M.H.Modi, B.Gowrishankar, R.Dhavan , G.S. Lodha

Synchrotron Utilization Division

Figure 1: Just before the sample position, a pin hole of 2.0mm size is set in reflectometer station Curve represented by filled circle is measured beam profile with the pinhole.  When pinhole is removed, the wings in either side of beam from central position rise up. The resultant increase in integrated count is ~1.7%.

Monochromatic radiation at sample point in reflectometer station comes through a post mirror after monochromatization by toroidal grating monochromator in beam line.  Any figure imperfection and surface roughness of these optical elements introduces scattering component in direct beam. This results in enhanced noise to signal ratio.  To reduce the stray light component, a pinhole assembly is mounted just before the sample. The resultant intensity profile of direct beam in angle scan becomes sharp (Figure 1). We have analyzed beam intensity profile in reflectometer in vertical plane.  The scan is carried out using Si photodiode detector mounted on detector arm.

The pinholes are made on a stainless steel plate. This pinhole plate is mounted on an extension plate. This extension plate is connected to the UHV compatible linear motion feed through by a connector and an Al rod. There is a provision to adjust the Al rod across the beam direction and to lock at the required position. Fixtures provide vertical /angular motion to the pinhole plate and extension plate. Z and angular motions are used to align the pinholes with synchrotron radiation.

Synchrotron Radiation in the optical range is used as a reference, for the alignment. Z motion is used to position the pinhole in the vertical direction. Locking position of Al rod in the connector is adjusted, for optimum use of feed through motion. After the alignment the feed through motion is used for positioning of pinhole and other components of the system in the beam path.

 Detector slit of 1mm size is used.  A pinhole of 2mm is mounted just before the sample.  When no pinhole is placed in beam path, the wings in intensity profile rises up significantly.  The results of beam profile measurement are shown in figure1.  When no pinhole is present, the noise to signal ratio is ~10^-3. 

Figure 2: Measured reflectivity profile of float glass sample for two successive measurements with a difference of pinhole before sample position in beam path is shown. When pinhole of 2mm is present (dotted line), the reflectivity profile decreases compare to without pinhole measurement.  Integrated reflectivity count decreased by ~2.4% in presence of pinhole. Percentage difference between two measurements is shown in (b). This difference become prominent as one goes on higher angle side because intensity is measured on logarithmic scale

With the pinhole just before the sample, signal ratio improves by two orders of magnitude i.e. ~10^-5.  The total integrated area under peak is 2% more compared to measurements with no pinhole; This scattered intensity, affects the reflectivity profile for any sample. 

In figure2, measured reflectivity profile of float glass sample with a pinhole before sample position in beam path is shown. When pinhole of 2mm is present (dotted line), the reflectivity profile decreases compared to without pinhole measurement (reduction in scattered intensity).  In absence of pinhole integrated reflectivity count increases by ~2.4%. The change in reflectivity profile basically arises from scattered count contents in direct beam as shown in figure 1.  The relative difference in intensity among two measurements is shown in figure 2-b.  This difference become prominent as one goes on higher angle side because intensity is measured on logarithmic scale.