US3374380A - Apparatus for suppression of ion feedback in electron multipliers - Google Patents

Description

March 19, 1968 G. w. GOODRICH 3,374,380

APPARATUS FOR SUPPRESSION OF ION FEEDBACK IN ELECTRON MULTIPLIERS Filed Nov. 10, 1965 I 7 z 42 7 Kg n Va li C VARIABLE VOLTAGE "\"15 SOURCE 54 5s INVENTOR.

GEORGE w. GOODRICH ATTORNEY United States Patent Office 3,374,380 Patented Mar. 19, 1968 ABSTRACT OF THE DISCLOSURE A pair of channel electron multipliers operating in series and disposed at an angle to minimize ion feedback.

This invention pertains to apparatus for suppression of feedback in electron multipliers and particularly those multipliers having a relatively unobstructed linear multiplying path.

In the prior art electron multipliers wherein a multiplying path is defined by a linear tube with secondary emissive resistive coating applied to the interior of the tube and a potential gradient applied across the ends of the tube, a problem has been experienced which is caused by ion or other type feedback in the tube. Such a multiplier is frequently used with a photocathode or other source of electrons which are to be multiplied. Positive ions are produced by the electrons striking residual gas molecules near the output end of the tube where the electron concentration is particularly high and these positive ions are accelerated toward the photocathode and either strike the photocathode or a portion of the tube near the photocathode producing spurious electrons which are multiplied along substantially the entire length of the multiplier. The presence of these spurious electrons can result in limited gain, added noise, or reduced sensitivity of the photocathode surface.

This invention overcomes these problems by providing apparatus to greatly reduce the effects of the ion feedback and the spurious electrons. This reduction is accomplished by forming the multiplier tube into at least two sections which may be called an input section and an output section which have nonlinear longitudinal axes so that the ions formed in the output section which are accelerated in a reverse direction will impact into the walls of the input section in an area just preceding the output section. This greatly reduces the number of electrons caused by the positive ion since the nearer the impact is to the output end, the fewer electrons will be produced that have a spurious origin.

The multiple sections of the multiplier tube can have separate potential gradients placed thereacross and can have surfaces of different conductivities. By making the conductivity of the output section higher than that of the input section, the high power dissipation is limited to only that section where the amplified signal is also at a high level.

Also, a switch may be placed between the input section and output section to switch off the multiplier. Further, three or more sections may be used with the longitudinal axes of adjacent sections being non-linear.

These and other objects will become more apparent when the following preferred embodiments are considered in connection with the drawings in which:

FIGURE 1 is a partly broken away elevational View of an image intensifier embodying this invention which has a large number of multiplier tubes in each of two multiplier arrays;

FIGURE 2 is a section taken at 22 of FIGURE 1 showing the angle made by the longitudinal axes of the multiplier tubes in adjacent arrays;

FIGURE 3 is a greatly enlarged view showing an electron multiplier of this invention having an input multiplier section and an output multiplier section with a photocathode at the input and a phosphor screen at the output;

FIGURE 4 is a greatly enlarged View of a portion of an array of multiplier tubes showing the field existing in the tubes and at the tube ends; and

FIGURE 5 is a cross-sectional view of another embodiment of this invention showing a multiplier having three multiplier sections and being joined at the section interfaces.

Looking at FIGURES 1 and 2, an image intensifier may be seen having a photocathode 20, input multiplying array 22, output multiplying array 24, and phosphor screen 26. Potential sources 28, 30, 32 and 34 provide increasing potential between photocathode 2t array 22, array .24 and phosphor screen 26. Source 35 provides during operation of the intensifier a zero voltage drop or a voltage that makes array 24 slightly more positive than array 22, thereby providing for electron travel from array 22 to array 24. If it is desired to interrupt operation, a potential making the input of array 24 more negative than the output of array 22 by an amount such as volts is applied by source 35 thereby preventing an electron signal from passing from array 22 to array 24.

Photocathode 20 may be made of materials known to the art such as antimony, cesium, potassium or sodium and phorphor screen 26 may also be made of materials known to the art such as activated sulfide of Zinc and cadmium. In this preferred embodiment, arrays 22 and 24 have an extremely large number of channels 23 having a length to diameter ratio of about 25 to 1 and having a small diameter. These channels 23 have a secondary emissive resistive coating on the inside thereof.

Array 22 has conductive coatings 36 and 38 deposited or otherwise coated on respectively the input and output surfaces and array 24 has coatings 40, 42 deposited or otherwise coated on respectively its input and output surfaces. The conductive coating may be made of chromium, platinum, gold, or silver. The purpose of the conductive coatings is to place the same voltage drop across each tube or channel 23 in array 22 and to place the same voltage drop across each channel 25 in array 24.

The tubes 23, 25 in arrays 22 and 24 form an included angle of 166 in this embodiment, shown in FIGURE 3. which has been found suitable to reduce ion feedback when the tubes 23, 25 have an overall length to diameter ratio of 50 to 1. Arrays 22 and 24 may be fabricated by placing a large number of small diameter glass tubes in a jig and heating until the tubes become fused to one another but before they'become distorted to form a billet. Arrays having the desired channel inclination can be made by appropriate angular cutting from this billet. By utilizing sufficient compounds of lead such as lead oxide in the glass, the secondary emissive resistive surface may be formed by passing hydrogen gas for 816 hours through the tubes while they are heated to a temper-ature of 300-500 centigrade. Silicate glass having about 30 percent of PhD on a molecular basis will produce a secondary emissive resistive surface.

The advantages of incliningthe channels relative one another will now be considered in more detail. in multiplying tubes of the prior art, as-illustrated in the patent to Goodrich and Wiley, Patent 3,128,408 issued Apr. 7, 1964, and entitled, Electron Multiplier, the input is disposed to receive incoming particles which cause secondary emission of electrons after striking the surface of the tube. These electrons are further caused to strike the walls of the tube resulting in additional secondary emission so that a forward progressing, ever-increasing avalanche of secondary electrons is produced. These secondary electrons become extremely numerous near the output end of the multiplier tube and the probability of an electron interacting with a residual gas molecule in the tube becomes relatively high, even if the residual gas pressure is very low.

The interaction of an electron with a residual gas molecule results in dislodging an electron from the molecule giving the molecule a positive charge so that it becomes a positive ion, indicated at 7 in FIGURE 3. The ion being opposite in sign to the electrons is caused to travel along the tube in a direction opposite to the electron travel direction and is likely to be accelerated in a straight path for nearly the entire length of the channel and impact against either the photocathode or a portion of the tube near the photocathode. The reason the ion is likely to travel in a direction which is nearly parallel to the axis of the tube is that its initial velocity transverse to the axis of the tube is very small and will be only on the order of thermal velocity and therefore about of an electron volt at room temperature. This energy is substantially less than the average energy of emission of an electron. The velocity energy imparted to the ion by the electric field within the multiplier is considerably higher and therefore the longitudinal distance traveled by the ion down the tube will be much greater than the transverse distance traveled. The ion path is shown at 72 in FIGURE 3 and due to the included angle between multiplier sections 23, 25, the ion strikes the section 23 at 74, near the output end of the tube, and hence, the amount of electrons produced by ion 70 will be much less than if ion 70 impacted against photocathode 20 or near the input of tube 23.

Arrays 22 and 24 preferably are separated from each other by a gap which is in the order of the channel tube diameter, in order to minimize Moire pattern beating effects. This spacing may be achieved by placing spacers between the arrays and then clamping, soldering or brazing the arrays together. Insulative spacers are shown in the embodimentof FIGURES l and 2 while a conductive spacer is shown for the embodiment of FIGURE 3.

It is not necessary that the channels 23, 25 at the array interface be perfectly aligned. Good resolution is obtainable if the channel diameter is kept reasonably small. The spread of electron signal from the output of the channels in the array 22 is proportional to the diameter of the channels in array 22 and by minimizing the channel diameter, the spread of the electron signal is minimized.

The included angle of the arrays may be increased, the limitation being to prevent ions formed in the output array 24 from being able to pass in a straight line to the photocathode 20 or to an area of the channels in input array 36 which is near photocathode 20. Preferably, the included angle should be small enough to prevent an ion from taking a linear path of about more than 1.5 channel section lengths before impacting on a channel wall. As explained, by limiting the ion impact to the output portion of array 22, the effect of the feedback ions is minimized. Also, it is desirable not to make the angle too small since the individual multiplier channel openings become highly oval and this imparts poor resolution to the tubes.

The electric field existing in an array is shown in FIG- URE 4 where field lines 44 are shown parallel to the walls of the channel in the array but bend upon leaving the array channels to assume a direction that is substantially perpendicular to the unipotential surfaces 36 and 38 in the case of array 22.

The number of array sections included in a multiplier assembly will generally increase as the required gain of the multiplier assembly increases. The two section multiplier shown in FIGURES 1 and 2 has provided a gain 10 to with voltages of 800 volts being placed across each of arrays 22, 24 respectively. Higher gains may be realized by using the three section multiplier of FIGURE 5 which has photocathode 50, phosphor screen 52, potential 4 sources 54, 56, 58 and multiplier 60, which has sections 62, 64 and 66. Sections 62, 64 and 66 may be constructed in the manner of sections 22 and 24 with the channel axes in adjacent sections being non-linear so that the multiplying path changes direction several times between photocathode 50 and phosphor screen 52. Also the device of FIGURE 5 has only one potential source 56 across all three multiplier sections 62, 64 and 66. The arrays 62, 64 and 66 are preferably spaced by conductive material which has a dimension in the order of a channel diameter and since the spacing material is conductive, one battery may be used for all three sections.

Although this invention has been disclosed and illus- -trated with reference to particular applications, the principles involved are susceptible of numerous other applications which will be apparent to persons skilled in the art. For example, light feedback is likewise prevented by this invention. The invention is, therefore, to be limited only as indicated by the scope of the appended claims. Having thus described my invention, I claim: 1. Apparatus comprising a plurality of electron multipliers each having an input end for receiving particles to be multiplied and having an output end for discharging electrons which are in a multiplied ratio of the received particles and having a longitudinal axis between said input and output ends, each of said electron multipliers having a substantially continuous resistive surface from its input end to its output end, against which said received particles impact to produce secondary emission of electrons and against which said secondary emission electrons impact to produce further secondary emission electrons thereby resulting in an avalanche of electrons, at least a first and a second of said multipliers being placed in series so that the output end of said first multiplier supplies the electron input of said second multiplier, particle emitting means at the input end of said first multiplier, voltage means connected across said multipliers to establish a longitudinal electrical current flow in said resistive surface and to establish a substantially longitudinal field with substantially no transverse field component for accelerating the electrons of secondary emission from the input of said first multiplier towards the output of said second multiplier, with the concentration of the avalanching electrons in said two multipliers being highest at the output end of said second multiplier, and with the probability of said accelerated electrons ionizing any residual gas molecules in the multipliers being higher at the output end of said second multiplier, and means for minimizing feedback output caused by ionization of the gas molecules in said second multiplier comprising means for supporting said first and second multipliers with the longitudinal axes thereof disposed to intersect at an angle for preventing substantially all of the ionized gas molecules in said second multiplier from passing completely through said first multiplier and for causing those ionized gas molecules to strike the surface of said first multiplier towards the output end of said first multiplier. 2. The apparatus of claim 1 with said first and second multipliers comprising individual tubes having longitudinal dimensions substantially larger than transverse dimensions, the output end of said first multiplier being spaced from the input end of said second multiplier by a distance substantially equal to the transverse dimension to minimize Moire beating effect. 3. The apparatus of claim 1 with said first multiplier comprising a multitude of individual multiplying channels having their longitudinal axes parallel and joined together to form a first multiplier array,

said second multiplier comprising a multitude of individual multiplying channels having their longitudinal axes parallel and joined together to form a second multiplier array.

4. The apparatus of claim 3 with photocathode means being placed adjacent the input end of said first multiplier array,

phosphor means being placed adjacent the output end of said second multiplier array.

5. The apparatus of claim 3 with a first set of voltage leads from said voltage means being placed across said first array and a second set of voltage leads from said voltage means being placed across said second array.

6. The apparatus of claim 1 with means being connected to said first and second multipliers for switching on and oil the electron stream.

7. The apparatus of claim 1 with the included angle between said longitudinal axes being approximately 166 and the overall length to diameter ratio of said multipliers being on the order of 50:1.

8. The apparatus of claim 1 with at least three multipliers in end to end relation with the longitudinal axes of adjacent multipliers being nonlinear so that gas molecules ionized in any of at least certain of said multipliers can not travel in a reverse direction more than one and one half multiplier lengths before impacting against a multiplier wall.

9'. The apparatus of claim 1 with at least three multipliers in end to end relation with the longitudinal axes of adjacent multipliers being nonlinear to minimize effects of ion feedbacks.

10. Apparatus comprising a plurality of electron multipliers each having an input end for receiving particles to be multiplied and having an output end for discharging electrons which are in a multiplied ratio of the received particles and having a longitudinal axis between said input and output ends,

each of said electron multipliers having a continuous substantially resistive surface from its input end to its output end, against which said received particles impact to produce secondary emission of electrons and against which said secondary emission electrons impact to produce further secondary emission electrons thereby resulting in an avalanche of electrons,

at least a first and a second of said multipliers being placed in series so that the output end of said first multiplier supplies the electron input of said second multiplier,

particle emitting means at the input end of said first multiplier,

voltage means connected across said multipliers to establish a longitudinal electrical current flow in said resistive surface and to establish a substantially longitudinal field with substantially no transverse field component for accelerating the electrons of secondary emission from the input of said first multiplier towards the output of said second multiplier, with the concentration of the avalanching electrons in said two multipliers being highest at the output end of said second multiplier, and with the probability of said accelerated electrons ionizing any residual gas molecules in the multipliers being higher at the output end of said second multiplier,

and means for minimizing feedback output caused by ionization of the gas molecules in said second multiplier comprising means for supporting said first and second multipliers with the longitudinal axes thereof disposed to intersect at an angle for preventing substantially all of the ionized gas molecules in said second multiplier from passing completely through said first multiplier and for causing those ionizing gas molecules to strike the surface of said first multiplier towards the output end of said first multiplier,

said first multiplier comprising a multitude of individual multiplying channels having their longitudinal axes parallel and joined together to form a first multiplier array.

said second multiplier comprising a multitude of individual multiplying channels having their longitudinal axes parallel and joined together to form a second multiplier array, and

a first set of voltage leads from said voltage means being placed across said first array and a second set of voltage leads from said voltage means being placed across said second array,

the conductivity of the resistive surfaces of said second array being greater than the conductivity of the resistive surfaces of said first array.

References Cited UNITED STATES PATENTS 2,872,721 2/1959 McGee 3l3-105 X 3,128,408 4/ 1964 Goodrich et al. 3l3103 X 3,197,663 7/1965 Norman et al. 313103 3,240,931 3/1966 Wiley et a1. 313-103 X JAMES W. LAWRENCE, Primary Examiner.

ROBERT SEGAL, Examiner. P. C. DEMEO, Assistant Examiner.

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