959:
1502:
occur in the gas space immediately round the anode which are numerically proportional to the number of original ionising events. Increasing the voltage further increases the number of avalanches until the Geiger region is reached where the full volume of the fill gas around the anodes ionised, and all proportional energy information is lost. Beyond the Geiger region, the gas is in continuous discharge owing to the high electric field strength.
31:
1459:
717:
141:. If the electric field is strong enough, then the free electron can gain sufficient velocity (energy) to liberate another electron when it next collides with a molecule. The two free electrons then travel towards the anode and gain sufficient energy from the electric field to cause further impact ionisations, and so on. This process is effectively a
437:
1083:
1498:
enters this region and creates a localised avalanche that is independent of those from other ion pairs, but which can still provide a multiplication effect. In this way, spectroscopic information on the energy of the incident radiation is available by the magnitude of the output pulse from each initiating event.
1015:
A Townsend discharge can be sustained only over a limited range of gas pressure and electric field intensity. The accompanying plot shows the variation of voltage drop and the different operating regions for a gas-filled tube with a constant pressure, but a varying current between its electrodes. The
1046:
In the presence of a magnetic field, the likelihood of an avalanche discharge occurring under high vacuum conditions can be increased through a phenomenon known as
Penning discharge. This occurs when electrons can become trapped within a potential minimum, thereby extending the mean free path of the
773:, is a term used where secondary ionisation occurs because the primary ionisation electrons gain sufficient energy from the accelerating electric field, or from the original ionising particle. The coefficient gives the number of secondary electrons produced by primary electron per unit path length.
1501:
The accompanying plot shows the variation of ionisation current for a co-axial cylinder system. In the ion chamber region, there are no avalanches and the applied voltage only serves to move the ions towards the electrodes to prevent re-combination. In the proportional region, localised avalanches
1497:
In the case of proportional counters, multiple creation of ion pairs occurs in the "ion drift" region near the cathode. The electric field and chamber geometries are selected so that an "avalanche region" is created in the immediate proximity of the anode. A negative ion drifting towards the anode
1493:
In the case of a GM tube, the high electric field strength is sufficient to cause complete ionisation of the fill gas surrounding the anode from the initial creation of just one ion pair. The GM tube output carries information that the event has occurred, but no information about the energy of the
208:
Townsend observed currents varying exponentially over ten or more orders of magnitude with a constant applied voltage when the distance between the plates was varied. He also discovered that gas pressure influenced conduction: he was able to generate ions in gases at low pressure with a much lower
121:
of the electron must allow free electrons to acquire an energy level (velocity) that can cause impact ionisation. If the electric field is too small, then the electrons do not acquire enough energy. If the mean free path is too short, then the electron gives up its acquired energy in a series of
949:
These two formulas may be thought as describing limiting cases of the effective behavior of the process: either can be used to describe the same experimental results. Other formulas describing various intermediate behaviors are found in the literature, particularly in reference 1 and citations
712:{\displaystyle {\frac {I}{I_{0}}}={\frac {(\alpha _{n}-\alpha _{p})e^{(\alpha _{n}-\alpha _{p})d}}{\alpha _{n}-\alpha _{p}e^{(\alpha _{n}-\alpha _{p})d}}}\qquad \Longrightarrow \qquad {\frac {I}{I_{0}}}\cong {\frac {e^{\alpha _{n}d}}{1-({\alpha _{p}/\alpha _{n}})e^{\alpha _{n}d}}}}
944:
1241:
145:
that generates free electrons. Initially, the number of collisions grows exponentially, but eventually, this relationship will break down—the limit to the multiplication in an electron avalanche is known as the
1038:
Discharges in vacuum require vaporization and ionisation of electrode atoms. An arc can be initiated without a preliminary
Townsend discharge, for example when electrodes touch and are then separated.
274:
760:
38:. The original ionisation event liberates one electron, and each subsequent collision liberates a further electron, so two electrons emerge from each collision to sustain the avalanche.
829:
125:
The avalanche mechanism is shown in the accompanying diagram. The electric field is applied across a gaseous medium; initial ions are created with ionising radiation (for example,
1274:
1131:
1310:
821:
413:
1397:
1370:
1343:
209:
voltage than that required to generate a spark. This observation overturned conventional thinking about the amount of current that an irradiated gas could conduct.
70:, collide with gas molecules, and consequently free additional electrons. Those electrons are in turn accelerated and free additional electrons. The result is an
1019:
At higher pressures, discharges occur more rapidly than the calculated time for ions to traverse the gap between electrodes, and the streamer theory of
201:
between the plates. However, this current showed an exponential increase as the plate gaps became small, leading to the conclusion that the gas
122:
non-ionising collisions. If the mean free path is too long, then the electron reaches the anode before colliding with another molecule.
1423:
of breakdown voltages is high, the above formula can only give a qualitative indication of what the real frequency of oscillation is.
1490:
in the gaseous medium to produce ion pairs, but different use is made by each detector type of the resultant avalanche effects.
1759:
962:
Voltage-current characteristics of electrical discharge in neon at 1 torr, with two planar electrodes separated by 50 cm.
1680:
380:
increases; two different effects were considered in order to better model the discharge: positive ions and cathode emission.
218:
169:
Townsend's early experimental apparatus consisted of planar parallel plates forming two sides of a chamber filled with a
1462:
Plot of variation of ionisation current against applied voltage for a co-axial wire cylinder gaseous radiation detector.
1098:
17:
1575:
1110:
823:, the average number of electrons released from a surface by an incident positive ion, according to the formula
939:{\displaystyle {\frac {I}{I_{0}}}={\frac {e^{\alpha _{n}d}}{1-{\epsilon _{i}}\left(e^{\alpha _{n}d}-1\right)}}.}
725:
1125:
whose schematic is shown in the picture on the right. The sawtooth shaped oscillation generated has frequency
142:
1521:
1467:
388:
Townsend put forward the hypothesis that positive ions also produce ion pairs, introducing a coefficient
158:
1236:{\displaystyle f\cong {\frac {1}{R_{1}C_{1}\ln {\frac {V_{1}-V_{\text{GLOW}}}{V_{1}-V_{\text{TWN}}}}}},}
1774:
1016:
Townsend avalanche phenomena occurs on the sloping plateau B-D. Beyond D, the ionisation is sustained.
1252:
312:
1471:
1288:
1691:
799:
1526:
391:
1420:
958:
1672:
1404:
1122:
1087:
75:
1636:
Principles of
Electron Tubes: Including Grid-controlled Tubes, Microwave Tubes and Gas Tubes
161:, magnitudes of currents flowing during this process can range from about 10 to 10 amperes.
1703:
1536:
1475:
1375:
1348:
1321:
781:
Townsend, Holst and
Oosterhuis also put forward an alternative hypothesis, considering the
190:
94:
8:
1764:
1511:
1431:
90:
71:
1744:
1715:
1707:
1603:
1719:
1647:
1634:
1479:
1102:
1032:
782:
79:
1589:
by A. von Engel. Biographical
Memoirs of Fellows of the Royal Society. 1957 3, 256-272
1779:
1723:
1676:
1571:
1313:
1280:
1072:
361:
93:, who discovered the fundamental ionisation mechanism by his work circa 1897 at the
1711:
1531:
1446:: achievable current is typically 10~20 times greater respect to that generated by
1028:
976:
965:
43:
205:
were multiplying as they moved between the plates due to the high electric field.
1623:
1541:
1113:
region of the S-type. The negative resistance can be used to generate electrical
1068:
1020:
212:
The experimental data obtained from his experiments are described by the formula
154:
35:
1277:
1094:
1065:
981:
369:
198:
178:
174:
118:
114:
83:
67:
1415:
Since temperature and time stability of the characteristics of gas diodes and
153:
The
Townsend avalanche can have a large range of current densities. In common
1769:
1753:
1619:
1439:
1082:
1024:
298:
147:
197:, and he found that the current flowing through the chamber depended on the
1516:
1027:, Meek, and Loeb is applicable. In highly non-uniform electric fields, the
992:
1400:
1114:
1615:
126:
106:
55:
1487:
1447:
1442:
charge generated by incident radiation (visible light or not) on the
1435:
1416:
1106:
30:
1458:
1118:
383:
353:
129:). An original ionisation event produces an ion pair; the positive
63:
1466:
Townsend avalanche discharges are fundamental to the operation of
34:
Avalanche effect in gas subject to ionising radiation between two
1443:
1408:
1061:
786:
428:
376:
rises faster than predicted by the above formula as the distance
339:
302:
182:
134:
1610:. Handbuch der Physik (Encyclopedia of Physics). Vol. XXI.
334:
pairs generated per unit length (e.g. meter) by a negative ion (
1611:
1060:
The starting of
Townsend discharge sets the upper limit to the
420:
360:
The almost-constant voltage between the plates is equal to the
424:
343:
335:
194:
186:
138:
1482:
or measuring its energy. The incident radiation will ionise
776:
181:
was connected between the plates, the lower plate being the
1483:
790:
202:
416:
372:
regime. Subsequent experiments revealed that the current
331:
170:
130:
110:
59:
1632:
Gewartowski, James W.; Watson, Hugh
Alexander (1965).
269:{\displaystyle {\frac {I}{I_{0}}}=e^{\alpha _{n}d},\,}
82:
requires a source of free electrons and a significant
1378:
1351:
1324:
1291:
1255:
1134:
832:
802:
728:
440:
394:
221:
105:
The avalanche occurs in a gaseous medium that can be
1071:
can withstand. This limit is the
Townsend discharge
189:. He forced the cathode to emit electrons using the
419:
pairs generated per unit length by a positive ion (
1745:Simulation showing electron paths during avalanche
1692:"Penning discharge in the KATRIN pre-spectrometer"
1666:
1646:
1633:
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1337:
1304:
1268:
1235:
938:
815:
754:
711:
407:
268:
1631:
1093:The occurrence of Townsend discharge, leading to
364:needed to create a self-sustaining avalanche: it
1751:
1001:; ionisation occurs, current below 10 microamps.
384:Gas ionisation caused by motion of positive ions
137:while the free electron accelerates towards the
1453:
1434:during Townsend discharge is naturally used in
86:; without both, the phenomenon does not occur.
1667:Kuffel, E.; Zaengl, W. S.; Kuffel, J. (2004).
1602:Little, P. F. (1956). "Secondary effects". In
1035:for further description of these mechanisms.
1570:, third edition 2000. John Wiley and sons,
762:, in very good agreement with experiments.
164:
1661:Glow- and Arc-discharge tubes and circuits
755:{\displaystyle \alpha _{p}\ll \alpha _{n}}
1689:
1649:Theory and applications of electron tubes
777:Cathode emission caused by impact of ions
265:
1457:
1081:
957:
795:Townsend's second ionisation coefficient
29:
1562:
1560:
1558:
1556:
989:I: unstable region: glow-arc transition
14:
1752:
1601:
1055:
1011:; large amounts of radiation produced.
100:
89:The Townsend discharge is named after
1669:High Voltage Engineering Fundamentals
1653:(2nd ed.). McGraw-Hill Co., Inc.
1644:
1587:John Sealy Edward Townsend. 1868-1957
328:first Townsend ionisation coefficient
286:is the current flowing in the device,
74:that permits significantly increased
1608:Electron-emission • Gas discharges I
1553:
1041:
973:D: self-sustained Townsend discharge
771:first Townsend avalanche coefficient
1568:Radiation Detection and Measurement
431:. The following formula was found:
24:
25:
1791:
1738:
1690:Frankle, FM, et al. (2014).
1426:
1086:Neon lamp/cold-cathode gas diode
1006:; the plasma emits a faint glow.
356:between the plates of the device.
1111:negative differential resistance
1269:{\displaystyle V_{\text{GLOW}}}
1050:
971:C: avalanche Townsend discharge
603:
599:
1657:Electrical conduction in gases
1580:
1305:{\displaystyle V_{\text{TWN}}}
1099:current–voltage characteristic
683:
653:
600:
588:
562:
524:
498:
490:
464:
13:
1:
1760:Electrical discharge in gases
1716:10.1088/1748-0221/9/07/P07028
1595:
953:
816:{\displaystyle \epsilon _{i}}
789:caused by impact of positive
368:when the current reaches the
1468:gaseous ionisation detectors
1454:Ionising radiation detectors
1109:in such a way that it has a
159:gaseous ionisation detectors
7:
1640:. D. Van Nostrand Co., Inc.
1522:Electric discharge in gases
1505:
1031:process is applicable. See
408:{\displaystyle \alpha _{p}}
330:, expressing the number of
10:
1796:
1696:Journal of Instrumentation
1645:Reich, Herbert J. (1944).
1312:is the Townsend discharge
1004:F-H region: glow discharge
999:A-D region: dark discharge
987:H: abnormal glow discharge
767:first Townsend coefficient
1009:I-K region: arc discharge
415:expressing the number of
301:current generated at the
1547:
1432:Avalanche multiplication
985:G: normal glow discharge
185:while the other was the
165:Quantitative description
157:, such as those used as
133:accelerates towards the
72:avalanche multiplication
1527:Field electron emission
193:by irradiating it with
1463:
1421:statistical dispersion
1393:
1366:
1339:
1306:
1270:
1237:
1097:breakdown, shapes the
1090:
1012:
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817:
756:
713:
409:
270:
66:are accelerated by an
39:
27:Gas ionization process
1673:Butterworth-Heinemann
1461:
1419:is low, and also the
1399:are respectively the
1394:
1392:{\displaystyle V_{1}}
1367:
1365:{\displaystyle R_{1}}
1340:
1338:{\displaystyle C_{1}}
1307:
1271:
1238:
1123:relaxation oscillator
1088:relaxation oscillator
1085:
969:B: saturation current
961:
941:
818:
769:( α ), also known as
757:
714:
410:
271:
78:through the gas. The
76:electrical conduction
33:
1537:Photoelectric effect
1494:incident radiation.
1478:in either detecting
1476:proportional counter
1376:
1349:
1322:
1289:
1253:
1132:
975:E: unstable region:
964:A: random pulses by
830:
800:
785:of electrons by the
726:
438:
392:
219:
191:photoelectric effect
95:Cavendish Laboratory
1708:2014JInst...9P7028F
1626:. pp. 574–663.
1512:Avalanche breakdown
1056:Gas-discharge tubes
101:General description
91:John Sealy Townsend
1659:" and chapter 12 "
1480:ionising radiation
1472:Geiger–Müller tube
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1362:
1335:
1302:
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1103:gas-discharge tube
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1033:Electron avalanche
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813:
793:. This introduced
783:augmented emission
752:
709:
405:
266:
52:Townsend avalanche
48:Townsend discharge
40:
18:Townsend avalanche
1775:Molecular physics
1682:978-0-7506-3634-6
1604:FlĂĽgge, Siegfried
1448:vacuum phototubes
1438:, to amplify the
1314:breakdown voltage
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1281:breakdown voltage
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1225:
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1196:
1073:breakdown voltage
1042:Penning discharge
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362:breakdown voltage
237:
16:(Redirected from
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1671:(2nd ed.).
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980:F: sub-normal
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1765:Ionization
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1596:References
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1405:resistance
1105:such as a
954:Conditions
56:ionisation
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344:anode
336:anion
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177:high-
139:anode
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