This model calculates an empirical functional form for SAS data characterized by two power laws.

Parameter Description Units Default value
scale Source intensity None 1
background Source background cm-1 0.001
coefficent_1 coefficent A in low Q region None 1
crossover crossover location -1 0.04
power_1 power law exponent at low Q None 1
power_2 power law exponent at high Q None 4

The returned value is scaled to units of cm-1 sr-1, absolute scale.


The scattering intensity \(I(q)\) is calculated as

\[\begin{split}I(q) = \begin{cases} A q^{-m1} + \text{background} & q <= q_c \\ C q^{-m2} + \text{background} & q > q_c \end{cases}\end{split}\]

where \(q_c\) = the location of the crossover from one slope to the other, \(A\) = the scaling coefficent that sets the overall intensity of the lower Q power law region, \(m1\) = power law exponent at low Q, and \(m2\) = power law exponent at high Q. The scaling of the second power law region (coefficent C) is then automatically scaled to match the first by following formula:

\[C = \frac{A q_c^{m2}}{q_c^{m1}}\]


Be sure to enter the power law exponents as positive values!

For 2D data the scattering intensity is calculated in the same way as 1D, where the \(q\) vector is defined as

\[q = \sqrt{q_x^2 + q_y^2}\]

Fig. 108 1D plot corresponding to the default parameters of the model.



Author: NIST IGOR/DANSE on: pre 2010

Last Modified by: Wojciech Wpotrzebowski on: February 18, 2016

Last Reviewed by: Paul Butler on: March 21, 2016