Corpuscular induced and X-ray or γ-ray induced

fluorescence

NON DESTRUCTIVE CHEMICAL ANALYSIS

Notes by:Dr Ivan Gržetić, professorUniversity of Belgrade – Faculty of Chemistry

Dual nature of the microscopic material

In 1924, Louis de Broglie suggested that just as light exhibits wave and particle properties, all microscopic material particles such as electrons, protons, atoms, molecules etc. have also dual character. They behave as a particle as well as wave.

Radiation with enough energy to knock electrons out of atoms and produce ions is called ionizing RADIATION and includes particles (protons, alpha particles, beta particles, accelerated ions) and rays (x-rays and gamma rays).

Induced X-ray florescence

Primary (incidet) radiation that can induce X-ray fluorescence could be dividet into three major groups:◦Corpuscular induced fluorescence◦X-ray induced fluorescence (wave

induced)◦γ-ray induced fluorescence (wave

induced)

Corpuscular induced

fluorescence

Electrons, protons, alpha particles or ionsThe radiation associated with

subatomic particles, such as electrons, protons, alpha particles or ions, which travel in streams at various velocities. All such particles have definite masses and have radiation properties very different from those of electromagnetic radiations, which have no mass and travel as waves at the speed of light.

Electron induced X-ray fluorescence

Common particles used for induced fluorescence are electrons and protons.

Electron induced X-ray fluorescence is tightely bond to instruments like electron microscopes equiped with appropriate spectrometers for x-ray detection.

Scanning electron microscope - SEM

SEM with EDS and WDS

EDS – Energy dispersive spectrometerWDS – Wavelength dispersive spectrometer

Electron sample intereaction

SAMPLE and SEM SIGNALSBackscattered electrons are electrons which

have been emitted from the primary beam and have reacted elastically with the sample's atoms. They are sent back almost in the same direction as the one they came from, and with very little energy loss. Chemical elements that have a high atomic number produce more backscattered electrons than the ones that have a low atomic number. The areas of the sample with a high atomic number will thus look whiter than the ones with a low atomic number (high phase contrast).

SAMPLE and SEM SIGNALSSecondary electrons are emitted when

the primary beam that has lost a part of its own energy, excites atoms of the sample. The secondary electrons have a little energy (about 50 eV) divided up on a large spectrum. Pictures obtained with the secondary electrons mostly represent the topography of the sample and they have very little phase contrast.

SAMPLE and SEM SIGNALSX-ray emision is electromagnetic radiations

induced by primary (incident) radiation. The energy of the secondary X-rays produced from atomes present in the sample in SEM are usualy between 0.5 et 30KeV.

Auger electronsContinuum...

SAMPLE and SEM SIGNALSIntereaction volumen, depth of signal

and resolution

X-ray induced fluorescence

Sample X-ray Intereaction

Saturation Depth or Penetration Depth

The deeper the X-ray enters the sample, the more it interacts with the sample’s atoms. An increasing portion of the X-ray is absorbed by the sample until a specific depth is reached beyond which the primary X-ray light can no longer penetrate. This also applies to the fluorescent light that must exit the sample in order to be detected.

The lowest detectable sample layer is called the saturation depth. The saturation depth depends on the intensity of the X-rays, the wavelength (i.e., the type of detected atom) and the density of the sample’s surroundings (the matrix). If different elements are analyzed in the same surroundings, the saturation depth increases according to the atomic number of the element in question.

Saturation Depth or Penetration Depth

Table of Saturation Depth or Penetration Depth

28000 μm = 0,028 m = 28 mm ; (1 μm = 10-6 m) ; 3 mm = 3000 μm

Difference between electron and X-ray impact volumen

RecommondationThe thicknes of the bulk sample

should be minimum 3 mm (5 mm).Thicker bulk samples are prefared for

samples made of light elements (10-15 mm minimum).

Thin layers and film samples are treated totaly different from bulk samples.

γ-ray induced fluorescence

Radioactive sourcesγ-ray induced fluorescence is usualy used for Field-

portable x-ray fluorescence (XRF) also known as Hand held X-ray analyser.

The X-ray emitting sources usually contain nuclides that decay by means of the electron-capture mechanism. During the decay, a inner shell electron is captured by the neutron-deficient nucleus, transforming a proton in a neutron. This results in a daughter nuclide that has a vacancy in one of its inner shells, which results in the emission of corresponding characteristic radiation.

For example, when a 55Fe-nucleus (26 protons and 29 neutrons) captures a K-electron and becomes a 55Mn nucleus, a Mn K-L3,2 (Mn-Kα) or KM3,2 (Mn-Kβ) photon will be emitted.

Radioactive sourcesAn instrument may contain one or more

of the following radioactive isotopes:

Cadmium (Cd-109) sourceThe 109Cd source emits X-rays with energies of

22 and 25 keV when it decays to 109Ag in the two-step nuclear process of electron capture and internal conversion. In addition, a small portion (3.8%) of its photon output is emitted at 88 keV. The 22- and 25-keV emissions of this isotope may be used for the K-shell excitation of atomic numbers 22 (Ti) through 44 (Ru) and L-shell excitation of atomic numbers 67 (Ho) through 94 (Pu).Additionally, an 88-keV co output will excite K-shells up to lead. The isotope has a half-life of 1.24 years, meaning that after this time the source activity is half its original value.

Americium (Am-241)The 241Am is a source of 59.5 keV X-rays

allowing for the excitation of higher atomic number elements. This isotope has generally been used for the K-shell excitation of atomic numbers 45 (Rh) through 66 (Dy). However, 241Am is also a source of lower excitations at approximately 13.95 and 17.7 keV (the neptunium L-lines). This permits all elements from about atomic number 22 (Ti) through 94 (Pu) to be analyzed effectively with only one radioisotope source. Furthermore, the americium isotope has a half-life of 432 years and therefore never requires replacement.

Useful range of X ray radioactive sources

The range of elements that can be usefully analyzed by means ofvarious radioactive X-ray sources.

THE END