Radiation detection & measurement

1. Training Course on Radiation Protection for Radiation
Workers and RCOs of BAEC, Medical Facilities & Industries
24 - 28 October 2021
Training Institute
Atomic Energy Research Establishment, Savar, Dhaka
Institute of Nuclear Science and Technology
Atomic Energy Research Establishment
Ganakbari, Savar, Dhaka
E-mail: dpaulbaec@yahoo.com
RADIATION DETECTION
AND MEASUREMENT

2. Introduction
We can’t detect or measure Ionizing Radiations with our senses.
To detect or measure Radiations we must need instruments.
Design, construction, type and use of Detectors depends upon
the properties and nature of the Radiation.
Ionizing
Radiation

5. Purpose of Measurement
 for measuring the activity
(Curie, Becquerel)
 for measuring the rate of radiation
(mR/h, Sv/h)
 for personnel monitoring
(mrem, mSv)

8. Gas-filled detectors
A Gas-filled detector consists of two electrodes to
which a certain electrical potential is applied. The
space between the electrodes is filled with a gas.
Ionizing radiation, passing through the space between
the electrodes, dissipates part or all of its energy by
generating electron-ion pairs.
Electrons move toward the positive electrode, positive
ions toward the negative one.

9. Common of Gas filled detectors are:
 Ionization Chamber
 Proportional Counters
 Geiger-Muller Counters
Gas Filled Detectors

10. Ionization Chambers
 Operating voltage 60-300 V.
 No ion recombination and
acceleration.
 Saturation current region.
 Low sensitivity since measure
only primary ionization.
 Not suitable for discriminate
between several types of
radiations.
 Used for X,  & high energy .
Fig. Ionization
Chamber
DC Amplifier
Micro-
Amp.
Power Supply

11. Ionization Chambers [contd…]
The ionization chamber is the simplest of all gas-filled
radiation detectors, and is widely used for the detection and
measurement of certain types of ionizing radiation.

12. Proportional Counter
 If voltage is increased above saturation current flow, the
ions gain sufficient energy to cause secondary
ionization, give larger number of ions.
 Pulse size is proportional to voltage and number of
primary ions.
 Pulse size gives an indication of difference of ionization
and hence the type of radiation.
 High sensitivity, suitable for energy discrimination.
 Used to determine energy of particles and photons.
 Methane-argon mixture are used and multiplication
factors 105 possible. Operating range 500 to 5000V.

13. Geiger-Muller Counters
 If voltage is further increased, every ion accelerates to cause
amplification greater than proportional counter.
 Pulse height is independent of ionization of particle or .
 Gas amplification factor can be 1010, extremely sensitive
and can detect radiation which produces even one ion pair.
 Output pulse of ion current has the same magnitude for
photon or particle energy and thus it is not possible to
distinguish radiation of different energies.
 Filled with an inert gas (argon or helium) and ethyl alcohol
vapor or other organic gas is added for quenching purpose.
 Dead time is 10 to 500 s.
 Operate at 1000 to 3000 V.

14. Operating Characteristics of a G-M Counter
 A Geiger tube has a wide
range of operating voltage
over which counting rate is
approximately independent
of operating voltage.
 This extended region of
operation is called “Geiger
plateau”.
 Preferable to operate in the
centre of geiger plateau.
Plateau length
Threshold
Voltage
Starting
Voltage
Operating
Voltage
Counting
Rate
Volts
Fig. Operating Characteristics of a Geiger Counter

15. Semiconductor Radiation Detector
Silicon and germanium, are by far the most widely used
semiconductors.
Example of Semiconductor Detector:
i. Ge(Li) (Germanium Lithium) Detector
ii. Si(Li) (Silicon Lithium) Detector
iii. HPGe (High Purity Germanium) Detector.

16. Scintillation Counter
 Radiation falls on certain
materials, known as scintillators,
produces a flash of light.
 Light flash gives number of
incident particles or gamma-rays.
 Using photomultiplier, light flash is
turned into an electronic pulse
which can be measured.
 Much more efficient for detecting
gamma-rays than Geiger counter.
 Amount of light produced is
dependent on energy of radiation.
A typical Probe used in
Contamination Monitor
Type : Scintillation

17. Personnel Dosimetry
 Measure the dose accumulated by individuals over any
given period of time.
 Dosimetry systems are designed to measure the dose
from radiations of greatest external hazard, gamma- and
X-radiation, - radiation and neutron.
 Following can be used for personnel dosimetry:
 film badge
 thermoluminescent dosimeters (TLD) and
 Pocket Dosimeter (or Electronic dosimeter)

18. Film Badge
 The badge consists of two parts:
photographic film and a holder.
 Photographic emulsions exposed to radiation appear
blackened after development.
 The degree of blackening is dependent on the
absorbed dose received by the film and hence dose
can be read off from a dose-calibration curve.
 The film is worn in a specially designed holder, which
contains a number of strips of plastic and metal, called
filters, so that beta, gamma and X-radiation doses can
be measured.
 The dose from thermal neutrons can also be measured.

19. Thermoluminescent Dosimeters (TLD)
 Certain materials after being exposed to radiation
emit light when heated. This fact is made use of
in thermoluminescent dosimeters.
 The intensity of light emitted is dependent upon the
radiation exposure.
 The dose received is obtained from the quantity of
light emitted, when the material is heated.

20. The dose can be read directly and pocket dosimeters
are used in addition to film badges or TLD. The
dosimeter are found with a variety of sensitivities ranging
from 0 - 9.99 Sv.
Modern electronic dosimeters
EXTERNAL DOSIMETRY : Direct Measurement
Pocket Dosimeter

21. Instruments for Measuring Dose Rate
Choose correct instrument for
each type of radiation.
Make sure that instrument is
functioning properly.
A Geiger counter use to
distinguish between beta- and
gamma- radiation simply by
taking measurements with the
blind open and closed.
Do not detect  since can’t
penetrate wall of counter.
A Beta and Gamma Geiger
Probe. This feature can be used
to distinguish between gamma-
and beta-radiation.

22. Instruments for Measuring Contamination
Used to detect the presence
of radiation (contamination).
Calibration is achieved by
measuring the count rate per
second from sources of known
strength.
The detection system in these
instruments is usually a Geiger
counter, a Scintillation counter
or a Solid-state detector.
A typical Contamination
Monitor. Depending on the
measurements to be made
different probes are used.
(alpha, beta and alpha/beta +
gamma probes available)

23. Neutron Monitors
Neutron require special detection
method.
To detect fast neutrons, allow
them to collide with atoms in light
material to slow down, after which
their presence is detected by letting
them collide with boron. Alpha
particles are produced which can
be counted.
5
10B + 0
1n 3
7Li + 2
4He
For slow neutron, gas counter are
used directly with fillings of
hydrogen or boron trifluoride.
Neutron Counter

24. Calibration of Radiation Monitors
Radiation monitors must be calibrated regularly
for the radionuclides or the radiation being used.
Many detectors are highly energy dependent or
may only be capable of detecting certain types
of radiation.
Care must be taken to ensure that the monitor
to be used is calibrated for the type and energy
of radiation to be measured.

25. THANK YOU
For Your Kind Attention