Radioactivity measurement Dose Calibrator 20130314 Ionization Chambers Several

核子醫學技術學實驗 Radioactivity measurement, Dose Calibrator 李辰衍 2013/03/14

Ionization Chambers • Several of the oldest and most widely used types of radiation detectors are based on the effects produced when a charged particle passes through gas. • The primary modes of interaction involve ionization and excitation of gas molecules along the particle track. • The majority of gas-filled detectors are based on sensing the direct ionization created by the passage of the radiation.

Ionization Chambers • Ion chambers in principle are the simplest of all gasfilled detectors. • Their normal operation is based on collection of all the charges created by direct ionization within the gas through the application of an electric field. As with other detectors, ion chambers can operated in current or pulse mode. • In most common applications, ion chambers are used in current mode devices. • In contrast, proportional counter or Geiger tubes are almost used in pulse mode.

MODES OF DETECTOR OPERATION : Current Mode • Pulse mode is easily the most commonly applied of these, but current mode also finds many applications. • All detectors used to measure the energy of individual radiation quanta must be operated in pulse mode. Such applications are generally categorized as radiation spectroscopy. • At very high event rates, pulse mode operation becomes impractical or even impossible.

MODES OF DETECTOR OPERATION : Current Mode • a current-measuring device (an ammeter or, more practically, a picoammeter) connected across the output terminals of a radiation detector.

MODES OF DETECTOR OPERATION : Current Mode • If we assume that the measuring device has a fixed response time T, then the recorded signal from a sequence of events will be a timedependent current given by • Because the response time T is typically a fraction of a second or greater, the effect is to average out many of the fluctuations in the intervals between individual radiation interactions and to record an average current that depends on the product of the interaction rate and the charge per interaction. • In current mode, this time average of the individual current bursts serves as the basic signal that is recorded.

The Ionization Process In Gases • As a fast charged particle passes through a gas, the types of interaction create both excited molecules and ionized molecules along its path. • After a neutral molecule is ionized, the resulting positive ion and free electron are called an ion pair • It serves as the basic constituent of the electrical signal or “delta ray” developed by the ion chamber. • The practical quantity of interest is the total number of ion pairs created along the path of radiation.

Principles of operation • The major components of a typical radioisotope calibrator include a detector (usually an ionization chamber), a high voltage source, a current-to-voltage amplifier / electrometer and a display unit with power supply • ( fig. 1 for a schematic diagram of a typical device).

Principles of Operation Figure 1. Radionuclide calibrator

Principles of Operation • Radioisotope calibrators use a well-type ionization chamber to measure the total amount ionization produced by the (liquid) sample to be administered to the patient. • A volume of gas (usually Argon under high pressure) is contained in a sealed chamber with two electrodes having a voltage difference between them. • The electrodes (cathode and anode) can be parallel plates (or cylinders), a pair of wires, or a single wire inside a cylinder.

Principles of Operation • No electrical current flows between the electrodes until the gas is ionized when the vial or syringe containing the radionuclide is put into the chamber. • The electrical potential between the electrodes causes the positively charged gas molecules to flow toward the cathode and the negatively charged gas molecules to flow to the anode. • The high voltage source must be sufficient to allow most of the ions produced to collect at the electrodes; called the saturation voltage, it usually ranges from 100 to 300 Volts, depending on the chamber.

Principles of Operation • If the potential across the chamber electrodes is too low, recombination of the charged particles will occur, reducing detection efficiency when measuring high activities. • The movement of charged particles toward the cathode and anode produces an ionization current that is proportional to the activity of the measured radioisotope – the higher the activity, the more photons pass through the chamber.

Principles of Operation • The magnitude of this current is usually very small (0. 1 pico Amp to 100 µ Amp), even for large amounts of radionuclides , and its measurement requires the use of an electrometer, a sensitive device for quantifying minute electric currents. • The electrometer output is electronically manipulated to produce a digital output expressed in multiples of either Curies (Ci) or Becquerels (Bq)

Principles of Operation • The degree to which ionization occurs is not only a function of the amount of a radionuclide, but also a function of the energy level of the radionuclide. • The chamber’s response is different for 1 Bq of Tc-99 m (140 ke. V) than for 1 Bq of I-131 (364 ke. V). • A correction factor is necessary so that the calibrator reads the same for equal activities of different isotopes. • This is either accomplished by using adjustable resistors to regulate the amplifier gain (analog method) or by multiplying the digital output with an isotope specific calibration factor (digital method).

Principles of Operation • Ion chambers are relatively inefficient for detecting X-ray and gamma radiation, because only a small percentage of photons passing through the chamber interact with the gas molecules. • Detection efficiency can be improved by increasing the pressure of the gas, thus increasing the gas density and the interaction probability. • Slower collection at the electrodes increases response time and requires a higher saturation voltage to prevent recombination. • Well type ion chambers also are relatively insufficient for detecting Beta radiation, but for a different reason.

Principles of Operation • Beta particles are stopped by the solution itself, the vial and in the end by the chamber wall, before they reach the gas inside of the ionization chamber. • A readout is still obtained however because the secondary radiation (bremsstrahlung) produced by the interaction with those materials consist of photons which ionize the filling gas. • The efficiency is a factor of 10 to 100 lower than for gamma radiation however.

PERFORMANCE CHECK PROCEDURES • Check source response • Objective. Measurement of the check source response establishes the constancy of the system’s response by examining the reproducibility in measuring a constant source over a long period of time, which is an indicator of the reproducibility of the electrometer and the integrity of the ionization chamber gas pressure. • Ideally, at least one relatively long lived source in a reproducible geometry will be measured each day before the calibrator is used. Caesium-137 is a good option because of its long half-life and radionuclidic purity, although other radionuclides such as 57 Co, 60 Co or 226 Ra can be used.

PERFORMANCE CHECK PROCEDURES • Linearity • Objective. This check confirms that, for an individual radionuclide, the same calibration setting can be used to indicate the correct activity over the range of use of that calibrator. • It is important that the linearity of the radionuclide activity calibrator be ascertained over the range of its use between the maximum activity administered and 1 MBq. (Typically, the maximum activity for 99 m. Tc will be about 150 GBq. )

PERFORMANCE CHECK PROCEDURES • Accuracy • Objective. This check is to ensure that the activity values determined by the radionuclide activity calibrator are traceable to national or international standards of radioactivity within the indicated uncertainties. • Sources to be used for this purpose must be traceable to an NMI and be provided with both a certificate to this effect and an uncertainty statement. • At each confirmation of accuracy, at least two radionuclides, selected from the list of commonly assayed radionuclides, are to be used.

Reference • RADIATION DETECTION AND MEASUREMENT, 3 RD EDITION GLENN F. KNOLL • HTTP: //EN. WIKIPEDIA. ORG/WIKI/IONIZATION_CHAMBER • IAEA Human Health Campus http: //nucleus. iaea. org/HHW/Technologists/Nuclear. Medicine/Educationa lresources/Nuclear. Medicine. Physicsfor. NMT/Equipment/Dose_Calibrator/i ndex. html • Dose Calibrators: Principles of operation http: //www. dosecalibrator. com/index. php? page=information&sub=princi ples • IAEA Technical Reports Series No. 454 Quality Assurance for Radioactivity Measurement in Nuclear Medicine