GB2106708A - Focusing structure for photomultiplier tubes - Google Patents

Abstract

In an electron discharge photomultiplier tube comprising an evacuated envelope (12) having therein an electron emissive photocathode (20) and a primary dynode (22) with a substantially flat base (29) spaced from the photocathode, the focusing means comprises a substantially flat electrically conductive structure (28) including an outer electron impermeable annular support ring and an inner annular mesh member substantially transmissive to electrons. The mesh member has a centrally disposed aperture therethrough which is substantially coextensive with the flat base of the primary dynode. <IMAGE>

Description

SPECIFICATION Improved focusing structure for photomultiplier tubes This invention relates to photomultiplier tubes and, particularly, to an improved focusing structure for a teacup photomultipliertube.

Electron emissive electrodes are used in photomultiplier tubes to emit electrons in response to impinging photons or to emit a plurality of secondary electrons for each impinging primary electron. The primary electrons can be photoelectrons from a photocathode or secondary electrons from another electrode or dynode. A problem that has been encountered in the construction of phototubes has been now to efficiently collect electrons emitted from one stage of the electron multiplier by another stage. In particular, the problem has been how to maximize the collection of electrons at the input stage of the electron multiplier, i.e., photoelectrons from the photocathode to the first or primary dynode of an electron multiplier.

U.S. Patent 4,112,325, issued to Faulkner September 5, 1978, describes a primary dynode having a relatively large area which provides a high collection efficiency for electrons emitted from the photocathode and incident thereon. This collection efficiency may be improved by the addition of a focusing electrode disposed between the photocathode and the primary dynode for focusing photoelectrons emitted from the photocathode onto the primary dynode. Such a focusing electrode is described in U.S. Patent 4,306,171, issued to Faulkner et al. on December 15, 1981. The focusing electrode operates in conjunction with an aluminized coating on an upper portion of the envelope sidewall, which operates at photocathode potential, to focus substantially all of the photoelectrons from the photocathode onto the primary dynode.

The photomulitpliertube structures disclosed in the above-cited patents comprise an envelope having a substantially uniform cylindrical sidewall. A photocathode is formed on the tube faceplate and on a portion of the tube sidewall adjacent to the faceplate. This tube structure, in conjunction with the disclosed focusing electrode, provides a relatively strong focusing or photoelectron extraction field to assure maximum collection efficiency of electrons by the primary dynode.

In many photomultiplier tubes uses, such as scintillation counting in general and gamma-ray camera systems in particular, a large number of photomultiplier tubes are used. In early gamma-ray camera systems, nineteen photomultipliertubes were arranged in a hexagonal array. By adding additional tubes to create hexagonal arrays of thirty-seven, sixty-one, or even ninety-one tubes, improved camera system resolution can be achieved. Different photomultiplier tube dimensions are also used to provide instruments with appropriate portability or coverage. Resolution of the gamma-ray camera depends principally upon the pulse-height resolution of the photomultiplier and, hence, upon the quantum efficiency of the photocathode and the collection efficiency of the primary dynode.

Large diameter photomultipliertubes, e.g., tubes having a nominal 133.4mm (5.25 inch) diameter faceplate, utilize the same size electron multiplier, operating at the same voltages, as tubes having a nominal 50.8mm (2 inch) diameter faceplate. Additionally, the longitudinal spacing between the faceplate of the tube and the focusing electrode is greater for large diameter tubes than for small diameter tubes. The transition from the large faceplate diameter to the small electron multiplier diameter is accomplished by using a modified funnel-shaped tube envelope such as that shown in Figure 1. The aluminized coating on the funnel-shaped envelope extends from the sidewall adjacent to the faceplate into the neck of the envelope.

The combination of the tapered envelope sidewall and the increased longitudinal spacing between the faceplate and the focusing electrode provides a lower electron extraction or focusing field at a given photocathode potential and focus voltage than that achieved at the same operating potentials with the straight cylindrical sidewall tube envelope disclosed in the above-cited patents. The lower extraction field means that fewer photoelectrons are focused from the periphery of the photocathode, including the sidewall portion of the photocathode, onto the first dynode. Some of the peripheral photoelectrons strike the focus electrode and do not impinge upon the primary dynode. This results in lower collection efficiency for tubes having large diameterfaceplates and funnel-shaped tube envelopes.

An electron discharge tube according to the present invention comprises an evacuated envelope having therein an electron emissive photocathode and a primary dynode having a substantially fiat base spaced from the photocathode. The primary dynode has an active area capable of emitting secondary electrons therefrom in response to the electrons impinging thereon. A secondary dynode is located adjacent to the primary dynode for receiving secondary electrons therefrom. Improved focusing means is disposed between and spaced from the photocathode and the primary dynode. The focusing means comprises a substantially flat electrically conductive structure including an outer electron impermeable annular support ring and an inner annular mesh member substantially transmissive to electrons.The mesh member has a centrally disposed aperture therethrough which is substantially coextensive with the flat base of the primary dynode.

In the drawings: Figure lisa partial cross-sectional view of a photomultipliertube incorporating a focusing structure in accordance with the invention.

Figure 2 is a plan view of the focusing structure along line 2-2 of Figure 1.

Figure 3 is a graph of a typical pulse-height distribution obtained with a thallium-doped, sodium iodide crystal and cesium 137 source.

Referring to the drawings there is shown in Figure 1 a photomultipliertube 10 comprising an envelope 12 including a funnel portion 13 having a small end, which is joined to a substantially cylindrical neck portion 14, and a large end, which is sealed to a faceplate 16 that closes one end of the envelope. A stem (not shown) closes the neck portion of the envelope. An aluminized coating 18 is disposed on the interior surface of the funnel 13 and on a portion of the interior surface of the neck 14. Within the tube 10 is a photocathode 20 on the faceplate 16 and also along a portion of the aluminized coating 18 on the funnel 13. The portion of the photocathode 20 on the faceplate 16 is semitransparent, while the portion of the photocathode 20 along the aluminized coating 18 is light-reflective.The photocathode 20 may be a potassium-cesium antimonide type, for example, or any of of a number of photoemissive types well known in the art.

Inside the tube 10 is a primary or first teacup dynode 22 preferably of a beryllium-copper material having an active oxide secondary emissive surface 24, such as beryllium oxide, which faces the faceplate 16. A substantially uniform layer 26 of an alkali antimonide compound, such as potassium-cesium-antimonide type, for example, or any of of a number of photoemissivetypes well known in the art.

Inside the tube 10 is a primary or first teacup dynode 22 preferably of a beryllium-copper material having an active oxide secondary emissive surface 24, such as beryllium oxide, which faces the faceplate 16. A substantially uniform layer 26 of an alkali antimonide compound, such as potassium-cesium-antimonide, overlies the coating 24 (as disclosed in the above-cited U.S. Patent 4,306,171). A novel apertured focusing electrode 28 is disposed in spaced relation between the teacup dynode 22 and the transparent portion of photocathode 20 on the faceplate 16. The teacup dynode 22 (as described in U.S. Patent 4,306,171) has a substantially flat base 29 and an output aperture 30 adjacent to a second dynode 32.

The second dynode 32, preferably made from beryllium-copper, acts as a receiving member for secondary electrons emitted from the teacup dynode 22. The second dynode 32 has an input aperture 34 and an output aperture 36. Secondary electrons emitted from the beryllium oxide secondary emissive surface of the second dynode 32 pass through the output aperture 36, and serve as primary electrons which impinge upon a chain or array 38 of eight beryllium-copper dynodes, consecutively numbered 40 through 47 inclusive, and an anode 48. The anode 48 is partially surrounded by an anode shield or ultimate dynode 47 of the array 38.

Each of the dynodes 40 through 47 has a beryllium oxide secondary emissive surface. Alternatively, the penultimate dynode 46 and the ultimate dynode 47 may be formed from Nichrome (as disclosed in U.S.

PatentApplication Serial No. 134,276, filed on March 26,1980 byTomasetti etal.) While a total often dynodes may be utilized in the above-described embodiment for propagating and concatenating electron emission from the photocathode 20 to the anode 48, it should be clear to one skilled in the art that additional dynodes may either be included between the second dynode 32 and the anode 48 or dynodes may be eliminated from the array. The total number of dynodes is governed, among other things, by the final gain desired from the tube.Evaporator assemblies (not shown) are provided to activate the secondary emissive surfaces of the dynodes and to form the photocathode. (Such evaporators are described, for example, in the above-cited U.S. Patent4,306,171.) The dynodes 22, 32 and 40 through 47, the focusing electrode 28 and the anode 48 have conductive wires attached thereto for placing electrostatic charges thereon. Electrical connection to the photocathode 20 is made by means of a spring contact (not shown) which is urged against the aluminzed coating 18 that extends into the neck 14 of the envelope 12. The spring contact is attached to one of the connecting wires.

The wires (not shown) terminate at the metal pins 50 located at the base 52 of the tube 10.

As shown in Figure 2, the novel focusing electrode 28 comprises a substantially flat electrically conductive structure including an outer electron impermeable annular support ring 62 and an inner annular electron transmissive mesh member 64. A large centrally disposed electron aperture 66 is formed in the mesh member 64. While the conductive structure may be formed from a single piece of metal, it is more convenient to form the structure by attaching, for example by welding, the mesh member 64 to the support ring 62. In the preferred embodiment, the annular support ring 62 comprises stainless steel having an outside diameter of about 46.74mm (1.84 inch), an inside diameter of about 28.58 mm (1.125 inch) and a thickness of about 0.38mm (0.015 inch). The mesh member 64 comprises stainless steel having a thickness of about 0.05mm (.002 inch).The mesh member 64 has a centrally disposed aperture 66 with a diameter of about 17.5mm (0.69 inch) and an electron transmissive portion, comprising a plurality of arcuate apertures 67 bounded by 3 plurality of radial supports 68 and a plurality of concentric circular members 70. The mesh member 64 is approximately 92 percent opticallytransmissive. The outermost circular member (not shown) as a width greater than the inner circular members 70 to facilitate attachment to one surface of the support ring 62. The mesh member 64 may be formed by conventional etching techniques well known in the art.The focusing electrode 28 therefore comprises a large centrally disposed electron aperture 66 having a diameter of about 17.5mum, and a substantially transmissive mesh 64 which ext ndsfrom the aperture 66 to the inside diameter of the support ring 62. While the mesh member 64 is disclosed to have a large centrally disposed aperture 66, the aperture diameter can be reduced from the preferred diameter to a smaller diameter by inwardly extending the radial supports 68 and providing additional concentric circular members 70. (The focusing structure disclosed in U.S. Patent 4,306,171 has a centrally disposed aperture equal in diameter to the present aperture 66; however, the remaining portion of the prior focusing structure is impermeable to electrons.) The novel focussing structure 28, having the electron transmissive mesh extending from the centrally disposed aperture 66 to the inside diameter of the support ring 62, provides the maximum practical electron transmission and permits the collection of substantially all of the photoelectrons emitted from the photocathode, 20, despite the weak extraction or focusing caused by the funnel-shaped configuration and size of the envelope.

The improvement in photoelectron collection efficiency is demonstrated by comparing the pulse height and pulse-height resolution of five tubes which utilized the novel (improved) focusing structure 28 and three tubes which had a prior art (standard) focusing structure in which the focusing structure is impermeable to electrons except at the central aperture.

Test Method The parameters of pulse height and pulse-height resolution are measured by optically-coupling the faceplate of the photomultiplier tube to a thallium-doped, sodium iodide crystal scintillator. A cesium 137 source provides monoenergetic (662keV) gamma rays which lose all of their energy by photoelectric conversion in the crystal. An operating voltage of about 1100 to 1500 volts is applied to the multiplier tube by means of a voltage divider of a type well known in the art. The output of the photomultipliertube is connected to and displayed on a multichannel analyzer. Atypical pulse-height distribution from a cesium 137 source and a sodium iodide crystal is graphically shown in Figure 3.A detailed description of scintillation counting may be found in The RCA PhotomultiplierHandbook (PMT-62) pp.69-72(1980). In Figure 3 herein, the energy, i.e., the pulse height (PH), is plotted along the abscissa. The photopeak of Figure 3 is associated with and centered about 662keV, the energy of the cesium 137 gamma rays.

Pulse-height resolution (PHR), in percent, is defined as 100 times the ratio of the width, A, of the photopeak at half the maximum count rate in the photopeak height, to the pulse height at maximum photopeak count rate, B, as shown in Figure 3. The smaller the value of the pulse-height resolution, the better the tube can resolve the photopeak height. The test results are summarized in the following table.

TABLE Serial No. Focus Structure PH (millivolts) PHR (percent) C33576 IMPROVED 580 6.75 C33577 IMPROVED 741 6.97 C33578 IMPROVED 847 6.76 C33579 IMPROVED 577 6.66 C33580 IMPROVED 745 6.85 Z005657 STANDARD 1054 6.89 Z005658 STANDARD 486 7.02 Z005659 STANDARD 315 7.57 The pulse-height resolution of the five tubes having the improved focusing structure 28 ranges from 6.66 to 6.97 percent, whereas the pulse-height resolution of the three tubes having the standard prior art focusing structure ranges from 6.89 to 7.57 percent. The focus electrode of each of the above tubes operated at a potential equal to the potential on the primary dynode.The test shows that the improved focusing structure 28, having the substantially electron transmissive mesh 64 disposed circumferentially around the central aperture 66, improves the pulse-height resolution of large diameter photomultiplier tubes having funnel-shaped envelopes by about 0.2 to about 0.6 percent. The addition of the substantially transmissive mesh member 64 provides a larger entrance aperture for the photoelectrons onto the active area of the primary dynode 22 than that provided by the standard focusing electrode. Furthermore, the mesh member 64 extends the electrostatic field generated by the potential on the focusing electrode 28 across the aperture 66, thereby offsetting the weak electrostatic cathode focusing field provided by the aluminum coating on the interior surface of the funnel 13. The mesh member 64 thus prevents the penetration of the cathode field into the teacup-shaped first dynode, where it would act to suppress the emission of secondary electrons from the heel area of the primary dynode 22 which is located opposite the output aperture 30. The same focusing effect cannot be obtained in the standard focusing structure at the same potential by enlarging the diameter of the central aperture. Furthermore, enlarging the aperture would be detrimental to overall tube performance, since the mesh member 64 is required to prevent the suppression of secondary electrons from the primary dynode by screening out the cathode electrostatic field. Such suppression of secondary electrons tends to decrease the gain and signal-to-noise ratio of the tube.

Claims (7)

1. An electron discharge tube comprising an evacuated envelope having therein an electron emissive photocathode, a primary dynode having a substantially flat base spaced from said photocathode, said primary dynode having an active area capable of emitting secondary electrons therefrom in response to the electrons impinging thereon, a secondary dynode adjacent to said primary dynode for receiving said secondary electrons, and focussing means disposed between and spaced from said photocathode and said primary dynode; wherein said focusing means comprises a substantially flat electrically conductive structure including an outer electron impermeable annular support part and an inner annular mesh part substantially transmissive to electrons, said mesh part having a centrally disposed electron aperture therethrough, and said electron aperture being substantially coextensive with said flat base of said primary dynode.

2. An electron discharging tube comprising an evacuated envelope including a cylindrical neck portion, a funnel portion having a small end and a large end, said small end being joined to said neck portion and said large end being closed by a faceplate, an electron emissive photocathode on an interior surface of said faceplate, a primary dynode having a substantially flat base spaced from said photocathode, said primary dynode having an active area capable of emitting secondary electrons therefrom in response to the electrons impinging thereon, a secondary dynode adjacent to said primary dynode for receiving said secondary electrons, and focusing means disposed between and spaced from said photocathode and said primary dynode; wherein said focusing means comprises a substantially flat electrically conductive structure including an outer electron impermeable annular support part and an inner annular mesh part substantially transmissive to electrons, said mesh part having a centrally disposed electron aperture therethrough, and said electron aperture being substantially coextensive with said flat base of said primary dynode.

3. Atube as claimed in Claim 1 or 2, further comprising an anode in communication with said secondary dynode and spaced therefrom, and wherein said primary dynode comprises a teacup dynode.

4. A tube as claimed in Claim 1,2 or 3, wherein said mesh part is fixedly attached to one surface of said support ring.

5. Atube as claimed in any preceding claim, wherein said mesh part has an optical transmission of approximately 92 percent.

6. A tube as claimed in any preceding claim wherein said support part of the focussing structure is a ring member having an annular mesh member attached thereto as said mesh part.

7. An electron discharge tube substantially as hereinbefore described with reference to the accompanying drawings.