Study of Thermoluminescence
of F-centers in Alkali Halide Crystals
Pure alkali halide crystals are transparent
throughout the visible region of the spectrum. The crystals
may be colored in a number of ways
(1) by the introduction of chemical impurities
(2) by introducing an excess of the metal ion
(3) by X-ray, g-ray, neutron
and electron bombardment
(4) by electrolysis
A colour centre is a lattice defect that
absorbs visible light. The simplest colour center is an F-centre.
The name comes from the German word for colour, Forbs. We
usually produce F-centres by heating the crystal in excess
alkali vapours or by irradiation. The new crystals show an
absorption band in the visible or ultraviolet, whereas the
original crystals are transparent in that region. This absorption
band is called F-band.
An F-centre can be regarded as a negative
ion vacancy and an electron which is equally shared by the
positive ions, surrounding the vacant lattice site. Conversely
a hole may be trapped at a +ve ion vacancy or at a -ve ion,
giving rise to V- and H-centres respectively.
Color centres produced by irradiation
When a X-ray quantum passes through an ionic
crystal, it will usually give rise to a fast photo-electron
with an energy of the same order as that of the incident quantum.
Such electrons, because of their small mass, do not have sufficient
momentum to displace ions and therefore loose their energy
in producing free electrons, holes, excitons and phonons.
Evidently these while moving near the vacancies form trapped
electrons as well as trapped holes.
The trapped-electron or trapped-hole centres
so formed can be destroyed (bleached) by illuminating the
crystal with light of the appropriate wavelength or warming
Important information about the colour centres can
be obtained by measuring the changes that occur when a coloured
crystal is gradually heated. As the temperature is raised
electrons and holes escape from their traps at an increasing
rate. The freed charges can recombine with each other or with
other defects and give out luminescence by recombination.
The resulting thermoluminescence or ‘glow’ reaches
maximum and then decreases to zero as the supply of trapped
electrons or holes becomes exhausted. The plot of luminescence
intensity verses temperature, taken at a constant heating
rate, is called the ‘glow curve’. It may contain
one or many glow peaks, depending upon whether there are one
or several different kinds of traps.
From the glow curve one determine the trap
depth; the deeper the trap, the higher the temperature of
the glow peak. A correlation between the temperature at which
thermoluminescence occurs and the temperature at which particular
band bleach can give valuable information about specific centres.
Typical results obtained from this set-up
for KCl crystal are shown in figure.
The experiment consists of the
Set-Up for creating Thermolumnescece
- Sample: KBr or KCl single crystal
- Thermolumniscence Temperature Meter, TL-02
* Digital Thermometer with RTD sensor
* Oven Power Supply
- Sample Holder
- Thermolumniscence Oven (upto 423K)
- Black Box
For Measurement of Luminescence
Complete in all respect,
only X-ray facilities are required to create F-centers in
Specifications of Photomultiplier tube 931A
300 to 650(nm)
Structure/No. of Stages
Average Anode Current
to Cathode Supply Voltage
Dark Current (after 30min)
The housing is designed to provide a shielding
from stray light and magnetic field. The slit arrangement
at the window is provide to adjust the size of the window
according to the incident beam and for the safety of photomultiplier
tube when not in use. For E.H.T. input and current output,
amphenol connectors are provided. A general purpose biasing
circuit, using low noise, metal film resistors, is mounted
on the base. The housing can be mounted in any position.
The biasing circuit can be changed to customer’s
specification. Minor modification in design of PMT Housing
is also possible.