GB2086648A - Photomultiplier tube - Google Patents

Abstract

In a photomultiplier tube having a circular-cage type dynode arrangement, a first-stage curved dynode 31 comprises a first curved portion 32 closer to a photocathode 3 and a second curved portion 33 having a smaller radius of curvature than the first arc and smoothly joined to the first arc. With this arrangement secondary electrons from the first stage dynode can be directed onto the second stage dynode 12 and are less likely to be diverted to the fourth stage dynode 14 so that the anode current is more uniform for a given intensity of incident light over the range of positions at which light can impinge onto the photocathode. <IMAGE>

Description

SPECIFICATION Photomultiplier tube This invention relates to a photomultiplier tube, and more particularly to a photomultiplier tube having a circular-cage type dynode arrangement.

A photomultiplier tube comprises a photocathode which emits electrons when exposed to light, multistage dynodes each providing amplified secondary emission of electrons upon receiving electrons from a prior stage, and lastly an anode collecting the multiplied electrons, arranged in that order within an evacuated glass envelope. When a faint incident light impinges on the photocathode, with the individual electrodes positively charged, an amplified current is drawn from the anode whose magnitude varies depending upon the intensity of the incident light.

In a photomultiplier tube with a circularcage type dynode arrangement, the photocathode comprises a slightly curved plate extending along the tube axis. The dynodes can be classified into two types; curved dynodes whose concave surfaces face the tube axis and flat dynodes which are flat plates. The first-stage dynode is of the curved type, facing the photocathode, and the second-stage dynodes onward comprise flat and curved dynodes distributed in a generally circular arrangement about the tube axis and arranged alternately so that the flat dynodes are located on the inside of the curved cynodes. The laststage dynode is again of the curved type, and is positioned to enclose an anode comprising mesh or wire directed parallel to the tube axis.

Electrons from the photocathode impinge on the first-stage curved dynode, then on the subsequent, alternately arranged flat and curved dynodes one after another, increasing their number as they more forward, and ultimately reach the anode.

The photomultiplier tube is generally required to have a uniform photoelectric conversion efficiency. That is to say, in whichever part of the photocathode the incident light may hit, the number of electrons collected by the anode or the strength of current obtained from the output end of the anode must be constant. Consider the photo-elecric conversion in two directions of the photocathode; one being parallel and the other perpendicular to the tube axis. The geometric arrangement of the elctrodes is regular in the cross-sectional plane parallel to the tube axis, so that the desired uniformity of photo-electric conversion can easily be obtained if the photoelectric emission efficiency of the photoelectric surface on the photocathode and that of the secondary electron emission efficiency of secondary electron surface on the dynode are uniform.However, the geometric arrangement of the electrodes is not regular in the crosssectional plane perpendicular to the tube axis and a complex potential gradient results.

Therefore, uniformity of photoelectric emission efficiency and secondary emission efficiency is not in itself enough to produce the desired uniform photoelectric conversion efficiency.

An object of this invention is to provide a photomultiplier tube having a circular-cage type dynode arrangement which is capable of producing a more uniform anode current for a given intensity of incident light over the range of positions at which the light can impinge on the photocathode.

In a photomultiplier tube with a circularcage type dynode arrangement according to this invention, the first stage dynode has a larger radius of curvature on the side which faces the photocathode or receives primary electrons and a smaller radius of curvature on the side which faces the second-stage dynode or lies closer to the third-stage dynode, and extends along the tube axis maintaining the same cross-sectional shape.

It is particularly preferable to make the side facing the photocathode of a curved plate obtained by dividing a cylinder which two planes passing through the axis thereof and having a central angle of 55 to 65 degrees and the side closer to the third-stage dynode of a curved plate obtained by dividing a cylinder, whose radius being 0.4 to 0.6 times larger than that of the aforesaid one, with two planes passing through the axis thereof and having a central angle of 85 to 95 degrees, with one end of the one divided curved plate inscribed with that of the other.

With the first-stage dynode thus formed, all of the secondary electrons emitted from that part thereof which has the smaller radius of curvature are collected by the second-stage dynode, and none of the electrons spills over the edge of the second-stage dynode and flies direct to the fourth-stage dynode. Thus, anode current according to the intensity of incident light is obtained irrespective of the position at which the light strikes the photocathode.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a front view, with a part thereof cut away, showing a common photomultiplier tube having a circular-cage type dynode arrangement; Figure 2 is a cross-sectional view taken along the line Il-Il of Fig. 1; Figure 3 is an enlarged cross-sectional view showing a conventional first-stage dynode and other components in the vicinity thereof; Figures 4 and 5 are graphs showing the relationship between photoelectric conversion efficiency and the position at which light impinges on the photocathode (Fig. 4 is for the conventional photomultiplier tube, and Fig. 5 is for the ideal case); Figure 6 is an enlarged cross-sectional view showing a first-stage dynode and other components in the vicinity thereof according to this invention;; Figure 7 is a graph representing the relationship between photoelectric conversion efficiency and the position at which light impinges on the photocathode for the first-stage dynode of Fig. 6; and Figure 8 is an enlarged cross-sectional view of another embodiment of first-stage dynode according to this invention.

To begin with, we will describe the structure of a typical photo-multiplier tube having a circular-cage type dynode arrangement. Fig. 1 is a front view showing the internal structure of such a photomultiplier tube, with part of an evacuated glass envelope facing the photocathode cut open. Fig. 2 is a cross-sectional view taken along the line ll-ll in Fig. 1. A mesh-type electrode 2 has a rectilinear crosssection, with both ends thereof constituting two vertexes of a regular triangle, the third vertex being at point 0 (Fig. 2) where the horizontal cross-sectional plane and axis of the tube intersect.A photocathode 3 is electrically connected to the mesh-type electrode 2, and extends, when viewed in cross-section, from one end of the mesh-type electrode 2 toward the axis of the glass tube, bending in the vicinity of its mid-point approximately 1 5 degrees toward the mesh-type electrode 2 and extending substantially the length thereof.

A first-stage dynode 11 is made of a curved plate, whose cross-sectional shape will be set out below.

A A second-stage dynode 1 2 is a fiat dynode having a rectilinear cross-section. It is one-half the length of the mesh-type electrode 2, and is directed at an angle of 80 degrees with respect to said mesh-type electrode 2, and positioned where the second-stage dynode 1 2 itself is bisected by a perpendicular drawn down from that end of the first-stage dynode 11 which is closer to a third-stage dynode 1 3.

A third-stage dynode 13, a fifth-stage dynode 1 5 and a seventh-stage dynode 1 7 are identical in shape with the first-stage dynode 11, and turned around the tube axis by 60, 1 20 and 1 80 degrees, respectively, with respect to the first-stage dynode 11.

A fourth-stage dynode 14, a sixth-stage dynode 1 6 and an eight-stage dynode 1 8 are identical in shape with the second-stage dy node 12, and turned around the tube axis by 60, 1 20 and 1 80 degrees, respectively, with respect to the second-stage dynode 1 2.

Two adjacent curved dynodes (for example, dynodes 11 and 13) are so positioned that the adjacent end 13' of the subsequent-stage dynode 1 3 lies on the inside of the adjacent end 11' of the prior-stage dynode 11, or closer to the center of the tube. Two adjacent flat dynodes are so positioned that the adjacent end of the prior-stage dynode lies on the inside of the adjacent end of the subsequentstage dynode. The adjacent two ends (for example 11' and 13') face a flat dynode 1 2 that forms an intermediate stage between the adjacent curved dynodes 11 and 1 3.

A last-stage dynode 1 9 has an ellipse-like cross-section having an opening facing the eighth-stage dynode 18.

An anode 20 is of the mesh type to permit efficient transmission of electrons entering the last-stage dynode 1 9 in which the anode 20 is enclosed. Where simple structure is preferred, one or two wires may be used as the anode 20.

With the anode 20 grounded through a resistor, the photocathode 3, dynodes 11 through 1 9 are positively applied with increasing voltages at regular intervals, such as--800V,,-720V,... -160V and -80V.

When, under such condition, a faint light impinges on the photocathode 3, as indicated by the arrow A in Fig. 2, electrons are emitted and hit the first-stage dynode 11 with a kinetic energy of 80 eV, following the gradient of a potential space formed by the mesh electrode 2, photocathode 3, dynodes 11 and 1 2. The dynode 11 emits approximately five times as many electrons as received, which strike the second-stage dynode 1 2 with a kinetic energy of 80 eV, following the gradient of a potential space formed by the dynodes 11 to 1 3. The second-stage dynode 1 2 alos emits approximately five times as many electrons as received.Similarly, the resultant electrons continue to strike the dynodes 1 3 through 1 9 one after another, as indicated by a broken line, emitting approximately five times as many electrons each time. Therefore, the anode 20 collects approximately 59 times or 2,000,000 times as many electrons as those initially emitted from the photocathode 3.

Fig. 3 is an enlarged cross-sectional view of the first-stage dynode and other componets in the vicinity thereof in a conventional photomultiplier tube.

When viewed in a horizontal cross-section, the first-stage dynode 21 is an arc having a chord of a length approximately five-sevenths that of the mesh-type electrode 2 and a central angle of 150 degrees. A line connecting one end 22 thereof with point 0 where the horizontal cross-sectional plane and axis of the tube intersect parallels the mesh-type electrode 2. The distance between the intersection point 0 and the other end 23 of the first-stage dynode 21 is equal to the length of said arc chord.

The dynodes 13, 1 5 and 1 7 constituting the other stages are identical with the firststage dynode 11 in shape.

When the first-stage dynode is shaped as shown in Fig. 3, not all of the electrons emitted from the photocathode 3 upon im pingement of light are multiplied and, ultimately, collected by the anode. Namely, electrons emitted from a certain position of the photocathode 3 are not collected by the anode, as shown in Fig. 4. In this diagram, the abscissa represents the position of the photocathode (which is indicated by the distance measured along the photocathode from the left end 25 thereof in Fig. 3), and the ordinate represents the photoelectric conversion effici ency (the proportion in which the photons received by the photocathode are collected as an anode current by the anode). In Fig. 4, most of the photons striking that part of the photocathode 3 which is closer to the meshtype electrode 2 are not collected by the anode.Fig. 5 is a graph similar to Fig. 4, except that Fig. 5 shows an ideal condition in which most of the incident photons are collected by the anode irrespective of the striking position on the photocathode.

Using the first-stage dynode, as described above, even if the photocathode 3 exhibits a uniform photoelectric emission efficiency, when viewed in terms of anode current, the photoelectric conversion efficiency is high only in a narrow area approximately to the right of the centre of the photocathode dropping sharply on the left.

To determine the cause of this phenomenon, the path of electrons was analyzed by the measurement of electric field by means of electrolytic tank and computer simulation. As a result, it has been pos.ulated that the electrons emitted from the left end 25 of the photocathode 3 strike a part 26 of the firststage 21 which is closer to the third-stage dynode 13, as indicated by a path a. Then, secondary electrons emitted from said part 26 are affected by the electric potentials of the third-stage dynode 1 3 and the fourth-stage dynode 1 4 with the result that they miss the second-stage dynode 1 2 but are attracted towards the fourth-stage dynode 14.

Fig. 6 is an enlarged cross-sectional view of the first-stage dynode and other components in the vicinity thereof according to this invention.

The first-stage dynode 31 has a curved cross-section which comprises an arc 32 having a central angle of 60 degrees and an arc 33 having a radius 0.4 times that of the arc 32 and a central angle of 90 degrees, the two arcs being disposed so that the arc 43 constitutes part of an inscribed circle within the circle of which are 42 forms part.

The normal from a part 35 of the first-stage dynode 31 which is closer to the third-stage dynode 1 3 points to an end 36 of the firststage dynode 1 3 which faces towards photocathode 3 or a gap between said end 36 and the second-stage dynode 1 2.

Curved dynodes 13, 1 5 and 1 7 following the first-stage dynode 31 are identical in shape with the curved dynode 21 shown in Fig. 3. Likewise, flat dynodes 12 and 14 and an anode are the same as those shown in Fig.

2. Therefore, such parts in Fig. 6 as are similar to those in Figs. 2 and 3 are designated by similar reference numerals.

The electron emitted from the left end 37 of the photocathode 3 strike a point 37' on the first-stage dynode 31 which is close to the third-stage dynode 1 3 and those from the middle 38 strike a point 38' in the middle, and those from the right end 39 strike a point 39' close to the photocathode 3, emitting secondary electrons. The secondary electrons thus emitted from the individual points of the first-stage dynode 31 are disjributed at an initial energy of not more than several eV's and in a direction of H (where 0 represents an angle of emission with respect to the normal and the emitting of probability in a direction 8 is expressed by F = cos 0), and consequently path of the electrons is spread as indicated by broken lines.As stated before, the arc between the points 37' and 38' points to the gap between one end of the arc 32 and one end of the second-stage dynode 12, the secondary electrons from said arc mainly strike left part of the second-stage dynode 1 2.

(Namely, the secondary electrons from said arc do not follow the path indicated by a in Fig. 3). Therefore, substantially all of the secondary electrons are received by the second-stage dynode 12.

Fig. 7 shows a photoelectric conversion efficiency of a photo-multiplier tube having the first-stage dynode shown in Fig. 6. As apparent, uniformity in the photoelectric conversion efficiency with the position on the photocathode where incident light strikes is greatly improved, as compared with the conventinal type in Fig. 4, approaching the ideal pattern in Fig. 5.

Fig. 8 is an enlarged cross-sectional view of another embodiment of the first-stage dynode according to this invention. Fig. 8 shows only the first-stage dynode, since the photocathode dynodes of other stages, and anode are similar to those described before.

This first-stage dynode comprises an arc 42 having a central angle a of 60 degrees and a radius R and an arc 43 having a central angle P of 90 degrees and a radius r which is 0.6 times as large as R, the arcs 43 being in inscribed relationship to the arc 42. In this embodiment, all the secondary electrons from the first-stage dynode are received by the second-stage dynode, though those emitted from a point thereon closest to the third-stage dynode run closer to the fourth-stage dynode than in the case of Fig. 6.

Accordingly, the photomultiplier tube having this type of electrode structure offers a similarly uniform photoelectric conversion efficiency to that of Fig. 7.

If the ratio of radius between the arc 32, having a central angle of 60 degrees, and the arc 33, having a central angle of 90 degrees, in the above-described two embodiments exceeds 0.6, a fraction of the electrons emitted from between the points 37' and 38' on the first-stage dynode 1 3 31 which extends toward the third-stage dynode 1 3 spills over the edge of the second-stage dynode 12, and is diverted in the direction of the fourth-stage dynode 14.

If, on the other hand, the ratio of radius between said two arcs 32 and 33 is smaller than 0.4, some of the electrons emitted from between the points 37' and 38' on the firststage dynode 31 which extends toward the point 39' fly back toward the photocathode 3 through the gap between the first-stage dynode 31 and the second-stage dynode 12 making no contribution to the emission of secondary electrons.

In cross-sectional view, the above-described curved dynodes comprise two arcs, one having a central angle of 60 degrees and the other 90 degrees, smoothly joined together.

In the vicinity of the joint, the extension of one arc substantially coincides with the other arc. Namely, the direction of the normal in the vicinity of the joint scarcely changes even if the arc 32 is expanded or contracted within the limit of 5 degrees and, at the same time, the arc 33 is contracted or expanded within the limit of 5 degrees in the embodiment in Fig. 6. This part corresponds substantially to the center of the dynode 31, and the eiec- trons emitted from the point 38' strike a relatively narrow area around the center of the second-stage dynode 1 2. Thus, the same effect can be obtained when the central angle of the arc 32 (60 degrees) varies within the range of 55 to 65 degrees and that of the arc 33 (90 degrees) varies within the range of 85 to 95 degrees.

For the same reason, the central angle of the arcs 42 and 43 in Fig. 8 can vary within the range of 85 to 95 degrees and 55 to 65 degrees, respectively.

As will be understood from the above, this invention improves the uniformity of photoelectric conversion efficiency at the anode by providing a first-stage dynode comprising a curved plate which, in turn, comprises a part facing a photocathode with a larger radius of curvature and a part facing a second-stage dynode with a smaller radius of curvature, in a photomultiplier tube having a circular-cage type dynode arrangement. More particularly, the first-stage dynode comprises, in crosssection, an arc having a centra! angle of 55 to 65 degrees and an arc having a central angle of 85 to 95 degrees, with the radius thereof ranging between 0.4 to 0 6 times as large as that of the narrow-angled arc which are inscribed to each other.

A portion of ellipse approximating the above described shape can produce the same effect, too. Therefore, this invention is not limited to the shape given by the abovedescribed geometric expression, but includes other similar shapes that may have different geometric expressions.

Claims (3)

1. In a photomultiplier tube having a photocathode directed parallel to the tube axis and comprising a generally curved or otherwise formal plate having a concave surface facing the tube envelope, a plurality of curved dynodes each directed parallel to the tube axis and comprising a curved plate whose concave surface faces the tube axis, the curved dynodes being disposed at equal angular and radial spacings around the tube axis, a plurality of flat dynodes each comprising a flat plate directed parallel to the tube axis, the flat dynodes being disposed at equal angular and radial spacings, alternately with, and on the inside of, the curved dynodes, a last-stage dynode comprising an elliptically curved plate directed parallel to the tube axis and having an opening facing the prior-stage dynodes, and an anode comprising a mesh or wire extending directed parallel to the tube axis, the anode being enclosed in the last-stage dynode, the improvement which comprises forming the first-stage curved dynode of a first curved portion and a second curved portion which is more sharply curved than the first curved portion and is smoothly joined therewith, the first curved portion being positioned far closer to the photocathode.

2. A photomultiplier tube according to Claim 1, in which said first arc has a central angle of 55 to 65 degrees and a second arc has a central angle of 85 to 95 degrees and a radius of curvature 0.4 to 0.6 times as large as that of the first arc.

3. A photomultiplier tube including a first stage dynode having first and second curved portions of different curvature and arranged so that the tube is capable of producing a more uniform anode current for a given intensity of incident light over the range of positions at which the light can impinge on the photocathode, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.