2. Because human senses cannot sense
radiation, instruments that detect radiation are
essential tools.
After a nuclear disaster detecting radiation becomes
particularly invaluable, as high levels of radiation can
become hazardous to life.
Regular monitoring while using radioactive substances
are critical to the safety of personnel.
2
3. Detection of radioactivity is necessary to ascertain
their
presence and
Intensity
Detection indirect (based on the effects of
radioactivity)
• Darkening of photographic plates
• Ionization of atoms
3
4. Film badges
Based on the fact that radiation affects photographic films.
Used to monitor the level of radiation that personnel working
with radioactive materials or x-rays are exposed to.
Typically worn on the outside of clothing, around the chest or
torso. This location monitors exposure of most vital organs.
The badge is worn over the protection to monitor the dose
actually received to unprotected parts (parts not covered with
lead, etc).
Film periodically removed and developed to measure exposure.
4
5. Film surrounded by case preventing light and moisture
from damaging it.
Reacts to radiation and becomes dark.
Other badges placed on the finger as a ring. For
detecting ionizing radiation, such as a narrow beam
directed toward a work area.
Multiple badges can be used to cover different areas, if
radiation exposure is a high risk.
5
7. Uses an air-filled chamber to detect X-ray and gamma
rays.
When radioactive particles form ion pairs inside the
chamber, the anode collects electrons produced from
this process.
The anode then produces a small electric current.
7
8. Electrometer measures this current and this in turn
displays and records the level of radiation present.
Typically measures radiations in units of
milliroentgens per hour (mR/hr) and roentgen per
hour (R/hr).
Construction: Gas filled enclosure between two
conducting electrodes, with a voltage across them.
8
9. Electrodes in the form of parallel plates or coaxial
cylinders.
Gas atoms in chamber ionized by radiation.
Electrons from the ionisation attracted to the positive
voltage plate (anode).
9
10. Positively charged atoms attracted to negative voltage
plate (cathode).
Resulting ionization current is measured.
This current is directly proportional to the radiation
being detected.
10
11. Geiger-Muller (GM) counter or Geiger Counter (GC)
The G.C. creates an electric pulse when radiation
interacts with a gas within it.
It converts and records the electric pulse into a
radiation reading.
Radiation is typically measured in Counts per minute
(CPM).
G. C.s detect alpha and beta particles and gamma rays.
11
12. Construction: Diode filled with inert gas at low
pressure, in which the anode is a metal rod fixed along
the axis of a cylindrical cathode.
Anode - insulated wire in the tube connected to the
positive terminal of a d.c. Source.
Cathode - metal tube connected to the negative
terminal of d.c. source.
High p.d. maintained between anode and cathode.
12
14. Some atoms of gas ionised by radiation passing
through thin window.
Released electrons accelerated by the field between
the wire and cylinder ionise other atoms.
Avalanche of free electrons released.
14
15. Avalanche creates a pulse of current at the output of
the tube.
Radiation is directly proportional to avalanches.
Pulses are amplified, and used to trigger an electronic
counter or delivered to speaker which clicks each time
a particle enters the detector.
15
16. Proportional counter
Used for quantifying alpha and beta radiation, neutron
detection, and x-ray spectroscopy.
Pulses produced larger than those produced by ion
chamber.
Pulse size reflects the energy deposited by the incident
radiation in the detector gas.
16
17. This makes it possible to differentiate the larger pulses
produced by alpha particles from the smaller pulses
produced by beta or gamma rays.
Count particles result from ionising radiation and
measure their energy.
Construction and operation similar to that of a GM
counter, except that it uses a lower operating voltage.
17
18. Made of tube filled with a mixture of an inert gas and
a quench gas.
Creates ion pairs, made up of an electron and a
positively charged atom.
Electrons attracted to anode, under the influence of an
applied field.
Positive ions attracted to the cathode.
18
19. The gas in the counter does not contain
electronegative components such as oxygen.
Otherwise, electrons heading towards the anode will
combine with the electronegative gas.
If this happens, a negative ion goes to the anode rather
than an electron, and unlike the electron, the negative
ion will fail to produce an avalanche.
19
20. Scintillation counters
Based on the property of certain chemical compounds
of emitting short flashes of light when excited by
charged particles or photons of high energy.
Counts flashes of light resulting from phosphor
material struck by radiation.
Consists of transparent crystal or organic liquid that
fluoresces when struck by ionizing radiation.
20
22. Flash from the material is measured by a Photo
Multiplier Tube.
PMT detector is attached to an amplifier and other
electronic equipment to count signals
The photons emitted by the scintillator are converted
to an electrical signal.
P.M.T. Is made up of several electrodes, known as
dynodes, whose potentials are increased in succession
along the length of the tube.
22
23. One electron entering the PMT ejects several other
electrons creating an avalanche.
A large electric pulse emerges at the output of the P.M.T.
which pulse is sent to an electronic counter.
In summary, alpha or beta particles or gamma rays produce
a flash of light in a crystal (zinc sulphide and silver for
alpha and beta radiation, sodium iodide and tellurium for
gamma-rays).
This is detected by a photomultiplier tube and the
electrical pulse recorded.
23
24. Other detectors
(a) Photographic plates. Though these are not good
for gamma-radiation, as there is insufficient
ionization, alpha and beta particles produce visible
tracks in the plate where they pass.
(b) Electroscope. The leaf falls, due to ionization of
the surrounding air. This is rather a crude method and
is not good for gamma radiation.
24
25. (c) Bubble chamber. Radiation creates tracks of
bubbles in a superheated liquid such as hydrogen or
propane. The bubble chamber is a much more effective
detector of radiation than the cloud chamber, because
of the much greater number of atoms per unit volume
of the liquid in it. This means that there is a much
greater chance of a collision occurring between an
incoming particle and a nucleus.
25
26. (d) Solid-state detector. This is a reverse biased p-n
junction of semiconductor material, and when
ionizing radiation falls on it ion pairs are formed at the
junction, thus producing a current through it.
(e) Cloud chambers show the tracks of radioactive
particles rather than measure the intensity of the
radiation. Two types exists: the expansion type and the
diffusion type. Their final results are similar but they
use different methods to achieve it.
26
27. (f) The spark counter measures the range of alpha-
particles. In nuclear research a stack of spark counters
is used to show the track of a particle as a line of
sparks.
27
