US2929949A - Method of and apparatus for electron multiplication - Google Patents

Description

March 22, 1960 c. H. VINCENT 2,929,949

APPARATUS FOR ELECTRON MULTIPLICATION METHOD OF AND Filed Feb. 8, 1957 2 Sheets-Sheet l Attorneys March 22, 1960 c. H. VINCENT METHOD OF AND APPARATUS FOR ELECTRON MULTIPLICATION' Filed Feb. 8, 1957 2 Sheets-Sheet 2 0 v I, .AHII r C l I- T 4 r IF M /\.n M 4! 0 I lvllllll l G A l/l/ n run \\W o E E 0 ..||.|l l 0 run w a lllll II o r W m F w? y M W .dynode, and so on.

United States fiatent METHOD OF AND APPARATUS FOR ELECTRON MULTIPLICATION Charles Holmes Vincent, Basingstoke, England, assignor to National Research Development Corporation, London, England, a British corporation Application February 8, 1957, Serial No. 639,013

Claims priority, application Great Britain February 11, 1956 13 Claims. (Cl. 313-105) This invention relates to a method of and apparatus for'electron multiplication, and more particularly relates to an electron multiplier device constructed in such manner as to utilized the Townsend electron avalanche process while at the same time suppressing feedback at spaced electrodes in order to provide better amplification of an electronic signal.

Electron multipliers are known in which electrons emitted from a cathode are directed on to an electrode, referred to as a dynode, and eject further electrons from this dynode as secondary emission. These secondary electrons are directed at the next stage to a further Since the number of secondary electrons ejected from each dynode may be several times the number of primary electrons causing the ejection, the emission of electrons from the cathode causes the arrival of many times that number of electrons at the final anode or collector electrode, and the device thus constitutes a means of amplifying a current or charge of free electrons by a large factor, if sufficient stages are incorporated.

In accordance with the present invention I provide a method of electron multiplication in which an output current variable at all times in accordance with a variable input current is obtained, comprising, causing an electron stream to pass in succession through a series of electrodes in a gas, using the Townsend electron avalanche process in the gas at the stages between the electrodes, and attenuating at the interposed electrode or electrodes the effects which may cause feedback between any two stages.

The Townsend electron avalanche process or efiect, otherwise known as the Townsend alpha (a) process, is a process in which electrons moving in an electric field in a gas acquire sufiicient energy to produce further electrons by ionizing molecules of the gas on impact therewith.

The electron multiplier in accordance with the invention may include a cathode and an anode and a number of intermediate reticular electrodes of low electron permeability spaced apart in succession, a gas-tight envelope for the parts and a gas for example air, inside said envelope, so that an electron stream passing through the said intermediate electrodes successively in the direction from cathode to anode, under the influence of potential difierences between the successive electrodes in the same sense is urged through the multiplier in the appropriate direction, and so that the Townsend electron avalanche process occurs in the spaces between the successive electrodes without breakdown occurring therein. The term breakdown herein and in the claims means a gaseous electric discharge which does not at all times vary in accordance with the input current. The reticular electrodes of low electron permeability are a convenient means for:

(a) Applying the necessary electric field to the space in which the electrons move, so that the Townsend electron avalanche process occurs therein, and the electrons move through the multiplier in the appropriate direction.

. field when it is desired to reduce the ice (b) Permitting the passage of electrons from one stage to the next in the forward direction (i.e. from cathode to anode) through their perforations.

(c) Attenuating the fiow of feedback-causing agents (such as positive ions and photons) in the backward direction, by intercepting a sufficient proportion of these at each electrode.

The term reticular is intended herein and in the claims to embrace a construction of electrodes provided in any way with a substantial number of small holes.

Furthermore it is to be noted that the electrodes under this invention do not require to be specially prepared to give them a high secondary emission ratio, as is the case with the aforesaid electron multiplier. Such preparation involves considerable difficulties and disadvantages.

Normally the cathode, anode, and intermediate electrodes will be used in a vacuum-tight envelope containing a gas. In some cases parts or the whole of some of these electrodes (including cathode and anode) may form part of the said vacuum-tight envelope. The potential between successive electrodes to ensure occurrence of the Townsend alpha process at each stage depends upon the gas used, the pressure thereof, and the spacing (S) between the successive electrodes. A desirable characteristic of the gas is that it should have a low Townsend gamma coefiicient between electrodes of the same material as the electrodes of the multiplier. The Townsend gamma coefficient is a factor measuring, for a specified gas and electrode material, the extent to which feedback accompanies the Townsend a process in that gas between two electrodes, the cathode being of that material. Another desirable characteristic of the gas is that at suitable pressures and with appropriate field strengths applied it should have a high value of the ratio:

where a is the Townsend alpha coefficient, and X is the electric field strength in volts per centimetre. The Townsend alpha coefiicient is defined as the number of new electrons created by ionization per electron per centimetre of path in the (negative) direction of the electric field, and it depends upon the gas, the density of the gas at the pressure used, and the electric field strength. In particular, for a specified gas, the ratio a/d is a function of the ratio X/d where d is the density of the gas in say, grams per litre.

The gamma coefficient ('y) is similarly a function of X/d for a specified gas and electrode material.

It is to be noted that the direction of the general movement of the electrons, referred to as the forward" direction herein, is from cathode to anode, since electrons are negatively charged particles. This direction is the negative direction of the electric field.

A high value of the above ratio p is desirable, since it enables a greater overall gain to be obtained for a given total voltage applied to the multiplier. The gas density and the operating field chosen for a particular gas may be such as to make the above ratio ,0 a maximum. The use of such a condition reduces the total voltage needed, as stated, and also reduces fluctuations of the multiplier gain with any small changes that may occur in the density of the gas. The voltage per stag'e'may, however, be re duced to give a field less than this designed operating gain of the multiplier.

Air is an example of a gas that has been used suc-' cessfully, but other gases meeting the above requirements may also be used. The gas used should obviously be free from any tendency to attack the electrodes or other parts of the structure chemically, or, particularly that at which it approaches breakdown.

3,, in asealed-off envelope, to be absorbed by them. It is undesirable that the gas should have an excessive tendency to form metastable states or negative ions. Metastable states arefound in the noble gases-suchas neon andargon, when they are pure. The molecules of'such gases consist of single atoms, and in a metastable state these store energy which may result in the undesired release. of. electrons when they reach a solid surface later. This undesirable property may be destroyed easily by the admixture of another gas in which the molecules contain more than one atom so that the metastable atoms are deenergised by collisions with the latter. A negative ion is formed when a molecule of the gas-captures an electron which collides with it, and this is an undesirable process since the negative ions move through the multiplier much more slowly than the electrons, and they do not participate in the Townsend avalanche and cause multiplication. However, the tendency of most gases to form negative ions is small at the relatively high ratios of field strength to gas density (that is, the ratio X/ d) used. in the multiplier.

Theinvention further comprises a circuit having an electron multiplier asv aforesaid (Le. a gas multiplier) in combination with means for supplying voltage difierences between the successive stages thereof of an order suflicient to obtain the Townsend electron avalanche process substantially while not being high enough to involve breakdown. The said Townsend process is not effective substantially until a certain voltage difierence occurs, but the process then increases with increasing voltage. At a certain voltage the said Townsend alpha process in conjunction with the aforesaid gamma feedback process causes breakdown. At a voltage slightly below this, there is a high electron multiplication factor, butthis it not a desirable condition of operation, since the multiplication factor changes rapidly with changes ofivoltage, and a small changeof. voltage or other conditions may initiate breakdown. Hence the elfect isobtained usefully over a certain range of voltage difference, i.e. above that at which it becomes substantial but below In my invention, a very large overall gain, may be obtained, with a' sufficient number of stages, by means of a moderate gross gain in each stage, and that in spite of losses due to electrons being'intercepted by the electrodes.

The gross gain in astageis the total electron multi: plication factor in thegas ofthat stage, and must be reduced by a factor to allow for the proportion of electrons intercepted by each electrode, to give the actual gain per stage.

Each stage may therefore be operated with a voltage across, it which is. substantially less than the voltage which would cause breakdown ifapplied to that stage.

Moreover, owing to the previously mentioned attenuation of the flow of'feedback agents from one stage to another, the operating voltage across each stage, even for a high overall gain, may be substantially less than the voltage which would cause breakdown if applied to all stages of the multiplier simultaneously. In my invention, therefore, the gain may be smoothly increased, by increasing the voltage per stage, from a low value up to a very high value, without approaching the conditions for breakdown or reaching acondition where the gain is excessively sensitive to voltage or other changes, anal the range of useful operating voltages is relatively wr e.

It may be shown that the logarithmic current gain per stage, under suitable conditions of operation, may approach the value nepers, where p is the ratio a/X aforesaid, and V is the voltage difference per stage between adjacent electrodes.

This value makes due allowance for the electrons lost by interception at the electrodes. Byway of example, the logarithmic current gain per stage might be 024W" nepers. It may also be shown that for a given value of p the gain approaches nepers per stage more closely the higher the gain per stage is. This is desirable, in order to obtain the greatest overall gain for a given overall voltage.

By way of example, the operating voltage difference at each stage may be of the order of to 200 volts in different cases, and this voltage might be reduced by an amount of up to tenor twentyvvolts or more if it was desired to reduce the gain. The voltage difference per stage may in some cases be different for particular stages and, in particular, for the stage comprising the cathode and the first intermediate electrode, for the stage comprisingthe collector or final anode andthe last intermediate electrode, and for stages adjacentzto these. of the order of one-tenth to one-half millimetre of mercury, for an electrode spacing (S) of the order of 2 cm. per stage. The optimum gas pressure in a design: will normally vary approximately inversely as the value of the spacing S chosen, while theoptimum voltageper stage will remain approximately constant, for a given gas and electrode material.

As a result of the invention an electron multiplication is obtained in a stage by stage system having a. high stable electron multiplication factor, and this is suitable for current amplification purposes. The device may be described as a. gas multiplier. The invention thuspenmits electron multiplication by the Townsend alpha process, while restricting the accompanying processes which cause positive feedback by releasing further elec trons in the input region. As is well known, the latter processes, particularly photoelectric effects and positive ion bombardment of the cathode, haveset an upper limit of about 1O -to the amplification in gases which has-been practicable in single stage devices such as-proportionat counters and gas-filled photoelectric cells, owning-to the imminence of breakdown.

The gas multiplier provided under the present invention may thus be freefrom this limitation by operation under conditions that are stable for any number of stages.

The current of free electrons to'be amplified enters the space between the cathods and the first perforatedelectrode, and/or other early stages of the gas-multiplier These electrons may be emitted from the cathode, and/or released by ionization of the gas, and/or 1 may be=i.n

sorted into the appropriate region of the gas-multiplier fore gives a uniform field throughout the working" volume of the multiplier. Theonly communication be tween one stage and the next is through holes in a centrah circular area in the intervening electrode, and these holes are small, closely spaced, and uniformly distributed, and" occupy afraction B of the total working area, that-is; of the total perforated area 1 of each electrode. The thickness of the electrodes is made small compared with the diameter of the holes or with the mean free path-of the electrons in the gas,.whichever is the greater. This is toavoid excessive losses of the electrons at theelec- By way of example, the gas pressure may betrodes due to weakening and distortion of the field within the holes.

As an approximation, each electrode may betaken to intercept all but a constant fraction (for each type of particle under given conditions) of all particles or photons falling upon it. Because of the low gas pressure and the fine hole patterns used, the scattering action of the gas molecules normally spreads the electrons or ions from a particular hole over at least several hole-spacings in crossing a stage. Hence the approximately constant fraction of any such particles is intercepted. A corresponcling optical effect arises as a result of diffraction with fine hole patterns. Moreover, a random method of orienting the hole patterns in successive electrodes may be used to avoid any systematic alignment of the holes through the multiplier. A reasonably low coefficient of light reflection at the electrode surfaces is desirable, so that the optical transmission through the multiplier is not unduly increased by the effect of multiple reflection within each stage. The light mainly concerned is in the visible and ultraviolet range. The constant fractions transmitted by each electrode, as aforesaid, are normally all of the order of B, for electrons, ions, and photons.

Each stage firstly should have a suitable margin of stability by itself, that is, with no voltage on the others, and this requires e" l, where a and 'y are the usual Townsend coefficients, and S is the spacing between electrodes.

Moreover, secondly, the flux of positive ions and of photons reaching a stage from a later stage should decrease markedly as the number of stages intervening in creases, so that the total feedback does not greatly exceed that in the stage itself. Assuming that there is no appreciable ionization in the gas by the positive ions, which is known to be the case at the field strengths and gas pressures used in the gas multiplier, the ratio of total flux to internal flux can be shown not to exceed where G is the gain factor, per stage. It is therefore desirable that GB is not greater than say one half. Provided that both the above conditions are satisfied, the feedback present has only a small effect on the gain, and neglecting edge effects G'= B e where 13 is the fraction of incident electrons which is transmitted by each electrode. Thus, for example, if the design gain is four per stage, B may suitably be one-eighth, in which case the holes will occupy approximately one-eighth of the electrode working area. The gross approximately e" is then 32. In this case log, 4 log, 32

as compared with the maximum possible value of 0.5pV aforesaid.

The table below shows a set of operating conditions used and the results observed. These results are not designed to provide accurate measurements under closely controlled conditions. The rise and decay time is considered to be mainly due to the collection of the positive ions in the last stage. No appreciable delay or distortion other than this was observed as the number of stages was increased, provided that the output current did not exceed the value stated, which is of the order at which space-charge effects are to be expected with an input pulse longer than the collection time. Larger currents show a greater gain than normal, and may initiate breakdown if excessive, with this particular type of electrode.

gain corresponding Diameter of perforated areas 3.81 cm. Electrode spacing (S) 1.92 cm. Hole diameter 0.079 cm. Hole spacing (square pattern) 0.198 cm. Electrode thickness (brass) 0.013 cm. Area ratio (B) 0.125. Gas pressure (room air) 0.25 mm. Hg. Breakdown voltage, single stage 500 v. Breakdown voltage per stage (8 stages,

no input) 250 v. Operating voltage per stage 160 v. Input pulse length 40/. S. Pulse repetition frequency 50 8- Gain per Stage (G) 3.2. Rise and decay time less than 10g S.

Maximum undistorted peak output c. 20, A.

By the way of example, and referring to the aforesaid table, the gain is reduced smoothly as the voltage per stage is reduced from 160 volts, and at say volts might be too small to be of use. The gain per stage G- becomes unity when the voltage per stage is so reduced that the electron multiplication is just sufiicient to offset the loss of electrons due to interception by the electrodes, and below this voltage the device attenuates the input current. A useful amplifier of this type should preferably be capable of giving a gain of at least two per stage at a suitable operating voltage. In the table, with the gain at 3.2 per stage, a multiplication to ten would occur in two stages or to one million in twelve stages, approximately.

Increase in the voltage difference per stage causes a greater gain per stage, but it is undesirable that the voltage should be so increased that the condition for breakdown is approached. As aforesaid, and referring to the above example, the gain G should not exceed avalue at which the product BG equals, say, one half. In the example in the table, B equals one-eighth (0.125) and the designed maximum gain is four. A gain of four per stage, or more, may be obtained without approaching the breakdown condition. In general, the condition that the product BG should be less than one-half will be fulfilled provided that B is made equal to 20... where G is the maximum gain per stage at which the multiplier is designed to be operated. This condition can therefore be met by a suitable choice of B .for any specified value of G and ensures that the feedback action is not excessively increased by mutual action between all the stages.

Since, however, the internal feedback action may lead to breakdown or the imminence of breakdown when an excessive voltage is applied to a single stage, the value of G specified should not be so high that this breakdown condition is approached. Referring to the example of the type of multiplier described with reference to the aforesaid table, it can be shown that when the overall effect is considered with both conditions duly taken into account, the gain per stage G cannot exceed should be such as to give a low value of the coeflicient gamma (7). Values of gamma which permit a satisfactory gain per stage can in fact be obtained with suitable choices of gases and electrode materials.

The rise and decay time mentioned in the aforesaid table may be shortened, if desired, to permit the amplification of shorter pulses of current or of currents fluc- 7 mating ata higherfrequencies; by "placing" an a open mesh conducting; grid betweenthe last-reticular electrode and? the finaianode or-collector plate, and closeto; the latter, this grid being held at a fixed potential, normally'intermediate" between that of the last'reticular electrode and that of the-final anode. This gridshields the final anode from-thepositive ions'inthelast stage, for the wholeor a part of the time-that they take to cross the stage, depending on the positionwithin'the stage where the ion concerned is formed, andthereby shortens the time that they contributeto the outputcurrent. However, most of the electrons formed inthestagestillreachthe anode, and they cause anoutputcurrent as they pass from the grid to, the anode; Themovementof the electrons through themultiplier is muchfaster-than the movement of the positiveions; and if-means'are used to utilise the output current duesto the'passage of theelectrons through the last stage, as by the-use of a grid, as aforesaid, and/or hy;the;.use of1aishort-time-constant in the anode circuit, suchiasqisobtained-z with a small series condenser connecting-1oamesistiveaload, very short pulses may be amplified that is, pulses'which have a duration of a small fraction of a microsecond.

Wherrthere isa ,sufiiciently large current flowing in a stageeof the 1' gas-multiplier, the conditions of operation become. modified bythe presenceof space charge, particularly-positive ion space charge, since the positive ions takemuch longer thanzthe-electrons to move away, when they are-v formed by ionization. This space-charge modifies the gain of the gas-multiplier in a number of ways, some ,of which may increase the gain and others decrease it.v Theyeffects, which increase the gain are particularly undesirable, since theyvmay alter the operating conditions fromthosea applied, as satisfactory for low currents in sucha-wayas to-cause. breakdown. There are two main eiiectsiof this ,sort. The first etfectis that a column of positiveiionsiendstto form on the cathode side of each hole-'inftheelectrodes; thereby focussing the electrons on-ihesholes and:increasing thegain. This effect may be QYercomeby, using a very-fine hole pattern in which the spacing bietweenthe holes is comparable with or smaller than the mean free'path of the electrons in the gas. For example, a hole spacing of 0.0062 cm. has been used for this purpose. The second etfect is due to the fact that under low current conditions some of the electrons are lost-byltheir. beingscattered'outside the edge of the perforated region. The space-charge draws these in to the perforated region, when it forms, and again causes an increase: in the gain. This elfect may be reduced by methods which reduce the edge loss at low-currents, for example by using a low ratio of the spacing (S) to the diameter oftheperforated area, and/or by applying a magnetiofieldin-adirection'parallel to the axis of the,-

multiplier-(that is, parallel -to'the electric field).

Tires-envelope ofthe multiplier may, for example, comprisea tube of glass, ceramic ware or other insulating material, with electrodes, including cathode and anode, spaced inside therein and having internal resistor tappings tmgivethe voltage-differences at successive stages, for example from cathode to anode. the negative potential at each electrode may be reduced successively by 100 volts, so that there is a potential difference of 100 volts across each stage.

Moreover a window may be provided adjacent the cathode to allow entryof incident light to impact on the cathode Thereticular electrodes may be formed, for example, as'wire mesh, by punching holes, by a photoengraving process; or the electrodes may be apertured in any appropriate manner.

In a modification the reticular electrodes and the re sistors connecting them may alternatively bereplaced by a sequence ofcloselyzspaced. fine, open-meshed wire grids,

mountedswith resistive;,,spacers. at .their, edgesto givea amass uniform incrcasein potential along'the multiplier-where tionsof applying an appropriate electric-field, permitting the passage-of particles through the gas, and causing: an appropriate degree of attenuation by interception of, some of the particles.

Embodiments ofthe invention will now be described simply'by way of example, with reference to the accompanying drawings in which:

Fig. l is a diagrammatic view of the main part of an. electron multiplier in longitudinal mid-section;

Fig. 2 is a transverse section on the line IIII of Fig. 1;.

Fig. 3 is a view similar to Fig. 1, but of a modification and showing the enclosing envelope;

Fig. 4 is a circuit diagram for the gas multiplier of Figs. 1 and 2.

In the respective figures the same references indicate. like parts.

Referring to Fig. 1, the cathode 10 forms one end of the assembly and has a thin window 11A to permit high energy entering particles to impact on the cathode. There then follows a succession of reticular electrodes in the form of grids 11 mounted on conductor rings 12, for ex-. ample of brass, and inter-spaced by insulating rings 13 which may be for example ceramic. The anode or collector plate is shown at 14, while in the mid-part of the assembly the part shown in chain-dotted lines could be.

utilised'for further similar electrodes forming further. intermediate stages as required. The periphery of the conducting rings 12 will be tapped or otherwise prepared for electrical connections. S indicates the distance between'successive electrodes.

An inlet tube is shown at 15 for supply of the gaswhich fills the assembly. There may be an outlet to permit the gas to circulate, or the inlet may be sealed after filling.

A cylindrically wound coil 25 may surround the assembly in order to permit the application of a magnetic fieldparallel to the axis of the multiplier. This reduces electron scattering as well as the efiect the space charge has onthe passage of electrons;

Referring to Fig. 2, an example of a pattern of holes in the electrodes 11 is shown and this pattern may extend to the periphery of the electrodes or it may be limited to a predetermined circular margin. The actual pattern in practice should preferably be to a much finer scale than that shown, in the drawing.

i In Fig. 3 the whole is shown enclosed in a vacuumtype envelope or tube 16- of electrically insulating material, the closed ends of which are not shown. The

cathode 10 has a negative voltage connection at 17 and alternate conductor rings 12 and resistive spacing rings 13 are shown. The rings 12 mount the electrodes, while the rings 13 provide uniform potential drops between theelectrodes. The rings 13 may be semi-conducting material. The anode 14 has a terimnal 18 for the out-.

put current, and a lead out from the adjacent grid is shown. at 19 for conducting to .earth through a resistor.

The whole electrode and spacerstructure is preferably finer and more closely spaced than it is possible. to show in the drawing. As before the mid-part indi cated in chain-dotted lines is provided for further stages; as required, and the whole assembly is gas filled.

In, Fig. 4 the envelope 16, the cathode and anode 10,,

14,, and, the electrodesare'shown as before with a space:

at the middle indicated in chain-dotted lines for further stages. An openmesh conducting grid 20 is shownplaced between the last reticular electrode and the anode 1.4. Asswas mentioned previously this allows pulsesoft' supply to give the desiredoverall voltage. The series resistors r which are not connected between electrodes are provided as a safety measure to limit the current if the multiplier should break down but they may not be necessary. The bleeder resistors r at the stages are designed according to normal practice for the voltages to be applied at the stages. Condensers C are provided across the stages at the anode end for the purpose of supplying short pulses of current when necessary, and to avoid or reduce fluctuations of potential across these stages with the signal. The output current is taken from the anode and is shown fed into a load resistance R to generate output voltage.

A condenser may be inserted between anode 14 and load resistance R to provide a short time-constant in the anode circuit, for the application of very short pulses. If the value of this condenser is C; the product C R is the time in seconds, normally a small fraction of a second, with appropriate values. With choice of an appropriate value of QR the slowly varying output current due to the positive ion movements may be reduced without greatly reducing the rapidly varying output current due to the electron movements, for a short input pulse.

It will be evident from the foregoing that the potential differences between successive electrodes are applied in the same sense, that is to say the successive electrodes are energised with voltages which follow the gradient between cathode and anode, so that the electron stream is between cathode and anode through the electrodes in succession.

I claim:

1. An electron multiplier comprising a cathode and an anode and a number of intermediate electrodes, each defining a plurality of small holes, said intermediate electrodes being spaced apart in succession, a gas tight envelope for the parts and a gas for example air, inside said envelope, so that an electron stream passing through the said intermediate electrodes successively in the direction from cathode to anode under the influence of potential differences between the successive electrodes in the same sense is urged through the multiplier in the appropriate direction, so that the Townsend electron avalanche process occurs in the spaces between the successive electrodes, without breakdown occurring therein.

2. An electron multiplier according to claim 1, in which said gas has a low Towsend gamma coefiicient.

3. An electron multiplier according to claim 2, in which said gas has a high value of the ratio of the Townsend alpha coeffieient to the electric field strength.

4. An electron multiplier including a cathode, an anode, a number of intermediate electrodes, each defining a plurality of small holes, said intermediate electrodes being spaced apart in succession, a gas-tight envelope, a gas, for example air, at a low sub-atmospheric pressure in said envelope, and means for applying potential difierences in the same sense between successive electrodes of an order high enough to obtain the Townsend electron avalanche process substantially but not high enough to involve breakdown.

5. An electron multiplier according to claim 4, in which the potential difierence between successive electrodes is of the order of to 200 volts.

6. An electron multiplier according to claim 5, in which the gas pressure is of the order of one-tenth to one-half millimetre of mercury.

7. An electron multiplier according to claim 5, in which the intermediate electrodes are of a thickness small compared with the width of the holes therein or with the mean free path of the electrons in the gas, whichever is the greater.

8. An electron multiplier according to claim 7, in which the patterns of holes in adjacent intermediate electrodes are differently disposed relative to each other to avoid systematic alignment of their holes.

9. An electron multiplier according to claim 4, including an open mesh conducting grid close to the anode or collector and between it and the adjacent intermediate electrode, and means for maintaining said conducting grid at a fixed potential, in order to facilitate amplification of short pulses.

10. An electron multiplier according to claim 4, in which the spacing between holes is comparable with or smaller than the mean free path of the electrons in the gas.

11. An electron multiplier according to claim 4, in which the electrodes have their holes disposed in a circular area, and the ratio of the spacing between these electrodes to the diameter of the circular area is small.

12. An electron multiplier according to claim 4, including means for superimposing a magnetic field in a direction parallel to the electric field of the multiplier.

13. A method of electron multiplication in which an output current, variable at all times with a variable input current is obtained, comprising causing an electron stream to pass in succession through a series of electrodes in a gas, using the Townsend electron avalanche process in the gas at the stages between the electrodes, and attenuating at the interposed electrode or electrodes the effects which may cause feedback between any two stages.

References Cited in the file of this patent UNITED STATES PATENTS 2,375,830 Spencer May 15, 1945 2,504,231 Smith Apr. 18, 1950 2,550,089 Schlesman Apr. 24, 1951 OTHER REFERENCES

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