X-Ray Fluorescence Sources

							                     X-Ray Fluorescence Sources

                                       Wenbo Yan

                                        09 Spring



Brief Introduction

X-ray is an electromagnetic radiation that is used to quantitatively and qualitatively

analysis the element composition in a material. When an X-ray excitation source emits a

beam of X-ray and impinges on a sample, the energy of the X-ray can be either absorbed

by the atom or scattered through the material. If absorption is the case, the so-called

photoelectric effect occurs. During this process, the inner shell electrons of atoms absorb

the energy transferred by the X-ray, then they can be ejected to the outer shell and leave

electron holes in their former shell. In order to keep the atomic state stable, the electrons

on the outer shell will jump to the inner shell to fill in the vacancies, which give off a

characteristic secondary X-ray whose energy is the difference of the two shells. This

                         [1]
process is called XRF.



The secondary x-rays can be labeled as K, L, M and N, with the subscript α, β, and γ, to

denote the shells where they are produced. For example, Kα x-ray represents the

electron transition form L to K, while Kβ stands for the electron transition from M to K.

Figure 1.[2] is the diagram depicts the production of K series and L series. Figure 2.    [3]



shows the characteristic x-rays for K, L and M.


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 Figure 1*. Production of K series and L series.( Figure is reproduced from Reference 2)




Figure 2*. Characteristic x-rays for K, L and M. (Figure is reproduced form Reference 3)




XRF sources

There are three types x-ray sources used for XRF analysis: (a) radioactive sources, which

is a type of discrete source (b) x-ray tubes, (d) synchrotron radiation facilities. [4] Each of


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those three types has advantages and disadvantages. Thus, the selection of a x-ray source

depends on the type, energy as well as strength. The energy of the primary x-ray that

applied should be higher than, but close to the energy difference of the two shells. For

example, to produce a Kα1 transition of a uranium atom, which means the K electron is

excited and the electron from L will fill in that vacancy, the energy needed from the

primary x-ray to do this job is “98.428 keV”* (figure is cited form ref. 4). This value is

the difference between the K and L shell of a single uranium atom. As a result, a

secondary x-ray radiation is emitted, which is called bremsstrahlung. [4]



The radioactive source is commonly used in XRF analysis, including Fe-55, Co-57, Cd-

109 and Am-241. [4] Because those are radioisotopes, they can only offer definite energy,
                                                                            [4]
which means they can be used to excite certain atoms. Table 1.                    lists the three

radioisotopes used in x-ray source. In order to extend the analysis range of atoms, those

three radioisotopes should be combined.         Another shortcoming of the radioactive

materials is that the safety should be concerned. One application is the portable devices,

for example, for the determination of the sulphur content in gasoline and coal or of the

metal content in scrap.




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                     Table 1* (Figure reproduced from Reference 4)

On the other hand, the x-ray tubes can be used to produce bremsstrahlung by heating the

electrons off a filament and accelerating the electrons to impinge the specimen. This

process is activated by applying a high voltage, therefore, the emission spectrum band can

be controlled by adjusting the applied voltage. Compared to the radioactive source, x-ray

tubes can be switched on and off, also, it provide a more intense source of photons. There

are three commercial types of x-ray tubes that can be used today. They are: Low-power

X-ray tubes (<1 kW); High-power X-ray tubes (1-5 kW); and High power X-ray tubes

with rotating anode (>5 kW). Table 2 [4] shows several x-ray tubes.




                   Table 2.* (Figure is reproduced from Reference 4)


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Reference:

1. http://www.fas.org/sgp/othergov/doe/lanl/lib-www/la-pubs/00326405.pdf

2.http://www.amptek.com/xrf.html,

3. http://www.vigyanprasar.gov.in/dream/mar2001/mar22.jpg

4. Handbook of Practical x-Ray Fluorescence Analysis (eds.)B.Beckhoff; B. KanngierBer;

N. Langhoff; R.Wedell; H. Wolff 2006, XXIV, 863 p., 385 illus., 12 in colour, Hardcover

ISBN: 3-540-28603-9




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