Università degli Studi di Palermo. Centro Grandi Apparecchiature SAXS. Small Angle X-Ray Scattering. Prof. Delia Chillura Dr.

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1 Università degli Studi di Palermo Centro Grandi Apparecchiature SAXS Small Angle X-Ray Scattering Prof. Delia Chillura Dr. Giorgio Nasillo

2 SAXS Instruments The system consists of the following five major units: 1) A source to produce X-rays; 2) X-ray optics to condition the primary X-ray beam; 3) A sample stage to establish and manipulate the geometric relationship between primary beam, sample, and detector; 4) A detector (HI-STAR Area Detector System) to record the scattered X-rays and to save and display the scattering pattern into a two-dimensional image frame. 5) Accessories (pumps, high voltage generator, cooling units and so on )

3 1 - The Source Produces X-rays with the required radiation energy, focal spot, and intensity: rotating anode generator (RAG); X-rays generated by sealed tubes or rotating anode generator have an X-ray spectrum, which consists of continuous radiation (also called white radiation, or Bremsstrahlung) and a number of discrete characteristic lines; Generally, a monochromatic radiation is selected for most of experimental purpose. The characteristic lines are K α1, K α2 and K β ; if the two K α lines cannot be resolved, they are simply referred to as K α line.

4 1 - The Source The wavelengths of characteristic lines are determined by the target (anode) materials of the X-ray generator. Sealed tube and rotating anode generators produce X-rays by bombarding the target sample with electrons generated from the filament (cathode); The target area is called focal spot, and the angle between the primary X-ray beam and the anode surface is called takeoff angle.

5 2 - The X-ray Optics They condition the primary X-ray beam into the required wavelength, beam focus size, beam profile, and divergence: a pinholes set; cross-coupled Göbel mirrors; The beam consists of three components: parallel, divergent, and convergent X-rays; to increase the parallel ones we have to make β and α as smaller as possible. The smaller the collimator, the lower the photon flux that strikes the sample, and the longer the count time to acquire statistically significant data.

6 2 - The X-ray Optics Göbel mirrors offer greater intensity than conventional optics; The Göbel mirror is a parabolic-shaped graded multilayer mirror; Multilayer mirrors reflect X-rays in the same way as Bragg diffraction from crystals, so multilayer mirrors can be used as a monochromator; Göbel mirrors are manufactured so that the d-spacing between the layers varies in a controlled manner (depending on wavelength, the location of the mirror with respect to the source, and the applications); In Fig(a) a single parabolically bent mirror transforms the divergent beam from source into a parallel beam; (b) in the cross-coupled Göbel mirrors, the second mirror turned 90 collimates the beam in the direction perpendicular to the first mirror; With Bragg diffraction, the radiation is monochromatized to K α, while K β and Bremsstrahlung are suppressed; The combination of 2 mirrors allows an easy switch between line focus geometry and point focus geometry without changing the X-ray tube.

7 3 - The Sample Stage The sample stages establish and control the geometric relationship between primary beam, sample and detector; It is mounted inside the sample chamber that can hold up to 15 samples and 4 reference samples; Sample alignment systems assist you in positioning the sample into the instrument center and in monitoring the sample s state and position before and during data collection; Through the Nanography analisys is possible to find the exact area of the sample that has to be investigated.

8 4 - The Detector The HI-STAR Area Detector is a two-dimensional Multiwire Proportional Counter (MWPC); The area detector has a large imaging area (11.5 cm diameter) for X-ray detection sensitive to X-ray wavelengths corresponding to the 3-15keV; It can collect a data frame of 1024x1024 (or 512x512) pixels with the pixel size 105 μm (210 μm for 512x512 frames); For most X-ray diffraction applications, the HISTAR system can be 104 times faster than a scintillation counter and 100 times faster than a linear position sensitive detector (PSD). of

9 4 - The Detector The chamber is filled with a Xe/methane gas mixture pressurized to approximately 4 atmospheres; when an X-ray photon enters the detector,it interacts with the Xe near the front window, ionizing the gas and creating a cloud of electrons; An electric field accelerates these electrons from the near-window region through a drift region; The electron cloud passes through the first cathode and is amplified by a factor of 2000 as it is collected at the anode wire surface; Analog signal processing electronics, located directly behind the detector, produce very low noise signals, permitting high spatial resolution (200 μm) to be achieved at low charge gains of 2000; The position decoding circuit (PDC) converts the analog signals from the detector into digital values representing the X-Y position of each X-ray photon.

10 5 - The Accessories For the production of x-rays a current has to be applied to cathode; this current is produced by an high voltage generator up to 6 KW; Since the rotating anode reaches very high temperature during the production of x-rays a cooling system is needed; A vacuum system all long the x-rays path is necessary to avoid lost of intensity due to the impact of photons with air molecules and to avoid additional scattering produced by same molecules; A beam stop (a small cilinder of Pb of few mm) is necessary to avoid that the direct beam (very intense) hits the surface of detector, damaging it; A personal computer equipped with a 32-bit wide parallel data link is necessary to control the instrument and, to display the experiment in real time as a 512 x 512 or a 1024 x 1024 pixel frame (with 32-bit data for each pixel) and to store data.