Gas Detectors

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Gas Detectors

  1. 1. NMT 215 - Non-Imaging Instrumentation Gas Detector Systems Jimmy L. Council
  2. 2. 2 Radiation Detection Devices Devices that detect the presence of ionizing radiation through the physical ways that radiation interacts with matter
  3. 3. 3 Classification of Detectors  Medium used for physical interaction  Type of interaction produced  Nature of the electronic pules generated, secondary to the physical interaction of radiation
  4. 4. 4 Categories of Detectors  Gas Detectors  Scintillation Detectors
  5. 5. 5 Gas Detectors  Definition  General principles of operation  Types of gas Detectors
  6. 6. 6 Definition - Gas Detector  Devices used to identify ionizing radiation by counting the ions produced as radiation passes through a gas volume
  7. 7. 7 Principles of Operation  Gas is enclosed within a chamber made of a conducting material that acts as a cathode  A central core acts as an anode  Cathode and Anode are connected by a resistance/capacitance circuit and have a voltage applied
  8. 8. 8 Principles of Operation  Radiation passes through the gas producing ion pairs  Positive ions move toward the negative cathode  Negative ions move toward the positive anode
  9. 9. 9 Principles of Operation  The number of ion pairs collected at the anode and cathode is a function of the voltage applied  The change in the charge on the capacitor is proportional to the number of ions collected  This charge is expressed as the Pulse Height
  10. 10. 10 Principles of Operation  Pulse Height is a function of the voltage applied to the detector electrodes  A characteristic gas detector curve can be obtained by plotting pulse height Vs voltage.  Six regions can be identified
  11. 11. 11 Characteristic Gas Detector Curve  Recombination Region  Ionization Region  Proportional Region  Nonproportional Region  Geiger-Mueller Region  Continuous Discharge Region
  12. 12. 12 Characteristic Gas Detector Curve
  13. 13. 13 Characteristic Gas Detector Curve - Recombination Region  Voltage is too weak to attract the ions  The ions recombine  No pulse is produced
  14. 14. 14 Characteristic Gas Detector Curve - Ionization region  Voltage is increased and eventually all ions are collected at the electrodes with no recombination - called the saturation voltage Pulse height is no longer a function of voltage - curve flattens into a plateau Voltage is sufficient to collect all ion pairs
  15. 15. 15 Characteristic Gas Detector Curve - Proportional Region  Voltage exceeds saturation voltage  Pulse height increases because of avalanche or gas amplification Ions move at high speeds and bump into other atoms and ionize them More than one ion is collected per ionizing event - increases pulse height Pulse height maintains proportionality
  16. 16. 16 Characteristic Gas Detector Curve - Nonproportional Region  Increase in voltage results in loss of proportionality between number of primary and secondary ions  Pulse height is more a function of voltage
  17. 17. 17 Characteristic Gas Detector Curve - Geiger-Mueller Region  High voltage causes the spread of ionization throughout the entire gas  One ionization causes an avalanche of all the gas atoms  Pulse height is no longer proportional to the ionizing event
  18. 18. 18 Characteristic Gas Detector Curve - Continuous Discharge  Voltage exceeds the Geiger-Mueller region  Collection of high energy ions results in a continuous discharge of the electrodes  Electrodes could be damaged or destroyed
  19. 19. 19 Types of Gas Detectors  Ionization Chambers “Cutie Pie” Pocket dosimeter Dose Calibrator  Proportional Counters  Geiger-Mueller Counters
  20. 20. 20 Ionization Chambers  Measure radiation using detectors that operate in the ionization region of the gas detector curve  The flow of electrons is a direct measure of the total number of ion pairs produced  One electron produced per event, uses amplification of the pulse height
  21. 21. 21 Ionization Chambers - “Cutie Pie”  Used as a survey meter  Measures roentgens per hour (R/hr)  Not used much in Nuclear Medicine  Usually for high dose rate areas
  22. 22. 22 Ionization Chambers - Pocket Dosimeter  Measures amount of radiation over time in rads  Precharged dosimeter gradually reduces voltage in response to the radiation exposed to it  Uses a quartz fiber
  23. 23. 23 Ionization Chambers - Dose Calibrator  Used to determine the activity of nuclides  Readout is in uCi or mCi
  24. 24. 24 Proportional Counters  Measures radiation in the proportional region of the gas detector curve  Maintains proportionality between the radiation energy and pulse height  May be used to detect low level of alpha and beta radiation in gas chromatography
  25. 25. 25 Geiger-Mueller Counters  Measures radiation using the Geiger-Mueller region of the gas detector curve  Basic survey meter used in nuclear medicine  Used in measuring low levels on all types of ionizing radiation
  26. 26. 26 Geiger-Mueller Counters  Uses a quenching mechanism to prevent continuous discharge  Dead time reduces efficiency - time in which the counter cannot respond to another event  Life of a G-M tube is 1-10 billion counts, as tube ages the plateau becomes shorter until operation cannot be maintained

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