According to Wikipedia, An ionisation chamber is defined as a device used for two major purposes: detecting particles in air (as in a smoke detector), and for detection or measurement of ionizing radiation.
An ionisation chamber is an instrument constructed to measure the number of ions within a medium (which we will consider to be gaseous, but can also be solid or liquid). It consists of a gas filled enclosure between two conducting electrodes. The electrodes may be in the form of parallel plates (parallel plate ionisation chambers: PPIC) or coaxial cylinders to form a convenient portable detector in which case, one of the electrodes may be the wall of the vessel itself.
When gas between the electrodes is ionized by any means, such as by alpha particles, beta particles, X-rays, or other radioactive emission, the ions and dissociated electrons move to the electrodes of the opposite polarity, thus creating an ionisation current which may be measured by a galvanometer or electrometer.
Each ion essentially deposits or removes a small electric charge to or from an electrode, such that the accumulated charge is proportional to the number of like-charged ions. A voltage potential that can range from a few volts to many kilovolts is applied between the electrodes, and allows the device to work continuously by mopping up electrons and preventing the device from becoming saturated. The current that originates is called a bias current, and prevents the device from reaching a point where no more ions can be collected.
Ionisation chambers are widely used in the nuclear industry as they provide an output that is proportional to dose and have a greater operating lifetime than standard Geiger tubes (in Geiger-Müller tubes the gas eventually breaks down). Ionisation chambers are used in nuclear medicine to determine the exact activity of radioactive therapeutic treatments. Such devices are called 'radioisotope dose calibrators'. Ion chambers are sometimes microphonic as they are very sensitive devices and non ion related charges can be set up inside due to the piezoelectric effect.