3. Gas Filled Detectors
Lecture 02
Volume of gas between two electrodes with an electric potential
difference (voltage) applied between the electrodes.
4. Gas Filled Detectors
Lecture 02
Commonly used Gas-filled detectors in
Nuclear Medicine
1. Survey Meter
2. GM Survey Meter
3. Dose Calibrator
5. Gas Filled Detectors
Lecture 02
Types of Gas Filled Detectors:
1. Ionization Chambers
2. Proportional Counters
3. GM counters
7. Gas Filled Detectors
Lecture 02
Regions of Gas Filled Detectors
1. Recombination Region
2. Ionization Chamber Region
3. Proportional Region
4. GM Region
5. Continuous Discharge
9. Gas Filled Detectors
Lecture 02
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.
10. Physics of Detectors
Lecture 02
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.
11. Physics of Detectors
Lecture 02
1. As voltage is increased, more ions are collected and fewer recombine.
2. Current increases as voltage is raised.
3. Recombination Region
12. Physics of Detectors
Lecture 02
Do you know the suitable energy range for gas filled detectors?
30 keV to 500 keV
13. Physics of Detectors
Lecture 02
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.
14. Physics of Detectors
Lecture 02
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.
15. Physics of Detectors
Lecture 02
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
16. Detection Efficiency
Lecture 02
Detection efficiency (sensitivity) is the ability to detect radiation.
Probability
Efficiency = Number detected / Number emitted
18. Detection Efficiency
Lecture 02
Detection efficiency (sensitivity) is the ability to detect radiation.
Probability
Intrinsic Efficiency
Number reaching Detector * Number detected
Number detected Number reaching Detector
Geometric Efficiency
19. Detection Efficiency
Lecture 02
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.
20. Detection Efficiency
Lecture 02
Do you know that geometric and intrinsic efficiencies are both
probabilities.
Could you guess the maximum and minimum value of this
probability.
22. Detection Efficiency
Lecture 02
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
23. Ionization Chamber
Lecture 02
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.
24. Ionization Chamber
Lecture 02
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.
26. Ionization Chamber
Lecture 02
Ion chambers are widely used in Radiology and Nuclear Medicine to measure
exposure.
Do you know why these are preferred?
28. Proportional Counters
Lecture 02
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".
29. Proportional Counters
Lecture 02
Thus one single incoming particle will
cause many electrons to arrive at the
wire, creating a pulse which can be
amplified and counted.
30. Proportional Counters
Lecture 02
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.
31. Proportional Counters
Lecture 02
1. Gas multiplication requires large
value of electric field.
2. Less volume of gas- multiplication-
region.
3. Seldom used in Medical Centers.
32. Proportional Counters
Lecture 02
Air is the ideal gas for Ionization Chambers.
Do you know which is the ideal gas for Proportional & GM Counters?
33. Proportional Counters
Lecture 02
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.
