X-ray Absorption and Fluorescence

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Background

Practical tips on how to deal with samples exhibiting strong x-ray absorption can be found on the Wiki page Samples with Strong X-Ray Absorption.

X-Ray Absorption

A brief discussion on X-Ray absorption is given below.

Absorption Length

The X-ray beam intensity I(x) at depth x in a material is a function of the attenuation coefficient mu, and can be calculated by the Beer-Lambert law:

I(x) = Io e^(-mu * x)

The attenuation coefficient mu is typical given in inverse length units of 1/cm, and is a function of the incident wavelength, material chemistry and density. It can be calculated or estimated using resources below.

The Absorption Length (or Attenuation Length) is defined as the distance into a material where the x-ray beam intensity has decreased to a value of 1/e (~ 40%) of the incident beam intensity (Io).

Recall that Euler's number e = 2.72.

This is a convenient description, as absorption length x = 1/mu, as shown below:

(1/e) = e^(-mu * x)

ln(1/e) = ln(e^(-mu * x))

1 = mu * x

x = 1/mu

as a simple example, consider a solid Nickel metal sample at room temperature probed by X-rays of energy = 30 KeV (Lambda = 0.41 A).

For Ni with density = 8.908 g*cm-3, we can calculate (using resources below) that mu ~ 85.0 cm-1.

Then absorption length x = 1/85 = 0.011 cm = 110 microns.

Capillary Transmission

Continuing with the above example for capillary transmission X-ray diffraction experiments, we can consider a cylindrically shaped solid Nickel metal sample of radius R = 0.055 mm (or 0.0055 cm). The diameter of this sample is then 2*R = 0.011 cm (see absorption length above)

Since mu = 85 cm-1, then mu*R = 0.935, therefore the % total incident x-rays transmitted through the sample is = e^(-2*muR) = ~ 40%

In general, a mu*R of ~ 1.0 is desired for capillary transmission x-ray samples.

Web Resources

WebAbsorb

a web based calculator to estimate X-ray absorption for powder XRD capillary samples (11BM page) http://11bm.xor.aps.anl.gov/absorb/absorb.php

MuCal

calculate X-ray absorption, fluorescence and more (by C. Segre @ IIT/ANL) http://csrri.iit.edu/mucal.html

NIST X-Ray Mass Attenuation Coefficient Tables

Tables of X-Ray Mass Attenuation Coefficients and Mass Energy-Absorption Coefficients http://www.nist.gov/pml/data/xraycoef/index.cfm

Software

The two python GUI programs described below compute approximate x-ray scattering cross sections (f, f' and f") for individual elements using the Cromer & Liberman algorithm.

downloaded both here:

https://subversion.xor.aps.anl.gov/trac/pyFprime/

Fprime

computes and plots elemental scattering factors.

Absorb

computes scattering and absorption for a given composition and makes an attempt to estimate density as well. WebAbsorb provides a web based utility based on this program (see http://11bm.xor.aps.anl.gov/absorb/absorb.php).