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Nuclear Medicine Instrumentation 242 NMT. Dr. Abdo Mansour Assistant Professor of radiology E-mail : a_mansour@inaya.edu.sa. Lecture No. 2 Applications of Gas-Filled Detectors. Characteristics of the Major Voltage Regions Applied Across a Gas-Filled Detector.
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Nuclear Medicine Instrumentation 242 NMT Dr. AbdoMansour Assistant Professor of radiology E-mail : a_mansour@inaya.edu.sa
Characteristics of the Major Voltage Regions Applied Across a Gas-Filled Detector • In the gas-filled detector, the magnitude of the voltage between the electrodes determines the type of response to each charged particle or photon.
At a higher voltage, most of the ions that are formed reach the electrodes. • When the voltage between the electrodes is relatively low, the field within the gas is weak and many of the ions simply recombine, leaving only a small fraction to reach the electrodes.
In small detectors, some of the ionization from beta radiation and photon radiation may escape detection. • A sensitive meteruse to measure the current of ions.
Dose calibrator • The dose calibrator is most frequently used in the nuclear medicine department as a table-top ionization chamber to confirm that the correct amount of activity has been dispensed before a dose of radiopharmaceutical is administered. • The dose calibrator consists of an ionization chamber surrounding an open well.
A dose calibrator is essentially a well-type ionization chamber capable of measuring quantities in the millicurie(37 MBq) range. • It does not contain a sodium iodide crystal. • The chamber is cylindrical and holds a defined volume of pressurized inert gas (usually argon). • Within the chamber is a collecting electrode.
Because it is extremely important to calibrate a dose of isotope before injection, the dose calibrator is an essential piece of equipment in any nuclear medicine laboratory. • The upper limit of sample activity that can be measured accurately is in the microcurie(37 kBq) range.
The selector buttons on the control panel and display require the user to select the appropriate radionuclide in order to display the correct activity.
As radiation emanates from the radiopharmaceutical in the syringe, it enters the chamber and interacts with the gas, causing ionization. • An electrical differential applied between the chamber and the collecting electrode causes the ions to be captured and measured. • This measurement is used to calculate the dose contained in the syringe.
Example • With radiopharmaceuticals, the activity of positron emitters may be measured in a typical dose calibrator before injected into the patient.
A dose calibrator with relatively high specified maximum activity is preferred. In addition, more lead shielding around the dose calibrator is required.
Survey meter: • When the ionization chamber is used as a survey meter, the current reading is usually interpreted as the average intensity of radiation in roentgens (R) per hour.
Function of Survey meter • The ionization chamber is not ordinarily used to count discrete radiation events but rather to measure the average number of ionizations per minute occurring within the gas.
Sensitivityof Survey meter • The lower limit of sensitivity for an ionization chamber is determined by the sensitivity of the meter used to measure the current. • In terms of radiation exposure, sensitivity down to 1 mRis low - and moderate-energy photons (10 keV to 1 MeV) is available in standard survey meters and dosimeters.
Notes: • The current produced in the circuitry is proportional to the number of primary ionizations in the chamber. • The amount of current is registered as radioactivity in mega-becquerelor milli-curie.
When used survey meter to measure the exposure rate, the meter reading is strongly affected by the energy of the photons. • Its principal uses are the monitoring of areas such as nuclear medicine laboratories for radiation and the detection of contamination.
