radiation detection and measurement
TRANSCRIPT
RADIATION DETECTION AND MEASUREMENT
Lecture 02
Shahid Younas
Previous
Lecture 02
1- Detection Principle
2- Types of Detectors
3- Modes
4- Dead Time
Gas Filled Detectors
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Volume of gas between two electrodes with an electric potential
difference (voltage) applied between the electrodes.
Gas Filled Detectors
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Commonly used Gas-filled detectors in Nuclear Medicine
1. Survey Meter
2. GM Survey Meter
3. Dose Calibrator
Gas Filled Detectors
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Types of Gas Filled Detectors:
1. Ionization Chambers
2. Proportional Counters
3. GM counters
Gas Filled Detectors
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Do you know how to determine type of gas filled detector?
Applied Voltage
Gas Filled Detectors
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Regions of Gas Filled Detectors
1. Recombination Region
2. Ionization Chamber Region
3. Proportional Region
4. GM Region
5. Continuous Discharge
Gas Filled Detectors
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Regions of Gas Filled Detectors
Gas Filled Detectors
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1. Ionization chambers have wide range of physical shapes ( parallel
plates, concentric cylinders or a wire within cylinder).
2. Proportional counters and GM counters must have thin wire
anode.
Physics of Detectors
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1. Ionizing Radiation produces ion pairs in the gas of the detector.
2. No voltage, no current flow .
3. Small voltage applied, some of the cations (-) and anions (+) are
attracted before they recombine.
Physics of Detectors
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1. As voltage is increased, more ions are collected and fewer recombine.
2. Current increases as voltage is raised.
3. Recombination Region
Physics of Detectors
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Do you know the suitable energy range for gas filled detectors?
30 keV to 500 keV
Physics of Detectors
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1. As voltage is increased further, a plateau is reached in the curve.
2. The applied electric field is sufficiently strong to collect almost all
ion pairs.
3. This is Ionization Chamber Region.
Physics of Detectors
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Proportional Region.
1. Amplification increases as the voltage increases.
2. Electrons / anions are accelerated to such high K.E. that they cause
additional ionization.
3. Double Tea-bag
4. Electrical charge collected from each interaction ~ energy
deposited in the gas by interaction.
Physics of Detectors
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GM Region
1. Charge collected from each event in the same regardless of the
amount of energy deposited by the interaction.
2. Higher Applied Voltage.
3. Discharge
Detection Efficiency
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Detection efficiency (sensitivity) is the ability to detect radiation. Probability
Efficiency = Number detected / Number emitted
Detection Efficiency
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In the equation given below which parameter is variable?
Efficiency = Number detected / Number emitted
Detection Efficiency
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Detection efficiency (sensitivity) is the ability to detect radiation. Probability
Intrinsic Efficiency
Number reaching Detector * Number detected
Number detected Number reaching Detector
Geometric Efficiency
Detection Efficiency
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Efficiency = Geometric Efficiency x Intrinsic Efficiency
Geometric efficiency is the fraction of emitted particles that reach the detector. Intrinsic efficiency is the fraction of detected particles reach the detector.
Detection Efficiency
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Do you know that geometric and intrinsic efficiencies are both probabilities.
Could you guess the maximum and minimum value of this probability.
Detection Efficiency
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Analyze the Geometric Efficiency in a well type detector
Detection Efficiency
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Intrinsic efficiency is also called as Quantum Detection Efficiency. It depends upon,
i. Energy of incident photon
ii. Atomic number
iii. Density
iv. Thickness of Detector
Intrinsic efficiency = 1- e-ux
Ionization Chamber
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If gas is air and walls of
chambers are of material whose
effective atomic number is
similar to that of air the amount
of current produced is
proportional to the exposure
rate.
Ionization Chamber
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1. Low intrinsic efficiency because of the low densities and low atomic
number of common gases.
2. Sensitivity increased by adding Argon (Z=18) or Xenon (Z=54) and
pressurizing for density.
3. No secondary ions are produced.
4. Current Mode.
Ionization Chamber
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1. Used in QA /QC of X ray machines.
2. Dose Calibrators filled with Argon.
Ionization Chamber
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Ion chambers are widely used in Radiology and Nuclear Medicine to measure
exposure.
Do you know why these are preferred?
WELCOME TO AVALANCH
Lecture 02
Proportional Counters
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1. Incident particle pulls an electron from an
Argon atom.
2. Electron rushes towards the wire; it will
knock other electrons from Argon atoms.
3. Causing an "avalanche".
Proportional Counters
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Thus one single incoming particle will
cause many electrons to arrive at the
wire, creating a pulse which can be
amplified and counted.
Proportional Counters
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1. Constructed with a cylindrical geometry.
2. Look at the position of anode and cathode in
figure.
3. Polarity of applied voltage is important in this
configuration because electrons must be
attracted to central axial wire.
Proportional Counters
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1. Gas multiplication requires large
value of electric field.
2. Less volume of gas- multiplication-
region.
3. Seldom used in Medical Centers.
Proportional Counters
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Air is the ideal gas for Ionization Chambers.
Do you know which is the ideal gas for Proportional & GM Counters?
Proportional Counters
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1. Noble gases either pure or binary mixtures.
2. Provided that gas multiplication factor < 100.
3. Quench gases can be used to prevent instability and proportionality losses
caused by UV photons.
4. Krypton & Xenon increase detection efficiency.
5. Hydrocarbon gases may also be used.
Proportional Counters
Lecture 02
P-10 is widely used gas for proportional counters.
Do you know the real name of the gas.
90% Argon + 10 Methane
Assignment
Lecture 02
Assignment 02
Find Geometric Efficiency of the Injection Room Dose-calibrators.