Principles of operation
The major components of a typical radioisotope calibrator include a detector (usually an ionisation chamber), a high voltage source, a current-to-voltage amplifier / electrometer and a display unit with power supply (see fig. 1 for a schematic diagram of a typical device). Radioisotope calibrators use a well-type ionisation chamber to measure the total amount ionisation 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.
No electrical current flows between the electrodes until the gas is ionised 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 to9 collect at the electrodes; called the saturation voltage, it usually ranges from 100 to 300 Volts, depending on the chamber. 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 ionisation 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. 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) (1 mCi = 37 MBq).
The degree to which ionisation 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).
Radioisotope calibrators have a separate resistor or (digital) setting for each isotope to be calibrated, based on a chamber response curve determined for the calibrator. The settings for selected isotopes are normalised to this curve, which in its turn has been confirmed using national standard traceable calibration sources. Where traceable standards are not available, settings are determined by interpolation.
Most calibrators have either a LED or LCD readout. In more sophisticated calibrators the readout is taken care of by a (standard) PC. The more sophisticated dose calibrators allow calculations like the assay of radionuclidic impurities such as Mo-99, the sample concentration, and the volume to deliver for a particular dose. The connection to a PC (RS-232 interface) opens up the possibility to integrate the dose calibrator in Hotlab Management Systems.
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. Most detectors have sealed, pressurised (as much as 20 atmospheres) ionisation chambers. The use of inert gasses such as Argon further enhances detection by helping the negative charged particles to collect at the anode as free electrons rather than recombine with large, slower-moving molecules such as oxygen. 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. 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 ionisation chamber. A readout is still obtained however because the secondary radiation (bremsstrahlung) produced by the interaction with those materials consist of photons which ionise the filling gas. The efficiency is a factor of 10 to 100 lower than for gamma radiation however.