Ionization chamber
The ionization chamber (engl. Ionization chamber , abbr. IC) is a radiant and particle detector , the measurement of ionizing radiation , ie, alpha , beta and gamma rays as well as for the measurement of ion beams can be used. The ionization chamber belongs to a series of gas-filled detectors or counter tubes , which differ from one another in terms of the high voltage applied and the various modes of operation that follow.
If one of the two electrodes is coated with fissile material, the ionization chamber can serve as a fission chamber for the detection of free neutrons (see also neutron detector ).
construction
The structure of the ionization chamber corresponds to a capacitor . The anode and cathode are designed either centric (cylindrical, hemispherical, spherical) or plane-parallel in order to enable the most uniform detection possible over the entire area. Between the anode and the cathode there is a counting gas (e.g. air or argon) in which charge carriers are generated by ionization when radiation is incident. The counting gas is selected to match the quantum energy of the measured radiation. If the counting gas is not air, the chamber is closed by windows that allow the radiation under investigation to pass through. Polyimide windows are suitable for low quantum energies . For high quantum energies windows are glassy carbon (English: glassy carbon ).
Mode of action
At the ionization chamber there is a high voltage between the anode and the cathode, which creates an electric field between the poles. This voltage is selected so high that the " lifetime " of the free electrons and ions that are created is greater than the flight time to the respective electrode until recombination ( saturation voltage ; see also counter tube function ). Ionizing radiation that enters the chamber ionizes the gas, the electrons reach the anode and can be measured as a current pulse.
Depending on the filling gas used, 30 to 40 eV of the energy of the radiation are absorbed per ionization . Is it z. B. to monoenergetic radiation with an energy of 1 MeV, the energy of a single particle of this radiation is completely absorbed after approx. 30,000 ionizations. This detector can therefore be used to measure the absorbed dose or the dose absorbed per unit of time, the dose rate .
use
For example, an ionization chamber is installed in the Hamburg Synchrotron Radiation Laboratory (HASYLAB) at DESY in the setup of experiments with synchrotron beams in order to "scan" the beam , which is only a few nanometers wide, and thus determine its exact position.
In the design of the fountain pen dosimeter , the ionization chamber is used as a dosimeter , i.e. as a measuring device for measuring the radiation dose in the context of radiation protection . Another application is activimeters .
With a high radiation flux and when the energy of the individual particle or quantum does not have to be measured, the pulses from the ionization chambers are not registered and analyzed individually ( pulse mode ), but the signal generated is measured as a current that changes over time ( current mode ). In this mode of operation, the law of decay is examined in teaching by placing a gaseous radioactive preparation (e.g. 220 Rn ) directly into the ionization chamber.
Measured variable and measuring range
For the previously use described chamber HASYLAB applies:
- Local dose rate : 0.0001 mSv / h - 0.01 Sv / h
- Absorbed dose rate : 0.1 mGy / h - 0.01 mGy / h
literature
- Glenn F. Knoll: Radiation detection and measurement . Wiley New York 1979, ISBN 0-471-49545-X .
