x rayby sushil kumar
TRANSCRIPT
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.
THANK YOU