Radiation detection principles involve using detectors to determine if radiation is present, how much radiation exists, and characteristics of the radiation such as the isotope or energy. Common detector types include gas-filled detectors like proportional and Geiger-Muller counters, scintillation detectors using materials like sodium iodide, and solid state detectors made of semiconductors. Gas detectors work by ionizing gas when radiation passes through, while scintillation detectors use radiation-induced flashes of light and solid state detectors measure charge carriers freed in semiconductor materials.

1. Radiation Detection Principles
Dr. Munir Ahmad
MSc Physics, PU
MS Nuclear Engineering, QU
PhD Medical Physics, UCL, UK
Postdoc Software Development Engineer, UoH, UK
munir.ahmad@uclmail.net

2. Radiation Detection
Generally by radiation detection we mean to
know that…….?
• Is radiation present around?
• How much radiation is there?
• Can we recognize the isotope?
• Can we guess the energy?

3. Radiation Detector

4. Good Detectors

5. Radiation Interactions
• We know from previous class that radiation
in matter can….
– Produce Ionization..
– Excitation…
• Both of these interactions can be used to
detect radiation….

6. Radiation Interactions

7. Radiation Detection
• Objective - Detection of ionizing radiations
• Main Process - Ionization Process
• Electrical Current Detection
– No applied voltage, no current flow,
recombination
– Small applied voltage, small current flow, before
recombination
– High applied voltage, high current flow, may be no
recombination

8. Radiation Interaction

9. Interaction Mechanisms
• The main reason for all above is charge and mass.

10. Charged Particle Interactions

11. Excitation
• Depending on the energy of
the incoming radiation or
the interaction level, UV, IR
or light may be emitted.

12. Ionization
• Secondary electrons may result.
• Characteristic x rays may be the
result along with Auger electrons.
• Auger Electron: An electron ejected by an electron excited
by the incoming radiation.

13. Bremsstrahlung
• When fast moving electrons
gets bended near a heavy
nucleus, the resulting change
in moment is converted into
radiations known as
bremsstrahlung.

14. Summary Physics of Detection

15. Ideal Detector Characteristics
• There is no ideal/universal radiation detector.
Compact & strong
Independent (Energy & Rate)
Efficient
Fast response
No radiation damage
Low cost
Convenient - usage
Stable – time & env.

16. Detectors
…………Some of the Radiation Detectors……...

17. Types of Radiation Detectors
• Gas filled detectors (Proportional, GM Counters)
• Scintillation detectors (NaI(Tl) Scintillators)
• Solid state detectors (Photodiode & Silicon)
Gas filled detectors Scintillation detectors Solid state detectors

18. Gas Filled Detectors
• Gas filled detectors are
instruments used to detect
and measure ionizing
radiations.
• They work by utilizing a gas
filled chamber that becomes
ionized when radiation
passes through.
• Ionization creates charged
particles which can be
detected and measured to
determine the intensity and
type of radiation present.
• These detectors are capable of
detecting various types of ionizing
radiations, including alpha beta
and gamma radiations.
• Each radiation interacts
differently with the gas inside the
detector allowing for
identification of radiation type.

19. Gas Filled Detectors

20. Regions of Operation
1) Recombination 2) Ionization 3) Proportional 4) Geiger

21. Ionization Chambers
• An ionization chamber is a type of radiation detection device.
In an ionization chamber, two opposing electrodes are placed
in a container filled with gas, and a high voltage is applied. As
the charged particles (radiation) pass through the gas, the gas
molecules are ionized to produce ions and electrons.

22. Ion Chambers
Only used for very basic radiation detection.

23. Types of Ion Chambers
Free air
chamber
Vented
chamber
High
pressure
chamber
Sealed low
pressure
chamber

24. Proportional Counters
• In proportional counter the voltage in the ionization chamber
is increased above a certain level to accelerate and not only
collect the particles.
• The accelerated electrons secondarily ionize the gas
molecules, resulting in a current that is larger than the primary
ionization current (amplification).

25. Proportional Counters
Better and more accurate radiation detection.

26. GM Counter
• Geiger-Muller (GM)
counter is a gas filled
detector.
• It operates on the
principal of gas
amplification with
following steps.
• Gas filling
• Electrodes
• Voltage application
• Ionization
• Avalanche effect
• Detection
• GM counter contains a low pressure noble
gas.
• There is a central electrode with a surrounding
electrode.
• A high voltage is applied across the electrodes.
• Radiation ionizes the gas and produces ion
pairs. These pairs are attracted towards
electrodes and creates current.
• Avalanche effect – cascade ionization
• The amplified current pulse is detected and
measured.

27. Townsend Avalanche Effect
• The free electrons are attracted towards the
positively charged central wire.
• They gain enough energy to ionize further gas atoms
due to energy gained.
• This leads to an avalanche of electrons causing rapid
ionization and a measurable current.

28. Scintillation Detectors
• A scintillation detector uses
the effect known
as scintillation.
• Scintillation is a flash of
light produced in a
transparent material by
passing a particle (an
electron, an alpha particle,
an ion, or a high-energy
photon).
• Scintillation occurs in the
scintillator, a key part of a
scintillation detector.

29. Various Radiation Effect
• Alpha Particles and Heavy Ions. Due to the very high ionizing power of heavy ions, scintillation counters are usually not
ideal for the detection of heavy ions. For equal energies, a proton will produce 1/4 to 1/2 the light of an electron, while alpha
particles will produce only about 1/10 the light. Where needed, inorganic crystals, e.g., CsI(Tl) and ZnS(Ag) (typically used in
thin sheets as α-particle monitors), should be preferred to organic materials. Pure CsI is a fast and dense scintillating
material with a relatively low light yield that increases significantly with cooling. The drawbacks of CsI are a high-temperature
gradient and a slight hygroscopicity.
• Beta Particles. For the detection of beta particles, organic scintillators can be used. Pure organic crystals include crystals of
anthracene, stilbene, and naphthalene. The decay time of this type of phosphor is approximately 10 nanoseconds. This type
of crystal is frequently used in the detection of beta particles. Organic scintillators, having a lower Z than inorganic
crystals, are best suited for detecting low-energy (< 10 MeV) beta particles.
• Gamma Rays. High-Z materials are best suited as scintillators for the detection of gamma rays. NaI(Tl) (thallium-doped
sodium iodide) is the most widely used scintillation material. The iodine provides most of the stopping power in sodium
iodide (since it has a high Z = 53). These crystalline scintillators are characterized by high density, high atomic number, and
pulse decay times of approximately 1 microsecond (~ 10-6 sec). Scintillation in inorganic crystals is typically slower than in
organic ones. They exhibit high efficiency for the detection of gamma rays and are capable of handling high count rates.
Inorganic crystals can be cut to small sizes and arranged in an array configuration to provide position sensitivity. This feature
is widely used in medical imaging to detect X-rays or gamma rays. Inorganic scintillators are better at detecting gamma rays
and X-rays. This is due to their high density and atomic number, which gives a high electron density.
• Neutrons. Since the neutrons are electrically neutral particles, they are mainly subject to strong nuclear forces but not
electric ones. Therefore, neutrons are not directly ionizing and usually have to be converted into charged particles before
they can be detected. Generally, every type of neutron detector must be equipped with a converter (to convert neutron
radiation to common detectable radiation) and one of the conventional radiation detectors (scintillation detector, gaseous
detector, semiconductor detector, etc.). Fast neutrons (>0.5 MeV) primarily rely on the recoil proton in (n,p) reactions.
Materials rich in hydrogen, such as plastic scintillators, are best suited for their detection. Thermal neutrons rely on nuclear
reactions such as the (n,γ) or (n,α) reactions to produce ionization. Materials such as LiI(Eu) or glass silicates are therefore
particularly well-suited for detecting thermal neutrons.

30. Scintillators

31. Liquid Scintillators

32. Semi Conductor Detectors
• In semiconductor
detectors, ionizing
radiation is measured by
the number of charge
carriers set free in the
detector material which is
arranged between two
electrodes, by the
radiation.
• Ionizing radiation
produces free electrons
and electron holes.

33. Semi Conductor Detectors

34. Thermo-luminescent Detectors

35. Band Theory of TLDs

36. Advantages/Disadvantages of TLD

37. Film Dosimetry

38. Question 1
‘When nuclear radiations pass through, gas
ionization is produced.’ This is the principle
of which of the following detectors?
a) Proportional counter
b) Flow counter
c) Geiger Muller counter
d) Scintillation counter

39. Question 2
 Which of the following is not a type of
radiation detectors?
a) Geiger Muller counter
b) Proportional counter
c) Semiconductor detector
d) Flame emission detector

40. Question 3
Which of the following acts as quenching gas
in Geiger Muller counter?
a) Alcohol
b) Argon gas
c) Krypton
d) Hydrogen

41. Question 4
Which of the following acts as ionizing gas
in Geiger Muller counter?
a) Alcohol
b) Argon gas
c) Krypton
d) Hydrogen

42. Question 5
• Which of the following is the main
disadvantage of solid state semiconductor
detector?
a) Low accuracy
b) Low sensitivity
c) It should be maintained at low
temperature
d) High pressure has to be produced

43. Question 6
Scintillation detector is a large flat crystal
of which of the following materials?
a) Sodium chloride
b) Sodium iodide
c) Sodium sulphate
d) Sodium carbonate

44. Assignment II
Describe Townsend Avalanche Effect in
your own words?
Describe difference between scintillation
and ionization processes?
 How energy of the incoming radiation can
possibly be differentiated using detectors?

45. Thanks