How Electrons are Emitted in an Ionisation Chamber

In some types of smoke detectors, you can find artificial radionuclides such as americium-241, a source of alpha particles. This radioactive source emits alpha particles into two ionization chambers, one open to the air and a reference chamber that does not allow particles to enter. The open air chamber allows smoke particles to enter the sensitive volume and changes the attenuation of alpha particles. If any smoke particles enter the open air chamber, some ions will bind to the particles and will not be available to carry current in that chamber.

An electronic circuit detects that a current difference has developed between the open and sealed chambers and sounds the alarm. All the energy of the primary electrons produced in the sensitive volume of the chamber must dissipate in the chamber. To operate as an ionization chamber, an applied voltage is used that is large enough to collect all ion pairs (positive ion and removed electron) produced in the gas by a radioactive source, but not large enough to cause any amplification of the gas. There are two basic configurations; the integral unit with the camera and electronics in the same housing, and the two-piece instrument having a separate ion chamber probe attached to the electronics module by a flexible cable.

The alpha particle causes ionization inside the chamber, and the ejected electrons cause additional secondary ionizations. When atoms or gas molecules between the electrodes are ionized by incident ionizing radiation, ion pairs are created and the resulting positive ions are created and dissociated electrons move to electrodes of opposite polarity under influence of electric field.

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