- Email: [email protected]
[18] Gas proportional detectors
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previously, is its ability to bin neighboring pixels together into larger pseudo-pixels. Binning is a standard capability of CCDs. Rather than individual rows of pixels being shifted into the serial readout register, two rows are shifted together to combine their charges. The readout register also is shifted out two at a time, so the signal is four times larger than that of an individual pixel. Although binning quadruples the pixel area, it only doubles the dynamic range, halves the read noise, and halves the measurement uncertainty, because the readout pixels in the linear registers are only twice the size of ordinary pixels. Binning also quadruples the speed with which each data frame can be read out, decreasing the dead time between frames, and reduces the size of the computer file representing each data frame, reducing the complexity of the computer system needed to process data and the time needed for processing. This APS-1 detector is a significant part of an integrated technical facility that also features an undulator X-ray source on the APS, carefully designed X-ray optics, modern networking (VME; HIPPI; ATM), and a powerful computing environment (multiple processors, fast, large RAID disk arrays). The complete facility is now being commissioned and will be fully operational during summer 1997. We believe that the SBC facility will be among the most effective crystallographic data collection centers ever built, and its CCD detector will be an important reason for its success. Acknowledgment This work has been supported by The U.S. Department of Energy, Office of Health and Environmental Research, under Contract W31-109-ENG-38 and by The National Institutes of Health, National Center for Research Resources, under grant R R 06017.
[ 18] G a s P r o p o r t i o n a l
Detectors
By RICHARD KAHN and ROGER FOURME As X-ray counters, gas proportional detectors provide unrivaled dynamical range and sensitivity for photons in the 5 to 13 keV range. This makes them attractive for the acquisition of diffraction data in macromolecular crystallography. Initial developments were on multiwire proportional chambers (MWPCs), which are widely used as X-ray area detectors with laboratory X-ray sources: two systems based on this kind of detector are commercially available. The advent of dedicated synchrotron radiation sources has led to the design of new generations of gas detectors. At LURE, we have developed several systems based on a large MWPC with a spherical drift METHODS IN ENZYMOLOGY,VOL. 276
Copyright © 1997 by Academic Press, Inc. All rights of reproductionin any formreserved.
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space and a fast position encoder; the count rate capability of these wire detectors is limited ultimately by space-charge effects and discharges. Microgap or microstrip devices, whose count rate capabilities are, up to now, limited by the associated electronics rather than by the detector itself, provide an interesting opportunity for future developments. Introduction We shall restrict the following discussion to the case of data collection with monochromatic radiation, since the Laue technique has a specific and comparatively limited field of application. Since the mid-1980s, two major experimental developments have led to a revolution in this field: area detectors, which produce quasi-immediate digital output of the diffraction pattern, and synchrotron radiation sources, which provide intense, parallel, and tunable X-ray beams. We have discussed 1 the characteristics one might have to attain for an area detector for biological crystallography using insertion devices on high-energy synchrotron sources of the third generation. Even in the case of a much weaker X-ray source, such as a bending magnet of a high emittance storage ring, there is yet no single detector that can meet all requirements for fast and accurate measurements of intensities of closely spaced Bragg reflections with a very large dynamic range (about six decades). Since the improvement of details of a three-dimensional structure relies on the weakest part of the diffraction pattern, a crucial point in the design and operation of such devices is the optimization of the signalto-noise ratio. In this context, gas detectors have a number of interesting characteristics: (i) their dynamic range is practically unlimited; (ii) their sensitivity, or more precisely their detective quantum efficiency ( D Q E ) , 2 can be close to 1 (the stopping power of properly designed gas detectors for photons in the energy range 5-13 keV is nearly 100%, the intrinsic noise is close to zero, and the point spread function is not relevant as far as integration in two-dimensional boxes [boxels] is concerned; (iii) gas detectors can be large, allowing long crystal-to-detector distances, which are beneficial with respect to signal-to-background ratio; (iv) since each photon is processed individually, the readout time at the end of each frame can be made negligible, allowing a quasicontinuous data collection. Accordingly, the experi1R. Fourme, A. Bahri, R: Kahn, and R. Bosshard, in "Proceedingsof the European Workshop on X-Ray Detectors for Synchrotron Radiation Sources" (A. H. Walenta, ed.), pp. 16-25. Center for Sensor Systems, University of Siegen, Germany, 1991. 2The DQE involves not only the transmission of windows and electrodes and the stopping power of the detector, but also its intrinsic noise and its point spread function, i.e., the spatial response to a punctual illumination.
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ment is shorter and the duty cycle, defined as the ratio of the total exposure time to total elapsed time during the diffraction experiment, can be very close to 1. Biological crystals are degraded during X-ray irradiation, and this degradation, once it has been initiated, continues even when irradiation is interrupted. Thus, crucial parameters in the maximization of the information derived from a given sample are the sensitivity of the detector and the duty cycle of the whole data collection system. This is still valid in the case of samples kept at cryogenic temperatures during data collection, a technique that slows down but does not suppress completely degradation effects, as shown in experiments using the powerful beams from undulators at the ESRF.
Working Principle of Multiwire Proportional Chamber Since the pioneering work of G. Charpak at CERN, 3 MWPCs have been largely described in the literature (see, for instance, Ref. 4 for their application in macromolecular crystallography and Ref. 5 for their use in synchrotron radiation experiments). The basic principle of their operation is summarized hereafter. Gas proportional detectors use as a first step the absorption of an X-ray photon in a gas mixture high in xenon or argon. This photoabsorption produces one electron-ion pair whose total energy is just the energy of the initial X-ray photon. The ion returns to its fundamental state either by emission of Auger electrons or by fluorescence. Since the kinetic energy of these first electrons is far greater than the energy of the first ionization level of the chemical species present in the gas (12.13 eV for Xe), fast successive collisions with atoms (or molecules) in the gas very quickly produce a cascade of new electron-ion pairs in a small region extending over a few hundred micrometers around the conversion point. The total number of electrons (primary electrons) that are produced during this process is proportional to the energy of the absorbed X-ray photon, and is thus a few hundred for - 1 0 keV photons. Driven by the local electric field, these electrons drift at a moderate speed (a few cm/zs -1) along an electric field line. During this travel, the energy acquired between two successive collisions is too low to allow the production of new electron-ion pairs. The primary electrons then reach, without amplification, the anode 3G. Charpak, R. Bouclier,T. Bressani,J. Favier, and C. Zupan6i~,NucL lnstrum. Methods 62, 262-268 (1968). 4R. Hamlin,Methods Enzymol. 114, 416-452 (1985). 5R. A. Lewis,J. Sync. Rad. 1, 43-53 (1994).
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wire located on the electric field line along which the primary electrons were drifting. In the immediate vicinity of this very thin anode wire (a few tens of micrometers diameter), the electric field is very high ( - 1 0 V/xm -1) and the electron energy increases enough between two successive collisions to allow the production of new electron-ion pairs. This leads to the exponential "avalanche" phenomenon from which each primary electron gives rise to a few times 104 secondary electron-ion pairs. The resulting charge cloud is quasi-neutral but the charge movements in the strong electrostatic field induce, simultaneously, a negative pulse on the anode wire and positive pulses on the nearest cathode wires. After amplification, the amplitudes of these signals are high enough to allow the encoding of the avalanche position (see Fig. 1). Owing to the rapid movement of ions close to the anode wires, a large fraction of the charges are collected within a few tens of nanoseconds, but the positive ions are neutralized on the nearest cathode wire after a much t
t
Y
FIG. 1. E x p a n d e d view of a multiwire proportional c h a m b e r showing the anode plane sandwiched between the two cathode planes. A is the position of an avalanche. T h e pulses V(t) induced on the electrodes are drawn in front of each plane. T h e negative anode pulse is generated on the anode wire where the avalanche occurs while the positive cathode pulses are induced over several wires. T h e centers of these distributions are used to determine the coordinates, x and y, of the avalanche.
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longer time (a few tens of microseconds for standard MWPCs). This limits the local count rate because the presence of positive ions lowers the local electric field in the region where the avalanche took place: a new avalanche occurring at the same place before the complete evacuation of the previously created positive ions induces pulses of reduced amplitude which may not be detected. This phenomenon limits the local counting rate of standard MWPCs to ~ 1 0 4 counts sec -1 mm -2. Moreover, for extremely high local X-ray fluxes, a continuous ion column may be created between the anode and the cathodes, leading to a spark and a rapid breakdown in the chamber high voltage. Therefore, one of the major effort in the design of new gas detectors has been to reduce the time needed to evacuate positive ions. For MWPCs, this is obtained by reducing the distance between anode and cathode planes (gap) to a few millimeters. New designs based on microgap 6 and microstrip gas detectors, as introduced by Oed, 7 have led to further improvements. It is important to limit the gaseous amplification so that the detector works in the proportional regime: since the pulses are thus proportional to the energy of the incident photon (with an energy resolution of ~15% for 10 keV photons), a parasitic event whose energy does not correspond to the expected one can be rejected easily. In addition, in order to keep a high local counting rate, the number of ions produced during the avalanche must be kept as low as possible, thus limiting the space-charge effects. On the other hand, the amplitudes of the induced pulses must be high enough to allow a good positional resolution (see, for instance, Boie et al. 8 on the balance between these two requirements in the case of a linear detector). During the conversion from the incident X-ray photon to electrons, and during the avalanche process, numerous UV photons are produced by nonionizing collisions. In a pure noble gas these long-range photons would induce discharges in the chamber; they are absorbed by a UV quencher (e.g., CO2, CH4, C2H6) added to the gas mixture. Encoding Electronics T h e e n c o d i n g electronics p e r f o r m s two functions: it d i s c r i m i n a t e s the d e t e c t e d e v e n t s a c c o r d i n g to their e n e r g y a n d t h e n e n c o d e s the p o s i t i o n 6 R. A. Lewis, N. S. Fore, P. Clifford, C. Hall, W. Helsby, A. Jones, B. Parker, J. Sheldon, I. Sumner, J. S. Worgan and C. Budtz-Jorgensen, in "Proceedings of the European Workshop on X-Ray Detectors for Synchrotron Radiation Sources" (A. H. Walenta, ed.), pp. 61-68. Center for Sensor Systems, University of Siegen, Germany, 1991. 7 A. Oed, Nucl. Instrum. Methods A263, 351-359 (1988). 8 R. A. Boie, J. Fischer, Y. Inagaki, F. C. Merritt, V. Radeka, L. C. Rogers, and D. M. Xi, Nucl. Instrum. Methods 201, 93-115 (1982).
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of the accepted events. In most systems, multiple events that lead to false coordinates are rejected. The first function is carried out by measurement of the amplitude of the anode pulse. A discriminator rejects events with amplitudes that are too high--induced by multiple events, by cosmic rays, or by the harmonics of the monochromatized beam--and events with amplitudes that are too low, such as those generated by electronic noise. An accepted event generates a logical signal to trigger the position encoding. Position encoding electronics can be either analog or digital. Analog electronics are generally cheaper and easier to implement; they are used in commercial detectors. However, digital electronics are usually faster. They can accommodate any detector size and can be more easily pipelined and parallelized. LURE Radial-Drift MWPC We shall describe the two latest gas proportional detectors that have been developed at LURE in collaboration with G. Charpak, R. Bouclier, R. Million, and J. C. Santiard at CERN. Both systems are equipped with essentially identical radial-drift MWPCs. 9 One of these detectors was the chief component of the MARK II diffractometer, which was installed on the D23 station until July 1994, and was used mainly for multiwavelength experiments. The other one is part of the MARK III, a four-circle diffractometer that has been operated on the D41 station for accurate measurements of diffraction data from macromolecular crystals. A diagram of the detector is shown in Fig. 2. The MWPC features three parallel planes spaced by a gap of 4 mm. The central plane (anode plane), at a potential between +2650 V and +2800 V, is made of 260 parallel and very thin gold-plated tungsten wires (diameter 20/.~m) with a 2-mm pitch. Two cathode planes, hold at ground potential, are made of 512 berylliumbronze wires of a larger diameter (200/xm) with a 1-mm pitch. Wires of one cathode plane are parallel to the anode wires while those of the other cathode plane are orthogonal. The anode and cathode planes must be kept rigid, and at fixed distances from each other; to achieve this, wires are mounted on epoxy resin frames, which are sandwiched between two thick aluminum frames. The radial drift space is a device consisting of two electrodes that are concentric spherical caps. The crystal is set at the common origin of the spheres defining these caps. One of these electrodes, at -17 kV, is made 9R. Kahn, R. Fourme, R. Bosshard,and V. Saintagne,Nucl. 603 (1986).
lnstrum. Methods
A246, 596-
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MWPC FIG. 2. Schematic diagram of the radial drift chamber, with the path of an X-ray photon and its resulting bunch of electrons. C is the center of the crystal, A is the point of interconversion of the X-ray photon into a bunch of electrons, and P is the spot position on the MWPC. The diagram shows the beryllium entrance window, B; the stainless steel grid, G; a portion of the series of conductive rings, R; and the multiwire planes of the MWPC.
of beryllium (thickness 500/xm) and has a bending radius of 410 mm. This electrode, which has a low absorption to X-rays of wavelengths below 2 A, is the entrance window of the detector. The second electrode is a stainless steel grid with a bending radius of 554 mm. This grid is at - 7 kV, making it transparent to electrons. Both electrodes are supported by a conical piece of stesalite, an insulating material with low resistivity that allows for the evacuation of surface charges. The inner surface of the conical edge is printed with a series of conductive rings whose potentials are adjusted with a chain of resistors ensuring a variation of ring potentials proportional to 1/r (r being the distance from the origin to a ring). The two spherical
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electrodes produce a radial field, and the rings correct field distortions in the vicinity of the insulator. In the region between the grid and the anode plane, electrons produced in the drift space are transferred to the MWPC. Since the grid and the first cathode plane are equipotential surfaces, field lines are normal to these surfaces. When an X-ray photon is scattered by the crystal, it is absorbed in the drift space and converted into an electron bunch that drifts through the gas. It passes through the stainless steel grid, and then curves in its path to enter the MWPC, creating an avalanche when it nears the anode. This avalanche produces on the anode and cathode planes electric pulses that are used to determine the coordinates of the event. Finally, this gives the angular coordinates of the original scattered X-ray photon. The radial drift space has several distinct useful effect: (1) It suppresses parallax very effectively. Parallax is a classical problem in MWPC detectors when X-ray beams are not orthogonal to the planes of wires, resulting in radially elongated spots. Two solutions have been proposed to overcome this problem without sacrificing detector efficiency. The first one is the use of pressurized xenon by which photons are converted to electrons in a thin layer of gas; for instance, in the Siemens detectors, 1° which are filled with a xenon gas mixture at 4 bars, photons are converted close to the spherical entrance window, thus limiting the parallax effect, at least for 8 keV (Cu Ka) photons. A similar configuration is described in Ref. 11. This solution is difficult to implement on large detectors. The second solution, and the one we have adopted, is the use of a drift space thick enough to give a probability of absorption close to unity and where electric field lines have radial symmetry. As such, all photons diffracted in a given direction by the crystal (placed at the origin of this radial field) are converted into electron bunches, which follow the same field line irrespective of where the conversion occurred, and thus are detected at a single point on the MWPC. (2) In a detector without a drift space, each electron bunch is concentrated into a small volume and the avalanche is produced in the immediate vicinity of the nearest anode wire. Pulse distributions induced on the cathode planes by the avalanche, and therefore the encoded position of the event, are thus centered along this anode wire. As a consequence, the resulting image reproduces an image of the anode plane structure. In contrast, with a drift chamber, the cross section of the electron branch is 10 R. M. Durbin, R. Burns, J. Moulai, P. Metcalf, D. Freymann, M. Blum, J. E. Anderson, S. C. Harrison, and D. C. Wiley, Science 232, 1127-1132 (1986). 11 S. P. Chernenko, A. B. Ivanov, S. A. Movchan, L. P. Smykov, and Yu. V. Zanevsky, in "Proceedings of the European Workshop on X-Ray Detectors for Synchrotron Radiation Sources" (A. H. Walenta, ed.), pp. 82-86. Center for Sensor Systems, University of Siegen, Germany, 1991.
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broadened to several mm 2 as it diffuses along the drift space and the corresponding event triggers avalanches over several anode wires. These avalanches induce pulses on cathode planes, which allow the positions of events that occur between anode wires to be determined correctly. The result is a continuous distribution of encoded positions in both directions of the anode plane, rather than a discrete one. Paradoxically, the blurring that accompanies the drift of the footprint of each event produces a more finely resolved diffraction pattern. (3) Owing to the low drift velocity of electrons, the drift space essentially cancels effects of the pulsed time structure of synchrotron radiation. Two photons scattered by the crystal during the same burst are counted as separate events because the respective drift times of the two bunches of electrons are likely to differ by a time interval larger than the dead time of the position encoder. (4) Because of the curved trajectory of the electron bunches, there must be a mapping function to give the distance, p, from the center of the detector to the located event as a function of the angle q~ between the scattered X-ray beam and the normal to the detector. For the radial drift chamber, the expression of p can be derived from the expression of the electric field between a sphere at potential V, and a plane at ground potential. A good approximation, which differs from the observed value by at most 0.2 mm (i.e., 0.2 pixel) at the periphery of the useful area, is p = 2D tan (q~/2), where D is the mean value between the crystal-to-grid distance and the crystal-to-first cathode plane distance. Because of this special mapping function, the spherical drift chamber has the unique property to give a stereographic projection of the reciprocal space. 12 The theoretical value of D for the LURE detectors is 571.5 mm; this parameter is refined by the data analysis software. The coverage of the detector is a cone with a semi-angle opening of 25 °. Field lines are undistorted up to 23.7 °, and reflections with beams up to 23 ° from the axis are kept in order to collect complete boxels. Thus, the useful area on the anode and cathode planes is a disk of diameter 486 mm partitioned into about 185,500 pixels of 1 × 1 m m 2. The fragile beryllium electrode at -17 kV is placed in a cylindrical compartment enclosed by a grounded aluminized Mylar window. A heliumfilled cone can be placed between the beam catcher and this window in order to minimize both absorption and scattering by air. The detector is filled with a mixture of 40% ethane, 58-59% argon/xenon, and 1-2% of ethanol. The percentage of xenon is adjusted according to wavelength in 12G. Bricogne, in "Computational Aspects of Protein Crystal Data Analysis" (J. R. Helliwell, ed.), pp. 107-145. Daresbury Laboratory, Warrington, U.K., 1987.
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order to absorb at least 99% of the diffracted beams. At the maximum percentage of xenon, the shortest wavelength compatible with an effective parallax suppression is about 0.95 A. Ethane acts as a UV quencher. Ethanol is also a quencher, 13 which improves the high count rate capability and the wire lifetime. Since the drift path of the electron bunch is long, the purity of gas is essential to minimize electron capture. For this purpose, materials used to build the detector are chosen to be vacuum-compatible so that the chamber is airtight. The gas is continuously circulated at atmospheric pressure and is purified by flowing through a removable Oxysorb cartridge.
Position Encoders and Data Acquisition System In both systems, the determination of x and y coordinates is made from the distribution of cathode pulses. The detector of the M A R K II diffractometer is equipped with a brute force digital position encoder, since the primary goal of the designers, J.-C. Santiard at C E R N and R. Bosshard at LURE, was a high count rate. Each cathode wire is equipped with an amplifier and a discriminator that labels the pulses as either 0 or 1. The binary patterns produced by the event on both cathode planes are analyzed by fast priority encoders that find boundaries of the cluster of pulses. The width of the cluster is probed and the few events with an abnormally large width are rejected. Then, coordinates of the cluster center are determined by two digital processors and used as the coordinates of the event. This procedure requires a set of amplifiers-discriminators with almost identical gains; this was obtained at the manufacturing stage by laser burning a carbon film resistor in each amplifier. The dead time of the encoder is 240 nsec, resulting in a fractional count loss of 13% at 300 kHz. This data acquisition system provides 512 x 512 pixels of 1 x 1 mm 2. For the latest detector on the M A R K III diffractometer, a new data acquisition system has been built. TMThe readout consists of 2 x 128 cathode strips (4 mm width) that are connected to shaping amplifiers coupled to a fast position encoder. Cathode amplitudes are digitized using 8-bit flash analog-to-digital converters. Valid cathode clusters lead to a few (3 to 5) nonzero consecutive digitized amplitudes. A specialized module that uses two digital signal processors calculates the center of gravity of the cluster and corrects the results according to the number of active strips. The 13 M. Atac, I E E E Trans. Nucl. Sci. NS-31(1), 99-102 (1984). 14 A. Bahri, R. Bosshard, J.-C. Santiard, R. Kahn, and R. Fourme, Rev. Sci. lnstrum. 63(1), 655-658 (1991).
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intrinsic resolution of this system is 1024 × 1024 pixels, but images are currently stored using 512 × 512 pixels in order to limit the volume of data. Each data acquisition system is built from VME electronic modules with dual 16-bit memories. For each accepted event with given x, y coordinates, the corresponding cell in the memory is incremented by 1. When this operation leads to an arithmetic overflow, the memory cell is cleared and the coordinates are stored in an overflow register (512 overflows can be stored per image). While the current electronic image (frame) is being recorded, the previous one is transferred to a magnetic disk for temporary storage. Frames are finally exported on an inexpensive 8-mm videotape. A standard 112 meter tape may store at least 4000 frames; since the crystal is rotated in most cases by 0.05° per frame, this corresponds to a total crystal rotation of 200°. The exposure time of each frame is typically 12-30 sec, and the dead time between two adjacent frames is at most 1.3 sec. Thus, the duty cycle of these detectors, as defined previously, is 91-96%. In both systems, the noise of the detector with its data acquisition system is negligible (10 -3 count pixe1-1 sec -1, on average).
Data Analysis Software The derivation of intensities from digital images is made off-line. The software package installed on the MARK II and MARK III diffractometers is based on MADNES. 15 This package does not require a prior knowledge of unit cell parameters and can work on a randomly oriented crystal. Corrections for beam intensity, count losses, and Lorentz factor are applied, together with the polarization correction suitable for the highly polarized synchrotron radiation beam. 16The background is estimated from the same pixels as those used for the integration of the peak: the counts used for this estimation are taken from sections in the three-dimensional profile (voxel), which were collected just before and just after the peak. The best results are obtained using MADNES to extract the corrected voxels, followed by the calculation of the final intensity data with the profile-fitting algorithm PROCOR. 17 15 A. Messerschmidt and J. W. Pflugrath, J. Appl. Cryst. 20, 306-315 (1987). The current package is the product of a series of EEC workshops on position sensitive detector software that have been organized by G. Bricogne.12 16R° Kahn, R. Fourme, A. Gadet, J. Janin, C. Dumas, and D. Andr6, J. Appl. Cryst. 15, 330-337 (1982). 17 W. Kabsch, J. Appl. Cryst. 21, 916-924 (1988).
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Performance
Detector with Digital Position Encoder: M A R K H Diffractometer The spot shape given by the detector is dominated by the point spread function (PSF) and not by the crystal volume. With a gas mixture providing an essentially complete absorption of photons in the drift space, the PSF is nearly Gaussian with a FWHM of 1.0-1.1 mm over the whole detector in the wavelength range 0.95-2 .&. The root mean square (rms) difference between observed and calculated spot positions is typically 300/zm (<½ pixel). These differences are due to cumulated effects of errors in crystal and detector centering, approximations in the mapping function, electrode distortions, construction defaults of the sensing planes, and positional shifts due to the encoder. The small error in spot position, together with the PSF, enables the routine use of 5 mm × 5 mm boxels on the detector. Since the detector has a discrete structure, the positional differential nonlinearity is fairly high, and frames exhibit the characteristic kilt-like pattern, which is observed on images produced by most MWPCs. This effect is due to dispersion in the response of discrete channels. Since the cluster boundaries are determined from the cathode pulses just above the threshold value, slight inhomogeneities in the gains of amplifiers may displace the coordinates of an event by _+1 pixel; this event is not lost, but its coordinates are shifted. As integration is done in a fairly large boxel, this effect does not alter the quality of intensity measurements. The boxelintegrated response is indeed quite uniform over the whole sensitive surface, and we do not apply a correction table obtained from uniform illumination of the detector as needed by several commercial models. The sensitivity is limited only by the transparency of the entrance window and by count losses. For a count rate of 200,000 counts sec -1, the DQEs at the wavelengths of 1 .& and 1.4 ,~ are 0.80 and 0.75, respectively. The count rate of the detector is in practice not limited by the encoder, but by the physical behavior of the detector at high local and total count rates. The local count rate is at most 20,000 counts sec -1 mm -2. The total count rate cannot currently exceed about 250,000 counts sec -I for comfortable and uninterrupted operation, even with the addition of ethanol in the gas mixture. At higher count rates, the detector becomes unstable, with discharges occurring between the anode and cathode planes. The detector is efficiently protected against the effect of these discharges by a device which switches off the high voltage on the anode plane in 1 txsec when the current delivered by the power supply exceeds a preset limit.
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Detector with Centroid Position Encoder: M A R K III Diffractometer With the centroid encoder, several characteristics are markedly improved with respect to the digital encoder: the differential nonlinearity is reduced; the local positional resolution is -150 ~m and the long range positional reproducibility is better than 200/~m so that the boxel size can be reduced to 3 x 3 mm 2. The useful count rate is somewhat decreased with respect to the count rate capability of the MARK II encoder and is currently 200,000 counts sec 1 with a fractional count loss of 20%.
Suitability for Macromolecular Crystallography Owing to their high sensitivity, the LURE detectors are photon counters that use, nearly optimally, every photon scattered by the sample. Counts accumulated in each boxel are highly significant, because of the multistep selection applied to each event and to the negligible detector noise. Other factors that contribute to a high signal-to-noise are relevant: (i) detector performance and design, boxels are relatively small compared to the total working area (0.013 and 0.005% for MARK II and MARK III, respectively); the crystal-to-detector distance is large; paths of diffracted beams in air are short, thus reducing parasitic scattering; (ii) x-ray source, the quasiparallel and highly monochromatic radiation ensures that the Bragg reflections have a narrow angular width; and (iii) data collection mode, due to the good duty cycle of the detector, the crystal rotation per frame, A~b,can be made comparable to the angular width of Bragg reflections so that the integrated background is minimal. There are two further advantages in using the smallest A~b compatible with the angular width of Bragg reflections: (i) crystals with longer unit cell parameters can be tackled: close reflections that overlap when using a large A~b are thus well separated because they are usually recorded on different frames; and (ii) a smaller z~b allows for a more accurate determination of unit cell parameters, orientation matrix, and detector parameters: after refinement, the typical rms deviation between observed and calculated ~b angles of reflection centroids is 0.0030.008 °. The stability of both positional and intensity responses of the detectors is very good at the time scale of a data collection. This is due to the photoncounting principle and to the mechanical stability of the detector, which are reflected in the quasi-invariance of the mapping function and of the pixel response. These detectors have also limitations. They are not suitable for wavelengths shorter than 0.95 .~ (anomalous dispersion experiments at the K-absorption edge of selenium are still feasible). Their count rate capability
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GAS PROPORTIONAL DETECTORS TABLE I COLLAGENASE DATA COLLECTION STATISTICSAT 1.71 .A RESOLUTIONa
Crystal
A (,~)
A~
Observed
Rejected
Unique
Completeness
gmerge
1 2
1.3857 1.3790
94 ° 96 °
101,675 109,495
1667 (1.6%) 2263 (2.1%)
32,496 42,754
59% 79%
0.046 0.038
a
Two crystals of good size (0.4 x 0.4 x 0.8 mm 3) from the native protein (space group 1422, a = 111.7, c = 165.8 A) were used. They were mounted in a random orientation. For both data collections, the total angular range, Aq~, was covered using 0.05 ° frames with an exposure time of 30 sec. The swing angle of the detector was 24 °, which corresponds to a maximum resolution of 1.71 -A. The diffraction is strong at low and medium resolution and decays rapidly below about 2.4 * . The useful resolution of the merged data set is 1.79 ,~, with 40,064 unique reflections (85% completeness) from which 41.5% have their intensity I > 3 o-(I). The final Rmerge at this resolution is 0.039.
is severely limited. Finally, they are more complex to operate and require more maintenance than imaging plates or CCD detectors.
Results Some results obtained with the MARK II data collection system for a few applications are summarized hereafter. They include single-wavelength and multiple-wavelength experiments. As described previously, intensities are integrated with MADNES. Data are binned into batches of a few degrees and scaled with the program R O T A V A T A from the CCP4 suite. 18 Scale factors are applied and a few outlier measurements are rejected by the CCP4 program A G R O V A T A . These results have been obtained by various groups and are representative of the quality of data that can be obtained routinely under the various temporal and experimental constraints of a synchrotron radiation instrument. An example of a high-resolution data collection at a single wavelength using native crystals of normal size is summarized in Table I. Collagenase f r o m Hypoderma lineatum 19 is one of our standard test crystals for the evaluation of various data collection systems using conventional and synchrotron sources. 2° With a net data collection speed of 5.80 ° per hour, the 18 CCP4, The SERC (UK) Collaborative Computing Project No. 4, a Suite of Programs for Protein Crystallography, Daresbury Laboratory, Warrington WA4 4AD, U K (1979). 19 I. Broutin, B. Arnoux, and A. Ducruix, ICSN, Gif sur Yvette, France. 2o I. Broutin, doctoral thesis, Universit6 Paris-Sud, Orsay, France, 1993.
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T A B L E II 2.5 .& RESOLUTION FOR CRD OF R A T
MAD
D A T A COLLECTION STATISTICS TO
MANNOSE-BINDING PROTEIN a
Wavelength
Number of reflections
Completeness (%)
Rmerge
Remote Peak Edge
13,812 13,461 13,421
93.7 91.3 91.0
0.031 0.043 0.043
Three nominal wavelengths were used: 1.4400 ,~ (remote), 1.5356 ,~ (HoLni absorption peak), 1.5363 A (HoLni absorption edge). The exposure time per 0.10 ° frame was 30 sec for the first 72 ° of data and 40 sec for the remainder, where a total rotation of 106 ° was collected around b*, followed by 45 ° around a*. Rmerge values are calculated for truly equivalent reflections. The program A G R O V A T A was modified by W. Weis in order to reject outlying measurements only with respect to redundant copies of the same Bijvoet mate, rather than with respect to the mean of all I(+_h). The higher values of Rmerge at the peak and edge wavelengths are probably due to shifts in the monochromator setting between various batches of reflections. The effects of these shifts are negligible at the wavelength remote from the anomalous scatterers absorption edge.
final Rmerg e value 2a obtained up to a resolution of 1.79 A with a completeness of 82.9% is 0.039. Tables II and III summarize statistics from two M A D data collections done at three wavelengths. In both cases they were completed using only one sample. The first one is the data collection from the carbohydraterecognition domain of rat mannose-binding protein, where two native Ca 2+ ions were substituted for two H o 3+ ions. 22 The second one is the data collection from cutinase from Fusarium solani pisi 23 with a single mercury atom per molecule, covalently attached to an engineered cysteine residue. The cutinase structure had been previously solved and refined to high resolution by the M I R method, 24 thus providing an accurate model. Multiple-wavelength data have been used as a development tool in the use of conventional heavy atoms as anomalous scatterers and the development of a statistical analysis of M A D data. 25 In Table IV, a comparison 26 of two data collections from nucleoside 21 Rm~rge = ~ ~ h
i
Ill(h) - ( l ( h ) ) l / ~ ~ li(h), where li(h) is the ith measurement and (l(h)) h
i
is the weighted mean of all measurements of I(h). 22 W. I. Weis, R. Kahn, R. Fourme, K. Drickamer, and W. A. Hendrickson, Science 254, 1608-1615 (1991). 23 E. de la Fortelle, R. Kahn, and R. Fourme, LURE, Orsay; C. Martinez and C. Cambillau, LCCMB, Marseille, France. 24 C. Martinez, P. de Geus, M. Lauwereys, G. Matthyssens, and C. Cambillau, Nature 356, 615-618 (1992). 25 M. Chiadmi, R. Kahn, E. de la Fortelle, and R. Fourne, Acta Cryst. D49, 522-529 (1993). 26 Data are from Refs. 1 and 6.
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T A B L E III MAD
DATA COLLECTION STATISTICS TO 1.65 FOR CUTINASE a
,~
RESOLUTION
Wavelengths
Batch 1
Batch 2
Batch 3
Batch 4
Total
Remote Peak Edge
0.025 0.035 0.030
0.027 0.036 0.032
0.029 0.035 0.035
0.031 0.039 0.033
0.028 0.036 0.032
aSpace group P2z, a = 35.1, b = 67.4, c = 37.05, /3 = 94 ° . The three nominal wavelengths for M A D data collection are 1.0143 ,~ (remote), 1.0061 ,~ (HgLni absorption peak), and 1.0093 A ( H g L m absorption edge). T h e exposure time per 0.05 ° frame was 12 sec. Data at the three wavelengths were m e a s u r e d successively in four batches of 48 ° with an overlap of 1° between each batch (total 189°). A s the n u m b e r of truly equivalent reflections is low, the quoted R values are calculated in the space group P21, so that Bijvoet mates are not distinguished. For this reason, R values, which incorporate the a n o m a l o u s signal, are largest and smallest at the peak and remote wavelengths, respectively.
diphosphate kinase (NDPK) from Dictyostelium discoideum 27 is presented. The first data collection was done using the LURE W32 wiggler line equipped with an EMBL type II imaging plate scanner and the second one using the D23 instrument equipped with the radial drift MWPC. Despite an X-ray flux about 50 times higher, the net data flow rate is only two times higher with the wiggler instrument because of the much lower duty cycle of the imaging plate scanner. The internal consistency of data is better for the MWPC data set.
Future Developments As already mentioned, improvements in the counting rate capability of gas detectors have led to the design of new detectors to increase the local counting rate capability by limiting space-charge effects, and of new electronics to increase the overall counting rate capability of the positionencoding system. On the detector side, a first approach which is developed at the Daresbury Laboratory is the microgap detector. 6 It is similar to a conventional chamber but its anode-cathode spacing is very much smaller (--300 ~m). This design effectively reduces the space-charge effect: at a local 27 Crystals are supplied by C. D u m a s , LBS, Orsay, France.
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T A B L E IV COMPARISON OF N D P K DATA COLLECTIONS USING T w o DIFFERENT EXPERIMENTAL SETUPS AT L U R E
Characteristic
W32 instrument
D23 instrument
Source Beam intensity (arbitrary scale) Detector type Rotation/frame Exposure time/frame Elapsed time/frame Duty cycle Rotation rate Rmerge (2.2 ,~ data)
5-pole superconducting wiggler 100 EMBL type II image plate scanner 1° 30 sec 172 sec 0.17 20.9 ° hour -~ 0.046
Bending magnet -2 Radial drift M W P C 0.05 ° 15 sec 16 sec 0.94 11.2 ° hour -1 0.028
counting rate of 3 × 10 5 counts s e c - 1 mm -2, amplitudes of the pulses are reduced by only 15%, and just by relaxing slightly the energy discrimination, an excellent counting rate linearity can be achieved until rates are well in excess of 105 counts sec -~ mm -2. Originally introduced by Oed, 7 the microstrip gas chambers (MSGCs) look very promising and have led to a great deal of work stimulated by potential applications in high-energy physics and X-ray astronomy. They are made using photolithographic techniques to produce an electrode structure on an insulating substrate. A typical design is shown in Fig. 3. These devices can be mass produced in large sizes to very high (0.2/zm) tolerances with very small electrode spacings (a few tens of micrometers). This small spacing allows a rapid evacuation of the positive ions, therefore reducing the space-charge effects. Owing to the fine electrode structure, a very good spatial resolution can be obtained.
VD
400~m Vc
VA /
1~
Anode ~ . . ~ m V
WB
FIG. 3. Cross-sectional drawing of a microstrip chamber. Typical distances for the electrode structure are indicated. The voltages VA, Vc, and VB applied to the anode, the cathode, and the backplane, respectively, are usually VA -- 500 V, VB = Vc = 0 V. The drift voltage, Vo, is set to at a negative value in order to produce an electric field of 500 to 1000 V cm -~.
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Numerous studies have been performed to determine the optimum configuration for an MSGC. 28-32 The properties of the substrate play a key role when the electrode structure is directly deposited on it: it must exhibit sufficient bulk and surface conductivity to prevent charging that would affect the gain stability at high fluxes. However, the resistivity should be kept high enough to limit leakage currents and avoid heating. Alternate electrode structures have also been proposed to avoid these charge-up effects. Excellent long-term stability at high fluxes with microstrips, built up using deep X-ray lithography techniques on polymer substrate, have been reported. 33 Very high count rates with stable gains up to 8 × 10 6 counts sec 1 mm 2 have been obtained using an MSGC where the anodes are separated from a continuous conductive cathode by insulating strips. 34 When the substrate is thin enough (-100 tzm), detectable pulses are induced on the backplane of the MSGC. These pulses can be collected on strips deposited on the rear plane and orthogonal to the microstrip structure. This allows two-dimensional position encoding of the event by using a time correlation with signals simultaneously generated on the microstrip electrodes. This procedure is rather complex and, at the time being, takes at least a few tens of nanoseconds. Therefore, the encoding electronics must be segmented to handle counting rates in excess of 10 7 counts sec -1 over the whole surface of the detector. An ultimate state of segmentation could be achieved by using small pads deposited on the backplane of the MSGC and bounded to their own electronics, making them individual counters. Conclusion Gas proportional detectors are well suited for the accurate measurement of X-ray diffraction data. The high signal-to-noise ratio and low background 28 C. Budtz-Jorgensen, A. Bahnsen, C. Olesen, M. M. Madsen, P. Jonasson, H. W. Schnopper, and A. Oed, Nucl. Instrum. Methods A310, 82-87 (1991). 29 j. E. Bateman, J. F. Connolly, R. Stephenson, and J. Morse, in "Proceedings of the European Workshop on X-Ray Detectors for Synchrotron Radiation Sources" (A. H. Walenta, ed.), pp. 87-91. Center for Sensor Systems, University of Siegen, Germany, 1991. 3o F. Angelini, R. Bellazzini, A. Brez, M. M. Massai, G. Sprande, and M. R. Torquati, Nucl. lnstrum. Methods A315, 21-32 (1992). 31 R. Bouclier, J. J. Florent, J. Gaudaen, G. Millon, A. Pasta, L. Ropelewski, F. Sauli, and L. I. Shekhtman, Nucl. Instrum. Methods A323, 240-246 (1992). 32 Yu. N. Pestov and L. I. Shekhtman, Nucl. Instrum. Methods A338, 368-374 (1994). 33 M. Lemonnier, A. Bahri, M. Bordessoule, F. Bartol, A. Labeque, Z. Liu, S. Megtert, M. Roulliay, M. F. Ravet, F. Rousseaux, and J. Perrocheau, NucL Instrum. Methods A349, 274-276 (1994). 34 F. Angelini, R. Bellazzini, A. Brez, M. M. Massai, R. Raffo, G. Sprande, and M. A. Spezziga, Nucl. Instrum. Methods A335, 69-77 (1993).
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achieved by these systems make them particularly suitable for collecting quality data at high resolution. New developments are taking place to improve their counting rate capability, which currently limits their use for very demanding synchrotron radiation experiments.
[19]
Diffraction-Data Processing for Electronic Detectors: Theory and Practice
By J A M E S
W. PFLUGRATH
Introduction With the advent of electronic area detectors, software was developed both to drive data acquisition and to process their data. There are many examples of software packages to analyze images from area detectors. These include the UCSD software,1 BUDDHA, 2 XENGEN, 3 XDS, 4 SAINT, 5 and MADNES. 6 All of these software packages use some variant of the rotation or oscillation method of data acquisition. This method was developed originally for data collection with X-ray film and currently is used almost universally. The geometry is simple: a crystal is rotated around a single axis while the detector remains fixed. The main feature to distinguish electronic area detectors from film and imaging-plate detectors is the readout time. Electronic detectors have a relatively fast readout, so in any given experiment a smaller volume of reciprocal space can be sampled on each image without the ratio of readout time to exposure time becoming significant. In a typical experiment the crystal is rotated from 0.1 to 0.25 degrees per image, a small enough rotation increment so that reciprocal lattice points are found on a few adjacent images, all of which must be considered when integrating the inten1 A. J. Howard, C. Nielsen, and Ng. H. Xuong, Methods Enzymol. 114, 452 (1985). 2 M. Blum, P. Metcalf, S. C. Harrison, and D. C. Wiley, J. Appl. Cryst. 20, 235 (1987). 3 m. Howard, G. L. Gilliland, B. C. Finzel, T. L. Poulos, D. H. Ohlendorf, and F. R. Salemme, J. Appl. Cryst. 20, 383 (1987). 4 W. Kabsch, J. Appl. Cryst. 21, 916 (1988). 5 Siemens, SAINT Software Reference Manual, Pub. No. 269-014200, Siemens Industrial A u t o m a t i o n Inc., Madison, W I (1993). 6 A. Messerschmidt and J. W. Pflugrath, J. Appl. Cryst. 20, 306 (1987).
METHODS IN ENZYMOLOGY,VOL. 276
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previously, is its ability to bin neighboring pixels together into larger pseudo-pixels. Binning is a standard capability of CCDs. Rather than individual rows of pixels being shifted into the serial readout register, two rows are shifted together to combine their charges. The readout register also is shifted out two at a time, so the signal is four times larger than that of an individual pixel. Although binning quadruples the pixel area, it only doubles the dynamic range, halves the read noise, and halves the measurement uncertainty, because the readout pixels in the linear registers are only twice the size of ordinary pixels. Binning also quadruples the speed with which each data frame can be read out, decreasing the dead time between frames, and reduces the size of the computer file representing each data frame, reducing the complexity of the computer system needed to process data and the time needed for processing. This APS-1 detector is a significant part of an integrated technical facility that also features an undulator X-ray source on the APS, carefully designed X-ray optics, modern networking (VME; HIPPI; ATM), and a powerful computing environment (multiple processors, fast, large RAID disk arrays). The complete facility is now being commissioned and will be fully operational during summer 1997. We believe that the SBC facility will be among the most effective crystallographic data collection centers ever built, and its CCD detector will be an important reason for its success. Acknowledgment This work has been supported by The U.S. Department of Energy, Office of Health and Environmental Research, under Contract W31-109-ENG-38 and by The National Institutes of Health, National Center for Research Resources, under grant R R 06017.
[ 18] G a s P r o p o r t i o n a l
Detectors
By RICHARD KAHN and ROGER FOURME As X-ray counters, gas proportional detectors provide unrivaled dynamical range and sensitivity for photons in the 5 to 13 keV range. This makes them attractive for the acquisition of diffraction data in macromolecular crystallography. Initial developments were on multiwire proportional chambers (MWPCs), which are widely used as X-ray area detectors with laboratory X-ray sources: two systems based on this kind of detector are commercially available. The advent of dedicated synchrotron radiation sources has led to the design of new generations of gas detectors. At LURE, we have developed several systems based on a large MWPC with a spherical drift METHODS IN ENZYMOLOGY,VOL. 276
Copyright © 1997 by Academic Press, Inc. All rights of reproductionin any formreserved.
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space and a fast position encoder; the count rate capability of these wire detectors is limited ultimately by space-charge effects and discharges. Microgap or microstrip devices, whose count rate capabilities are, up to now, limited by the associated electronics rather than by the detector itself, provide an interesting opportunity for future developments. Introduction We shall restrict the following discussion to the case of data collection with monochromatic radiation, since the Laue technique has a specific and comparatively limited field of application. Since the mid-1980s, two major experimental developments have led to a revolution in this field: area detectors, which produce quasi-immediate digital output of the diffraction pattern, and synchrotron radiation sources, which provide intense, parallel, and tunable X-ray beams. We have discussed 1 the characteristics one might have to attain for an area detector for biological crystallography using insertion devices on high-energy synchrotron sources of the third generation. Even in the case of a much weaker X-ray source, such as a bending magnet of a high emittance storage ring, there is yet no single detector that can meet all requirements for fast and accurate measurements of intensities of closely spaced Bragg reflections with a very large dynamic range (about six decades). Since the improvement of details of a three-dimensional structure relies on the weakest part of the diffraction pattern, a crucial point in the design and operation of such devices is the optimization of the signalto-noise ratio. In this context, gas detectors have a number of interesting characteristics: (i) their dynamic range is practically unlimited; (ii) their sensitivity, or more precisely their detective quantum efficiency ( D Q E ) , 2 can be close to 1 (the stopping power of properly designed gas detectors for photons in the energy range 5-13 keV is nearly 100%, the intrinsic noise is close to zero, and the point spread function is not relevant as far as integration in two-dimensional boxes [boxels] is concerned; (iii) gas detectors can be large, allowing long crystal-to-detector distances, which are beneficial with respect to signal-to-background ratio; (iv) since each photon is processed individually, the readout time at the end of each frame can be made negligible, allowing a quasicontinuous data collection. Accordingly, the experi1R. Fourme, A. Bahri, R: Kahn, and R. Bosshard, in "Proceedingsof the European Workshop on X-Ray Detectors for Synchrotron Radiation Sources" (A. H. Walenta, ed.), pp. 16-25. Center for Sensor Systems, University of Siegen, Germany, 1991. 2The DQE involves not only the transmission of windows and electrodes and the stopping power of the detector, but also its intrinsic noise and its point spread function, i.e., the spatial response to a punctual illumination.
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ment is shorter and the duty cycle, defined as the ratio of the total exposure time to total elapsed time during the diffraction experiment, can be very close to 1. Biological crystals are degraded during X-ray irradiation, and this degradation, once it has been initiated, continues even when irradiation is interrupted. Thus, crucial parameters in the maximization of the information derived from a given sample are the sensitivity of the detector and the duty cycle of the whole data collection system. This is still valid in the case of samples kept at cryogenic temperatures during data collection, a technique that slows down but does not suppress completely degradation effects, as shown in experiments using the powerful beams from undulators at the ESRF.
Working Principle of Multiwire Proportional Chamber Since the pioneering work of G. Charpak at CERN, 3 MWPCs have been largely described in the literature (see, for instance, Ref. 4 for their application in macromolecular crystallography and Ref. 5 for their use in synchrotron radiation experiments). The basic principle of their operation is summarized hereafter. Gas proportional detectors use as a first step the absorption of an X-ray photon in a gas mixture high in xenon or argon. This photoabsorption produces one electron-ion pair whose total energy is just the energy of the initial X-ray photon. The ion returns to its fundamental state either by emission of Auger electrons or by fluorescence. Since the kinetic energy of these first electrons is far greater than the energy of the first ionization level of the chemical species present in the gas (12.13 eV for Xe), fast successive collisions with atoms (or molecules) in the gas very quickly produce a cascade of new electron-ion pairs in a small region extending over a few hundred micrometers around the conversion point. The total number of electrons (primary electrons) that are produced during this process is proportional to the energy of the absorbed X-ray photon, and is thus a few hundred for - 1 0 keV photons. Driven by the local electric field, these electrons drift at a moderate speed (a few cm/zs -1) along an electric field line. During this travel, the energy acquired between two successive collisions is too low to allow the production of new electron-ion pairs. The primary electrons then reach, without amplification, the anode 3G. Charpak, R. Bouclier,T. Bressani,J. Favier, and C. Zupan6i~,NucL lnstrum. Methods 62, 262-268 (1968). 4R. Hamlin,Methods Enzymol. 114, 416-452 (1985). 5R. A. Lewis,J. Sync. Rad. 1, 43-53 (1994).
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wire located on the electric field line along which the primary electrons were drifting. In the immediate vicinity of this very thin anode wire (a few tens of micrometers diameter), the electric field is very high ( - 1 0 V/xm -1) and the electron energy increases enough between two successive collisions to allow the production of new electron-ion pairs. This leads to the exponential "avalanche" phenomenon from which each primary electron gives rise to a few times 104 secondary electron-ion pairs. The resulting charge cloud is quasi-neutral but the charge movements in the strong electrostatic field induce, simultaneously, a negative pulse on the anode wire and positive pulses on the nearest cathode wires. After amplification, the amplitudes of these signals are high enough to allow the encoding of the avalanche position (see Fig. 1). Owing to the rapid movement of ions close to the anode wires, a large fraction of the charges are collected within a few tens of nanoseconds, but the positive ions are neutralized on the nearest cathode wire after a much t
t
Y
FIG. 1. E x p a n d e d view of a multiwire proportional c h a m b e r showing the anode plane sandwiched between the two cathode planes. A is the position of an avalanche. T h e pulses V(t) induced on the electrodes are drawn in front of each plane. T h e negative anode pulse is generated on the anode wire where the avalanche occurs while the positive cathode pulses are induced over several wires. T h e centers of these distributions are used to determine the coordinates, x and y, of the avalanche.
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longer time (a few tens of microseconds for standard MWPCs). This limits the local count rate because the presence of positive ions lowers the local electric field in the region where the avalanche took place: a new avalanche occurring at the same place before the complete evacuation of the previously created positive ions induces pulses of reduced amplitude which may not be detected. This phenomenon limits the local counting rate of standard MWPCs to ~ 1 0 4 counts sec -1 mm -2. Moreover, for extremely high local X-ray fluxes, a continuous ion column may be created between the anode and the cathodes, leading to a spark and a rapid breakdown in the chamber high voltage. Therefore, one of the major effort in the design of new gas detectors has been to reduce the time needed to evacuate positive ions. For MWPCs, this is obtained by reducing the distance between anode and cathode planes (gap) to a few millimeters. New designs based on microgap 6 and microstrip gas detectors, as introduced by Oed, 7 have led to further improvements. It is important to limit the gaseous amplification so that the detector works in the proportional regime: since the pulses are thus proportional to the energy of the incident photon (with an energy resolution of ~15% for 10 keV photons), a parasitic event whose energy does not correspond to the expected one can be rejected easily. In addition, in order to keep a high local counting rate, the number of ions produced during the avalanche must be kept as low as possible, thus limiting the space-charge effects. On the other hand, the amplitudes of the induced pulses must be high enough to allow a good positional resolution (see, for instance, Boie et al. 8 on the balance between these two requirements in the case of a linear detector). During the conversion from the incident X-ray photon to electrons, and during the avalanche process, numerous UV photons are produced by nonionizing collisions. In a pure noble gas these long-range photons would induce discharges in the chamber; they are absorbed by a UV quencher (e.g., CO2, CH4, C2H6) added to the gas mixture. Encoding Electronics T h e e n c o d i n g electronics p e r f o r m s two functions: it d i s c r i m i n a t e s the d e t e c t e d e v e n t s a c c o r d i n g to their e n e r g y a n d t h e n e n c o d e s the p o s i t i o n 6 R. A. Lewis, N. S. Fore, P. Clifford, C. Hall, W. Helsby, A. Jones, B. Parker, J. Sheldon, I. Sumner, J. S. Worgan and C. Budtz-Jorgensen, in "Proceedings of the European Workshop on X-Ray Detectors for Synchrotron Radiation Sources" (A. H. Walenta, ed.), pp. 61-68. Center for Sensor Systems, University of Siegen, Germany, 1991. 7 A. Oed, Nucl. Instrum. Methods A263, 351-359 (1988). 8 R. A. Boie, J. Fischer, Y. Inagaki, F. C. Merritt, V. Radeka, L. C. Rogers, and D. M. Xi, Nucl. Instrum. Methods 201, 93-115 (1982).
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of the accepted events. In most systems, multiple events that lead to false coordinates are rejected. The first function is carried out by measurement of the amplitude of the anode pulse. A discriminator rejects events with amplitudes that are too high--induced by multiple events, by cosmic rays, or by the harmonics of the monochromatized beam--and events with amplitudes that are too low, such as those generated by electronic noise. An accepted event generates a logical signal to trigger the position encoding. Position encoding electronics can be either analog or digital. Analog electronics are generally cheaper and easier to implement; they are used in commercial detectors. However, digital electronics are usually faster. They can accommodate any detector size and can be more easily pipelined and parallelized. LURE Radial-Drift MWPC We shall describe the two latest gas proportional detectors that have been developed at LURE in collaboration with G. Charpak, R. Bouclier, R. Million, and J. C. Santiard at CERN. Both systems are equipped with essentially identical radial-drift MWPCs. 9 One of these detectors was the chief component of the MARK II diffractometer, which was installed on the D23 station until July 1994, and was used mainly for multiwavelength experiments. The other one is part of the MARK III, a four-circle diffractometer that has been operated on the D41 station for accurate measurements of diffraction data from macromolecular crystals. A diagram of the detector is shown in Fig. 2. The MWPC features three parallel planes spaced by a gap of 4 mm. The central plane (anode plane), at a potential between +2650 V and +2800 V, is made of 260 parallel and very thin gold-plated tungsten wires (diameter 20/.~m) with a 2-mm pitch. Two cathode planes, hold at ground potential, are made of 512 berylliumbronze wires of a larger diameter (200/xm) with a 1-mm pitch. Wires of one cathode plane are parallel to the anode wires while those of the other cathode plane are orthogonal. The anode and cathode planes must be kept rigid, and at fixed distances from each other; to achieve this, wires are mounted on epoxy resin frames, which are sandwiched between two thick aluminum frames. The radial drift space is a device consisting of two electrodes that are concentric spherical caps. The crystal is set at the common origin of the spheres defining these caps. One of these electrodes, at -17 kV, is made 9R. Kahn, R. Fourme, R. Bosshard,and V. Saintagne,Nucl. 603 (1986).
lnstrum. Methods
A246, 596-
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MWPC FIG. 2. Schematic diagram of the radial drift chamber, with the path of an X-ray photon and its resulting bunch of electrons. C is the center of the crystal, A is the point of interconversion of the X-ray photon into a bunch of electrons, and P is the spot position on the MWPC. The diagram shows the beryllium entrance window, B; the stainless steel grid, G; a portion of the series of conductive rings, R; and the multiwire planes of the MWPC.
of beryllium (thickness 500/xm) and has a bending radius of 410 mm. This electrode, which has a low absorption to X-rays of wavelengths below 2 A, is the entrance window of the detector. The second electrode is a stainless steel grid with a bending radius of 554 mm. This grid is at - 7 kV, making it transparent to electrons. Both electrodes are supported by a conical piece of stesalite, an insulating material with low resistivity that allows for the evacuation of surface charges. The inner surface of the conical edge is printed with a series of conductive rings whose potentials are adjusted with a chain of resistors ensuring a variation of ring potentials proportional to 1/r (r being the distance from the origin to a ring). The two spherical
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electrodes produce a radial field, and the rings correct field distortions in the vicinity of the insulator. In the region between the grid and the anode plane, electrons produced in the drift space are transferred to the MWPC. Since the grid and the first cathode plane are equipotential surfaces, field lines are normal to these surfaces. When an X-ray photon is scattered by the crystal, it is absorbed in the drift space and converted into an electron bunch that drifts through the gas. It passes through the stainless steel grid, and then curves in its path to enter the MWPC, creating an avalanche when it nears the anode. This avalanche produces on the anode and cathode planes electric pulses that are used to determine the coordinates of the event. Finally, this gives the angular coordinates of the original scattered X-ray photon. The radial drift space has several distinct useful effect: (1) It suppresses parallax very effectively. Parallax is a classical problem in MWPC detectors when X-ray beams are not orthogonal to the planes of wires, resulting in radially elongated spots. Two solutions have been proposed to overcome this problem without sacrificing detector efficiency. The first one is the use of pressurized xenon by which photons are converted to electrons in a thin layer of gas; for instance, in the Siemens detectors, 1° which are filled with a xenon gas mixture at 4 bars, photons are converted close to the spherical entrance window, thus limiting the parallax effect, at least for 8 keV (Cu Ka) photons. A similar configuration is described in Ref. 11. This solution is difficult to implement on large detectors. The second solution, and the one we have adopted, is the use of a drift space thick enough to give a probability of absorption close to unity and where electric field lines have radial symmetry. As such, all photons diffracted in a given direction by the crystal (placed at the origin of this radial field) are converted into electron bunches, which follow the same field line irrespective of where the conversion occurred, and thus are detected at a single point on the MWPC. (2) In a detector without a drift space, each electron bunch is concentrated into a small volume and the avalanche is produced in the immediate vicinity of the nearest anode wire. Pulse distributions induced on the cathode planes by the avalanche, and therefore the encoded position of the event, are thus centered along this anode wire. As a consequence, the resulting image reproduces an image of the anode plane structure. In contrast, with a drift chamber, the cross section of the electron branch is 10 R. M. Durbin, R. Burns, J. Moulai, P. Metcalf, D. Freymann, M. Blum, J. E. Anderson, S. C. Harrison, and D. C. Wiley, Science 232, 1127-1132 (1986). 11 S. P. Chernenko, A. B. Ivanov, S. A. Movchan, L. P. Smykov, and Yu. V. Zanevsky, in "Proceedings of the European Workshop on X-Ray Detectors for Synchrotron Radiation Sources" (A. H. Walenta, ed.), pp. 82-86. Center for Sensor Systems, University of Siegen, Germany, 1991.
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broadened to several mm 2 as it diffuses along the drift space and the corresponding event triggers avalanches over several anode wires. These avalanches induce pulses on cathode planes, which allow the positions of events that occur between anode wires to be determined correctly. The result is a continuous distribution of encoded positions in both directions of the anode plane, rather than a discrete one. Paradoxically, the blurring that accompanies the drift of the footprint of each event produces a more finely resolved diffraction pattern. (3) Owing to the low drift velocity of electrons, the drift space essentially cancels effects of the pulsed time structure of synchrotron radiation. Two photons scattered by the crystal during the same burst are counted as separate events because the respective drift times of the two bunches of electrons are likely to differ by a time interval larger than the dead time of the position encoder. (4) Because of the curved trajectory of the electron bunches, there must be a mapping function to give the distance, p, from the center of the detector to the located event as a function of the angle q~ between the scattered X-ray beam and the normal to the detector. For the radial drift chamber, the expression of p can be derived from the expression of the electric field between a sphere at potential V, and a plane at ground potential. A good approximation, which differs from the observed value by at most 0.2 mm (i.e., 0.2 pixel) at the periphery of the useful area, is p = 2D tan (q~/2), where D is the mean value between the crystal-to-grid distance and the crystal-to-first cathode plane distance. Because of this special mapping function, the spherical drift chamber has the unique property to give a stereographic projection of the reciprocal space. 12 The theoretical value of D for the LURE detectors is 571.5 mm; this parameter is refined by the data analysis software. The coverage of the detector is a cone with a semi-angle opening of 25 °. Field lines are undistorted up to 23.7 °, and reflections with beams up to 23 ° from the axis are kept in order to collect complete boxels. Thus, the useful area on the anode and cathode planes is a disk of diameter 486 mm partitioned into about 185,500 pixels of 1 × 1 m m 2. The fragile beryllium electrode at -17 kV is placed in a cylindrical compartment enclosed by a grounded aluminized Mylar window. A heliumfilled cone can be placed between the beam catcher and this window in order to minimize both absorption and scattering by air. The detector is filled with a mixture of 40% ethane, 58-59% argon/xenon, and 1-2% of ethanol. The percentage of xenon is adjusted according to wavelength in 12G. Bricogne, in "Computational Aspects of Protein Crystal Data Analysis" (J. R. Helliwell, ed.), pp. 107-145. Daresbury Laboratory, Warrington, U.K., 1987.
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order to absorb at least 99% of the diffracted beams. At the maximum percentage of xenon, the shortest wavelength compatible with an effective parallax suppression is about 0.95 A. Ethane acts as a UV quencher. Ethanol is also a quencher, 13 which improves the high count rate capability and the wire lifetime. Since the drift path of the electron bunch is long, the purity of gas is essential to minimize electron capture. For this purpose, materials used to build the detector are chosen to be vacuum-compatible so that the chamber is airtight. The gas is continuously circulated at atmospheric pressure and is purified by flowing through a removable Oxysorb cartridge.
Position Encoders and Data Acquisition System In both systems, the determination of x and y coordinates is made from the distribution of cathode pulses. The detector of the M A R K II diffractometer is equipped with a brute force digital position encoder, since the primary goal of the designers, J.-C. Santiard at C E R N and R. Bosshard at LURE, was a high count rate. Each cathode wire is equipped with an amplifier and a discriminator that labels the pulses as either 0 or 1. The binary patterns produced by the event on both cathode planes are analyzed by fast priority encoders that find boundaries of the cluster of pulses. The width of the cluster is probed and the few events with an abnormally large width are rejected. Then, coordinates of the cluster center are determined by two digital processors and used as the coordinates of the event. This procedure requires a set of amplifiers-discriminators with almost identical gains; this was obtained at the manufacturing stage by laser burning a carbon film resistor in each amplifier. The dead time of the encoder is 240 nsec, resulting in a fractional count loss of 13% at 300 kHz. This data acquisition system provides 512 x 512 pixels of 1 x 1 mm 2. For the latest detector on the M A R K III diffractometer, a new data acquisition system has been built. TMThe readout consists of 2 x 128 cathode strips (4 mm width) that are connected to shaping amplifiers coupled to a fast position encoder. Cathode amplitudes are digitized using 8-bit flash analog-to-digital converters. Valid cathode clusters lead to a few (3 to 5) nonzero consecutive digitized amplitudes. A specialized module that uses two digital signal processors calculates the center of gravity of the cluster and corrects the results according to the number of active strips. The 13 M. Atac, I E E E Trans. Nucl. Sci. NS-31(1), 99-102 (1984). 14 A. Bahri, R. Bosshard, J.-C. Santiard, R. Kahn, and R. Fourme, Rev. Sci. lnstrum. 63(1), 655-658 (1991).
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intrinsic resolution of this system is 1024 × 1024 pixels, but images are currently stored using 512 × 512 pixels in order to limit the volume of data. Each data acquisition system is built from VME electronic modules with dual 16-bit memories. For each accepted event with given x, y coordinates, the corresponding cell in the memory is incremented by 1. When this operation leads to an arithmetic overflow, the memory cell is cleared and the coordinates are stored in an overflow register (512 overflows can be stored per image). While the current electronic image (frame) is being recorded, the previous one is transferred to a magnetic disk for temporary storage. Frames are finally exported on an inexpensive 8-mm videotape. A standard 112 meter tape may store at least 4000 frames; since the crystal is rotated in most cases by 0.05° per frame, this corresponds to a total crystal rotation of 200°. The exposure time of each frame is typically 12-30 sec, and the dead time between two adjacent frames is at most 1.3 sec. Thus, the duty cycle of these detectors, as defined previously, is 91-96%. In both systems, the noise of the detector with its data acquisition system is negligible (10 -3 count pixe1-1 sec -1, on average).
Data Analysis Software The derivation of intensities from digital images is made off-line. The software package installed on the MARK II and MARK III diffractometers is based on MADNES. 15 This package does not require a prior knowledge of unit cell parameters and can work on a randomly oriented crystal. Corrections for beam intensity, count losses, and Lorentz factor are applied, together with the polarization correction suitable for the highly polarized synchrotron radiation beam. 16The background is estimated from the same pixels as those used for the integration of the peak: the counts used for this estimation are taken from sections in the three-dimensional profile (voxel), which were collected just before and just after the peak. The best results are obtained using MADNES to extract the corrected voxels, followed by the calculation of the final intensity data with the profile-fitting algorithm PROCOR. 17 15 A. Messerschmidt and J. W. Pflugrath, J. Appl. Cryst. 20, 306-315 (1987). The current package is the product of a series of EEC workshops on position sensitive detector software that have been organized by G. Bricogne.12 16R° Kahn, R. Fourme, A. Gadet, J. Janin, C. Dumas, and D. Andr6, J. Appl. Cryst. 15, 330-337 (1982). 17 W. Kabsch, J. Appl. Cryst. 21, 916-924 (1988).
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Performance
Detector with Digital Position Encoder: M A R K H Diffractometer The spot shape given by the detector is dominated by the point spread function (PSF) and not by the crystal volume. With a gas mixture providing an essentially complete absorption of photons in the drift space, the PSF is nearly Gaussian with a FWHM of 1.0-1.1 mm over the whole detector in the wavelength range 0.95-2 .&. The root mean square (rms) difference between observed and calculated spot positions is typically 300/zm (<½ pixel). These differences are due to cumulated effects of errors in crystal and detector centering, approximations in the mapping function, electrode distortions, construction defaults of the sensing planes, and positional shifts due to the encoder. The small error in spot position, together with the PSF, enables the routine use of 5 mm × 5 mm boxels on the detector. Since the detector has a discrete structure, the positional differential nonlinearity is fairly high, and frames exhibit the characteristic kilt-like pattern, which is observed on images produced by most MWPCs. This effect is due to dispersion in the response of discrete channels. Since the cluster boundaries are determined from the cathode pulses just above the threshold value, slight inhomogeneities in the gains of amplifiers may displace the coordinates of an event by _+1 pixel; this event is not lost, but its coordinates are shifted. As integration is done in a fairly large boxel, this effect does not alter the quality of intensity measurements. The boxelintegrated response is indeed quite uniform over the whole sensitive surface, and we do not apply a correction table obtained from uniform illumination of the detector as needed by several commercial models. The sensitivity is limited only by the transparency of the entrance window and by count losses. For a count rate of 200,000 counts sec -1, the DQEs at the wavelengths of 1 .& and 1.4 ,~ are 0.80 and 0.75, respectively. The count rate of the detector is in practice not limited by the encoder, but by the physical behavior of the detector at high local and total count rates. The local count rate is at most 20,000 counts sec -1 mm -2. The total count rate cannot currently exceed about 250,000 counts sec -I for comfortable and uninterrupted operation, even with the addition of ethanol in the gas mixture. At higher count rates, the detector becomes unstable, with discharges occurring between the anode and cathode planes. The detector is efficiently protected against the effect of these discharges by a device which switches off the high voltage on the anode plane in 1 txsec when the current delivered by the power supply exceeds a preset limit.
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DATA
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Detector with Centroid Position Encoder: M A R K III Diffractometer With the centroid encoder, several characteristics are markedly improved with respect to the digital encoder: the differential nonlinearity is reduced; the local positional resolution is -150 ~m and the long range positional reproducibility is better than 200/~m so that the boxel size can be reduced to 3 x 3 mm 2. The useful count rate is somewhat decreased with respect to the count rate capability of the MARK II encoder and is currently 200,000 counts sec 1 with a fractional count loss of 20%.
Suitability for Macromolecular Crystallography Owing to their high sensitivity, the LURE detectors are photon counters that use, nearly optimally, every photon scattered by the sample. Counts accumulated in each boxel are highly significant, because of the multistep selection applied to each event and to the negligible detector noise. Other factors that contribute to a high signal-to-noise are relevant: (i) detector performance and design, boxels are relatively small compared to the total working area (0.013 and 0.005% for MARK II and MARK III, respectively); the crystal-to-detector distance is large; paths of diffracted beams in air are short, thus reducing parasitic scattering; (ii) x-ray source, the quasiparallel and highly monochromatic radiation ensures that the Bragg reflections have a narrow angular width; and (iii) data collection mode, due to the good duty cycle of the detector, the crystal rotation per frame, A~b,can be made comparable to the angular width of Bragg reflections so that the integrated background is minimal. There are two further advantages in using the smallest A~b compatible with the angular width of Bragg reflections: (i) crystals with longer unit cell parameters can be tackled: close reflections that overlap when using a large A~b are thus well separated because they are usually recorded on different frames; and (ii) a smaller z~b allows for a more accurate determination of unit cell parameters, orientation matrix, and detector parameters: after refinement, the typical rms deviation between observed and calculated ~b angles of reflection centroids is 0.0030.008 °. The stability of both positional and intensity responses of the detectors is very good at the time scale of a data collection. This is due to the photoncounting principle and to the mechanical stability of the detector, which are reflected in the quasi-invariance of the mapping function and of the pixel response. These detectors have also limitations. They are not suitable for wavelengths shorter than 0.95 .~ (anomalous dispersion experiments at the K-absorption edge of selenium are still feasible). Their count rate capability
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GAS PROPORTIONAL DETECTORS TABLE I COLLAGENASE DATA COLLECTION STATISTICSAT 1.71 .A RESOLUTIONa
Crystal
A (,~)
A~
Observed
Rejected
Unique
Completeness
gmerge
1 2
1.3857 1.3790
94 ° 96 °
101,675 109,495
1667 (1.6%) 2263 (2.1%)
32,496 42,754
59% 79%
0.046 0.038
a
Two crystals of good size (0.4 x 0.4 x 0.8 mm 3) from the native protein (space group 1422, a = 111.7, c = 165.8 A) were used. They were mounted in a random orientation. For both data collections, the total angular range, Aq~, was covered using 0.05 ° frames with an exposure time of 30 sec. The swing angle of the detector was 24 °, which corresponds to a maximum resolution of 1.71 -A. The diffraction is strong at low and medium resolution and decays rapidly below about 2.4 * . The useful resolution of the merged data set is 1.79 ,~, with 40,064 unique reflections (85% completeness) from which 41.5% have their intensity I > 3 o-(I). The final Rmerge at this resolution is 0.039.
is severely limited. Finally, they are more complex to operate and require more maintenance than imaging plates or CCD detectors.
Results Some results obtained with the MARK II data collection system for a few applications are summarized hereafter. They include single-wavelength and multiple-wavelength experiments. As described previously, intensities are integrated with MADNES. Data are binned into batches of a few degrees and scaled with the program R O T A V A T A from the CCP4 suite. 18 Scale factors are applied and a few outlier measurements are rejected by the CCP4 program A G R O V A T A . These results have been obtained by various groups and are representative of the quality of data that can be obtained routinely under the various temporal and experimental constraints of a synchrotron radiation instrument. An example of a high-resolution data collection at a single wavelength using native crystals of normal size is summarized in Table I. Collagenase f r o m Hypoderma lineatum 19 is one of our standard test crystals for the evaluation of various data collection systems using conventional and synchrotron sources. 2° With a net data collection speed of 5.80 ° per hour, the 18 CCP4, The SERC (UK) Collaborative Computing Project No. 4, a Suite of Programs for Protein Crystallography, Daresbury Laboratory, Warrington WA4 4AD, U K (1979). 19 I. Broutin, B. Arnoux, and A. Ducruix, ICSN, Gif sur Yvette, France. 2o I. Broutin, doctoral thesis, Universit6 Paris-Sud, Orsay, France, 1993.
282
DATA
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T A B L E II 2.5 .& RESOLUTION FOR CRD OF R A T
MAD
D A T A COLLECTION STATISTICS TO
MANNOSE-BINDING PROTEIN a
Wavelength
Number of reflections
Completeness (%)
Rmerge
Remote Peak Edge
13,812 13,461 13,421
93.7 91.3 91.0
0.031 0.043 0.043
Three nominal wavelengths were used: 1.4400 ,~ (remote), 1.5356 ,~ (HoLni absorption peak), 1.5363 A (HoLni absorption edge). The exposure time per 0.10 ° frame was 30 sec for the first 72 ° of data and 40 sec for the remainder, where a total rotation of 106 ° was collected around b*, followed by 45 ° around a*. Rmerge values are calculated for truly equivalent reflections. The program A G R O V A T A was modified by W. Weis in order to reject outlying measurements only with respect to redundant copies of the same Bijvoet mate, rather than with respect to the mean of all I(+_h). The higher values of Rmerge at the peak and edge wavelengths are probably due to shifts in the monochromator setting between various batches of reflections. The effects of these shifts are negligible at the wavelength remote from the anomalous scatterers absorption edge.
final Rmerg e value 2a obtained up to a resolution of 1.79 A with a completeness of 82.9% is 0.039. Tables II and III summarize statistics from two M A D data collections done at three wavelengths. In both cases they were completed using only one sample. The first one is the data collection from the carbohydraterecognition domain of rat mannose-binding protein, where two native Ca 2+ ions were substituted for two H o 3+ ions. 22 The second one is the data collection from cutinase from Fusarium solani pisi 23 with a single mercury atom per molecule, covalently attached to an engineered cysteine residue. The cutinase structure had been previously solved and refined to high resolution by the M I R method, 24 thus providing an accurate model. Multiple-wavelength data have been used as a development tool in the use of conventional heavy atoms as anomalous scatterers and the development of a statistical analysis of M A D data. 25 In Table IV, a comparison 26 of two data collections from nucleoside 21 Rm~rge = ~ ~ h
i
Ill(h) - ( l ( h ) ) l / ~ ~ li(h), where li(h) is the ith measurement and (l(h)) h
i
is the weighted mean of all measurements of I(h). 22 W. I. Weis, R. Kahn, R. Fourme, K. Drickamer, and W. A. Hendrickson, Science 254, 1608-1615 (1991). 23 E. de la Fortelle, R. Kahn, and R. Fourme, LURE, Orsay; C. Martinez and C. Cambillau, LCCMB, Marseille, France. 24 C. Martinez, P. de Geus, M. Lauwereys, G. Matthyssens, and C. Cambillau, Nature 356, 615-618 (1992). 25 M. Chiadmi, R. Kahn, E. de la Fortelle, and R. Fourne, Acta Cryst. D49, 522-529 (1993). 26 Data are from Refs. 1 and 6.
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283
T A B L E III MAD
DATA COLLECTION STATISTICS TO 1.65 FOR CUTINASE a
,~
RESOLUTION
Wavelengths
Batch 1
Batch 2
Batch 3
Batch 4
Total
Remote Peak Edge
0.025 0.035 0.030
0.027 0.036 0.032
0.029 0.035 0.035
0.031 0.039 0.033
0.028 0.036 0.032
aSpace group P2z, a = 35.1, b = 67.4, c = 37.05, /3 = 94 ° . The three nominal wavelengths for M A D data collection are 1.0143 ,~ (remote), 1.0061 ,~ (HgLni absorption peak), and 1.0093 A ( H g L m absorption edge). T h e exposure time per 0.05 ° frame was 12 sec. Data at the three wavelengths were m e a s u r e d successively in four batches of 48 ° with an overlap of 1° between each batch (total 189°). A s the n u m b e r of truly equivalent reflections is low, the quoted R values are calculated in the space group P21, so that Bijvoet mates are not distinguished. For this reason, R values, which incorporate the a n o m a l o u s signal, are largest and smallest at the peak and remote wavelengths, respectively.
diphosphate kinase (NDPK) from Dictyostelium discoideum 27 is presented. The first data collection was done using the LURE W32 wiggler line equipped with an EMBL type II imaging plate scanner and the second one using the D23 instrument equipped with the radial drift MWPC. Despite an X-ray flux about 50 times higher, the net data flow rate is only two times higher with the wiggler instrument because of the much lower duty cycle of the imaging plate scanner. The internal consistency of data is better for the MWPC data set.
Future Developments As already mentioned, improvements in the counting rate capability of gas detectors have led to the design of new detectors to increase the local counting rate capability by limiting space-charge effects, and of new electronics to increase the overall counting rate capability of the positionencoding system. On the detector side, a first approach which is developed at the Daresbury Laboratory is the microgap detector. 6 It is similar to a conventional chamber but its anode-cathode spacing is very much smaller (--300 ~m). This design effectively reduces the space-charge effect: at a local 27 Crystals are supplied by C. D u m a s , LBS, Orsay, France.
284
DATA
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T A B L E IV COMPARISON OF N D P K DATA COLLECTIONS USING T w o DIFFERENT EXPERIMENTAL SETUPS AT L U R E
Characteristic
W32 instrument
D23 instrument
Source Beam intensity (arbitrary scale) Detector type Rotation/frame Exposure time/frame Elapsed time/frame Duty cycle Rotation rate Rmerge (2.2 ,~ data)
5-pole superconducting wiggler 100 EMBL type II image plate scanner 1° 30 sec 172 sec 0.17 20.9 ° hour -~ 0.046
Bending magnet -2 Radial drift M W P C 0.05 ° 15 sec 16 sec 0.94 11.2 ° hour -1 0.028
counting rate of 3 × 10 5 counts s e c - 1 mm -2, amplitudes of the pulses are reduced by only 15%, and just by relaxing slightly the energy discrimination, an excellent counting rate linearity can be achieved until rates are well in excess of 105 counts sec -~ mm -2. Originally introduced by Oed, 7 the microstrip gas chambers (MSGCs) look very promising and have led to a great deal of work stimulated by potential applications in high-energy physics and X-ray astronomy. They are made using photolithographic techniques to produce an electrode structure on an insulating substrate. A typical design is shown in Fig. 3. These devices can be mass produced in large sizes to very high (0.2/zm) tolerances with very small electrode spacings (a few tens of micrometers). This small spacing allows a rapid evacuation of the positive ions, therefore reducing the space-charge effects. Owing to the fine electrode structure, a very good spatial resolution can be obtained.
VD
400~m Vc
VA /
1~
Anode ~ . . ~ m V
WB
FIG. 3. Cross-sectional drawing of a microstrip chamber. Typical distances for the electrode structure are indicated. The voltages VA, Vc, and VB applied to the anode, the cathode, and the backplane, respectively, are usually VA -- 500 V, VB = Vc = 0 V. The drift voltage, Vo, is set to at a negative value in order to produce an electric field of 500 to 1000 V cm -~.
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285
Numerous studies have been performed to determine the optimum configuration for an MSGC. 28-32 The properties of the substrate play a key role when the electrode structure is directly deposited on it: it must exhibit sufficient bulk and surface conductivity to prevent charging that would affect the gain stability at high fluxes. However, the resistivity should be kept high enough to limit leakage currents and avoid heating. Alternate electrode structures have also been proposed to avoid these charge-up effects. Excellent long-term stability at high fluxes with microstrips, built up using deep X-ray lithography techniques on polymer substrate, have been reported. 33 Very high count rates with stable gains up to 8 × 10 6 counts sec 1 mm 2 have been obtained using an MSGC where the anodes are separated from a continuous conductive cathode by insulating strips. 34 When the substrate is thin enough (-100 tzm), detectable pulses are induced on the backplane of the MSGC. These pulses can be collected on strips deposited on the rear plane and orthogonal to the microstrip structure. This allows two-dimensional position encoding of the event by using a time correlation with signals simultaneously generated on the microstrip electrodes. This procedure is rather complex and, at the time being, takes at least a few tens of nanoseconds. Therefore, the encoding electronics must be segmented to handle counting rates in excess of 10 7 counts sec -1 over the whole surface of the detector. An ultimate state of segmentation could be achieved by using small pads deposited on the backplane of the MSGC and bounded to their own electronics, making them individual counters. Conclusion Gas proportional detectors are well suited for the accurate measurement of X-ray diffraction data. The high signal-to-noise ratio and low background 28 C. Budtz-Jorgensen, A. Bahnsen, C. Olesen, M. M. Madsen, P. Jonasson, H. W. Schnopper, and A. Oed, Nucl. Instrum. Methods A310, 82-87 (1991). 29 j. E. Bateman, J. F. Connolly, R. Stephenson, and J. Morse, in "Proceedings of the European Workshop on X-Ray Detectors for Synchrotron Radiation Sources" (A. H. Walenta, ed.), pp. 87-91. Center for Sensor Systems, University of Siegen, Germany, 1991. 3o F. Angelini, R. Bellazzini, A. Brez, M. M. Massai, G. Sprande, and M. R. Torquati, Nucl. lnstrum. Methods A315, 21-32 (1992). 31 R. Bouclier, J. J. Florent, J. Gaudaen, G. Millon, A. Pasta, L. Ropelewski, F. Sauli, and L. I. Shekhtman, Nucl. Instrum. Methods A323, 240-246 (1992). 32 Yu. N. Pestov and L. I. Shekhtman, Nucl. Instrum. Methods A338, 368-374 (1994). 33 M. Lemonnier, A. Bahri, M. Bordessoule, F. Bartol, A. Labeque, Z. Liu, S. Megtert, M. Roulliay, M. F. Ravet, F. Rousseaux, and J. Perrocheau, NucL Instrum. Methods A349, 274-276 (1994). 34 F. Angelini, R. Bellazzini, A. Brez, M. M. Massai, R. Raffo, G. Sprande, and M. A. Spezziga, Nucl. Instrum. Methods A335, 69-77 (1993).
286
DATA
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achieved by these systems make them particularly suitable for collecting quality data at high resolution. New developments are taking place to improve their counting rate capability, which currently limits their use for very demanding synchrotron radiation experiments.
[19]
Diffraction-Data Processing for Electronic Detectors: Theory and Practice
By J A M E S
W. PFLUGRATH
Introduction With the advent of electronic area detectors, software was developed both to drive data acquisition and to process their data. There are many examples of software packages to analyze images from area detectors. These include the UCSD software,1 BUDDHA, 2 XENGEN, 3 XDS, 4 SAINT, 5 and MADNES. 6 All of these software packages use some variant of the rotation or oscillation method of data acquisition. This method was developed originally for data collection with X-ray film and currently is used almost universally. The geometry is simple: a crystal is rotated around a single axis while the detector remains fixed. The main feature to distinguish electronic area detectors from film and imaging-plate detectors is the readout time. Electronic detectors have a relatively fast readout, so in any given experiment a smaller volume of reciprocal space can be sampled on each image without the ratio of readout time to exposure time becoming significant. In a typical experiment the crystal is rotated from 0.1 to 0.25 degrees per image, a small enough rotation increment so that reciprocal lattice points are found on a few adjacent images, all of which must be considered when integrating the inten1 A. J. Howard, C. Nielsen, and Ng. H. Xuong, Methods Enzymol. 114, 452 (1985). 2 M. Blum, P. Metcalf, S. C. Harrison, and D. C. Wiley, J. Appl. Cryst. 20, 235 (1987). 3 m. Howard, G. L. Gilliland, B. C. Finzel, T. L. Poulos, D. H. Ohlendorf, and F. R. Salemme, J. Appl. Cryst. 20, 383 (1987). 4 W. Kabsch, J. Appl. Cryst. 21, 916 (1988). 5 Siemens, SAINT Software Reference Manual, Pub. No. 269-014200, Siemens Industrial A u t o m a t i o n Inc., Madison, W I (1993). 6 A. Messerschmidt and J. W. Pflugrath, J. Appl. Cryst. 20, 306 (1987).
METHODS IN ENZYMOLOGY,VOL. 276
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