameter of the source was found to be 40 pm FWHM at 46 Watts power, i.e., 47 kv voltage and 0.95 ma current. The spatial stability of the anode spot was found to be less than 2 l.trn over a period of 16 hours. The polycapillary optic optimized for use with the microfocus source was manufactured by X- ray Optical Systems, Inc. The specifications of the polycapillary x-ray optic are listed in Table I. To integrate the polycapillary optic into the microfocus system, the optic was installed on a miniature XY stage and the input of the optic was placed close to the x-ray source. In order to align the optic with respecto the anode, this assembly was also placed on a translation stage so that the distance between the input of the optic and the x-ray source could be adjusted. The

4 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol alignment of the optic to the x-ray source was made by maximizing the x-ray flux. To collimate the x-ray beam produced by the polycapillary optic coupled with microfocus x- ray generator, the 550 pm diameter pinhole installed at additional miniature XY stage was placed on the same translation stage at 30 mm from the output of the optic. The pinhole collimator was then similarly aligned with the x- ray beam. Table I. The specifications of the polycapillary x-ray optic. Input diameter, mm 1.30 Output diameter, mm 4.36 Input focal distance, mm 5.50 Length, mm Transmission efficiency for 8 kev x-ray photons, % 28 Sample crystals were mounted on a $-axis goniometer at 90 mm from the output of the polycapillary optic. Diffraction data were recorded using a RAXIS-IIC imaging plate detector from Molecular Structure Corporation. EXPERIMENTAL RESULTS A 10 pm thick nickel foil was installed at the output of the optic, in order to monochromatize the x-ray beam produced by the microfocus x-ray source. That filter provided a beam with 86 f 2 % of the x-ray photons being at the energy of the Cu Ka line [5]. High resolution Kodak TekPan x-ray film was placed at different distances from the output of the optic in order to measure the beam diameter. The exposed films were scanned through a 25 x 25 pm aperture PDS microdensitometer. The beam diameter at 90 mm from the output of the optic was found to be 288 f 15 pm PWHM. Beam diameters were also measured using a RAXIS-IIC image plate detector placed 190 mm, 290 mm, 390 mm and 490 mm from the output of the optic. The beam divergence inferred from these measurements is 2.55 f 0.15 mrad PwI-w X-ray flux measurements were made at a power level of 46.5 Watts (990 pa tube current and 47.0 kv tube voltage). A 500 pm diameter pinhole was installed in the place of the crystal, i.e., at 90 mm from the output of the optic, and a Macromolecular Structure Corporation PIN diode detector was placed after the pinhole. Prior to the x-ray flux measurement, the PIN diode detecto response for Cu Ka x-ray photons was calibrated at lower tube current using an Amptek

5 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol CdZnTe detector. The Cu Ka count rate produced by the microsource-polycapillary optic system through 10 pm nickel filter and 500 pm pinhole measured at a power level of 46.5 Watts was found to be (1.3 f 0.1) x lo8 counts per second. The intensity of the beam produced by the microfocus x-ray source coupled with the polycapillary optic was compared with that from rotating anode generator systems. Two rotating anode x-ray sources, FR591 (Nonius) and Rigaku RU-200 (Molecular Structure Corporation) were used for these measurements. The rotating anode x-ray generators were coupled with two different sets of confocal multilayer mirrors aligned according to the company s optimal specifications. The FR591 rotating anode generator was equipped with confocal graded multilayer monochromator (Green optic; CMF24-48Cu6, Osmic, Inc.), while the RU-200 rotating anode generator was coupled with the newest generation confocal multilayer optic (Blue optic; CMF12-38Cu6, Osmic, Inc.)..Both rotating anode systems were equipped with a 300 pm collimator placed after the optic, so the beam diameter at the point where the crystal is mounted is comparable to the size of the beam produced by microfocus x-ray system. A 500 pm diameter pinhole was installed in the place of the crystal, i.e., at 25 mm from the end of the collimator, and the PIN diode detector was placed after the pinhole. The results of flux measurements are summarized in Table II. Table II. Comparison of the x-ray flux produced by different generator-opticonfigurations. Generator type Operating parameters Optic Cu Ka x-ray flux, counts per second FR59 1 (Nonius) 5000 Watt Multilayer (5.OztO.l) x lo7 rotating anode (100 ma tube current monochromator and 50 kv voltage) CMF Cu6 RU-200 (Rigaku; 5000 Watt Multilayer (2.X 0.1) x lo8 MSC) rotating anode (100 ma tube current and 50 kv voltage) monochromator CMF Cu6 Microfocus source 46.5 Watt Polycapillary optic, (1.3ti.l) x lo8 UltraBright (Oxford) (0.995 ma tube current and 47 kv voltage) 10 pm Ni filter

6 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol Further evaluation of the x-ray diffraction system with microfocus source and polycapillary optic consisted of a series of measurements of diffraction from crystals of two proteins, lysozyme and thaumatin, followed by processing of diffraction data with HKL program [6]. Both crystals are tetragonal; space group P&212 with cell dimensions a=b=78.12 A, c=37.94 A for lysozyme, and P4i212 with cell dimensions a=b=58.6 A, c=151.6,& for thaumatin. Figure 2 shows examples of single diffraction images from both crystals. The diffraction data collected with our microfocus x-ray source and collimating optic have shown good quality. For example, 42,540 reflections (11,264 unique) collected at ambient temperature from lysozyme crystal yielded Rsym =5.0% data extracting to 1.70 A and an average I/o(I) was Diffraction experiment with a thaumatin crystal resulted in 88,669 observations (18,333 unique) processed to 2.0 A. The Rsym was 10.8% (96% completeness). Figure 2. Representative diffraction images for the lysozyme (left) and thaumatin (right) crystals collected with the microfocus x-ray system. The recorded diffraction spots extended to 1.7 a for lysozyme and to 1.95 A for thaumatin. CONCLUSIONS We present results from a microfocus x-ray system coupled with a collimating polycapillary optic. The Cu Ka x-ray flux in 500 pm diameter pinhole is 7.0 times greater than the Cu Ka x- ray flux achieved from a prototype system reported previously [5]. This x-ray flux from the microfocus x-ray system is 2.6 times higher than that achieved from a rotating anode generator

7 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol operating at 5000 Watts equipped with a graded confocal multilayer monochromator (Green optic), and 60% of the x-ray flux that can be achieved using the newest graded confocal multilayer optic (Blue optic). The microfocus x-ray system achieved these flux levels with power dissipation more than 100 times less than the power required by the rotating anode generators. Diffraction data collected with the microfocus x-ray system are of high quality, and can be used for evaluation of diffraction from crystals of biological macromolecules. Additional enhancement of this x-ray system is expected through improvement of the microfocus x-ray generator. One example of such improvement is the following. The size of the anode spot of the x-ray source could be decreased to 15 pm FWHM in order to allow a polycapillary optic to collect a greater fraction of the x-rays emitted from the source, and thus to increase the x-ray flux delivered to the sample crystal. Decreasing the anode take-off angle from the current 22.5 can also improve the anode power load, and thereby increase the x-ray output. With these modifications, we estimate that the flux can be increased by a factor of four. REFERENCES 1. Arndt, U.W. J. Appl. Crust., 1990,23, Shuster, H., & Abel, H. J. Phys. D: Appl. Phys., 1995,4A, Li, P.-W. & Bi, R.-C. J. Applied Cryst., 1998, 31, Kumakhov, M.A. & Komarov, F.F. Phys. Rep., 1990, 191(5), Gubarev, M., Ciszalc, E., Ponomarev, I., Gibson, W. & Joy, M. J. Appl. Cyst., 2000, 33, Otwinowski, 2. & Minor, W. Methods Enzymol,, 1997,276,