Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-12-08
2002-10-08
Allen, Stephone (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C313S532000
Reexamination Certificate
active
06462324
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to photomultiplier devices for the detection of radiant energy. More specifically, this invention relates to a photomultiplier having a field isolation mesh that is configured to improve the electron collection efficiency of its associated dynode and the pulse height resolution and magnetic sensitivity of the photomultiplier.
BACKGROUND OF THE INVENTION
A photomultiplier device conducts and amplifies radiant energy by way of a photocathode adapted to release electrons in response to radiation, such as light, incident thereon. The photomultiplier device amplifies the incident radiation by channeling the electrons released from the photocathode through an array of secondary dynodes. Each secondary dynode has an emission surface area that is responsive to electrons incident thereon by releasing a plurality of secondary electrons for each electron impinging on the emission surface of the secondary dynode. The secondary dynodes are arrayed or aligned in cascade such that the secondary electrons emitted in one dynode are transported sequentially to the other dynodes. In this way, multiplication of the photoelectrons released from the photocathode is accomplished, thereby amplifying the initially received radiation energy.
Photoelectrons are channeled first from the photocathode of the photomultiplier to the input of a primary dynode. The dynodes, including the primary dynode, have field isolating mesh or grid sections disposed about their input apertures. The field isolating mesh sections are typically formed of electrically conductive material and may be energized at the same electrical potential as the secondary emission surface of the dynode with which it is associated. The mesh functions to draw primary electrons toward the secondary mission surface while simultaneously electrostatically shielding the secondary mission electrons from the field of the photocathode or next preceding dynode. Thus, secondary emission electrons are channeled away from the input aperture of a dynode to the output surface of the dynode. The electrons passed from the last dynode output aperture of the array are collected by an anode, providing an amplified radiant energy signal.
As can be appreciated, field isolation among dynodes is necessary for the proper functioning of the photomultiplier device. Yet, the physical structure of the known dynode mesh sections limits the efficient performance of electron multiplication. For example, the conductive members of the mesh partially obstruct the path of traveling electrons to the input aperture, which adversely affects the electron collection efficiency of the dynode and the uniformity of secondary electron emissions produced. Moreover, this obstruction impedes the uniform coating of secondary emissive materials on a dynode surface during the manufacturing process.
It is known to utilize proximity-varied density configurations with secondary dynode mesh sections to increase electron transfer efficiency and to facilitate shielding of secondary dynode walls and output apertures, such an arrangement is shown and described in U.S. Pat. No. 4,112,326. However, such non-uniform configurations fail to address the poor collection efficiency between the photocathode and the first or “primary” dynode and the lack of uniformity in secondary electron emissions released therefrom. Moreover, the known mesh configurations do not facilitate the uniform coating of the primary dynode with secondary emissive materials during the manufacturing process.
Presently, a photomultiplier is desired wherein the primary dynode field isolation mesh is dimensioned to provide a significant improvement in the primary dynode electron collection efficiency, magnetic sensitivity, as well as the pulse height resolution of the photomultiplier.
SUMMARY OF THE INVENTION
In accordance with a first aspect of the present invention, there is provided a photomultiplier tube which includes an envelope having a faceplate. A photocathode disposed in the envelope receives radiant energy incident on the faceplate of the envelope and provides photoelectrons in response thereto in the known manner. A first dynode, also disposed in the envelope has an input aperture, an output aperture, and a secondary emissive surface formed between said input and output apertures. The input aperture faces the photocathode and the secondary emissive surface is oriented for receiving the photoelectrons from the photocathode. A field isolating mesh is positioned over the input aperture of the first dynode. The field isolating mesh includes a periphery formed of an electrically conductive material and a central opening. The periphery provides an isolating electric field in the vicinity of the input aperture of the first dynode when energized. The central opening is dimensioned to provide a maximum throughput of photoelectrons from said photocathode to the secondary emissive surface of the first dynode.
In accordance with a further aspect of this invention, the field isolating mesh is formed of two segments including a first segment that is positioned in the input aperture of the first dynode. The second segment is disposed in parallel spaced relation to the input aperture and said first segment. In use, the first segment is energized at the same electric potential as the first dynode and the second segment is energized at a different electric potential, usually that of the focusing electrode used in the photomultiplier tube for focusing the photoelectrons from the photocathode onto the first dynode.
Both of the foregoing arrangements provide a significant increase in the collection efficiency of the first dynode. As a result, the pulse height resolution of a photomultiplier tube in accordance with this invention is significantly improved relative to known devices. Furthermore, the magnetic sensitivity of the photomultiplier tube according to this invention is superior to the known photomultipliers.
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Iijima et al., “Study on fine-mesh PMTs for detection of aerogel Cherenkov light”,Nucl. Instr. and Meth. in Phys. Res. A 387, (1997) pp. 64-68).
Hearty, “Detection of small light pulses and pulse height resolution of a fine mesh photomultiplier tube”,Nucl. Instr. and Meth. in Phys. Res. A 376, (1995), pp. 83-87.
Suzuki et al., “New Mesh PMTS For High Magnetic Environments”,IEEE Transactions on Nuclear Science, vol. 33, No. 1, Feb. 1986, pp. 377-380.
McComsey Sanders Carl
Venkatarao Anita Sreepadraj
Wright Joseph Warren
Allen Stephone
Burle Technologies, Inc.
Dann Dorfman Herrell and Skillman, P.C.
Hill Bradford
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