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The Townsend discharge is a gas ionization process where free electrons, accelerated by a sufficiently strong electric field, give rise to electrical conduction through a gas by avalanche multiplication caused by the ionization of molecules by ion impact. When the number of free charges drops or the electric field weakens, the phenomenon ceases. The Townsend discharge is named after John Sealy Townsend, who discovered the fundamental ionization mechanism by his work between 1897 and 1901. It is also known as a Townsend avalanche. ==General description of the phenomenon== The avalanche is a cascade reaction of electrons in a gaseous medium that can be ionized, such as air where there is a sufficiently high electric field applied. Following an original ionisation event, due to such as ionising radiation, an ion pair is produced and the positive ion drifts towards the cathode while the free electron drifts towards the anode of the device. If the electric field is strong enough, the free electron gains sufficient energy to liberate a further electron when it next collides with another molecule. The two free electrons then travel towards the anode and gain sufficient energy from the electric field to cause impact ionisation when the next collisions occur, and so on. This process is effectively a chain reaction of electron generation, and it depends on the free electrons gaining sufficient energy between collisions to sustain the avalanche.〔 The total number of electrons reaching the anode is equal to the number of collisions, plus the single initiating free electron. The limit to the multiplication in an electron avalanche is known as the Raether limit. The Townsend avalanche can have a large range of current densities. In common gas-filled tubes, such as those used as gaseous ionization detectors, magnitudes of currents flowing during this process can range from about 10−18 amperes to about 10−5 amperes. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Townsend discharge」の詳細全文を読む スポンサード リンク
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