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Invariant Calculation


The scattering, or Porod, invariant (Q*) is a model-independent quantity that can be easily calculated from scattering data.

For two phase systems, the scattering invariant is defined as the integral of the square of the wave transfer (Q) multiplied by the scattering cross section over the full range of Q from zero to infinity, that is


where g = q for pinhole geometry (SAS) and g = qv (the slit height) for slit geometry (USAS).

The worth of Q* is that it can be used to determine the volume fraction and the specific area of a sample. Whilst these quantities are useful in their own right they can also be used in further analysis.

The difficulty with using Q* arises from the fact that experimental data is never measured over the range 0 =< Q =< infinity. At best, combining USAS and WAS data might cover the range 1e-5 =< Q =< 10 1/|Ang| . Thus it is usually necessary to extrapolate the experimental data to low and high Q. For this

High-Q region (>= Qmax in data)

  • The power law function C/Q4 is used where the constant C (= 2.|pi|.(|bigdelta||rho|).Sv) is to be found by fitting part of data within the range QN-m to QN (where m < N).

Low-Q region (<= Qmin in data)

  • The Guinier function I0.exp(-Rg2Q2/3) where I0 and Rg are obtained by fitting as for the high-Q region above. Alternatively a power law can be used.

Using invariant analysis

  1. Select Invariant from the Analysis menu on the SasView toolbar.

  2. Load some data with the Data Explorer.

  3. Select a dataset and use the Send To button on the Data Explorer to load the dataset into the Invariant panel.

  4. Use the Customised Input boxes on the Invariant panel to subtract any background, specify the contrast (i.e. difference in SLDs - this must be specified for the eventual value of Q* to be on an absolute scale), or to rescale the data.

  5. Adjust the extrapolation range as necessary. In most cases the default values will suffice.

  6. Click the Compute button.

  7. To include a lower and/or higher Q range, check the relevant Enable Extrapolate check boxes.

    If power law extrapolations are chosen, the exponent can be either held fixed or fitted. The number of points, Npts, to be used for the basis of the extrapolation can also be specified.

  8. If the value of Q* calculated with the extrapolated regions is invalid, a red warning will appear at the top of the Invariant panel.

    The details of the calculation are available by clicking the Details button in the middle of the panel.



Volume Fraction

The volume fraction |phi| is related to Q* by


where |bigdelta||rho| is the SLD contrast.


Specific Surface Area

The specific surface area Sv is related to Q* by


where Cp is the Porod constant.


O. Glatter and O. Kratky Chapter 2 in Small Angle X-Ray Scattering Academic Press, New York, 1982


This help document was last changed by Steve King, 01May2015

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