Radiation detection and measurement

2. INTRODUCTION
 We cannot detect or measure ionizing radiation with
our senses
 To detect or measure radiation we must need
instruments
 Design, construction, type and use of detectors depends
upon the properties and nature of radiation

3. RADIATION DETECTION
 Suitable device is required to detect and measure the
amount and energy of radiation
 These devices consists
1. detector in which interaction takes place
2. measuring device to record the interaction

4.  For measuring the activity
 (curie, becquerel)
 For measuring the rate of radiation
 (mR/h, muSv/h)
 for personal monitoring
 (mrem, mSv)

5.  IMPORTANT EFFECTS on which detection of
radiation based are
 1. IONIZATION
 2. LUMINISCENCE
 3. PHOTOGRAPHIC EFFECT
 4. THERMOLUMINESCENCE
 5. CHEMICAL EFFECT
 6. BIOLOGICAL EFFECT

6. IONIZATION
 This effect consists of removing electrons from
originally neutral atoms or molecules, giving rise to
positive ions and negative ions
 Gaseous and solid media are used for ionization
 1. Ionization chambers
 2. Proportional chambers
 3. Geiger muller counters
 4. Semiconductor detectors

7. LUMINESCENCE
 Process in which radiation excites the atom of a
material and its energy is converted in to visible light
flash
 Using light sensitive photomultipliers , the light
flashes are converted in to electrical pulses
Scintillators
(scintillator coupled to a photomultiplier forms
scintillation detector)

8. PHOTOGRAPHIC EFFECT
 Film exposed by x rays or gamma rays, will form a
latent image with black metallic silver
 The degree of blackness can be measured by means of
optical density, which is proportional to radiation
exposure

9. THERMOLUMINISCENCE
 Radiation can impart energies to certain crystalline
materials (lithium fluoride,calcium sulfate ) or certain
glasses, which can store these energies for long time
 The energy stored can be later released in the form of
light or luminiscence by heating these materials
 The quantity of light released can be measured and
correlated to radiation dose
 TL DOSIMETERS

10. CHEMICAL EFFECTS
Radiation can cause chemical changes (oxidation of
ferrous sulfate to ferric sulfate)
 Radiation can also cause change of colouration in
certain plastics
 These changes can be measured and correlated to
radiation dose

11. BIOLOGICAL EFFECTS
 Radiation exposure to the body can be measured by
biological methods
 Eg; analysis of blood for chromosomal aberrations in
persons exposed to radiation dose of above 10-1000rem

12. DETECTORS
 Radiation interacts with detector materials and
deposits energy by ionization and excitation
 The energy deposited by single interaction is very
small and hence all detectors need signal
amplification
 In addition, the detector system performs signal
processing and signal storage with the help of
electronic circuit

13. SIGNAL PROCESSING
 It can be done by pulse mode or current mode
Pulse mode: the signal from each interaction is
processed individually
Current mode: the electrical signals from
individual interactions are averaged together and give
net current signal

14. PULSE MODE
 Two interactions must be seperated by a time
interval, so that it can produce two distinct
signals
 This time interval is called dead time of the
detector
 Dead time depends upon the components in
the detector system
 (eg. Multichannel annalyser in GM counter
detector)
 Dead times of different detectors vary widely

15. CURRENT MODE
 The electrical signal of each interaction is integrated
to give net electric signal
 Hence current mode operation is suitable for high
interaction rates to avoid dead time losses

16. DETECTOR EFFICIENCY
 It is a measure of its ability to detect radiation
 It is a product of geometric efficiency and intrinsic
efficiency

17. DETECTOR EFFICIENCY
GEOMETRIC EFFICIENCY- fraction of emitted photon that
reaches detector
no of photons reaching the detector
no of photons emitted by source
INTRINSIC EFFICIENCY-fraction of particular photons that
are detected
no of photons detected
no of photons reaching the detector
( intrinsic efficiency is often called quantum detection
efficiency(QDE) which is determined by energy of photon,
detector thickness, atomic number and density)

18. DETECTOR EFFICIENCY
 The probability of detector efficiency varies from 0 to 1
 It increases when the source is closer to the detector
 It is 0.5 for a point source placed under a flat surface
detector
 It is 1 for well type detector system

19. RADIATION DETECTORS
 Classified as based on their detection mode
1. GAS FILLED DETECTORS
2. SCINTILLATION DETECTORS
3. SEMICONDUCTOR DETECTORS

20. GAS FILLED DETECTORS
 These detectors has volume of gas between two
electrodes in which voltage is applied
 When exposed to radiation, the gas is ionized and ion
pairs are formed
 Positive ions move towards negative electrode and
negative ions move towards positive electrode
 The electron travels through the circuit and reaches
cathode and recombines with positive ions. This forms
an electric current

21. TYPES OF GAS FILLED DETECTORS
 Three types based on applied voltage
 1. IONIZATION CHAMBERS
 2. PROPORTIONAL COUNTER
 3. GEIGER MULLER COUNTER

23. IONIZATION CHAMBER
 It consists of an outer cylinder coated inside with
graphite and central electrode
 They are used in current mode
 They are used as survey meters and dosimeters in
radiotherapy
 High atomic number gases Argon (Z=18) or
xenon(Z=54) can be used to increase sensitivity
towards x and gamma rays. Used in isotope calibrator
and CT scans

25. IONIZATION CHAMBER
ADVANTAGES
1. Walls can be made tissue equivalent
2. All types of radiation can be measured
3. Can be calibrated to any energy
DISADVANTAGES
Small signal current which require high amplification
and has restricted sensitivity

26. PROPORTIONAL COUNTER
 Are designated with specific gas medium, operated
with higher voltages.
 In general, Nitrogen and Argon are used
 Krypton and Xenon are used at higher energies with
higher efficiencies
 The higher electric field accelerates the electrons to
high kinetic energy, producing secondary ionization
 They provide large surface area and serve as detector in
CT scans

27. GEIGER COUNTER
 The Geiger Muller(GM) counter consists of a
cylindrical cathode with a fine wire anode along its
axis
 It has high efficiency for charged particles and record
every particle seperately
 They are inefficient towards X rays and Gamma rays
 Voltage pulse size is independent of radiation energy
eg: 1Kev and 5Kev radiation give same pulse size.
Hence they cannot be used as spectrometers and dose
rate meters

29. SCINTILLATION DETECTOR
 It emits visible or ultraviolet light when exposed to
radiation
 However, signal needs to be amplified, hence all
scintillation crystals are provided with photomultiplier
tubes
 Scintillation material includes organic compounds and
inorganic crystals
 They have higher average atomic number and higher
density and widely used in radiology

30. SCINTILLATION DETECTOR
 CONSISTS of
1. LUMINISCENT MATERIAL
2. OPTICAL DEVICE TO FACILITATE THE
COLLECTION OF LIGHT
3. OPTICAL COUPLING BETWEEN THE
LUMINISCENT MATERIAL AND THE
PHOTOMULTIPLIER
4. PHOTOMULTIPLIER TUBE
5. ELECTRICAL CIRCUIT TO RECORD THE PULSE
APPEARING AT THE OUTPUT OF THE
PHOTOMULTIPLIER TUBE

32. SCINTILLATION DETECTOR
 When a photon interacts with the crystal, electrons are
raised to excited state. The excited electrons return
back to low energy with the emission of visible and UV
light. This is called luminiscence
 Operated at pulse mode and the electronic circuit can
identify individual interactions
 Commonly used in gamma cameras and CT scanners

33. RADIOLOGICAL SCINTILLATOR
 Sodium iodide(NaI) is used in all nuclear medice
applications
 It is used in gamma camera, thyroid probe and gamma
well counter under pulse mode operation
 It has high density (Iodine Z=53) and provides high
photoelectric absorption probability for X and Gamma
rays

34. RADIOLOGICAL SCINTILLATOR
 Bismuth germinate is used as detector in PET
scanners (Bismuth Z=83)
 Cesium iodide with Thallium activator is used in thin
film transister technology in digital radiography
 In CT scans, scintillator coupled with photodiodes are
used
 High resolution CT scans require crystals with less
after glow. Cadmium tungstate and Gadolinium
ceramics are commonly employed as scintillators

35. PHOTOMULTIPLIER TUBE
 It converts visible and UV light in to an electric signal
and does signal amplification of order of millions
 It mainly consists of an evacuated glass tube
containing and 10 to 12 dynodes and an anode
 The electron emitted by the photocathode falls on first
dynode and gets accelerated.

36.  Additional electrons ( 5 electrons per one incident
electron) are produced in the dynode and similar
process continues in the second dynode and so on.
 The total amplication is about 10^5 for a 10 minute
dynode pmt system

38. PHOTO DIODE
 It is a semiconductor device that converts light in to
electrical signal
 When it is exposed to light, an electrical current is
generated that is proportional to amount of incident
light
 Photodiodes with CdWO4 is used in CT scan as
detector and also used in digital radiography with thin
film transisters
 They are smaller in size and cheaper

39. THERMOLUMINISCENT DOSIMETER
 Organic scintillators used in personal monitoring and
patient dose estimation
 Amount of light emitted is proportional to the amount
of energy absorbed by TL material
 The peak light intensity is proportional to the
radiation dose received
 Measurement of peak light intensity forms the basis
for thermoluminescent dosimetry(TLD)

40. THERMOLUMINISCENT DOSIMETER
 Lithium fluoride is a useful TLD material with little
fading. Its effective atomic number is close to that of
tissue
 Available in the form of powder,rods,discs or chips made
of Teflon(PTFE) impregnated with lithium fluoride
 It can be worn by a person or inserted in to a body cavity
or pasted on a equipment
 Heating and measurement of peak light intensity done
in a special device called TLD reader after radiation
exposure

41. PHOTOSTIMULABLE PHOSPHORS
 These are Scintillators used in imaging plate
technology
 When exposed to radiation, fraction of excited
electrons is trapped as absorbed energy
 These electrons are released by scanning the plate by
lazer beam (700 nm)
 The laser light stimulates the trapped electrons and
release visible light

43. PHOTOSTIMULABLE PHOSPHORS
 The light may be collected by means of fiberoptic
guide tube and passed on a photomultiplier tube. The
PMT produces an electronic signal.
 This is the used in computed tomography
 Phosphors such as GD2O2S & BaFBr & BaFI can be
used as photostimulable phosphors in CR
 The imaging plate can be reused by erasing the entire
trapped electrons

44. SEMICONDUCTOR DETECTORS
A semiconductor diode with reverse bias voltage supply
can be used to detect visible and UV light
When the diode is exposed to light photons, the low
energy electrons (valence bond) are excited in the
depletion region and raised to higher energy state(
conduction band)
The hole move towards the p-type semiconductor and
electron move towards n-type semiconductor
This produces a momentary current flow in the circuit
and forms the voltage signal

45. SEMICONDUCTOR DETECTORS
 The amount of energy required is 3ev to create a
electron hole pair compared to that of 34ev in ion
chambers
 The voltage pulse is larger than ion chamber and the
pulse raise time is shorter, since the electron hole
moves rapidly. Hence they can be used in
spectrometers
 The intrinsic noise is higher due to semiconductor
resistance and thermal energy produced

46. SEMICONDUCTOR DETECTORS
 Silicon based P-N junction diode reduce noise
significantly than germanium at room temperature
 The pulse is narrower than ion chambers and hence
energy resolution is better compared to ion chambers
and scintillation detectors
 These are used in kvp meter, digital pocket
meter(silicon), gamma ray spectrometry
 They can be used as photodetectors in flat panel
detectors

47. PRACTICAL DOSIMETERS
 FREE AIR IONIZATION CHAMBER
 The whole chamber is provided with lead lining to
prevent the entry of external radiation in to chamber
 The beam size is controlled by diaphragm D
 The ionization is measured for a length L of the
collector plate C
 The lines are made straight & perpendicular to the
collector by a guard ring G

49. Temperature and pressure
correlation
 The response of ion chamber is affected by air
temperature and pressure, since the density of air
depends on temperature and pressure
 The density or mass of air in the chamber volume will
increase as the temperature decreases or pressure
increases.
 As a result, the chamber reading for a given exposure
will increase
 The chambers are usually calibrated under standard
atmospheric conditions (760 mm hg, 22C)

50.  THIMBLE IONIZATION CHAMBER
 It is basically an ionization chamber with small volume
of air
 Generally bakelite or plastic are used as wall material
 The inner surface of the wall is coated by a conducting
material like graphite
 The graphite acts as an outer electrode and participates
in charge collection
 The central cathode is made up of thin aluminium

51.  When radiation passes through the chamber, ion pairs
are produced in the air cavity as well as the walls of the
chamber
 These ion pairs are collected by the electrodes and it is
measured in terms of ionization charge Q
 It is very small in size and very much suitable for
routine measurements in hospitals, for calibrating X-
ray, telecobalt units and linear accelerators
 They needs to be calibrated once in 3 yrs

53. POCKET DOSIMETER
 It is an ion chamber with a quartz fiber suspended
with in an air filled chamber on a wire frame
 The movement of quartz fiber is proportional to
radiation exposure, which is measured in Roentgen
 The dosimeter are available in different ranges 0-
200mR, 0-500mR, 0-5R, 0-20R, 0-200R, 0-600R for
measurement of X & gamma rays
 For personal monitoring, smallest range (0-200mR)
should be used

54. POCKET DOSIMETER
 The main advantage of lies in its ability to provide
instant on the spot check of radiation dose received by
the personnal
 Film and TLD will not show accumulated exposure
immediately
 This is very useful in non routine work in which
radiation levels vary considerably & may be quite
hazardous eg: cardiac catheterization laboratory
 Small in size and easy to use & do not provide
permanent record

55. POCKET DOSIMETERS
 Now a days, this are avaiable with easy display of
instant radiation measurements
 Presently, semiconductor diode based pocket
dosimeters digital display are also available
 They have good energy & polar response, with reliable
readings, matching to TLD badges. This make loud
beep sounds for every 15 to 30 min.
 The sound becomes more frequent as dose rate
increases and becomes continuous sound at high
radiation fields

57. DOSE-AREA PRODUCT METER
 It is a flat radiolucent air chamber, fitted over the
collimater of the X-ray unit.
 Since air is used as medium, the attenuation is very
little
 It measures air dose and radiation field area
 It indicates how much patient area is exposed with a
given radiation dose. So that assessment of radiation
hazards and associated biological effect is made easy

58. DOSE-AREA PRODUCT METER
 It is used in fluoroscopic (angiogram) or cardiac
catheterization laboratory examinations, where the
procedure is long
 It is also useful in pediatric imaging to assess pt dose
 It is also called as Roentgen-area product (RAP) meter

59. AREA MONITORING
 The assessment of radiation levels at different
locations in the vicinity of radiation installation is
known as area monitoring or radiation survey
 The objective is to ensure radiation safety and
minimize personal exposure
 An ideal monitor should have uniform response to X
and gamma radiation over the range of 15KeV to 3meV

60.  Instruments used for area monitoring are called
radiation survey meters or area monitors
 Any survey meter consist of two main parts
 1) device which detect the radiation
 2) display system to measure radiation

61.  The different types of meters used
 1. Ionization type
 2. Geiger muller type
 3. Scintillation detector type

62. IONIZATION CHAMBER SURVEY
METER
 They are used to measure X-ray machine outputs,
estimate radiation levels in brachytherapy, in
monitoring radionucleide therapy pt and survey the
radioactive material packages
 They are capable of monitoring higher radiation
exposure rate levels and used in different ranges

63. GM TYPE SURVEY METER
 Very sensitive and useful for monitoring of low level
radiation
 The electronic circuit of GM is very simple and less
costly
 They are pulsed in nature. They should be used in X
ray units, that emits continuous X rays. Not used in X
ray units that emits pulsed X ray units(linear
accelerators)

64. GM TYPE SURVEY METER
 Used in nuclear medicine and suitable for radioactive
contamination & low level radiation survey
 They have long dead time (100 msec) and result in
20% loss. Hence ionization chamber survey meters are
preferred for accurate radiation survey

66. PERSONAL MONITORING SYSTEMS
1) To monitor and control individual doses regularly
2) Report and investigate overexposures and
recommend remedial measures urgently
3) Maintain lifetime cumulative dose records of the
users of service

67. PERSONAL MONITORING DEVICES
1) Film badges
2) TLD badges
3) Pocket dosimeter

68. PERSONAL MONITORING DEVICES
These devices provide
1) Occupational absorbed dose information
2) Assurance that dose limits are not exceeded
3) Trends in exposure to serve as check in working place
In India, country wide personal monitoring service is
offered by private agencies, accredited by BARC,
mumbai

69.  Film badge has some disadvantages such as
 -fading at high temperature and humidity
 -high sensitivity to light, pressure and chemicals,
 -complex dark room procedure and limited self life
 Hence, TLD badges are currently used in india instead
of film badges

70. THERMOLUMINISCENT DOSIMETER
 It is based on the phenomenon of
thermoluminiscence, the emission of light when
certain materials are heated after radiation exposure
 It is used to measure individual doses from X, beta and
gamma radiations
 It gives very reliable results since no fading is observed
under extreme climatic conditions

71. THERMOLUMINISCENT DOSIMETER
 The typical TLD badge consists of a plastic casette in
which nickel coated aluminium card is placed
 There are three filters in the cassette corresponding to
each disk namely Cu+AI, perspex and open
 The metallic filter is meant for gamma radiation,
perspex is for beta radiation

72. THERMOLUMINISCENT DOSIMETER
 When the TLD is exposed to radiation, the electrons in
the crystal lattice are excited more from the valence
band to conduction band.
 They form a trap just below the conduction band
 The number of electrons in the trap are proportional
to radiation exposure and thus it stores the absorbed
radiation energy in the crystal lattice

74. TLD READER
 After radiation exposure, the dose measurements are
made by using a TLD reader.
 It has heater, photo multiplier tube(PMT), amplifier and
a recorder
 The TLD is placed in the heater cup, where it is heated.
While heating, electrons return to their ground state
with emission of light
 This emitted light is measured by the PMT, which
converts light in to electrical current
 The PMT signal is the amplified & measured by a
recorder

76.  The disks are reusable after proper annealing up to 300
times
 The annealing process release the residual energy
stored from earlier exposure
 A typical annealing cycle consists of 400 degree
Celsius for 1hr followed by 300 degree celsius for 3 hrs

77.  GUIDELINES FOR USING TLD BADGE
 1. Worn at chest level, that is expected to receive
maximum radiation exposure
 2.Used only by persons directly working in radiation
 3.Pregnant radiation workers should wear a second badge
at waist level ( under lead apron) to assess radiation dose
 4.The name, personal number, type of radiation, period of
use, location on the body etc should be written legibily in
block letters on the front side of badge

78.  5. A TLD badge once issued to a person should not be
used by another person
 6. Each institution must keep one TLD card loaded in
TLD holder as control which is required for correct
dose estimation
 7. If lead apron is used, TLD badge should be worn
under lead apron
 8. while leaving the premises of the institute, the
workers should deposit their badges in the place where
control TLD is kept

79. THANK YOU