This slide discusses on Radio activiy counters, its types, applications and limitations.

1. Radio-activity Counters
Presented By
NIZAM ASHRAF
OST-2018-22-15

2. Radioactivity
• Radioactivity is a phenomenon that occurs naturally in a number of substances.
• Atoms of the substance spontaneously emit invisible but energetic radiations,
which can penetrate materials that are opaque to visible light.
• The effects of these radiations can be harmful to living cells but, when used in the
right way, they have a wide range of beneficial applications, particularly in
medicine.
• Radioactivity has been present in natural materials on the earth since its
formation.
• However, because its radiations cannot be detected by any of the body’s five
senses, the phenomenon was only discovered 100 years ago when radiation
detectors were developed.

4. Radio-active detectors
• A wide range of radioactivity detectors are in use today. The main detector
is the Geiger–Mueller counter or G-M counter.
• A Geiger counter is an instrument made of glass or metal tube used for
detecting and measuring ionizing radiation.
• It has a thin window, usually made of mica at one end to enclose the gas.
• It detects ionizing radiation such as α-particles, β- particles, and gamma
rays using the ionization effect produced in a Geiger–Müller tube.
• In wide and prominent use as a hand-held radiation survey instrument, it is
perhaps one of the world's best-known radiation detection instruments.
1. Geiger–Mueller counter

6. Principle of operation
• A Geiger counter consists of a Geiger–Müller tube (the sensing
element which detects the radiation) and the processing electronics,
which displays the result.
• The Geiger–Müller tube is filled with an inert gas such as helium,
neon, or argon at low pressure, to which a high voltage is applied.
• The tube briefly conducts electrical charge when a particle or photon
of incident radiation makes the gas conductive by ionization.
• The ionization is amplified within the tube by the Townsend discharge
effect to produce an easily measured detection pulse, which is fed to
the processing and display electronics.

7. • The electronics also generate a high voltage, typically 400–900 volts,
that has to be applied to the Geiger–Müller tube to enable its
operation.
• To stop the discharge in the Geiger–Müller tube a little halogen gas or
organic material (alcohol) is added to the gas mixture.

8. • The detected radiation can be readout as two: counts or radiation
dose.
• The counts display is the simplest and is the number of ionizing
events detected displayed either as a count rate, such as "counts per
minute" or "counts per second", or as a total number of counts over a
set time period. The counts readout is normally used when alpha or
beta particles are being detected.
• While the radiation dose rate is displayed in a unit such as the sievert
which is normally used for measuring gamma or X-ray dose rates.
• A Geiger–Müller tube can detect the presence of radiation, but not its
energy, which influences the radiation's ionizing effect.

9. Limitations
• There are two main limitations of the Geiger counter.
• Because the output pulse from a Geiger–Müller tube is always of the
same magnitude, the tube cannot differentiate between radiation
types.
• Secondly, the inability to measure high radiation rates due to the
"dead time" of the tube. This is an insensitive period after each
ionization of the gas during which any further incident radiation will
not result in a count, and the indicated rate is, therefore, lower than
actual.

10. Types and Application
• The GM counters can be generally categorized as "end-window",
windowless "thin-walled", "thick-walled“ and hybrids of this types.
• GM counters are mainly employed in the
1. Particle Detection.
2. Gamma and X-Ray Detection.
3. Neutron Detection – (Boron trifluoride or Helium-3)
4. Physical Design.

11. 2. Ionization Chamber
• The ionization chamber is the simplest of all gas-filled radiation
detectors.
• Widely used for the detection and measurement of certain types of
ionizing radiation such as X-rays, gamma rays, and beta particles.
• Ion chambers have a good uniform response to radiation over a wide
range of energies and are the preferred means of measuring high
levels of gamma radiation.
• They are widely used in the nuclear power industry,
research labs, radiography, radiobiology, and
environmental monitoring.

12. Principle
• An ionization chamber measures the charge from the number of ion pairs
created within a gas caused by incident radiation.
• It consists of a gas-filled chamber with two electrodes; known as anode and
cathode.
• A voltage potential is applied between the
electrodes to create an electric field.
• When gas between the electrodes is ionized
by incident ionizing radiation, ion-pairs are
created and the resultant positive ions and
dissociated electrons move to the electrodes
of the opposite polarity under the influence
of the electric field.
• This generates an ionization current which is
measured by an electrometer circuit.

13. Types and Applications
• The following chamber types are commonly used:
1. Free-air Chamber
2. Vented Chamber
3. Sealed low pressure Chamber
4. High pressure Chamber
5. Parallel-plate Chamber
• The following are the Applications of Ionization Chamber
1. In Nuclear industry
2. Smoke detectors
3. Medical radiation measurement

14. Counter
3. Scintillation Counter
• A scintillation counter is an instrument for detecting and measuring
ionizing radiation by using the excitation effect of incident radiation on a
scintillating material, and detecting the resultant light pulses.
• It consists of a scintillator which generates photons in response to incident
radiation, a sensitive photodetector (usually a photomultiplier tube (PMT),
a charge-coupled device (CCD) camera, or a photodiode), which converts
the light to an electrical signal and electronics to process this signal.

15. • Scintillation counters are widely used in radiation protection, assay of
radioactive materials as they can measure both the intensity and the
energy of incident radiation.
• When high energy atomic radiations are incident on a surface coated
with some fluorescent material, then flashes of light (scintillations)
are produced.
• The scintillations are detected with the help of a photomultiplier
tube, that gives rise to an equivalent electric pulse.
Principle

16. Scintillator
• The Scintillator is made from a single crystal that should have
following characteristics:
• Available in proper form
• High efficiency
• Transparent to light
• Suitable value of refractive index
• High resolution power
• Stable under experimental conditions
Popular types of crystals used as Scintillators are:
Cesium Iodide, Zn Sulphide, Xenon, Organic Phosphors for detection of
Gamma rays.

17. Photomultiplier Tube
• Around 10 dynodes are specifically designed and properly positioned,
for automatic focusing of electrons.
• Each dynode have a particular function:
1. Collection of photoelectrons from previous dynode
2. Emission of low energy electrons

18. Working
1. The radiations are allowed to enter the scintillators through a
window of pyrex glass.
2. When high energy radiations strike the crystal, short duration
scintillations are emitted.
3. The photoelectrons emitted from cathode are directed towards 1st
dynode that give rise to secondary emission of electrons.
4. The secondary electrons, emitted from the surface of 1st dynode,
get accelerated towards 2nd dynode.
5. The process repeats and electron get much more multiplied in
number. A high energy pulse is delivered to the counting device
through the anode.
6. The electric pulse is then delivered to the electronic counting
device, through a discriminator.

19. Applications
• Used in hand held radiation survey meters, personnel and
environmental monitoring for radioactive contamination
• Medical imaging
• Radiometric assay
• The ability to accommodate samples of any type, including liquids,
solids and gels.
• The ability to count separately different isotopes in the same sample,
which means dual labelling experiments can be carried out.
• Scintillation counters are highly automated

20. Thank You