6. Need for monitoring
Detection is to know:
Whether the radiation is
present or not?
If so, what is the type of
radiation?
7. Measurement is to:
1. Estimate the dose or dose rate
[Survey meters]
2. Identify the radionuclides
[Spectrometer]
3. Quantify the radioactivity
[in Bq]
8. Effect Type of Instrument Detection medium
Electrical Ionization chamber
Proportional Counter
Geiger Counter
Solid state
Gas
Semi conductor
Light Scintillation Counter Crystal or Liquid
Chemical Film
Chemical dosimeter
Photographic
emulsion
Solid or Liquid
Thermo
Luminescence
TLD
OSL
Crystal
9. Type of Detectors
Gas filled Detectors
Ionization chamber
Proportional counter
Geiger Mueller counter
Scintillation Detectors
NaI:Tl, CsI:Tl, CsI:Na, LaBr:Ce, ZnS(Ag)
Semiconductor detectors
HPGe
12. When radiation is absorbed in the gas, ion pairs are
produced
Collection of ion pairs is a function of the applied
voltage
If there is no voltage across chamber wall (cathode)
and central wire (anode)the ion pairs will recombine and
no charge will flow in the external circuit
As voltage increases , some ion pairs will still
recombine , but others will flow to the electrodes. At
voltage 10 V or more recombination becomes negligible
13. Ionization Chambers
In this region, the number of electrons collected by the anode
will be equal to the number produced by the primary ionizing
particle
The ionization chamber region - there is no multiplication
of
ions due to secondary ionization
Gas amplification factor is equal to one
The amplitude of the signal is proportional to the quantity
of
energy deposited in the active region of the detector
Distinguish between radiations of different specific
ionization,
such as alphas and betas or gammas
15. Proportional Counter
As the voltage across the counter is increased beyond
the ionization chamber
region - secondary electrons are produced by collision.
This is the beginning of
the proportional region
The gas amplification factor is greater than one.
Multiplication of ions in the gas, which is called an
avalanche,
- is initially restricted to the vicinity
of the primary ionization.
Increasing the voltage,
- the avalanche to increase in size by
spreading out along the anode.
16. The no. of electron collected increases
roughly exponentially with voltage
At a given voltage on the detector the
ionization produced is amplified by a
constant amount
i.e., the number of ion pairs collected
is proportional to the initial ionization
17. PRINCIPLE :
Output pulse is proportional to
number of primary ion pairs
APPLICATIONS
Flow Counters, Neutron
detectors
Spectroscopy
18. Geiger Muller Tube
Continuing to increase the high voltage beyond the
proportional region
- cause the avalanche to extend along the
entire length of the anode.
An avalanche across the entire length of the anode is
called a Townsend
avalanche.
- here end of the proportional region is reached
and the Geiger region begins.
At this point, the size of all pulses is the same
—regardless of the nature of
the primary ionizing particle.
19. When operated in the Geiger region, therefore, a counter
cannot distinguish among the several types of radiations.
In the Geiger region,
the avalanche is already extended as far as possible
axially along the anode.
Increasing the voltage, therefore,
causes the avalanche to spread radially, resulting in an
increasing counting rate.
We therefore have a slight positive
slope in the plateau, Finally, if the voltage is increased even
further, the tube generates a series of
self-initiated discharges. It is then said to be in the REGION
OF REPETITIVE
DISCHARGE, and here it is no longer any use as a radiation
detector
20. GM counter has a slope
of about
3% per 100 V.
The operating voltage for
a GM tube is
about 1/3rd to 1/2nd the
distance from
the knee of the curve
25. Thermoluminescent Dosimeters
Many different crystals emit light if they are heated after having
been exposed to
radiation.
This effect is called thermoluminescence (TL), and dosimeters
based on this
effect are called thermoluminescent dosimeters (TLD).
Absorption of energy from the radiation excites the atoms in the
crystal, which
results in the production of free electrons and holes in the TL
crystal.
These are trapped by the activators or by imperfections in the
crystalline lattice,
thereby stored the excitation energy in the crystal as trapped
electrons.
26. Heating the crystal releases the excitation energy as
light. Measurement of the
emitted-light intensity leads to a glow curve.
The total light output is proportional to the number of
trapped, excited electrons,
which, is proportional to the amount of energy
absorbed from the radiation.
Light output is directly proportional
to the radiation
absorbed dose
27. Some of TLD crystals include LiF, CaF2 :Mn, CaSO4 : Tm,
CaSO4 : Dy, Li2B4O7 : Cu,
and LiF: Mg,Ti.
Thermoluminescent materials are found in the form of loose
powder, disks,
squares, and rods.
After being worn for the prescribed period of time, the TLD
material is heated and the intensity of the resulting
luminescence is measured with a photomultiplier tube whose
output signal, is proportional to absorbed dose
