Synchrotron beamlines

A rotating anode setup resembles a typical synchrotron beamline on a laboratory scale, and some progress concerning the optimum design of rotating setups was made by transferring sophisticated techniques for the optimization of beamline optics (Pedersen [72]) to rotating anode equipment. 

The ordinary user who carries out scattering experiments at a synchrotron beamline will rarely adjust the optics without help. Nevertheless, during the beam time one should be able to assess the quality of the adjustment. Thus the user will most probably have to readjust some slits or to adjust the flux according to the requirements of the experiment. 

Absorbers are found at many synchrotron beamlines. Two different principles are realized. Tilt-absorbers are operated continuously, whereas filters on a revolving disc offer step-wise attenuation of flux. Absorbers change the spectral composition of the primary beam. Thus the utilization of an absorber during scattering experiments should be avoided. 

Some experimental techniques require the sample to be studied in very well-defined orientations and positions with respect to the X-ray beam. In the corresponding experiments the structure of the samples is, in general, not changed. A synchrotron beamline is required, because it would take too much time to record the respective data with laboratory equipment or because a special beam shape (microbeam) is essential for scanning the part with high spatial resolution. 

Some experiments are aiming at the study of structure evolution. In general, the studied material is isotropic or exhibits simple anisotropy (e.g., fiber symmetry). Most frequently the material is irradiated in normal-transmission geometry. A synchrotron beamline is necessary, because in situ recording during the materials processing is requested with a cycle time of seconds between successive snapshots (time-resolved measurements). 

These features made gas-filled detectors the limiting factor for effective use of synchrotron beamlines. Nevertheless, they are still well-suited for laboratory equipment10. Gas-filled detectors are classified by their dimensionality. 

D Position Sensitive Detectors are multi-wire electrical-field detectors. The principal limitation of the total counting rate reduces the applicability at a synchrotron beamline in particular for 2D detectors. But even strong, narrow peaks pose a problem, because the whole image is distorted as soon as local saturation occurs. The detector response is changing, because the wires are worn out by use. 

The beamline scientist at a synchrotron beamline has some in-house beamtime of his own. The test-user may convince the staff scientist to study a test sample during this beamtime. It is possible that the user simply sends the sample, but it is better to join in the measurement. 

After having performed such tests the new user should be able to assess whether it appears reasonable to study the scientific problem at a synchrotron beamline. Sometimes one will simply be able to use devices (furnace, extensometer, sample recipient) provided at the beamline. Sometimes the researcher will have to adapt some own devices to fit in the beamline, to control it remotely and to record its output signals together with the scattering patterns. Sometimes special equipment will have to be constructed. 

In advance of the beamtime discuss the experiment with an expert. At a synchrotron beamline cooperate with the local beamline staff. 

The parameter values and patterns determined in this section are reasonably linked into adapted versions of data pre-evaluation procedures in order to permit automated pre-processing of complete series of scattering patterns. Novices at synchrotron beamlines will resort to on-site procedures and take home pre-processed data. 

For SAXS and USAXS experiments it is sufficient to measure the distance R by hand or by a laser distance-meter. For MAXS and WAXS, calibration by means of samples with known sharp reflection are required. Such calibration samples are available at synchrotron beamlines. Different materials are used for application in different angular regions (isotropic crystalline materials for WAXS, Ag-behenate for MAXS, diverse biological samples3 for SAXS and USAXS). Calibration sheets are frequently published on the home pages of synchrotron beamlines. 

This is most easily done at a laboratory source where the current of the X-ray tube is decreased to the lowest possible value. At a synchrotron beamline this is more complicated, because the measurement of the primary beam requires special adjustment. So, technically this should be done before the final optical adjustment of the device, as long as the slits can be narrowed for the purpose of intensity attenuation and as long as the primary beam stop is not yet mounted. It is not advised to use absorbers that are mounted behind the monochromator, because they change the spectral composition of the X-ray beam. 

Direct calibration to absolute intensity is not a usual procedure at synchrotron beamlines. Nevertheless, the technical possibilities for realization are improving. Therefore the basic result for the total scattering intensity measured in normal transmission geometry is presented. At a synchrotron beamline point-focus can be realized in good approximation and the intensity /(s) is measured. Then integration of Eq. (7.19) results in... 

Protection of the Detector. With all direct calibration methods the primary beam intensity must be measured. If the primary beam itself is attenuated, shape of the beam and spectral composition of the radiation may be altered. This problem is avoided if the load of the detector is reduced by scanning the beam using a slit or a perforated disc. On the other hand, in order to be useful at a powerful synchrotron beamline these devices should have very tiny and well-defined slits or holes. 

The measured SAXS curve of the calibration sample must have been pre-processed in the usual way (cf. Sects. 7.3 - 7.6). Therefore it is important to have calibration samples with a well-defined thickness27. Because synchrotron beamlines can be adjusted to a fairly wide range of radiation power, it is important to have thin calibration samples for a high-power adjustment (e.g., common SAXS with wide slit openings) and thick calibration samples for low-power adjustments (e.g., USAXS with microbeam). For calibration samples from synthetic polymers, thicknesses ranging between 0.2 mm and 3 mm are reasonable. It appears worth to be noted that not only polymers, but as well glassy carbon [88] can be used as a solid secondary standard for the calibration to absolute intensity. 

The treatment is rather involved, so the reader is asked to consider the WARREN s textbook. In addition, if the concept shall be applied to synchrotron beamline setups with flat 2D detectors some geometrical modifications must be carried out. 

Figure 6.19 Catalytic in situ reactor made of a quartz capillary, suitable for use at synchrotrons for the collection of EXAFS an XRD data. The scheme of the synchrotron beamline shows the positions of the mono-chromator, the ion chambers which measure the intensity of the X-rays before and after the sample, and the position-sensitive X-ray detector which records the XRD diffractogram (adapted from Clausen [44]).

The crystals of NCPs containing a-satellite DNA palindrome and chicken erythrocyte histones diffracted isotropically to 3.0 A using an in-house rotating anode X-ray source and to better than 2.5 A at a moderate intensity synchrotron beamline [30,31]. The crystals used for structure determination were grown in the microgravity environment using a counter-diffusion apparatus [32]. Ground-based... 

Rapid collection of diffraction data depends on access to such powerful X-ray sources. This chapter describes how high-quality, high-throughput data collection can be achieved. We use SGX-CAT, the SGX Collaborative Access Team beamline, located at the Advanced Photon Source of Argonne National Laboratory as an example to illustrate the concepts behind the design of, and the hardware used at, synchrotron beamlines. Many of these features are found, individually or in combination, at other beamlines. Data collection at synchrotron sources produces enormous quantities of data. We, therefore, also discuss the information technology infrastructure and software that is necessary for effective data management. 

Two related alternatives for mail-in use of synchrotrons have arisen. Several facilities permit users to send crystals to the synchrotron beamline, where local staff load them onto a robot that places the crystals into the X-ray beam (see Section 12.3.3). The user then operates the beamline remotely, during a specific period allocated for their experiments. Considerable effort has been expended by various synchrotron facilities to provide robust, secure internet connections between the beamline and the end user. 

Synchrotron beamlines are a complex hybrid of hardware and software. Although current designs have achieved a level of robustness inconceivable a decade ago, tight process control is essential. For example, at SGX-CAT the position of the beam is controlled to within 0.5 jxradians (0.000028°). This tolerance corresponds to keeping the X-ray beam centroid within a 25 xm diameter circle at a location 50 m from the undulator source. This performance, reflecting the combined capabilities of the synchrotron and the beamline, is impressive to say the least. 

We have in this chapter summarized various aspects of data collection at a synchrotron beamline. In this section, the operations of SGX-CAT are presented to illustrate these concepts in action. 

Cohen, A. E., EUis, P. J., Miller, M. D., Deacon, A. M. and Phizackerley, R. P. (2002). An automated system to mount cryo-cooled protein crystals on a synchrotron beamline. 

Further technological improvements at synchrotron beamlines promise to have a substantial effect on data quality for weakly diffracting crystals, enabling the measurement of higher resolution data and the collection of useful data from more challenging problems. 

XAFS measurements were performed at the HASYLAB synchrotron (beamline XI. 1) in Hamburg, Germany, and at the European Synchrotron Radiation Facility (beamline BM 29) in Grenoble, France. The measurements were done in transmission mode using ion chambers filled with a mixture of Ar and N2 to have a px of 20% in the first and a px of 80% in the second ion chamber. The monochromator was detuned to 50% of maximum intensity to avoid higher harmonics present in the X-ray beam. 

---