8. Radiation is the emission and transmission
of energy through space or through a
material medium.
Radiation can be in the form of sub-atomic
particles (protons, neutrons and electrons)
or electromagnetic waves
9. 9
What is Radiation?
Form of energy
Emitted by nucleus of atom or orbital electron
Released in form of electromagnetic waves
or particles
10. Ionizing Radiation (IR) causes ions to be
produced when radiation is absorbed in
matter
Non-Ionizing radiation (NIR) refers to
radiation energy that, instead of producing
charged ions, when passing through
matter, has sufficient energy only for
excitation.
15. Why We Measure
Personal Dosimetry vs Regulatory
requirements
Classification of safe and dangerous
zones
Safety vs Protective measures
Quality control and Quality assurance
Data consistancy and standardization
16. Importance of Radiation in Daily
Life
Medicine: Radiology, Radiation Therapy
Power Production and Space Exploration
In Agriculture to improve variety & yield
production
Weapons of mass destruction
Industrial applications: Gauges,
Radiography, Mineral exploration
17. Why is Radiation Detection
Difficult?
Can’t see it
Can’t smell it
Can’t hear it
Can’t feel it
Can’t taste it
We take advantage of the fact that radiation
produces ionized pairs to try to create an
electrical signal
18. How Do I Protect Myself?
Reducing the dose from any source radiation
exposure involves the use of three protective
measures:
– TIME
– DISTANCE
– SHIELDING
19. Time
− The amount of
exposure an individual
accumulates is directly
proportional to the time
of exposure
− Keep handling time to
a minimum
20. Distance
− The relationship
between distance and
exposure follows the
inverse square law.
The intensity of the
radiation exposure
decreases in
proportion to the
inverse of the distance
squared
− Dose2 = Dose1 x (d1/d2)2
21. Shielding
− To shield against beta
emissions, use plexiglass
to decrease the
production of
bremsstrahlung radiation.
− If necessary, supplement
with lead after the
plexiglass
− To shield against gamma
and x-rays, use lead,
leaded glass or leaded
plastic
22. Self Protection Measures
1st Avoid exposure and contamination
Detect radiation exposure
Back away when too high
2nd Use Personal Protective Equipment
(PPE)
3rd Decontaminate yourself
23. Simplified Radiation PPE
Protect your respiratory tract
Respirator, surgical mask, etc.
Protect your skin
Gloves!
Outer clothing
Chemical suit is not always needed
24. Self Decontamination
Wash it off
Hand washing
Tepid water, mild soap
No scrubbing
If showering, use shampoo
Remove and launder
clothing
Monitor after decon
26. ALARA
Goal of exposure management:
keep radiation exposures to a level
As Low As Reasonably Achievable
(ALARA)
27. 27
Radiation Units
Units of radiation include
• Curie
- measures activity as the number of atoms
that decay in 1 second.
• rad (radiation absorbed dose)
- measures the radiation absorbed by the
tissues of the body.
• rem (radiation equivalent)
- measures the biological damage caused
29. RADIATION DETECTORS
• Instruments used in the practice of health &
Medical physics serve a wide variety of
purposes
• one finds instruments designed specifically for
the measurement of a certain type of
radiation, such as Alpha Particles, Beta
Particles low-energy X-rays, high-energy
gamma rays. fast neutrons, and so on
30. • The basic requirement of any such
instrument is that its detector interact with
the radiation in such a manner that the
magnitude of the instrument's response
like development of current or voltage
pulse, Charging and Discharging of
Capacitors is proportional to the radiation
effect or radiation property being
measured
31. Ionizing Radiation 31
Overall View
Ionizing radiation interacts with matter in
various ways: ionization (photoelectric effect),
excitation, braking radiation, Compton effect,
pair production, annihilation etc.
Mechanisms of interaction are utilized for the
detection of ionizing radiation.
Function and principles of electroscope,
ionization chambers, proportional chambers,
Geiger-Muller counters, solid-state detectors,
and scintillation counters, depends on those
ways of interaction. and cloud chambers have
been describe.
33. How a Radiation Detector Works
The radiation we are interested in detecting all interact
with materials by ionizing atoms
While it is difficult (sometime impossible) to directly
detect radiation, it is relatively easy to detect
(measure) the ionization of atoms in the detector
material.
Measure the amount of charge created in a detector
electron-ion pairs, electron-hole pairs
Use ionization products to cause a secondary reaction
use free, energized electrons to produce light photons
Scintillators
We can measure or detect these interactions in many
different ways to get a multitude of information
34. General Detector Properties
Characteristics of an “ideal” radiation detector
High probability that radiation will interact with the
detector material
Large amount of charge created in the interaction
process
average energy required for creation of ionization pair (W)
Charge must be separated an collected by electrodes
Opposite charges attract, “recombination” must be avoided
Initial Generated charge in detector (Q) is very small
(e.g., 10-13C)
Signal in detector must be amplified
Internal Amplification (multiplication in detector)
External Amplification (electronics)
Want to maximize V
C
Q
V
35. Detection and
measurement includes
the following
components:
Detector
Preamplifier
Amplifier
Single channel analyzer
Multi-channel analyzer
Scalar-Timer
36. a detector produces a signal for every particle
entering in it.
Every detector works by using some interaction
of particles with matter.
Use characteristic effects from interaction of
radiation with matter to detect, identify and/or
measure properties of radiation.
Respond to radiation by producing various
physical effects
37. Detection Processes
Ionization: Gas/Liquid chambers and
Semiconductor Detectors
Scintillation: Scintillation counters and TLDs
Sparking: Sparking chamber
Blackening of photographic film: Nuclear
Emulsion detector and Film dosimetry
Bubbling/Clouding of supersaturated
liquids/vapors: Cloud Chambers and Bubble
Chambers
39. Following is the list of most common types of
detectors:
Gas-filled counters
ionization chamber
Proportional Counter
Geiger-Muller counter
Scintillation detectors
Semiconductor detectors
40. Gas Detectors
Most common form of radiation detector
Relatively simple construction
Suspended wire or electrode plates in a container
Can be made in very large volumes (m3)
Mainly used to detect b-particles and neutrons
Ease of use
Mainly used for counting purposes only
High value for W (20-40 eV / ion pair)
Can give you some energy information
Inert fill gases (Ar, Xe, He)
Low efficiency of detection
Can increase pressure to increase efficiency
g-rays are virtually invisible
41. Ionization Chambers
Two electric plates
surrounded by a metal
case
Electric Field (E=V/D) is
applied across
electrodes
Electric Field is low
only original ion pairs
created by radiation are
collected
Signal is very small
Can get some energy
information
Resolution is poor due to
statistics, electronic noise,
and microphonics
Good for detecting heavy charged
particles, betas
42. Proportional Counters
Wire suspended in a tube
Can obtain much higher electric
field
E 1/r
Near wire, E is high
Electrons are energized to the
point that they can ionize
other atoms
Detector signal is much larger
than ion chamber
Can still measure energy
Same resolution limits as ion
chamber
Used to detect alphas, betas,
and neutrons
43. Scintillator Detectors
Voltage is not applied to these types of
detectors
Radiation interactions result in the creation of
light photons
Goal is to measure the amount of light created
Light created is proportion to radiation energy
To measure energy, need to convert light to
electrical signal
Photomultiplier tube
Photodiode
Two general types
Organic
Inorganic
} light electrons
44. Ionizing Radiation 44
Scintillation Counters
The Key Components of a Typical Scintillation Counter
High voltage
supplier and
multi-channel
analyzer /
computer
system
Photomultiply tube
Photo-
cathode
Na(Tl)I
crystal
Thin Al
window
X- or
g rays
46. Ionizing Radiation 46
Fluorescence Screens
Fluorescence materials absorb invisible energy and emit visible light.
J.J. Thomson used fluorescence screens to see electron tracks in cathode ray
tubes. Electrons strike fluorescence screens on computer monitors and TV
sets give dots of visible light.
Röntgen saw the shadow of his skeleton on fluorescence screens.
Rutherford observed alpha particle on scintillation material zinc sulfide.
Fluorescence screens are used to photograph X-ray images using films
sensitive visible light.
47. Semiconductor Detector
A semiconductor detector acts as a solid-state
ionization chamber
The operation of a semiconductor radiation
detector depends on its having either an
excess of electrons or an excess of holes.
A semiconductor with an excess of electrons
is called an n-type semiconductor, while one
with an excess of holes is called a p-type
semiconductor
50. one of the oldest devices used to detect and
measure ionizing (nuclear) radiation
Named for Hans Geiger who invented the device
in 1908, and Walther Müller who collaborated
with Geiger in developing it further in 1928
one of the most sensitive, especially for the low
radiation levels typically found in most situations.
51. Following is the
assembly of the
components of GM
counter:
Power-supply
GM tube
Discriminator
Scaler/timer
52. GM Counter Assembly
Variable voltage source
Gas-filled counting chamber
Two coaxial electrodes well insulated from
each other
Electron-pairs
produced by radiation in fill gas
move under influence of electric field
produce measurable current on electrodes, or
transformed into pulse
53. GM Tube In Action
wall
fill gas
R
Output
Aor
Anode (+)
Cathode (-)
End
window
Or wall
57. Voltage versus Ions Collected
Voltage
Number
of Ion
Pairs
collected
Ionization region
Saturation Voltage
100 % of initial
ions are collected
Recombination
region
58. The characteristics curve
depends on three factors
Plateau: the part of the
curve where the number
of counts per second is
(almost) independent of
the voltage.
Threshold voltage:
always lies in the plateau
region and is a function
of the gas pressure and
the anode diameter
Figure of merit: is always
less than 1%
59. A process causing the discharge to
terminate
Two methods used for quenching are:
External quenching
Internal quenching
60. Used to restore the counter
to its quiescent state after
the passage of ionizing
radiation
An RC circuit is used for
reducing the high voltage
applied to the tube, for a
fixed time after each pulse,
to a value that is too low to
support further gas
multiplication
The voltage must be reduced
for a few hundred secs which
is greater than the transit
time of the positive ions
A counter with 98% pure
argon is used
61. The advantage of external quenching is
that it gives long life time to GM tube
The disadvantage is that it has long
recovery time
62. The quenching agent gas in the Geiger counter
stops the flow of electrical current after a few
microseconds.
the quenching gas is of low ionization potential
(halogens or organic vapors)
Halogens are preferably used because it
increases the life of GM tube
Organic quenched tubes usually have a flatter
plateau than halogen quenched tubes
63. The purpose of the quenching additive to the
gas is to effectively absorb UV-photons emitted
from the electrodes when the ions produced in
the multiplication process impact on the
electrodes.
Such photons otherwise liberate secondary
electrons (via the photo-electric effect) which
may initiate the avalanche process all over
again, thereby leading to catastrophic
breakdown of the tube (i.e. a spark).
64. Device serving as pulse height selector
able to make selection from output analogical
pulses, rejecting the impulses with voltage
amplitude inferior to a certain threshold voltage
Threshold voltage should neither too low nor too
high to avoid noise and data-loss respectively
Its function is double:
To eliminate the noise
To provide a standard shaped pulse to scaler
65.
66. Scalar: counts the number of pulses
Timer: measures the length of counting
time in a given measurement
Collectively used to:
Make measurement of pulses for a preset
length of time (set on timer) recording the
number of counts by the scaler
Determine the count-rate by measuring
duration with timer for a preset number of
counts
68. Resolving Time
The negative ions, being electrons, move very rapidly
and are soon collected, while the massive positive
ions are relatively slow-moving and therefore travel
for a relatively long period of time before being
collected
These slow-moving positive ions form a sheath
around the positively charged anode, thereby greatly
decreasing the electric field intensity around the
anode and making it impossible to initiate an
avalanche by another ionizing particle. As the positive
ion sheath moves toward the cathode, the electric
field intensity increases, until a point is reached when
another avalanche could be started
69. Resolving Time
dead time
The time required to attain this electric field
intensity
recovery time
the time interval between the dead time and the
time of full recovery
resolving time
The sum of the dead time and the recovery time
72. Objective: to extract the amplitude or timing information the
electrical signal is coupled to an amplifier, sent through gain
and filtering stages, and finally digitized to allow data storage
and analysis.
amplitude or timing information include the different
characteristics of the radiation, such as the type, the intensity
and energy of the radiation
The signal can be either processed entirely through analog
circuit or can be converted into digital form
73. The signal can be a continuously varying
signal
a sequence of pulses, occurring
periodically
at known times
randomly
All of these affect the choice of signal
processing techniques.
74. First steps in signal processing:
Formation of the signal in the detector
(sensor)
Coupling the sensor to the amplifier
Detectors use either
direct detection or
indirect detection
75. The detector pulse has a very low amplitude &
time duration i.e. narrow band width
To extract any kind of information requires
amplification of detector signal
Preamplifier is a simple and efficient amplifier
directly connected to detector
76. A preamplifier, in effect, acts as a capacitance
terminator thus preventing deterioration of
detector.
Matches the high electric impedance of detector
with low impedance of the coaxial cable
connected to subsequent signal processing
circuit
Basically plays a role as an impedance matcher
between the detector and the rest of the circuit
77. The function is to amplify the pulses from
detector via a preamplifier
Also used to shape a pulse for further
detection
78. High-voltage power supply typically
provides 800 to 1,200 volts to the PMT
Raising voltage increases magnitude of
voltage pulses from PMT
Preamp connected to PMT using very
short cable
Amplifies voltage pulses to minimize distortion
and attenuation of signal during transmission
to remainder of system
79.
80. SCA Contd.
Amplifier further amplifies the pulses and
modifies their shapes – gain typically adjustable
SCA allows user to set two voltage levels, a
lower level and an upper level
If input pulse has voltage within this range, output
from SCA is a single logic pulse (fixed amplitude and
duration)
Counter counts the logic pulses from the SCA
for a time interval set by the timer
81.
82. SCA energy modes
LL/UL mode – one knob directly sets the lower
level and the other sets the upper level
Window mode – one knob (often labeled E) sets
the midpoint of the range of acceptable pulse
heights and the other knob (often labeled E or
window) sets a range of voltages around this
value.
Lower-level voltage is E - E/2 and upper-level
voltage is E + E/2
83.
84. An MCA system permits an energy spectrum to
be automatically acquired much more quickly
and easily than does a SCA system
The detector, HV power supply, preamp, and
amplifier are the same as for SCA systems
The MCA consists of an analog-to-digital
converter, a memory containing many storage
locations called channels, control circuitry, a
timer, and a display
85.
86.
87.
88. G-M tube: Advantages
Variety of sizes and shapes
Inexpensive
The slightest radiation event strong enough to cause
primary ionization results in ionization of the entire gas
volume
Thus detector is highly sensitive, even in lowest intensity
radiation fields
Only simple electronic amplification of the detector signal is
required
Hardware lasts longer
Requires less power
Strong output signal means G-M needs less electrical noise
insulation than other detectors
NET 13088
89. G-M tube: Disadvantages
Incapable of discerning between type and energy
of the radiation event
Only counts events and yields output in events per
unit time or dose rate
A beta particle or gamma ray, high or low energy,
represents one event counted
Only capable of detecting fields to some upper limit
of intensity
Limited to lower intensity fields due to detector dead
time
NET 13089
90. Personnel Monitor Devices
The most common monitor devices to determine the personal exposure
history are:
Radiation Film Badges
Pocket Dosimeter
91. Radiation film badges are composed of two pieces of film,
covered by light tight paper in a compact plastic container. Various
filters in the badge holder allow areas to be restricted to X-ray, g-ray, b-
rays only.
Radiation causes a blackening (silver) of the film material
(mostly a silver bromide emulsion) The sensitivity of the film material is
limited
For g-radiation the sensitivity is in the range of 10 - 1800 mrem.
For b-radiation the sensitivity is in the range of 50 - 1000 mrem.
Special film material is used for neutron monitoring.
The badge is usually not sensitive for radiation because the
-particles are absorbed in the light-tight paper.
92. Pocket dosimeter
The pocket dosimeter or pen dosimeter is a common small sized
ion chamber which measures the originated charge by direct collection on a
quartz fiber electroscope.
The U-shaped fiber is close to a U-shaped wire. If the fiber is
charged it will be deflected away from the wire. The position of
deflection is a measure of the accumulated radiation dose.
93. The dosimeter records total exposure from the initial
charging to the time of reading.
It is an active device as the radiation exposure can be
read immediately as opposed to the passive film badge which is
only read after approximately six months.
94. Dosimeters, which are also available in high or low ranges, can be in the
form of a badge, pen/tube type, or even a digital readout and all measure exposure or
the total accumulated amount of radiation to which you were exposed. (The Civil
Defense pen/tube tube would show a reading like below when looking through it.) It's
also similar to the odometer of a car; where both measure an accumulation of units.
The dosimeter will indicate a certain total number of R or mR exposure received, just
as the car odometer will register a certain number of miles traveled.
95. Thermoluminescence
(TL) is the ability to convert energy from
radiation to a radiation of a different wavelength,
normally in the visible light range.
Two categories
Fluorescence - emission of light during or immediately
after irradiation
Not a particularly useful reaction for TLD use
Phosphorescence - emission of light after the
irradiation period. Delay can be seconds to months.
TLDs use phosphorescence to detect radiation.
96. Thermoluminescence
Radiation moves electrons into “traps”
Heating moves them out
Energy released is proportional to
radiation
Response is ~ linear
High energy trap data is stored in TLD for
a long time
97. TL Process
Valence Band (outermost electron shell)
Conduction Band (unfilled shell)
Phosphor atom
Incident
radiation
Electron trap (meta
stable state)
-
98. TL Process, continued
Valence Band (outermost electron shell)
Conduction Band
Phosphor atom
Thermo luminescent
photon Heat Applied-
99. Output – Glow Curves
A glow curve is obtained from heating
Light output from TL is not easily interpreted
Multiple peaks result from electrons in "shallow" traps
Peak results as traps are emptied.
Light output drops off as these traps are depleted.
Heating continues
Electrons in deeper traps are released.
Highest peak is typically used to calculate dose
Area under represents the radiation energy deposited in
the TLD
100. Trap Depths - Equate to Long
Term Stability of Information
Time or temperature
101. TLD Reader Construction
Power Supply
PMT
DC Amp
Filter
Heated Cup
TL material
To High
Voltage To ground
Recorder or meter
102.
103. NET 130
Module 5 - Radiation
Detection Principles and
Instruments 103
Crystal retains this energy until heat is applied.
The “trapped” energy is then released in the form of
light, as the atoms of the crystal return to their “ground
state”
The light emitted is then correlated to dose received
Once the TLD has been “read”, memory is cleared
TLD is then available for re-use
NET 130 Module 5 - Radiation Detection Principles and Instruments 103
Thermoluminescent Dosimeter
