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Questions and Answers
What type of dosimeters based on semiconductor technology are commonly used in patient dosimetry?
Why are p-type diodes preferred over n-type diodes in diagnostic radiology dosimeters?
Why is it essential for MOSFETs to be connected to a bias voltage during irradiation?
What is the type of current generated in a silicon diode when radiation falls on it?
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Which dosimeter type is a miniature silicon transistor?
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What should be ensured about the leakage (dark) current in semiconductor dosimeters?
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Why are MOSFETs convenient for use in many applications?
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Study Notes
Radiation Detection
- Radiation detection can be categorized into three types: gas ionization, solid state, and scintillation.
Types of Detectors
- Gas filled detectors:
- Ionization chambers
- Proportional counters
- Geiger-Mueller (GM) counters
- Solid state detectors:
- Scintillator
- Semiconductor detectors
Geiger-Muller Counter
- Measures radioactive decay by registering the ions and electrons produced as a radioactive particle passes through a gas-filled chamber.
- Creates a detectable electrical pulse that can be counted and measured.
Patient Dosimetry
- Primary responsibility of the medical physicist in diagnostic radiology.
- Radiation measurements are critical for exposure control of workers and the public.
Radiation Fields
- Vary from plain projection geometry, slit geometry, point geometry, and can be stationary or moving, including rotational.
Dose Measurements
- Essential in acceptance testing and quality control.
- Accuracy requirements are less stringent than for radiotherapy.
- Must not interfere with the examination.
Dosimeters
- Instrument that measures the dose of ionizing radiation.
- Comprises a measuring assembly and one or more detector assemblies.
- Can be classified as active or passive.
Active Dosimeters
- Display the dose value directly.
- Include ionization chambers and semiconductor detectors.
Passive Dosimeters
- Record a signal that must be subsequently retrieved and converted to dose (or air kerma) by a reading device.
- Include thermoluminescent (TLD), optically stimulated luminescence (OSL), and film (including radiochromic film).
Properties of Diagnostic Radiology Dosimeters
- Many types of ionization chambers and solid state detectors are commercially available for the measurement of air kerma (and its derivatives).
- Solid state detectors have found wide use in quality control measurements due to their small size, ruggedness, and convenience of use.
Scintillation Detector
- Measures the rate of radioactive decay by sensing flashes of light that the radiation produces in the detector.
Ionization Chambers
- Air-filled chamber with an electric field formed by the application of a polarizing voltage across two electrodes.
- Collect all charges liberated by the ionization of the air contained within the chamber.
- The number of ions collected or the rate of their collection is the recorded signal.
- Specialized chambers come in parallel plate, cylindrical, and spherical shapes.
Application Hints for Ionization Chambers
- Venting is necessary to prevent air pressure changes affecting the reading.
- Automatic temperature and/or pressure corrections should be considered when interpreting the reading.
- The chamber should be at a distance from all supporting devices to avoid backscatter radiation.
- The user should know the limitations and rated ranges for all quantities affecting the measurements.
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Description
Test your knowledge on radiation detection instruments used as survey monitors, including gas-filled detectors like ionization chambers, proportional counters, and Geiger-Mueller counters, as well as solid-state detectors like scintillator and semiconductor detectors. Learn about Geiger-Muller counters and how they measure radioactive decay through gas-filled chambers.
