Download Check source response Objective.

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 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-99m
(140 keV) than for 1 Bq of I-131 (364 keV).
• 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 57Co, 60Co or 226Ra 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 99mTc 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, 3RD EDITION GLENN F.
KNOLL
• HTTP://EN.WIKIPEDIA.ORG/WIKI/IONIZATION_CHAMBER
• IAEA Human Health Campus
http://nucleus.iaea.org/HHW/Technologists/NuclearMedicine/Educational
resources/NuclearMedicinePhysicsforNMT/Equipment/Dose_Calibrator/in
dex.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