Principles of X-ray

OutlineEM spectrumCharacteristics of X-rayAttenuation of X-rayX-ray interaction with matter

DispersionThe phenomenon due to which white light splits into seven colors (VIBGYOR) when passed through an equilateral prism/grating is called dispersion.

SpectrumThe band of seven colors obtained on the other side of the prism, on the screen when white light splits/ dispersed into, is called spectrum.

Electromagnetic waves are created by the vibration of an electric charge.This vibration creates a wave which has both an electric and a magnetic component. An electromagnetic wave transports its energy through a vacuum at a speed of light.

Electromagnetic radiation is made up of waves of energy that contain electric and magnetic fields vibrating transversely and sinusoidal to each other and to the direction of propagation of the waves. Electromagnetic waves can travel through the vacuum of outer space. Sound waves are examples of mechanical waves while light waves are examples of electromagnetic waves.

__________________________________________________________________________________ 0.01nm 10nm 200nm 400nm 800nm 0.4cm 25cm

Y-rays X-rays Far-UV UV Visible IR Microwaves RadiowavesEM can be expressed in Wave length (m/)Frequency (Hz)Energy (eV)X-rays Wavelengths : 0.01 to 10nmFrequency : 31016Hz to 31019Hz) Energy : 100 eV to 100 keVDefinationX-rays may be defined as electromagnetic radiation of wavelength (0.1-100), high energy end (100eV-over 100keV) produced by deceleration of high energy electrons and/or by electron transitions in the inner orbit of atoms. 1 nm (nanometer)=10-9 meter = 10

Absorption spectrum is the fraction of incident radiation absorbed by the material over a range of frequencies. The absorption spectrum is primarily determined by the atomic and molecular composition of the material.Emission Spectrum is formed due to transition of a electron from a high energy state to a lower energy state. The energy of the emitted photon is equal to the energy difference between the two statesAbsorptionEmission

Fluorescenceis a process whereby an atom or molecule emits immediate radiation in the course of shifting from a higher to a lower electronic state. Time interval is lowestPhosphorescence does not emits immediate radiation as it absorbs but re-emitted at a lower intensity for up to several hours.

Formation of X-Rays (emission)Produced by the deceleration of high-energy electrons. Electronic transition of electrons in the inner orbitals of atoms.

Formation of X-Rays (fluorescence)Exposure of a substance to x-ray radiation absorption and then fluorescence (secondary X-ray)

Formation of X-Rays (decay, synchroton)Radioactive decay X-ray emission (common in medicine)Synchrotron source radiation (accelerated particles)

X-rays are part of the EM spectrum lying between UV and -raysNature of X-raysWavelength expressed in Angstrom ( 1 = 10-8cm = 10-10 m )Energy expressed in kilo electron-volt (keV) (1 J = 1.602x10-16 keV)E (keV) = 12.36 / () Exhibits the duality wave property ( ) during diffraction process particle nature ( E ) during absorption and scattering Its energy given by E = h = hc /

where, h Plancks constant (6.625x10-34 J-s) frecuency ( in per sec.) wavelength ( in meters )c velocity of light ( 3 x 108 m/s)E Energy ( in Joules)

Origin of X-ray SpectraMoseleys Law : 1/ = K (Z-)2 K : constant of particular series (K, L, M) Z : Atomic Number, : wave length : shielding constantHigher the atomic number in the periodic table smaller is the wavelength of the element and higher is the energy of the x-ray photon The constant is equal to 1 for the K-lines and 7.4 for the L-lines

Thus when X-rays pass through matter, part of x-ray will be absorbed. The absorbed intensity is proportional to the path in the matter.

Taking an infinitesimal slab, dz, of sample: Iois the intensity entering the sample at z=0, I is the intensity of light leaving the sample dI is the intensity absorbed in the slab,

Primary X-ray intensity is attenuated. Attenuation is given by the decay law

I = Io exp (-z) z (in g/cm2) is the thickness of the medium traversed and is the mass attenuation coefficient ( in cm2/g). This is known as Lambert-Beer law LawLinear attenuation coefficient and Lambert-Beer law

Origin of X-rays1. When external radiation or particle beam knocks out inner shell electron Outer shell electrons jump down to fill up the vacancy Emission of mono-energetic X-rays (sharp lines)

X-rays nomenclature

X-rays nomenclature

Fluorescence yield The yield for the light elements is very lowElement wK wL O0.003 K0.118 Cu0.4250.006 Mo0.7490.039 Sm0.9150.180 U0.9600.478CaZrNdHg

If the energy is too high, many photons will pass the atom and only a few electrons will be removed. If the energy becomes lower than the binding energy, a jump or edge can be seen: the energy is too low to expel electrons from the corresponding shell, but is too high to expel electrons from the lower energetic shell.An absorption edge is a sharp discontinuity in the absorption spectrum of X-rays by an element that occurs when the energy of the photon corresponds to the energy of a shell of the atom (K,LI,LII,LIII, etc.).Absorption EdgeAbsorption Edge phenomenon has important analytical consequences.

The most effective X-ray tube photons for exciting a sample are those with energies just above the appropriate absorption edge.Analytical Consequence of Absorption edgeAbsorption EdgeThus an X-ray tube with a Cr anode that emits characteristic lines at 5.41 keV ( Cr K 1.2) and 5.95 keV (Cr K) will effectively excite Ca (absorption edge 4.04 keV) but can not excite Fe at all (absorption edge 7.11 keV). This element will only be excited by tube continuum radiation above this threshold.Photo electric ionizationTransmissionX ray PhotoEnergies of K and K photons always lies just below the K-line absorption edge, so these lines are not efficiently absorbed by the elements

X-rays interact with matter

Prominent Interaction AbsorptionRayleighPhotoelectricPair ProductionScatteringComptonPhoto electric Effect : When an x-ray photon has an energy equivalent to the K shell binding energy of an atom then the photon will be totally absorbed and an electron ejected from the atom. The atom is ionised and in this excited state higher orbital electrons return to K ground state with the emission of a x-rayline

Absorption EffectIodineEnergy levelsK -33.2keVL -4.3keVM -0.6keV Photon with energy 40keV enters Photoelectron from K-shell with energy (40-33.2)=6.8keV exits

Pair ProductionA photon with Energy > 1.02Mev might completely disappear under the production of an electron and a positron (electron with positive charge) with the annihilation of the x-ray photon.

Auger EffectMore pronounced in Low Z elements (4-20) where electrons are loosely bound and when present at lower concentration detection becomes difficult due to poor flourescence yield.

Auger vs. Fluorescence yieldFluorescence yield (w) is fraction of ionizations that yield characteristic X-ray versus Auger yield (a) within a particular family of X-rays. w + a =1 W values lie between 0 to 1 The fluorescent yield for the L shell is lowAuger electrons predominate at lower ZWhy EDXRF analyses starts from Na (Z=11) ?

ScatteringCompton ScatteringRayleigh scatteringElastic or Rayleigh scattering (coherent scattering) Low energy photons simply bounce off an atom with no change in momentum.The scattered and incident photons are coherent Only change in directionE = E Inelastic or Compton Scattering (incoherent scattering)When the energy of a photon is large compared with the binding energy of an electron then the weakly bound outer electron may be free from atom with transfer of energy and change in direction. E < E and >Effective for high Z elements

Low energy tube radiation is more likely to fall below the threshold for excitation of inner orbital electrons and will also therefore be more effectively scattered.Analytical Consequence

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