# guinier

Parameter | Description | Units | Default value |
---|---|---|---|

scale | Source intensity | None | 1 |

background | Source background | cm^{-1} |
0.001 |

rg | Radius of Gyration | Å | 60 |

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

**Definition**

This model fits the Guinier function

\[I(q) = \text{scale} \cdot \exp{\left[ \frac{-Q^2R_g^2}{3} \right]}
+ \text{background}\]

to the data directly without any need for linearisation (*cf*. the usual
plot of \(\ln I(q)\) vs \(q^2\)). Note that you may have to restrict the data
range to include small q only, where the Guinier approximation actually
applies. See also the guinier_porod model.

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

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

**References**

A Guinier and G Fournet, *Small-Angle Scattering of X-Rays*,
John Wiley & Sons, New York (1955)