X-Ray Fluorescence


   

Introduction

X-ray Fluorescence (XRF) is a nondestructive method for the elemental
analysis of solids and liquids. The sample is irradiated by an intense x-ray
beam, which causes the emission of fluorescent x-rays. The emitted x-rays
can either be detected using energy dispersive or wavelength dispersive
detector. Either the energies or wavelengths of the emitted x-rays are used
to identify the elements present in the sample while the concentrations
(how much) of the elements are determined by the intensity of the x-rays.
XRF is a bulk analysis technique with the depth of sample analyzed varying
from less than 1 mm to 1 cm depending on the energy of the emitted x-ray
and the sample composition. The elements commonly detected range from
sodium to uranium. Lighter elements from boron to fluorine may also be detected.

Principles of X-ray Energy Spectroscopy

X-ray Energy Spectroscopy (XES) is a technique for rapid, simultaneous multi-element
analysis. When excited by an appropriate source, a sample will emit x-rays of energies
that are characteristic for the elements composing the sample.  By measuring the
energies of x-rays that are emitted from an excited sample and counting the number
of x-rays of each energy, XRF allows us to identify which elements are present in a
sample, and also determine the relative concentration of these elements within the sample.

The sample can be excited by an x-ray source, by a radioisotope source, or by an electron
beam. Specifically, in X-ray Florescence Spectroscopy (XRF), generated x-rays are used
as the primacy source. The primary source "excites" the sample by removing/"knocking out"
tightly bound electrons from the inner-shell orbital of the excited atom in the sample.

Relaxation of the excited atom to the ground state is accompanied by the emission of
fluorescent x-rays.

Figure 1 illustrates the XRF process.


Figure 1: Schematic of the XRF process. Steps 1 & 2, incident x-ray knocks out an inner
shell electron, 3, higher shell electron fills the empty vacancy, 4, excess energy given
up as an x-ray (photon).




Figure 2: Kevex 7000 X-ray system in the Goldwater Laboratories at ASU.


Figure 2 shows the X-ray system at ASU that is used for the analysis of the specimens
in Images of Nature.

The X-rays emitted from the exited sample strike a detector, which is typically a
Solid- State detector in the case of Energy Dispersive Spectroscopy X-ray
Florescence (EDS XRF). The detector used in Kevex 7000, Goldwater Laboratories, is a
silicon drifted with lithium, Si(Li) detector (Figure 3a and 3b).



Figure 3(a):The Si(Li) detector schematic

 


Figure 3(b):The XRF sample chamber with Si(Li) detector

The elements that are present in a sample can be identified by the location of their
energy peaks along the horizontal axis. Since, in theory, the number of x-rays produced
is proportional to the number of atoms present in a sample, quantitative elemental
concentrations can be determined from the net intensities of the energy peak.

Sample Information
Most samples can be analyzed "as is" for most qualitative analysis and quantitative analysis.
Some samples may require sample preparation such as pelletizing or casting as a fusion disk.
Vacuum compatibility of the sample depends upon the element to be detected. The sample
need not be conductive or insulating. For quantitative analysis the samples should be
homogeneous with a flat smooth surface. Quantitative and qualitative elemental information
can be obtained from a bulk infinite thickness sample. Qualitative elemental concentrations
in solids/liquids are of the order of parts per million weight percent (ppm wt%) range and
qualitative elemental data from boron to uranium can be acquired. The quality of these results depends on the calibration standards.

References

1. The Kevex 7000 User's Manual, Kevex Corp. California, 1979.

2. www.analytical.phylips.com/products/xrf/2830/

3. Practical X-Ray Spectrometry by R. Jenkins and J.L. de Vries, published by
    The Macmillan Press Ltd.

4. Y. Yoneda and T. Horiuchi, Rev. Science Instruments 42:1069 (1971).

5. H. Aiginger and P. Wobrauschek, Nuclear Instrument Methods 114: 157 (1974).

6. Grieken, Rene E. and Andzrej Markowski. Handbook of X-ray Spectrometry:
    Methods and Techniques. Marcel Dekker, Inc., New York (19993) P. 453.