Beam-Position Monitoring

Optical devices are placed in the light path in order to shape the primary beam. Beam-position monitors, shutters, slits, monochromators, stabilizers, absorbers, and mirrors are utilized for this purpose. The effective beam shape and its flux are defined by these components. In particular, if mirrors are cooled, vibration must be avoided and thermal expansion should be compensated. 

Beam-position monitors are installed in the electron path of the synchrotron. They guarantee that the optics of each beamline sees the source of the synchrotron light at always the same position. 

THE MACHINE, BEAM LINE FRONT ENDS, BEAM POSITION MONITORING AND STABILITY... 

Photon beam position monitors are essential to ensure that after an injection the electron beam position is adjusted to allow the SR to strike the beam line optical components in a constant way. The wavelength output from a double crystal monochromator is especially sensitive to the vertical beam position. Also, the quality of the focus, from a toroid mirror, is especially sensitive to the horizontal beam position (figures 5.18(c) and (e)). On existing machines it is necessary to recalibrate the wavelength and the focussing of a beam line optical system after each injection. 

The simplest solution of the first problem is to place an ionization chamber somewhere between aperture slit and sample. Another possibility is to place there a thin Kapton foil, inclined 45° to the primary beam. The small portion of intensity, that is scattered by the foil, can then be registered by a scintillation counter. If a second foil is mounted close behind the sample, then the absorption of X-rays in the sample can be measured additionally by comparing the intensities registered at the two foils. This design has been realized at some SSRL beamlines in Stanford. At the polymer beamline at the HASYLAB it became evident, that fluctuations of the primary beam position can lead to fluctuations of the intensity at the sample, which cannot be registered by a monitor placed in front of the sample. This observation is obviously caused by restrictions of the beam directly at the sample holders. 

The entire microprobe setup is positioned on a movable granite table. Compound refractive lenses are used for focusing to a routinely achievable spot size of 1-2 pm vertically and 12-15 pm horizontally. The intensity of the incoming, the focused, and the transmitted beam is monitored by ionization chambers and photodiodes. A miniature ionization chamber with an aperture of 50 pm diameter as an entrance window was developed at the ESRF for measuring the intensity of the focused beam close to the sample (Somogyi et al. 2003). The characteristic X-ray line intensities are detected with a Si(Li) detector of 30 mm active area, 3.5 mm active thickness, and 8 pm thick Be window placed at 90° to the incoming linearly polarized X-ray beam. Fast scanning XRF measurements (>0.1 s live time/spectrum) are possible. 

A beam profile monitor (BPM) is provided before the slit. It provides the continuous display of the shape and position of the beam in both the X- and V-coordinates. In BPM, a helical wire on a rotatory drive crosses the beam vertically and then horizontally during each revolution. A cylindrical collector around the grounded wire collects beam-induced secondary electrons from the wire to provide a signal proportional to the intercepted beam at that instant. 

Fig. 3. Above The laser-driven photoacoustic system, consisting of a COj laser and an intracavity-positioned photoacoustic PA) cell. Ethylene concentrations are detected using three microphones placed on the resonator tube of the PA cell. Below Instead of a PA cell, a HeNe laser is used to measure ethylene concentrations near the plant tissue under normal atmospheric conditions. The deflection of this laser beam beam is monitored with a position-sensitive detector...

The Balance. Figure 14 shows a schematic of the Cahn electrobalance (38) most frequently used in thermogravimetiy work (arrangement 1). A photodetection system monitors the beam position. If the beam moves from the horizontal, enough current flows to the torque motor to move the beam back to its original... 

Most modern TGAs, however, in addition incorporate the principle of electromagnetic balances that have relatively little dependence on vibration (one of the common problems for weight measurements), have high sensitivity, and display little thermal drift. The beam position is monitored by a photodetection scheme. This concept was introduced by Cahn Instruments. As shown in Fig. 2, after taring the sample, the balance is assumed to be in equilibrium. Addition of the sample to the left side of the beam will cause the right side of the beam to be displaced upward. Sufficient current is... 

Fig. 14 The first experimental set-up with which it was possible to collect quasi-simulta-neously SAXS/WAXS/XAFS data. 1-position sensitive WAXS detector, 2-sample position and sample environment, 3-ion chamber for incoming beam intensity monitoring, 4-fluoresence detector, 5-optical bench, 6-SAXS detector, 7-beam stop in which a photo diode for transmitted beam intensity monitoring is integrated, 9-evacuated SAXS Sight tube. 

Tensile testing is done in a home-made [1] stretching-machine. The machine performs symmetric drawing in order to maintain the position of the beam on the sample. Signals from load cell and transducer are recorded during the experiment. The macroscopic deformation is determined close to the beam position to ensure accurate comparison of the mechanical data with the nanostructure evolution. For this reason a precise method has been developed [2]. In this method a pattern of fiducial marks is stamped on the sample. The sample is monitored by a TV-camera. Using the fiducial marks the local strain s = - o)/ o is computed automatically... 

In order to correlate the macroscopic response of the material to its structure it is necessary to assess the deformation of the sample close to the beam position. For this purpose a method has been devised by Stribeck et al. [2]. A pattern of parallel fiducial marks is stamped on the sample. A video camera monitors the sample during deformation (Fig. 3.1). This method gives precise values of the macroscopic elongation provided the sample is kept straight and the contrast among the fiducial marks is sufficient. The pseudo-color representation provides good visual contrast. The center of the X-ray beam on the sample is marked by a cross in the image. Close... 

As mentioned above, the thermal deactivation-induced acoustic waves were detected by sensitive microphones in a majority of PA studies. In order to improve the sensitivity of the teehnique, a new detection system with optical microphone was used to detect the acoustic wave generated from the sample. In this system, the acoustic wave-induced change in the position of a laser beam on a pellicle is used to detect the signal. Instead of a microphone to detect the signal, a laser beam positioned on a Mylar pellicle (10 /u-M thickness) monitors the acoustic waves. The vibrations of the pellicle caused by acoustic waves deflect the laser beam, which is in turn is detected by a silicon photodiode (Fig. 5). 

The encircling probe was characterised with its mirror in water. As we did not own very tiny hydrophone, we used a reflector with hemispherical tip with a radius of curvature of 2 mm (see figure 3c). As a result, it was possible to monitor the beam at the tube entrance and to measure the position of the beam at the desired angle relatively to the angular 0° position. A few acoustic apertures were verified. They were selected on an homogeneous criteria a good one with less than 2 dB of relative sensitivity variations, medium one would be 4 dB and a bad one with more than 6 dB. 

Detection of cantilever displacement is another important issue in force microscope design. The first AFM instrument used an STM to monitor the movement of the cantilever—an extremely sensitive method. STM detection suffers from the disadvantage, however, that tip or cantilever contamination can affect the instrument s sensitivity, and that the topography of the cantilever may be incorporated into the data. The most coimnon methods in use today are optical, and are based either on the deflection of a laser beam [80], which has been bounced off the rear of the cantilever onto a position-sensitive detector (figme B 1.19.18), or on an interferometric principle [81]. 

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