instrumentation review. direct and indirect ionization direct - charge particles that strip away...
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Instrumentation Review
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Direct and Indirect Ionization
• Direct - Charge particles that strip away electrons from atoms
• Indirect - uncharged that have to interact with electrons in order to remove them from the atom, these electrons go on to cause ionization.
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Ionization
• Theory of operation– Radiation enters the active volume of the detector
• Several gasses can be used
– Radiation causes ionization of gas– Negative half of the ion pair (electron) is attracted to the
anode wire• Anode wire is positively charged
– Wire is monitored by electronics and current is measured
– Change in electron density around wire will change current flow
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Radiation DetectionGas Filled DetectorsRadiation DetectionGas Filled Detectors
Air or Other Gas
Incident Ionizing Radiation
ElectricalCurrent
Measuring Device
+
-
Cathode -
Anode +
+ + +
- - -
+ -
Voltage Source
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Gas Filled Detectors
• Ion Chambers• Proportional Counters• GM tubes
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GAS IONIZATION
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Ion Chambers
• Good agreement between measurements and actual radiation level
• Constant response curve• Slow response• Low voltage• Advantage- not energy dependant• Disadvantage- subject to environmental
changes, slow
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Response Curve
Energy (MeV)
0.5 1.0 2.0
Meter reading
Actual reading
1
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Ion Chambers
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Uses
• Survey unknown radionuclides• Survey unknown levels• Survey if you are going to use info for dose
estimates
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Proportional Counters
• Highly directional (anisotropic)• Simple Energy spectroscopy• Very good at detecting charged particles• Medium Voltage• Able to detect Neutrons • Avalanche- when electrons produced by
primary ionization produce ionization themselves as they are accelerated toward the anode
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Proportional Counters
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GM Tubes
• Most sensitive gas-filled• High voltage• Able to detect gammas, med-high energy betas, and
alphas • Can detect charged particles in a gamma background• Overcompensation• Saturation ( not usually worry about)
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Overcompensation
Energy (MeV)
0.5 1.0 2.0
Meter reading
Actual reading
1
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Uses
• General area survey• Can be used to determine radiation type• Can be used to contamination surveys• Industrial gauging devices• Emergency response
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GM Tubes
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GM Tubes
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