What is Luminescence?

What is Luminescence?

The property of emission of light when a energetic particle impinges on  material (semiconductors)  leading to creation of electron hole pairs and excitation of carriers. When these carriers come to their equilibrium states they emit light.

Mechanism of Recombination

Radiative Recombination

When the excited excess carriers reach equilibrium positions by emission of photons it is said to be Radiative recombination  

Non-Radiative Recombination

When the excited excess carriers reach equilibrium positions by emission of phonons due to surface / bulk defects / other defects it is said to be non-radiative recombination.

 

What Is Activator?

Impurity atom occurring in relatively small concentrations in host material or a small stoichometric excess of one of constituents of material which exhibits the property of Luminescence. 

What Is Killer?

Presence of certain type of impurity may also inhibit Luminescence of other centers, in which case they are referred as killers.

After a particular level of concentration, the efficiency decreases as excess carrier returns to the ground state only if there is no other activator with in a sphere of

radius ‘R’ around central activator atom

Dependence of luminescence efficiency on Activator concentration

CHARACTERISTICS OF A PHOSPHOR

1. High concentration of carrier traps.
2. High emission efficiency.
3. Large trap depth.
4. Traps, Luminescence & lattice are not to be damaged by repeated irradiation & heating process.

BETA DECAY

There are 3 models of beta radioactivity

i) Negatron (β^-) Emission

ii) Orbital Electron Capture

iii) Positron (β⁺) Emission

BETA SPECTROSCOPY

Beta ray spectra is of two types:

i) Continuous Spectrum

ii) Sharp Line Spectrum

The continuous spectrum is due to Negatrons and Positrons.

There is a possibility of the daughter nucleus being created in an excited state, which may decay by gamma emission.

Rutherford suggested that part of gamma radiation is absorbed by outer electrons of same atom and as a result the secondary beta ray electrons are ejected.  The process is called “Internal Conversion”.

The sharp Line Spectrum is due to secondary beta rays emitted due to internal conversion.

The process of Beta disintegration differs from alpha disintegration in following two respects:

i) The alpha particles are already present in the initial nucleus while beta particles are not present in the initial nucleus. Beta particles are created at the time of emission.

ii) The energy spectrum of alpha particles is discrete while energy spectrum of beta particles is continuous.

iii) Beta decay is a three body problem while alpha decay is a two body problem.

In the Neutrino, the spin and angular momentum vector are oppositely directed and in anti neutrino these vectors are aligned together.

Since a matter-antimatter pair is formed whenever energy is converted into mass, in a conversion of a nuclear Neutron to a Proton, the negative electron (matter) should accompany anti neutrino (anti matter). Similarly Neutrino is emitted with positron emission and orbital electron capture.

FERMI THEORY OF BETA DECAY

In 1934, Fermi made a successful theory of beta decay. The theory is based on following assumptions:

1) The light particles, the electron and neutrino ar created by transformation of a neutron into a proton in a nucleus or vice versa.

Note:- Neutron or Proton that are transformed to beta particles are not free particles but are bound to Nucleus by Nuclear Forces.

2) The energy remains conserved in decay process, the available energy being shared among the electron and the neutrino.

3) The beta decay process is analogous to the emission of electromagnetic radiation by an atom, with electron-neutrino field acting in place of Electromagnetic field.

4) “Electron-Neutrino” field is weak in contrast to short range strong interactions which exist between Nucleons bound in the nucleus.

5) Time   Dependent Perturbation theory is a very good approximation because of smallness of coupling constants.

6) As Nucleons move with velocities of only nearly c/10 in nuclei calculations can be made with non relativistic nuclear wave functions.