US2922048A - High current coaxial photomultiplier tube - Google Patents

Description

Jan. 19, 1960 N. W. GLASS HIGH CURRENT COAXIAL PHOTOMULTIPLIER TUBE Filed Aug. 14, 1958 2 Sheets-Sheet 1 165K 6 01! 88K 45k 87k 32k Fig.3

INVENTOR. Neal W. Glass Jan. 19, 1960 N. w. GLASS 2,922,048

HIGH CURRENT COAXIAL PHOTOMULTIIPLIER TUBE Filed Aug. 14, 1958 2 Sheets-Sheet 2 I00 I I I v I I I. A 7.0 I

U) 2 4000 VOH's g on Tube E v Q 5.0 l 0 o 2: D O 4.

D o. S 3.0

2700 Volrs on Tube Relative Light INVENTOR. Neel W Glass avm 2,922,048 IHGH CURRENT COA IIJIAIIE PHOTOMULTIPLIER Neel Glass, Los Alamos, N. Mex., assignor to the United States of America as represented by the United States Atomic Energy Commission Application August 14, 1958, Serial No. 755,117

3 Claims. (Cl. 250207) This invention relates to photomultiplier tubes, and more specifically to a medium-gain photomultiplier tube having high current output, fast rise-time, and matched output impedance.

In some applications a need has arisen for a photomultiplier tube, the output of which could be fed into a long length of low impedance coaxial line and displayed directly on Oscilloscopes, the system thus maintaining fast response, large dynamic range, and good reliability, without the need for any amplifiers. One example of such an application is the measuring of brightness of an explosion by placing the phototube in the vicinity of the explosion site, and connecting the phototube by coaxial cable to instrumentation located a safe distance from the explosion site. be capable of delivering several amperes of approximately linearly rising output current at the low impedance necessary to match that of the cable, wtih a rise-time of about one millimicrosecond, and an electron current gain in the 10 to 10 region.

The principal limitation on high current output in the prior art conventional photomultiplier tubes arises as a result of space-charge limitations. The space-charge limited current density is proportional to the three halves power of the dynode voltage and to the inverse second power of the spacing, for plane parallel plates. Hence, dynode spacings must be as small, and voltages as large, as is feasible. In addition, instabilities and feedback problems become increasingly severe at higher current levels and fast pulse rise-times. Briefly, some undesirable effects may be as follows: (1) Modulation of dynode voltages during a current pulse due to insuflicient interdynode electrical capacity and/or excessive lead inductance from the dynodes to the interstage capacitors; (2) self oscillation due to resonances in the structure due to dynode lengths and sizes; (3) light feedback from the later stages and the collector to the photocathode due to fluorescence of the former under electron bombardment; (4) ion feedback from the later stages to the earlier. As will presently become apparent, the minimizing of effects (1) and (2) are obtained by built-in capacity between dynodes or providing very short leads to external capacitors; and compactness of structure. The present invention also eliminates or reduces to negligible value effects (3) and (4) by providing for a dynode configuration which not only focuses the electrons in their proper paths but also effectively baffles light and ion feedback.

It is, therefore, a prime object of the present invention to provide a photomultiplier tube having low output impedance to match that of a coaxial transmission line, large dynamic range, fast response to light stimuli, and good reliability.

This and other objectives and advantages will become apparent as this description proceeds with reference to the drawing made a part of this specification.

In the drawing: Figure 1 is a diagrammatic end plan view of a preferred embodiment, Figure 2 is a plot shownited States Patent To fill this need, the phototube should ing output characteristics of a representative tube in accordance with the present invention, and Figure 3 is a schematic circuit drawing showing one mode of connection of the tube elements.

The photomultiplier tube of the present invention com-' prises a generally elongated cylindrical housing having alternate circumferential light sensitive cathode areas and opaque areas, an elongated cylindrical anode on the axis of the housing, and a plurality of elongated dynodes and shield members in two types of arrays alternating in the annular space between the anode and the envelope. Referring to Figure 1, it is seen that the tube ofthe present invention comprises a cylindrical transparent vitreous housing 11 having alternate areas 13 and 15 arranged as light transmissive and opaque areas respectively, an anode 17 supported on the axis of the housing, and a plurality of light and ion shielding and/ or secondary 'emissive electrodes arranged in the shape between anode 17 and envelope 11. Although the elongation of the electrodes is not shown in the drawing, it is understood that such electrodes are elongated in the manner well known in the art and as shown, for example, in Patent 2,231,693, issued to R. L. Snyder on February 11, 1941. The electrodes between the envelope and the anode are arranged in two types of arrays designated A and B which are alternated throughout the annular space within the housing.

Array A is supported in a sector symmetrically about a vertical median plane passing through the axis of the anode and the center of an envelope window. This array may comprise a photo-cathode and two dynodes, or three active or target dynodes, and a shield member. Array B is supported in a sector symmetrically about a vertical median plane passing through the axis of the anode and an opaque housing area. This array also has three target dynodes and a shield member. The electrodes of arrays A and B are of such configurationthat complementary target electrodes of the two arrays ,PIO'. vide continuous electron impingement and emission paths from the cathode to the anode, as shown by electron path trajectories t and t while preventing light feedback and ion feedback from later stages and the anode to earlier stages or the photo-cathode.

For purposes of explanation, the active electrodes of array A are considered to comprise dynodes 21, 23, and 25 and shield member 27. Dynode 21 is of generally inverted trough shape having an apex portion 30 of sharp wedge or peaked shape merging into a pair of flared intermediate planar portions 32, which in turn merge into lateral wings 34. The electrons emittedfrom the photocathode 13 impinge on the outer surfaces of dynode 21 and the resulting amplified number of electrons emitted and reflected from dynode 21 are directed to the first dynode 21 of array B and from thence to the second dynode 23' of array B and then to the inner surface of dynode 23 of array A. Dynode 23 is of generally open trough shape having a bottom of spread M shape and diverging wings. The surface of the M portion facing the anode is of secondary emissive nature and cooperates with dynode 23 of array B for amplifying and directing the amplified electron beam in a generally radially inward direction to impinge on the innermost dynode 25 of array A. The wings on dynode 23 are for light and ion shielding purposes. Dynode 25 is similar in shape to dynode 21 but proportionately smaller. It has a peaked apex portion, two flaring planar portions and outwardly extending wing portions, and the configuration is such that electrons passing from dynode 23 impinge upon it, generating additional secondary electrons and the total beam current then passes to dynode 25' of array B. The configuration of dynode 25 of array B is similar to that of dynode 23 of array A and the parts'thereof serve equivalent functions. The

final totality of electrons reflected from and emitted by urated current outputs for various voltages measured between electrodes are as follows:

dynode 25 are projected to the collector electrode, i.e., the anode 17. ,The final electrode in array A is the shield member 27. This member shapes the electron trajectories between dynodefZS and anode 17, isolates dynode 25 from the anode, provides coaxial output geometry, and in addition provides illuminant shielding between the anode and the dynodes. The coaxial output geometry is obtained by connecting shield members 27 together, thereby providing aisubstantially continuous constant potential sheath with respect to anode 17 Thefinal electrode in array A is shielding member 29 which helps to shape the electron trajectories between the translucent photo-cathode window 13 and the .outwa'rdly directed surface of dynode 21.

The mode of connection for a photomultiplier tube in accordance with the present invention is shown in Figure 3. Of special note is the equivalent outer sheath ofra coaxial line formed by shield members 27. The inner conductorsof the coaxial line thus formed is anode 17 and its extending conductor.

Although the embodiment above described calls for a translucentphoto-cathode on the interior surface of the envelope, it is entirely feasible to utilize the first dynode as the photo-cathode. Where an envelope section is utilized as the photo-cathode, a conducting base is provided on the glass to reduce cathode resistivity effects. A suitable base is procurable under the trade name NESA and is manufactured by the Pittsburgh Plate Glass Company.

The dynode coating is preferably antimony-cesium in that it is believed the high current density behavior of antimony-cesium may be superior to that of silver magnesium, or copper-beryllium. s

Figure 2 shows a plot of current versus incident light for a typical tube. The light source is a xenon flash tube with'a light pulse rise-time of about a microsecond. The photomultiplier was situated end-on to the light source 'and equipped with a conical reflector.

Typical voltage configurations, electron gains, and sat- Max. Output 21 21 23 23 25 25' Shield Anode Total Total Saturation 27 17 Volts Gain Current (amps.)

Gains per stage at 2,000 volts It is seen that the optimum voltages per stage are nonuniform and that they follow an alternating pattern. At high current outputs rather large voltages are developed on the anode, hence element 27 provides a shield to prevent voltage feedback effects from the anode to the dynodes, particularly 25 and 25'.

While certain specific embodiments have been illustrated and described, it is understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A photomultiplier tube comprising an elongated cylindrical envelope, a cylindrical anode, supported at the axis of the envelope, a plurality of elongated spaced opaque areas on said enevelope and a plurality of light admitting windows therebetween, a photo-cathode supported adjacent each of said windows, a plurality of secondary emissive dynodes arranged in two types of radial arrays, which are alternately positioned to fill the annular space between the anode and the envelope, said dynodes in an array being radially staggered with respect to the dynodes in, the adjacent array, said dynodes each having a portion arranged at an angle with respect to the electron path such that electrons emitted by each cathode undergo multiplication upon impingement on a dynode and redirected flight to the next adjacent dynode.

2. The photomultiplier tube of claim 1 in which the photo-cathodes are translucent photo emissive coatings affixed to the envelope between the opaque areas.

3. The photomultiplier tube of claim lincluding a constant potential sheathsurrounding the anode and constituting with'the anode a coaxial line.

References Cited in the vfile of this patent UNITED STATES PATENTS 2,213,554 Steyskal Sept. 3, 1940 2,613,330 Bruining L. Oct. 7, 1952 2,676,282 Polkosky Apr. 20, 1954 2,702.8 Herzog Feb. 22, 1955

Images