Ionization chamber

The ionization chamber (English ionization chamber, abbr IC ) is a radiant and particle that can be used for the measurement of alpha, beta and gamma radiation, and for the measurement of ion beams. The ionization chamber is part of the series of gas-filled detectors or Geiger counters, which differ from one another in terms of the applied high voltage and the consequent different mode of action.

If one of the two electrodes 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 ionization chamber comprises a chamber in which there is a capacitor to generate an electric field. The anode and cathode in order to enable the entire area as uniformly as possible a detection, either centrally (cylindrical, hemispherical, spherical) or constructed in plane-parallel. Between the anode and the cathode is a counter gas (eg, air or argon), are generated upon incidence of radiation in the charge carrier by ionization. The counting gas is chosen to match the quantum energy of the measured radiation. Is the counting gas, no air, the chamber is closed by windows that are transparent to the examined radiation. For low quantum energies polyimide windows are suitable. For high quantum energies windows are (English: glassy carbon) of glassy carbon.

Ionization chamber for low quantum energies

Ionization chamber for high quantum energies (explosion drawing)

Operation

In the ionization chamber is located between anode and cathode, a high voltage creates an electric field between the poles. This voltage is so high that the "lifetime " emergent free electrons and ions is greater than the flight time to the respective electrode to recombination (saturation voltage; see also under counter tube # function). Ionizing radiation entering the chamber, the ionized gas, the electrons reach the anode, and can be measured as a current pulse.

Depending on the filling gas used per 30 to 40 eV ionization energy of the radiation is absorbed. Is it, for example, mono -energy radiation having an energy of 1 MeV, the energy of a single particle of this radiation is absorbed completely by 30,000 ionization. Thus, one can with this detector, the absorbed dose or the absorbed dose per unit time, to measure the dose rate.

Use

An ionization chamber, for example, at the Hamburg Synchrotron Radiation Laboratory (HASYLAB ) at DESY built into the structure of experiments with synchrotron radiation to "scan" the only a few nanometers wide beam to ( scan ) and thus determine its exact location. In the design of the Füllhalterdosimeters to use the ionization chamber dosimeter, ie as a measuring device for measuring the radiation dose in the context of radiation protection. Another application is activimeter.

At high radiation flux, and if the energy of the single particle / photon need not be measured, the pulses of ionization chambers are not registered individually and analyzed ( pulse mode), but the signal generated is measured as time-varying current (current mode).

Measured quantity and measuring range

For the described 'Using ' chamber at HASYLAB applies:

0.1 mGy / h - 0.01 mGy / h ( absorbed dose rate )