Introduction

• X-rays are a part of electromagnetic spectrum.

• X-rays have a wavelength in range of 10-5 Å to 100 Å; conventional X-Ray spectroscopy is largely confined to approximately 0.1 Å to 25 Å .

• X- Ray spectroscopy is based uponmeasurement of emission, absorption,scattering, fluorescence and diffraction ofelectromagnetic radiation.

• They are defined as short wavelengthelectromagnetic radiation produced bydeceleration of high-energy or

• Electronic transition of electrons in the innerorbitals of atoms.

X RAY CHARACTERSTICSEMISSIONProduced in three ways:1. By bombardment of a metal target with

a beam of high energy electrons2. By exposure to primary beam of X-rays

to generate secondary X-Rays3. By employment of radioactive source

whose decay process results in X-Rayemission.

X-Ray sources produce both continuousand line spectrum.

• Continuous radiation also called whiteradiation or Bremsstrahlung the lattermeaning radiation arising from retardation ofparticles.

• It is dependent on accelerating voltage V butindependent of target material.

• It results from collision between electrons ofbeam and atoms of target material.

• The maximum photon energy corresponds toinstantaneous deceleration of electron to zerokinetic energy in single collision.

• Duane-Hunt Lawhν0 = h c/ λ0= Ve where

Ve, product of accelerating voltage and charge onelectron, is K.E. of all electrons in the beam, h isPlanck’s constant, and c is velocity of light; ν0 ismaximum frequency of radiation that can beproduced at voltage V ( volts) and λ0 (Å) is lowwavelength limit for radiation.

• λ0= 12,398/V (after substituting numerical values)

• Characteristic line spectra result fromelectronic transitions involving innermostorbitals.

Characteristic Line Spectra• Line Spectra consists of two series of lines.

• The shorter wavelength is called K-seriesand longer wavelength L-series.

• The short wavelength produced when highenergy electrons from cathode removeelectrons from orbitals nearest to nucleusof target atom. The formation of excitedion results which loses quanta of X-radiation as electrons from outer orbitalsundergo transitions to vacated orbitals.

• The wavelengths of characteristic X-Rays isindependent of chemical combinationbecause transitions responsible for theselines involve electrons that take no part inbonding.

• Moseley’s Law (1914): A linearrelationship exists between reciprocalvalues of wavelength for each transitionseries and the square of the atomic no. Zof the excited atom.

ABSORBTION

• X rays are absorbed by matter and degree ofabsorbtion is determined by the nature andamount of absorbing material.

• A peculiarity of X-Ray spectra is appearance ofsharp discontinuities, called absorption edges,at wavelengths immediately beyondabsorption maxima.

• At absorption maxima the energy of X-Rayquantum corresponding to that wavelengthexactly matches the energy required to justeject the highest energy K electron of theelement; immediately beyond this wavelength,the energy of the radiation is insufficient tobring about removal of K electron and abruptdecrease in absorption occurs.

• The energy of radiation is partitioned betweenK.E. of electron and P.E. of the excited ion.

Where x is the sample thickness in cms and I and I0 are intensities of transmitted and incident beams. ρ is density of sample and μm is mass absorption coefficient, a quantity independent of physical and chemical state of element. It has unit cm2/g.

Mass absorption coefficients are additive functions of the weight fractions of elements contained in a sample. Thus,

μm= WA μA + WB μB+ WC μC +…….

where sample is containing weight fractions WA, WB, WC of elements A,B, and C. The terms μA , μB , μC are the respective mass absorption coefficients for each of the elements.

FLUORESCENCE

• The absorption of X-Rays produce electronically excitedions that return to ground state by transitions involvingelectrons from higher energy levels characterised byemission of X-radiation (fluorescence) of wavelengthsidentical to those that result from excitation produced byelectron bombardment.

• When fluorescence is to be excited by radiation from anX-Ray tube , the operating voltage must be sufficientlygreat so that cut off wavelength λ0 is shorter than theabsorption edge of the element whose spectrum is to beexcited.

DIFFRACTION

• Diffraction of x rays is used in analysis ofcrystalline materials with high degree of accuracyand specificity.

• When X-rays are scattered by orderedenvironment in a crystal, interference bothconstructive and destructive) takes place amongthe scattered rays because distance betweenscattering centers are of the same order ofmagnitude as the wavelength of the radiation.Diffraction is the result.

Bragg’s Law (1912)

• The X-Rays appear to be reflected from the crystal only if the angle of incidence ϴsatisfies the condition that

sin ϴ= n λ/ 2 d

where d is the interplanardistance of the crystal , n is an integer , λ is wavelength of X-radiation.

• At all other angles, destructive interference occurs.

INSTRUMENTATION FOR X-RAY SPECTROSCOPYComponents for X-ray spectroscopy are :(1) X-ray generating equipment (X-ray tube)(2) Collimator(3) Analyzer Crystal(4) Monochromators(5) Detectors

X-Ray TubeDetermining the energy of the X-Ray

Controlling the intensity of X-Ray

100KV!

X-Ray Tube

Collimators

• X-rays can be generated by an X-ray tube.

• X-rays tube is a vacuum tube that uses a high voltage to accelerate the electrons released by a hot cathode to a high velocity.

• The high velocity electrons collide with a metal target, the anode, creating the X-rays.

• Less than 1% of electrical power converted to radiant power so anode cooled

• A collimator is a device that narrows a beam of particles or waves.

• Narrow mean to cause the directions of motion to become more aligned in a specific direction (i.e., collimated or parallel).

• Collimation is achieved by using a series of closely spaced ,parallel metal plates or by a bundle of tubes ,0.5 or less in diameter.

Analyzer Crystal

• Analyzing crystal acts as diffraction grating (dispersingelement) scanning through entire range of goniometer(monochromator) and permits radiation at a particularposition to be correlated with wavelength through Bragg’scondition.

• The range of wavelengths usable with various crystals isgoverned by d spacings of the crystal planes and by thegeometric limits to which goniometer can be rotated.

• No crystal can be used over entire range. Ammoniumdihydrogen phosphate has much greater wavelength rangebut it slow dispersion prevents its use at low wavelegths.Topaz , LiF used at low wavelengths .

• Crystal is mounted on rotating table that permits variationand precise determination of angle ϴ between crystal faceand collimated incident beam.

Monochromator

• Need of Monochromator

• Types of Monochromator

• Working of Filters

• Filter v/s Monochromator

• Working of diffraction grating

• Advantages of monochrome X-Rays

Need of Monochromator

• Monochromator crystals partially polarize an unpolarized X-ray beam

• The main goal of a Monochromator is to separate and transmit a narrow portion of the optical signal chosen from a wider range of wavelengths available at the input.

Monochromators

Types of Monochromator

• Metallic Filter Type

• Diffraction grating type

Working of Filters

• Filters exploit the X-ray absorption edge of the particular element.

• At wavelengths longer than the absorption edge (i.e. just above the edge), the absorption of the X-rays is considerably less than for wavelengths shorter than the absorption edge (i.e. just below the edge) as shown for nickel metal:

Working of Filters

• The absorption edge of nickel metal at 1.488 Å lies between the Kα(λ = 1.542 Å) and Kβ (λ = 1.392 Å) X-ray spectral lines of copper. Hence nickel foil of an appropriate thickness can be used to reduce the intensity of the Cu Kβ X-rays as shown:

Choice of filter metal

• The choice of filter material depends upon the choice of anode material in the X-ray tube as shown in the following table:

Anode Cu Co Fe Mo Cr

Filter Ni Fe Mn Zr V

Limitations of Filter

• X-ray filters were used to reduce the unwanted white radiation from the X-ray source and to eliminate (as much as possible) the Kβ radiation.

• The drawback of filters is that the background radiation is still high and that the transmitted radiation is still not very monochromatic.

Working of diffraction grating

• Source (A)

• Entrance slit (B)

• Collimator(C)

• Grating (D)

• Another mirror (E)

• Exit slit (F)

Advantages of monochrome X-Rays

• Improved resolution

• Minimizing sample damage

• Improved signal to noise ratio

• Analyze of small samples

• Multispotting on samples

• Simplified data processing

X-ray Detectors

• Solid State Detectors

• Scintillation Detectors

• Gas-filled Detectors

Solid State Detectors

X-ray

Solid State Detectors

• The charge carriers in semiconductor are electrons and holes.

• Radiation incident upon the semiconducting junctionproduces electron-hole pairs as it passes through it. Electronsand holes are swept away under the influence of the electricfield, and the proper electronics can collect the charge in apulse.

Scintillation Detector

+

-

Scintillator

Photo Cathode

Focusing

cup

Dynodes

HV

Resistor

e-

e-

Scintillation detectors

• Scintillation detectors consist of a scintillator and a device, such as aPMT(Photomultiplier tubes), that converts the light into an electricalsignal

• Consists of an evacuated glass tube containing a photocathode,typically 10 to 12 electrodes called dynodes, and an anode.

• Electrons emitted by the photocathode are attracted to the firstdynode and are accelerated to kinetic energies equal to the potentialdifference between the photocathode and the first dynode.

Scintillation detectors

• When these electrons strike the first dynode, about 5 electrons are ejected from the dynode for each electron hitting it.

• These electrons are attracted to the second dynode, and so on, finally reaching the anode.

• Total amplification of the PMT is the product of the individualamplifications at each dynode.

• If a PMT has ten dynodes and the amplification at each stage is 5, the total amplification will be approximately 10,000,000.

• Amplification can be adjusted by changing the voltage applied to the PMT.

Gas-filled detectors

Gas-filled detectors

• A gas-filled detector consists of a volume of gas between twoelectrodes, with an electrical potential difference (voltage) appliedbetween the electrodes.

• Ionizing radiation produces ion pairs in the gas.

• Positive ions (cat-ions) attracted to negative electrode (cathode);electrons or anions attracted to positive electrode (anode).

• In most detectors, cathode is the wall of the container that holds thegas and anode is a wire inside the container.

33

Geiger Mueller Counter

Battery or

High Voltage

Resistor(-) Cathode

+ -

(+) Anodee-

+

Geiger Mueller Counter• GM counters also must contain gases with specific properties

• Most common type of detector

• Electrical collection of ions

• When the gas amplification factor reaches 108, the size of the outputpulse is a constant, independent of the initial energy deposit.

• In this region, the Geiger- Mueller region, the detector behaves like aspark plug with a single large discharge.

• Simple cheap electronics.

• Energy dependence.

• Large dead times, 100-300µs, result

• No information about the energy of the radiation is obtained or itstime characteristics.

X-ray Diffractometer• Diffraction is a phenomena of bending of light around the corners of an

obstacle ,when the size of an obstacle is of the order of wavelength of light.

• Bragg's law n = 2d Sin

where d= distance between similar atomic planes in mineral (inter-atomic

spacing

= angle of diffraction

= wavelength

n= an integer – 1,2,3.. etc (order of diffraction)

• A diffractometer is a measuring instrument for analyzing the crystallographicstructure of a material from the scattering (diffraction) pattern producedwhen a beam of radiation or particles (such as X-rays) interacts with it.

X-ray Diffractometer• A typical diffractometer consists of a source of radiation, a

monochromator to choose the wavelength, collimator to make the beamparallel, a sample and a detector.

• An x-ray diffractometer illuminates a sample of material with x-rays ofknown wavelength.

• A strip of X-Ray film is mounted in circular position around the sample.

• The undeviated central beam passes out through a hole E cut in the filmstrip P. Diffracted beam falls on the film at various points like d1, d2, d3 etc.

• Intensities of the diffraction peaks are proportional to the fraction of thematerial in the mixture.

References: Principles of Instrumental analysis, SkoogGoogle, Wikepedia