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The Intensity Problem

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.. index::
        single: intensity; problem
        single: intensity; absolute
        see: absolute intensity; intensity, absolute

The intensity (see :ref:`SASdata`) is permitted in three different forms:

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  1. absolute units: dΣ ⁄ dΩ(Q)

    differential cross-section per unit volume per unit solid angle (typical unit: 1/cm)

  2. absolute units: dσ ⁄ dΩ(Q)

    differential cross-section per unit atom per unit solid angle (typical unit: cm^2)

  3. arbitrary units: I(Q)

    usually a ratio of two detectors but unit is meaningless (typical unit: a.u.)

This presents a few problems for analysis software to sort out when reading the data. Fortunately, it is possible to analyze the unit attribute to decide which type of intensity is being reported and make choices at the time the file is read. But this is an area for consideration and possible improvement.

One problem arises with software that automatically converts data into some canonical units used by that software. The software should not convert units between these three types of intensity indiscriminately.

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.. index:: I(Q)

A second problem is that when arbitrary units are used, then the set of possible analytical results is restricted. With such units, no meaningful volume fraction or number density can be determined directly from I(Q) .

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