By PRACHI

X-RAYS

High energy photons.Short λ EM radiation.Produced by deceleration of high-energy e-s or by electronic transition of e-s in the inner orbitals of atoms.X-ray range 10-5Å to 100 ÅUsed 0.1Å to 25 Å

Characteristics of X-rays

Absorption of X-rays give information about the absorbing material.

Fluorescence emission of X-rays enables to identify & measure heavy

elements in the presence of each other & in any matrix.

Diffraction of X-rays enables analysis of crystalline materials with a high degree

of specificity & accuracy.

INSTRUMENTATION OF X-RAY SPECTROMETRY

COMPONENTS:1. Source: The X-Ray tubes, radioisotopes & secondary fluorescent

sources.2. Filters & Monochromators.3. A Sample holder.4. A Radiation detector or transducer.5. A Signal processor & read out.

X-RAY SPECTROMETER

X-RAY SPECTROMETER

MONOCHROMATORS

•Used for restricting the wavelength range of incident radiation.

•Used to absorb certain λs but permit desired λs.

• Consists of a pair of beam collimators & a dispersing element.

•Is placed between the X-ray source & the sample.

• Only one type of radiation is however desired.

GONIOMETER

- Rotatable table.- Is the analyzing crystal

positioning system- When the crystal rotates by θo

the detector rotates through 2θo

- The detector is therefore always in the correct position

(the Bragg angle) to detect dispersed & diffracted

radiation.

COLLIMATORS

- Consists of a series of closely spaced metal plates.

- Absorb all but parallel beams of radiation.

MONOCHROMATORS

BRAGG EQUATION

whereD spacing of the analyzing crystalӨ half the angle between the detector & the samplen order of diffraction

d2

nsin

MONOCHROMATORS

•A Single crystal mounted on a goniometer that permits variation & precise determination of angle Ө between crystal

face & collimated incident beam.

•By Bragg equation,for any given angular setting of the goniometer, only a few

wavelengths are diffracted.

As the crystal rotates through an angle Ө, the detector must simultaneously move through an angle 2Ө.

X-Ray Diffraction spectrum

DETECTORS

- Transform photon energy into electrical pulses.

- The pulses (photons) are recorded as counts per second (known as count rate).

- Count rate is a measure of the intensity of the X-ray beam.

Three classes of X-ray detectors

- Gas-filled detectors

- Scintillation detectors

- Semiconductor detectors

Gas-Filled Detectors

- Filled with inert gas.

- Gas is ionized by X-ray photons to produce Gaseous+ ions and e-

- Under the influence of an applied potential, e- move to central wire anode & are detected;

While slower moving cations attracted towards metal cathode.

DETECTORS

Gas-Filled Detectors

- Three types

Ionization Chambers- Number of electrons reaching the anode is

reasonably constant.

Proportional Counters- Number of electrons increases rapidly with

applied voltage.

Geiger Muller Tubes- Enormous amplification of the electrical

pulse.

DETECTORS

GAS AMPLIFICATION OF VARIOUS TYPES OF GAS- FILLED DETECTORS

THE GEIGER TUBE

•Operated in the voltage region V5 & V6.

•Tubes are filled with Ar.

•Amplification of electrical pulse is enormous in this region.

•Gas amplification > 10^9.

•Each photon produces an avalance of electrons & cations.

•Conduction of electricity is not continuous.

THE GEIGER TUBE

LIMITATIONS:

• Lacks the large counting range of other detectors.

• Has long dead time.

• Its use in X-ray spectrometers is limited by this factor.

• Quantitative applications have decreased.

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