Radiation detection and measurement

RADIATION DETECTION AND MEASUREMENT

Lecture 02

Shahid Younas

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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

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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

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P-10 is widely used gas for proportional counters.

Do you know the real name of the gas.

90% Argon + 10 Methane

Assignment

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Assignment 02

Find Geometric Efficiency of the Injection Room Dose-calibrators.