TR-USAXS

A true niche product

The TR-SAXS is to our knowledge the only marketed USAXS system for the home laboratory market.

Since the development of the canonical Bonse-Hart USAXS system almost 50 years ago, very few companies have tried to provide instrumentation for this niche-of-niche SAXS market.

As specialists in SAXS scattering, this remains a mystery to us. Any well equipped materials science lab should have an in-house USAXS system or a USAXS-option for their GANESHA system.

Why USAXS?

Small Angle X-ray Scattering (SAXS) measures the scattering intensity above background as close to zero angle as possible in order to probe structure at the largest length scales.

Conventional SAXS (GANESHA, Mat:Nordic, Bio-Nordic, etc. ) which relies on apertures to define the divergence of an incident x-ray beam, has a maximum probe length on order of several thousand Angstroms.

So in order to extend the probe length to greater distances, we adopt a different geometry (referred to as Bonse-Hart) in which the beam divergence is controlled by the angular diffraction width of a single crystal like Silicon(111) or Germanium (220).  For Cu Ka radiation (l = 0.154nm) and Si(111), this divergence is under 7 arc sec, which corresponds to a minimum momentum transfer qmin ≈ 4pq/l where q is the scattering angle corresponding to the half-width of the crystal rocking curve (7 arc sec).  The maximum real-space resolution of 2p/qmin ≈ 5 microns corresponds well to our observations.

This increased resolution over that of conventional SAXS is conventionally referred to as UltraSAXS or USAXS.

 

Schematic of USAXS

The Bonse-Hart geometry

Figure 1 is the side view schematic of the Bonse-Hart geometry.  The x-ray source, parallel beam conditioner, crystal surfaces, sample and detector are all extended along the y-axis, perpendicular to the page.  Multiple bounces of the crystal diffraction on both the monochromator and the analyzer reduce the tails of the crystal diffraction rocking curve and improve signal-to-noise of the sample scattering.  The UltraSAXS curve is generated by rotating and translating the analyzer crystal as indicated.  The scattered x-rays are collected at each angle by a scintillation detector  or area detector with a collection area large enough so that the scattering within the angular range of interest will be intersected by the active detection area.

A compact system

It can appreciated that since each crystal length is approx. 4cm long, the entire beam path can be relatively short.  By making the path length as short as possible, the incident and scattered intensity drop due to 1/r2 losses is minimized.

ITR-USAXS_spheres

 

Figure 2 shows an example USAXS curve of intensity vs. q(Å-1) taken on the prototype system. Oscillations due to a 1 micron sphere size are easily seen and the low q cutoff of 1.2×10-4 Å-1 corresponds very well with the prediction for the Si(111) diffraction rocking curve for the monochromator and the analyzer.  The slit-smeared scattering falls off as 10-3 which is precisely what one would expect for sample particles with smooth surfaces.

 

Unique Features of the TR-USAXS

  • Horizontal beam orientation
    • Allows the use of a conventional x-ray line source in standard vertical tube tower as a base configuration
    • The sample changer is vertical and more easily accommodates commercial sample cells for heating, cooling, liquids
    • High Resolution X-ray Reflectivity can be performed on nearly horizontal samples(X-ray Reflectivity module with linear detector is optional and can be added at a future time)
  • Wide crystal reflection surface – up to 20mm to collect a large solid angle from the x-ray source and sample scattering
  • Low friction crystal rotation – Sapphire bearing surfaces and piezo motion control with feedback reduce slip/stick on crystal rotation

Word of Caution

The TR-USAXS instrument employs a so-called slit geometry and as described, should be used for isotropic materials only, like powders, polycrystalline samples, liquids, amorphous solids, random media, etc.  For oriented or anisotropic samples, a 2-dimensional USAXS is available, but a synchrotron is advised unless an intense high resolution source such as Exillums Metal jet  is used.