X-ray Diffraction- Characteristics of X-rays Flashcards
What order of magnitude is the wavelength of x-rays?
Angstroms, 10^-10, similar to interatomic distances in crystals
What are electromagnetic waves?
They are a form of energy. They consist of oscillating perpendicular electric magnetic field. They have a frequency, wavelength and amplitude. Intensity is proportional to square of amplitude.
What gives rise to scattering of x-rays?
The electric field coupling to the charged electrons in atoms
Wavelength and energy range of x-rays?
- 01Å-100Å
0. 1keV-1000keV
How are x-rays generated?
When matter is bombarded with high energy particles/photons. In lab use an X-ray tube. Electrons are emitted from a filament and accelerated, by an applied potential, to impact on a suitable target. Collisions between electrons and atoms of target generates heat and x-rays.
Features of an X-Ray tube
Sealed evacuated tube. Heated tungsten filament in a focussing cup emits electrons. Target is Cu which is water cooled. X-rays generated from copper pass out through beryllium windows on opposite walls of tube. X-rays are emitted in all directions from the Cu
Efficiency of X-ray tubes
Highly inefficient. Less than 1% electron KE converted into x-rays. Rest is transformed to heat hence need for cooling.
Two components of x-ray spectrum from tube
Continuous radiation (broad range of wavelengths as curves) Characteristic radiation (specific wavelengths as peaks)
Continuous radiation
Also Bremsstrahlung radiation or white radiation. Arises from electron deceleration. Not all electrons decelerate in same way. Shortest wavelength radiation arises from single impact deceleration where all KE converted to x-rays.
E=hνmax=eV
λmin=c/νmax=hc/eV
So increasing tube voltage shifts λmin to lower wavelengths and increases intensity of spectra
What does an x-ray spectrum look like from a tube?
Relative intensity vs wavelength. Has curves for each voltage like Maxwell-Boltzmann distribution. Has peaks on each curve which go much higher than peak of curve.
Characteristic radiation
Arises from quantum transitions in the target. Characteristic lines appear on continuous background above a critical tube voltage and occur only at specific wavelengths which are characteristic of the target material. Several sets of characteristic lines can be observed labelled K, L, M. If voltage is increased further the characteristic wavelength doesn’t change.
Where is the characteristic wavelength for copper targets?
About 1.54Å
How does characteristic radiation arise?
An electron with KE, E, knocks out core shell electron from Cu atom in target. Ejected photo-electron leaves a core hole. Electron transfer from a higher energy level fills the core hole accompanied by emission of photon with energy ΔE=hν. The energy of emitted photon is quantised. Incident electron must have KE greater than φK to eject a core shell electron (binding energy of K-shell electron.
Where are the K, L, M electron shells?
K is core where two electrons can be
L is next level up where 8 electrons can be
M is 3rd level where 18 electrons can be
What are the transition groups?
K series: transition to the K shell
L series: transition to the L shell
M series: transition to the M shell
