Townsend discharge

The Townsend discharge is a gas ionization processwhere free electrons, accelerated by a suciently strongelectric eld, give rise to electrical conduction througha gas by avalanche multiplication caused by the ioniza-tion of molecules by ion impact. When the number offree charges drops or the electric eld weakens, the phe-nomenon ceases.The Townsend discharge is named after John SealyTownsend, who discovered the fundamental ionizationmechanism by his work between 1897 and 1901. It isalso known as a Townsend avalanche.

1 General description of the phe-nomenon

Avalanche eect between two electrodes. The original ionisa-tion event liberates one electron, and each subsequent collisionliberates a further electron, so two electrons emerge from eachcollision: the ionising electron and the liberated electron.

The avalanche is a cascade reaction involving electrons ina region with a suciently high electric eld in a gaseousmedium that can be ionized, such as air. Following anoriginal ionisation event, due to such as ionising radia-tion, the positive ion drifts towards the cathode, while thefree electron drifts towards the anode of the device. If theelectric eld is strong enough, the free electron gains suf-cient energy to liberate a further electron when it nextcollides with another molecule. The two free electronsthen travel towards the anode and gain sucient energyfrom the electric eld to cause impact ionisation when thenext collisions occur; and so on. This process is eec-tively a chain reaction of electron generation; it dependson the free electrons gaining sucient energy betweencollisions to sustain the avalanche.[1] The total number

of electrons reaching the anode is equal to the numberof collisions, plus the single initiating free electron. Thelimit to the multiplication in an electron avalanche isknown as the Raether limit.The Townsend avalanche can have a large range of cur-rent densities. In common gas-lled tubes, such as thoseused as gaseous ionization detectors, magnitudes of cur-rents owing during this process can range from about1018 amperes to about 105 amperes.

2 Quantitative description of thephenomenon

The basic setup of Townsends early experiments inves-tigating ionization discharges in gases consisted of pla-nar parallel plates forming two sides of a chamber lledwith a gas. A direct current high voltage source was con-nected between the plates, the lower voltage plate be-ing the cathode while the other was the anode. Forcingthe cathode to emit electrons using the photoelectric ef-fect, by irradiating it for example with an X-ray source,Townsend found that the current I owing through thechamber depends on the electric eld between the platesin such a way that gas ions seemed to multiply as theymoved between them. He observed currents varying ex-ponentially over ten or more orders of magnitude with aconstant applied voltage when the distance between theplates was varied. He also discovered the importanceof the pressure of the gaseous medium, and was able togenerate ions in gases at low pressure with a much lowervoltage than that required to generate a spark. This over-turned conventional thinking about the amount of currentthat an irradiated gas could conduct.[2]

The experimental data obtained from his experiments aredescribed by the following formula

I

I0= end;

where

I is the current owing in the device, I0 is the photoelectric current generated at thecathode surface,

e is Eulers number

1

2 3 CONDITIONS

n is the rst Townsend ionization coecient, ex-pressing the number of ion pairs generated per unitlength (e.g. meter) by a negative ion (anion) movingfrom cathode to anode,

d is the distance between the plates of the device.

The almost constant voltage between the plates is equal tothe breakdown voltage needed to create a self-sustainingavalanche: it decreases when the current reaches the glowdischarge regime. Subsequent experiments revealed thatthe current I rises faster than predicted by the above for-mula as the distance d increases: two dierent eectswere considered in order to explain the physics of thephenomenon and to be able to do a precise quantitativecalculation.

2.1 Gas ionization caused by motion ofpositive ions

Townsend put forward the hypothesis that positive ionsalso produce ion pairs, introducing a coecient p ex-pressing the number of ion pairs generated per unit lengthby a positive ion (cation) moving from anode to cathode.The following formula was found

I

I0=

(n p)e(np)dn pe(np)d

=) II0= e

nd

1 (p/n)end

since p n , in very good agreement with experi-ments.The rst Townsend coecient ( ), also known as rstTownsend avalanche coecient is a term used wheresecondary ionization occurs because the primary ioniza-tion electrons gain sucient energy from the acceleratingelectric eld, or from the original ionizing particle. Thecoecient gives the number of secondary electrons pro-duced by primary electron per unit path length.

2.2 Cathode emission caused by impact ofions

Townsend, Holst and Oosterhuis also put forward an al-ternative hypothesis, considering the augmented emissionof electrons by the cathode caused by impact of positiveions. This introduced Townsends second ionization coef-cient i ; the average number of electrons released froma surface by an incident positive ion, according to the fol-lowing formula:

I

I0=

end

1 i (end 1) :

These two formulas may be thought as describing limitingcases of the eective behavior of the process: either can

be used to describe the same experimental results. Otherformulas describing various intermediate behaviors arefound in the literature, particularly in reference 1 and ci-tations therein.

3 Conditions

Current (A)1015 1010 105 10 0

1000

800

600

400

200

0

Dark Discharge Glow Discharge Arc

Vol

tage

(V

)

A' A

B' B C D E

FG

H

I

J K

Voltage-current characteristics of electrical discharge in neon at1 torr, with two planar electrodes separated by 50 cm.A: random pulses by cosmic radiationB: saturation currentC: avalanche Townsend dischargeD: self-sustained Townsend dischargeE: unstable region: corona dischargeF: sub-normal glow dischargeG: normal glow dischargeH: abnormal glow dischargeI: unstable region: glow-arc transitionJ: electric arcK: electric arcA-D region: dark discharge; ionization occurs, current below10 microamps.F-H region: glow discharge; the plasma emits a faint glow.I-K region: arc discharge; larges amounts of radiation pro-duced.

A Townsend discharge can be sustained only over a lim-ited range of gas pressure and electric eld intensity. Theaccompanying plot shows the variation of voltage dropand the dierent operating regions for a gas-lled tubewith a constant pressure, but a varying current betweenits electrodes. The Townsend avalanche phenomena oc-curs on the sloping plateau B-D. Beyond D the ionisationis sustained.At higher pressures, discharges occur more rapidly thanthe calculated time for ions to traverse the gap betweenelectrodes, and the streamer theory of spark discharge ofRaether, Meek and Loeb is applicable. In highly non-uniform electric elds, the corona discharge process isapplicable. See Electron avalanche for further descrip-tion of these mechanisms.Discharges in vacuum require vaporization and ionizationof electrode atoms. An arc can be initiated without apreliminary Townsend discharge; for example when elec-trodes touch and are then separated.

4.3 Ionising radiation detectors 3

4 Applications

4.1 Gas-discharge tubes

The starting of Townsend discharge sets the upper limit tothe blocking voltage a glow discharge gas-lled tube canwithstand this limit is the Townsend discharge breakdownvoltage also called ignition voltage of the tube.

Neon lamp/cold-cathode gas diode relaxation oscillator

The occurrence of Townsend discharge, leading to glowdischarge breakdown shapes the current-voltage charac-teristic of a gas discharge tube such as a neon lamp in away such that it has a negative dierential resistance re-gion of the S-type. The negative resistance can be usedto generate electrical oscillations and waveforms, as in therelaxation oscillator whose schematic is shown in the pic-ture on the right. The sawtooth shaped oscillation gener-ated has frequency

f = 1R1C1 ln V1VGLOWV1VTWN

;

where

VGLOW is the glow discharge breakdownvoltage,

VTWN is the Townsend dischargebreakdown voltage,

C1 , R1 and V1 are respectively thecapacitance, the resistance and the supplyvoltage of the circuit.

Since temperature and time stability of thecharacteristics of gas diodes and neon lamps islow, and also the statistical dispersion of break-down voltages is high, the above formula canonly give a qualitative indication of what thereal frequency of oscillation is.

4.2 Gas phototubes

Avalanche multiplication during Townsend dischargeis naturally used in gas phototubes, to amplify the

photoelectric charge generated by incident radiation (vis-ible light or not) on the cathode: achievable current istypically 10~20 times greater respect to that generatedby vacuum phototubes.

4.3 Ionising radiation detectors

Plot of variation of ionisation current against applied voltage fora co-axial wire cylinder gaseous radiation detector.

Townsend avalanche discharges are fundamental to theoperation of gaseous ionization detectors such as theGeigerMller tube and the proportional counter in eitherdetecting ionizing radiation or measuring its energy. Theincident radiation will ionise atoms or molecules in thegaseous medium to produce ion pairs, but dierent useis made by each detector type of the resultant avalancheeects.In the case of a GM tube the high electric eld strengthis sucient to cause complete ionisation of the ll gassurrounding the anode from the initial creation of just oneion pair. The GM tube output carries information that theevent has occurred, but no information about the energyof the incident radiation.[1]

In the case of proportional counters, multiple creation ofion pairs occurs in the ion drift region near the cath-ode. The electric eld and chamber geometries are se-lected so that an avalanche region is created in the im-mediate proximity of the anode. A negative ion drift-ing towards the anode enters this region and creates a lo-calised avalanche that is independent of those from otherion pairs, but which can still provide a multiplication ef-fect. In this way spectroscopic information on the energyof the incident radiation is available by the magnitude ofthe output pulse from each initiating event.[1]

The accompanying plot shows the variation of ionisationcurrent for a co-axial cylinder system. In the ion chamberregion, there are no avalanches and the applied voltageonly serves to move the ions towards the electrodes to pre-vent re-combination. In the proportional region, localisedavalanches occur in the gas space immediately round theanode which are numerically proportional to the number

4 8 EXTERNAL LINKS

of original ionising events. Increasing the voltage furtherincreases the number of avalanches until the Geiger re-gion is reachedwhere the full volume of the ll gas aroundthe anodes ionised, and all proportional energy informa-tion is lost.[1] Beyond the Geiger region the gas is in con-tinuous discharge owing to the high electric eld strength.

5 See also Avalanche breakdown

Electric arc

Electric discharge in gases

Field electron emission

Paschens law

Photoelectric eect

Townsend (unit)

6 Notes[1] Glenn F Knoll. Radiation Detection and Measurement,

third edition 2000. John Wiley and sons, ISBN 0-471-07338-5

[2] John Sealy Edward Townsend. 1868-1957 by A. von En-gel. Biographical Memoirs of Fellows of the Royal Soci-ety. 1957 3, 256-272

7 References Little, P.F. (1956). Secondary eects. InFlgge, Siegfried. Electron-emission Gas dis-charges I. Handbuch der Physik (Encyclopediaof Physics) XXI. Berlin-Heidelberg-New York:Springer-Verlag. pp. 574663..

James W Gewartowski and Hugh Alexander Wat-son (1965). Principles of Electron Tubes: IncludingGrid-controlled Tubes, Microwave Tubes and GasTubes. D. Van Nostrand Co, Inc.

Herbert J. Reich (1939, 1944). Theory and applica-tions of electron tubes. McGraw-Hill Co, Inc. Checkdate values in: |date= (help) Chapter 11 "Electricalconduction in gases" and chapter 12 "Glow- and Arc-discharge tubes and circuits".

E.Kuel, W.S. Zaengl, J.Kuel (2004). HighVoltage Engineering Fundamentals, Second edition.Butterworth-Heinemann. ISBN 0-7506-3634-3.

8 External links Simulation showing electron paths during avalanche

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General description of the phenomenonQuantitative description of the phenomenonGas ionization caused by motion of positive ionsCathode emission caused by impact of ions

ConditionsApplicationsGas-discharge tubesGas phototubesIonising radiation detectors

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